Accessing the triplet state of perylenediimide by radical-enhanced intersystem crossing

Owing to their exceptional photophysical properties and high photostability, perylene diimide (PDI) chromophores have found various applications as building blocks of materials for organic electronics. In many light-induced processes in PDI derivatives, chromophore excited states with high spin multiplicities, such as triplet or quintet states, have been revealed as key intermediates. The exploration of their properties and formation conditions is thus expected to provide invaluable insight into their underlying photophysics and promises to reveal strategies for increasing the performance of optoelectronic devices. However, accessing these high-multiplicity excited states of PDI to increase our mechanistic understanding remains a difficult task, due to the fact that the lowest excited singlet state of PDI decays with near-unity quantum yield to its ground state. Here we make use of radical-enhanced intersystem crossing (EISC) to generate the PDI triplet state in high yield. One or two 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) stable radicals were covalently attached to the imide position of PDI chromophores with and without p-tert-butylphenoxy core substituents. By combining femtosecond UV-vis transient absorption and transient electron paramagnetic resonance spectroscopies, we demonstrate strong magnetic exchange coupling between the PDI triplet state and TEMPO, resulting in the formation of excited quartet or quintet states. Important differences in the S1 state deactivation rate constants and triplet yields are observed for compounds bearing PDI moieties with different core substitution patterns. We show that these differences can be rationalized by considering the varying importance of competitive excited state decay processes, such as electron and excitation energy transfer. The comparison of the results obtained for different PDI–TEMPO derivatives leads us to propose design guidelines for optimizing the efficiency of triplet sensitization in molecular assemblies by EISC.


Introduction
Excited states of organic chromophores with high spin multiplicities (S $ 1), such as triplet or quintet states, play a major role as reaction intermediates in a number of photophysical processes of technological relevance, including singlet ssion (SF), photodynamic therapy, and triplet-triplet annihilation photon up-conversion (TTA). [1][2][3][4][5][6][7] For instance, the performance of many optoelectronic devices, such as OLEDs, may be significantly improved if we know how to control (maximize or avoid) triplet formation and the subsequent regeneration of emissive singlet states (i.e., triplet harvesting). Recent reports have shown that the spin properties of the molecules may be crucial for the efficiency of these processes and thus the performance of optoelectronic devices relying on them. [8][9][10][11][12][13] Due to their exceptional photostability, low production cost, high molar absorption coefficients, and tunable energy levels, [14][15][16] perylenediimide (PDI, see Fig. 1a) chromophores have been suggested as alternatives to fullerene acceptors in organic solar cells and photovoltaics. 17 They have photophysical and redox properties that overcome some of the limitations of commonly employed organic solar cell materials, making them promising candidates for organic energy conversion applications, as demonstrated by the wealth of literature available on the topic. In a number of studies, PDIs have been shown to undergo singlet ssion, 18,19 which is promising for enhancing solar cell performance by creating two triplet excitons from a singlet exciton. 8,20,21 Recent reports on PDI dimers, trimers, and tetramers [22][23][24][25] show that even unsubstituted PDI has suitable energetic properties to undergo SF and that geometric arrangements for efficient SF can be found. However, it is likely that the device performance may still be considerably enhanced if we manage to understand (and control) the optical and magnetic properties of the PDI chromophores in their excited triplet or even quintet states, and optimize the covalent linkage mediating the electronic and spin communication between the PDI subunits.
While it seems evident that an investigation into the properties and formation conditions of the high-multiplicity excited states of PDI will increase our understanding, accessing these states is not straightforward due to the near-unity uorescence quantum yield of PDI. 26 Most approaches to access the PDI excited triplet state that have been presented so far have relied on bimolecular excitation energy transfer, radical pair recombination, or core substitution with groups bearing heavy atoms. [27][28][29][30][31][32] However, more recently it has been shown that the PDI excited triplet state can also be sensitized efficiently by covalent attachment of a stable radical to the chromophore, [33][34][35][36] making use of the process of radical-enhanced intersystem crossing (EISC). By varying the attachment position of the linker, linker type, and radical type, the interaction between chromophore and radical can in principle be tuned to optimize the triplet yield. However, guidelines for a rational design of such chromophore-radical systems still remain to be established. 37 Apart from the magnitude of the exchange interaction between chromophore triplet and radical (J TR ), the excited state energies may also play an important role and may limit the overall yield of EISC, e.g. by favoring other excited state decay pathways.
An overview of the relevant photophysical processes is shown in Fig. 1b. Possible excited state processes are excitation energy transfer (EET), electron transfer (ET), and enhanced internal conversion (EIC), which may effectively compete with EISC and limit its yield. In the presence of the stable radical, the chromophore triplet state is generated and may further interact with the stable radical, whereby the type of interaction depends primarily on the magnitude of the exchange interaction between chromophore triplet and radical, J TR . 4,34,[38][39][40][41] When referring to the different accessible spin states in such systems, we will adopt the Gouterman nomenclature 42 in which the rst part denotes the spin multiplicity of the chromophore and the second part the overall spin multiplicity of the chromophoreradical system. By spin coupling, (excited trip-doublet and) tripquartet states may be formed in chromophoreradical systems, while, when a second radical is attached to the chromophore, overall quintet spin states may become accessible if J TR is larger than any other magnetic interactions in the system. [43][44][45][46] Here, we use a systematic approach to explore the optical and magnetic properties of the excited singlet and triplet states of PDI chromophores with and without p-tert-butylphenoxy side groups on the PDI core. The chosen side groups can be synthetically introduced in a straightforward manner and improve the solubility of the compounds considerably. The respective chromophores are referred to as PDI 0sg and PDI 4sg and their structures are shown in Fig. 1a. Making use of radicalenhanced intersystem crossing, we show that the PDI excited triplet state can be sensitized efficiently by covalent attachment of either one or two 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) stable radicals to the imide position(s) of the PDI chromophores. These PDI-TEMPO compounds are shown in Fig. 1a and are referred to as PDI 0sg -TEMPO, PDI 4sg -TEMPO, and PDI 4sg -(TEMPO) 2 . Using transient electron paramagnetic resonance experiments (EPR), we demonstrate, that the interaction between chromophore triplet and radical falls within the strong coupling regime, where trip-quartet or trip-quintet states are formed. Femtosecond UV-vis transient absorption spectroscopy (fsTA) reveals important differences in the EISC rate constants and yields between the different PDI-TEMPO compounds with and without side groups on the PDI core. It becomes evident that faster EISC does not necessarily guarantee a higher EISC yield, highlighting the need to consider the excited state energies of the reaction partners and the varying yields of competing excited state processes for a complete description of the interaction between chromophore triplet and radical. The critical discussion and comparison of the results, leads us to propose strategies to optimize the yields of EISC in molecular three-spin systems. Fig. 1 Overview of the investigated compounds and their photophysics. (a) Chemical structures, (b) schematic overview of the photophysical processes expected to occur in covalently-linked chromophore-radical compounds assuming a non-negligible exchange interaction between chromophore triplet and radical (J TR ). Abbreviations: EICenhanced internal conversion; ETelectron transfer; EETexcitation energy transfer; EISCenhanced intersystem crossing.

Experimental
Steady state absorption and uorescence spectroscopy Steady-state absorption measurements of the samples in toluene were carried out on a Shimadzu UV-1601 UV-vis spectrometer. For all uorescence measurements, the samples were diluted substantially, corresponding to absorbances <0.1 at the excitation wavelengths. Steady-state uorescence spectra were recorded on a FluoroMax-4 uorimeter from Horiba. The raw spectra were corrected for the spectral sensitivity of the instrument and uctuations of the excitation light source.

Fluorescence quantum yields and lifetimes
Fluorescence quantum yields were determined using a C11347 absolute photoluminescence quantum yield spectrometer from Hamamatsu Photonics K. K., Japan. The same solutions as prepared for the uorescence measurements were used. Wavelengths of 480 nm or 520 nm, for compounds based on PDI 0sg or PDI 4sg , respectively, were chosen for excitation of the samples and the analysis was performed using the tools provided with the data acquisition soware. Fluorescence lifetime measurements were carried out using a FluoTime 100 uorescence lifetime spectrometer from Picoquant GmbH, Germany. The samples were excited at 470 nm and the scattering light from the excitation source was cut off with the help of a long-pass lter placed in the detection path. The instrument response function was collected (without any lters) using a solution of LUDOX® (colloidal silica) in distilled water. To obtain the uorescence decay times, iterative re-convolution of the instrument response function with a monoexponential decay function was performed in MATLAB. The model decay function was t to the experimental data using a least-squares tting approach (minimization of the residuals using a built-in trustregion-reective algorithm).

Femtosecond transient absorption
The setup has been described in more detail elsewhere. [47][48][49][50] In brief, a Ti:sapphire laser amplier system (Coherent Libra) with a repetition rate of 1 kHz, a pulse duration of 100 fs and a wavelength of 800 nm was used as the pulse source. Part of its output was used to pump a TOPAS-White non-collinear optical parametric amplier tuned to deliver pulses peaking at 530 nm. The pump power at the sample position amounted to 1 mW (i.e., 1 mJ per pulse). For probing (330-740 nm), a supercontinuum was generated in a CaF 2 plate. The pump beam diameter at the sample was 160 mm (FWHM), while the diameter of the probe beam was 100 mm. The relative polarization of pump and probe beams was set to the magic angle and the instrumental response time was $180 fs (FWHM).
Femtosecond transient UV-vis absorption (fsTA) spectra were recorded at 139 different time delays, where 50 linear time steps between À1 and 1 ps were followed by linear steps on a logarithmic scale up to roughly 4 ns. A total of 2000 spectra were acquired per time point and averaged over four successive scans. The chopper in the probe beam path was set to a frequency of 500 Hz, while that in the pump path was set to 250 Hz. For every time point, four different sets of data were collected: (i) only the white light (probe) reaches the sample, (ii) pump and probe both blocked, (iii) pump and probe both reach the sample, (iv) only the pump reaches the sample. Signals (i) and (iii) are used for the calculation of DA, while signals (ii) and (iv) are needed to account for dark signals and pump light scattering. The chirp of the white light was measured in a separate (optical Kerr effect, OKE) experiment and accounted for in the processing of the TA data. In addition, solvent spectra were recorded separately under identical conditions as the samples and subtracted from the sample data following the procedure detailed in ref. 51.
The TA data were analyzed using home-written MATLAB routines. Aer subtraction of the suitably scaled solvent background, the data were chirp-corrected by interpolation in the time domain using a function of the form where t D is the instrumental delay time and the parameters p n are obtained by tting the temporal peaks of the OKE response as a function of wavelength.
For the room temperature fsTA experiments the samples were prepared in toluene solutions with an absorbance between 0.1 and 0.4 at the excitation wavelength of 530 nm in a 1 mm cuvette. During the experiments, the sample solutions ($2 mL) were own continuously. In addition, UV-vis spectra were taken before and aer the measurements to verify the sample absorbances and conrm the absence of sample degradation during the measurement.

Electron paramagnetic resonance
For all transient EPR measurements, the samples were prepared with an absorbance of roughly 0.3 at the excitation wavelength of 535 nm, measured in a 2 mm cuvette. For the measurements at Q-band frequencies (34.0 GHz), the samples in toluene were transferred into quartz EPR tubes with an outer diameter of 1.6 mm (inner diameter of 1 mm). The solutions were rapidly frozen in liquid nitrogen before insertion into the EPR resonator for the measurements.
Transient EPR measurements were carried out on a Bruker ELEXSYS E580 spectrometer operated at the Q-band and equipped with a Bruker EN 5107D2 resonator. During the measurement, the sample was kept at a constant temperature of 80 K using an Oxford Instruments nitrogen gas-ow cryostat (CF 935). The samples were excited through the top of the sample holder with depolarized light at 535 nm using an optical ber. The excitation energy was 0.5 mJ at a repetition rate of 50 Hz (pulse duration $5 ns).
Echo-detected eld-swept EPR spectra were recorded using the sequence hn À DAF À p 2 À s À p À s À echo with a delay-aer-ash (DAF) of 9 ms, s ¼ 140 ns and a p pulse length of 40 ns. The DAF value was chosen to maximize the signal intensity. The repetition rate of the experiment is determined by that of the laser. Transient nutation measurements used the sequence hn À DAF À z À s À p À s À echo where the ip angle z was gradually increased by increasing the corresponding microwave pulse length in steps of 2 ns, starting at 20 ns, i.e., p 2 . At every magnetic eld position within the region of the transient spectrum, the integrated echo intensity was then recorded as a function of this pulse length. The magnetic eld step size was set to 0.1 mT (eld scan range of 70 mT, 701 points). The whole data set was background corrected using a polynomial background function. The (cross-term averaged) Fourier transform was then calculated aer dead-time reconstruction, windowing using a Hamming window, and zero lling to 2048 data points. The frequency spectra were normalized by division of the frequency axis by the reference frequency u 0 obtained for the same sample in the dark (doublet multiplicity).

UV-vis characterization
Fig . 2 shows the UV-vis absorption and steady state uorescence spectra of the two PDI chromophores, PDI 0sg and PDI 4sg , measured in toluene solution at room temperature. It can be seen that the UV-vis absorption spectrum of PDI 4sg is considerably red-shied due to the presence of the four p-tert-butylphenoxy side groups on the PDI core, as also predicted by TD-DFT calculations presented in the ESI. † The strongest absorption peak of the S 0 / S 1 transition shis from 527 nm in PDI 0sg to 571 nm in PDI 4sg . Compared to the UV-vis absorption spectrum of PDI 0sg , the transitions in the spectrum of PDI 4sg appear to be signicantly broadened, potentially indicating a larger conformational space or a stronger solute-solvent interaction. As known from the literature 15,16 and conrmed by DFT, the presence of the side groups also induces a signicant twist of the PDI core. Both PDI chromophores are highly uorescent with uorescence quantum yields of 0.97 measured for PDI 0sg and 0.92 for PDI 4sg . The maximum of the uorescence spectrum is located at 535 nm for PDI 0sg and 604 nm for PDI 4sg , which corresponds to Stokes shis of 284 cm À1 and 957 cm À1 , respectively.
Although PDI 4sg has a lower uorescence quantum yield, its uorescence lifetime, measured by time-correlated single photon counting, is signicantly increased compared to that of PDI 0sg . A monoexponential t to the data in toluene (Fig. S2 †) yielded values of 3.95 AE 0.05 ns for PDI 0sg and 5.94 AE 0.05 ns for PDI 4sg . Literature data are available for PDI 0sg and agree very well with the photophysical characterization presented here. 27,52 The uorescence lifetimes measured for the two PDI chromophores are also consistent with the values obtained from a Strickler-Berg analysis. 53,54 The analysis, presented in the ESI, † resulted in uorescence lifetimes of s SB F ¼ 3.9 ns for PDI 0sg and 7.9 ns for PDI 4sg , assuming molar absorption coefficients of 7.1 Â 10 4 M À1 cm À1 for PDI 0sg 52 and 3.1 Â 10 4 M À1 cm À1 for PDI 4sg at the maximum of the respective absorption spectrum in the visible region. The latter value determined here for PDI 4sg in toluene, is in good agreement with data published previously for similar PDIs. 14, 55 For PDI 0sg , the calculated uorescence lifetime is very close to the experimentally measured value of 3.95 ns, indicating that the assumptions implicit in the Strickler-Berg analysis are well satised and that the absorbing and emitting electronic states are the same. For PDI 4sg , the trend is predicted correctly but the agreement between the experimental (5.94 ns) and calculated (7.0 ns) values is less satisfying. A possible reason for this could be a relatively large geometry change in the excited state (i.e., Condon approximation only partially fullled), which would be consistent with the signicantly increased Stokes shi in PDI 4sg as compared to PDI 0sg .
Due to the low molar absorption coefficient of TEMPO (3 470nm ¼ 10.5 M À1 cm À1 ) 56 compared to PDI, the UV-vis absorption spectrum of PDI 0sg -TEMPO is virtually identical to that of its parent compound PDI 0sg . In the case of PDI 4sg -TEMPO and PDI 4sg -(TEMPO) 2 we observe a slight red-shi of the absorption bands compared to the spectrum of PDI 4sg (see Fig. S1 †). Although the absorption of TEMPO itself does not contribute to the spectral shape, the presence of the TEMPO radical substituents may inuence the solute-solvent interactions which may result in the observed spectral red-shi of up to 6 nm for PDI 4sg -(TEMPO) 2 in toluene.
While the uorescence quantum yields of the parent PDI chromophores are similar, the uorescence quantum yields F F of the three PDI-TEMPO compounds differ signicantly. PDI 4sg -TEMPO has a relatively high uorescence quantum yield of 0.23 (23%). In contrast, that of PDI 0sg -TEMPO is roughly a factor of ten lower ($0.02). The attachment of a second nitroxide radical to the second imide position of PDI in PDI 4sg -(TEMPO) 2 also has a major effect on the uorescence quantum yield, which from 0.23 in PDI 4sg -TEMPO to 0.04 in PDI 4sg -(TEMPO) 2 (see also Table 1).
In line with the uorescence quantum yields, also the uorescence lifetimes of the PDI-TEMPO compounds are reduced signicantly with respect to those of their parent chromophores. The uorescence lifetimes correspond to the S 1 state deactivation time constants and will be discussed in more detail further below. They reect the same trend as observed for the uorescence quantum yields.

Femtosecond transient UV-vis absorption
To characterize and compare the excited state kinetics of the different PDI systems, fsTA experiments were carried out for all compounds in toluene solution at room temperature. Contour plots of the data obtained for PDI 0sg and PDI 0sg -TEMPO are shown in Fig. 3, while the data for all remaining compounds can be found in the ESI (Fig. S4). † The photophysics of PDI derivatives is well-known and has been studied extensively. 28,34,[57][58][59][60] In agreement with previous results for unsubstituted PDIs, we observe an intense ground state bleach between $400 and 550 nm for PDI 0sg . In the center of the spectrum ($530 nm), the ground state bleach overlaps with stimulated emission extending from 500 to $660 nm, in line with the uorescence spectrum of PDI 0sg shown in Fig. 2. Ground state bleach and stimulated emission are accompanied by a broad excited state absorption extending almost over the entire visible range. The ground state bleach recovers only partially within the detection time window of $3 ns, corresponding to an S 1 state decay in the nanosecond range, in agreement with the measured uorescence lifetime of 4.0 ns.
When a TEMPO radical is attached to the imide position of the PDI in PDI 0sg -TEMPO, the decay of the excited singlet state is found to be markedly accelerated. Aer about $60 ps, the features corresponding to the S 1 state have disappeared and we observe the formation of new excited state absorption signals at 480 and 510 nm, which can be attributed to the excited triplet state of PDI. 27,28 Furthermore, the stimulated emission signal has vanished, indicating the depletion of the S 1 state. The signals present aer $60 ps are long-lived and do not decay within the detection time window, indicating an excited state lifetime of microseconds (or longer).
We can thus conrm that the PDI excited triplet state is formed rapidly in PDI 0sg -TEMPO aer photoexcitation by enhanced intersystem crossing in the presence of the covalently attached TEMPO radical. A similar observation is made for PDI 4sg -TEMPO and PDI 4sg -(TEMPO) 2 as shown in Fig. S4. † To determine the time constants of the excited state deactivation, a global kinetic analysis of the fsTA data of the PDI-TEMPO compounds was carried out and the results (including the decay associated spectra) are presented and discussed in the ESI. † We found that three time constants were necessary in all cases to reproduce the experimental data satisfactorily. The third time constant, representing the triplet state decay, was xed to 1 ms in the analysis of the spectra while the rst two time constants, attributed to (i) vibrational relaxation in the S 1 state and (ii) deactivation of the S 1 state, were le to vary freely. In the case of PDI 0sg -TEMPO, we obtained an S 1 state deactivation time constant of 27 ps, while time constants of 110 ps and 47 ps were obtained for PDI 4sg -TEMPO and PDI 4sg -(TEMPO) 2 , respectively.
Compared to PDI 4sg -TEMPO, the fsTA data of which was already published previously, 36 the deactivation of the excited   singlet state is found to be faster in PDI 0sg -TEMPO by a factor of about four. The S 1 state deactivation is also accelerated (by a factor of roughly two) when attaching a second TEMPO radical to the system in PDI 4sg -(TEMPO) 2 . When carefully analyzing the recovery of the ground state bleach, it is possible to determine the triplet/EISC yield from the fsTA data of the different PDI-TEMPO systems 37 as illustrated in Fig. S7 in the ESI † using PDI 0sg -TEMPO as an example. We accounted for the contribution of stimulated emission to the apparent amplitude of the ground state bleach by considering the shapes of the steady state UV-vis absorption and uorescence spectra. For the determination of the amplitude of the ground state bleach, the contribution from stimulated emission was subtracted from the spectra. The yields were then determined by considering the amplitude of the ground state bleach directly aer photoexcitation (0.2 ps) and comparing this value to the amplitude of the ground state bleach aer complete deactivation of the S 1 state, accounting also for partial overlap with excited state absorption signals. The deactivation of the S 1 state was considered to be complete at a time delay corresponding to 5 Â s S1 . The spectra used for this determination of the triplet yields are shown in Fig. 4 together with a visual comparison of the kinetics of triplet state formation for PDI 0sg -TEMPO, PDI 4sg -TEMPO, and PDI 4sg -(TEMPO) 2 . It can be seen that while triplet state formation is much faster in PDI 0sg -TEMPO, the triplet yield is comparable to that of PDI 4sg -TEMPO and PDI 4sg -(TEMPO) 2 . The corresponding calculated yields for the three compounds in toluene are listed in Table 1.

Discussion of the triplet state formation yields
The latter nding, that the triplet yield is not signicantly higher in PDI 0sg -TEMPO despite a much faster deactivation of the S 1 state, is unexpected and demands an explanation. As illustrated in Fig. 1b, different competitive decay processes are possible in the excited state and may limit the triplet state formation yield. In particular, we will need to consider the likelihood of electron and excitation energy transfer in the excited state and possible differences between PDI 0sg -TEMPO and PDI 4sg -TEMPO that may have an impact on the yield of these decay processes.
In a previous study on a similar PDI-TEMPO compound with two phenoxy sidegroups, 33 PDI 2sg -TEMPO, electron transfer was found to occur in tetrahydrofuran solution at room temperature with a time constant of 1.2 ps, followed by a fast decay of the excited state absorption to the ground state with a time constant of 86 ps (i.e., no triplet formation). In toluene, on the other hand, triplet formation by enhanced intersystem crossing was found to occur with a time constant of 45 ps, well in line with the observations made here for PDI 0sg -TEMPO and PDI 4sg -TEMPO. The absence of electron transfer in toluene was rationalized by the stabilization of the ion pair state in polar solvents.
To determine whether electron transfer is possible in PDI 0sg -TEMPO, PDI 4sg -TEMPO, and PDI 4sg -(TEMPO) 2 we investigated the inuence of the solvent polarity on the excited state reaction rate constants and yields. To this end, room temperature fsTA data were also recorded using 2-methyltetrahydrofuran as the solvent, which has a signicantly higher dielectric constant than toluene (3 r ¼ 7.0 vs. 2.4). 61 The corresponding data are shown in Fig. S8 † and the spectra and kinetics of the three compounds are compared in Fig. S9. † The formation of the PDI anion by photoinduced electron transfer should be detectable in fsTA experiments, since the corresponding signatures are well known. The radical anion of PDI 0sg (PDI À 0sg ) has its rst absorption peak near $710 nm, 62,63 while PDI À 2sg absorbs around $725 nm. 64 In PDI À 4sg this peak is further red-shied to $780 nm. 58 If electron transfer occurred for PDI 0sg -TEMPO and was followed by recombination to the ground state, the sharp anion absorption band around 710 nm should be clearly visible in the spectra. For PDI 4sg -TEMPO, the maximum of the PDI anion absorption peak at 780 nm is outside the standard detection window of our UV-vis fsTA setup. For this reason, we acquired additional fsTA data using a setup with an extended detection window. These data for PDI 4sg -TEMPO were already published earlier 36 but are shown again in Fig. S12 † for completeness. When comparing the shapes of the spectra of the two PDI-TEMPO compounds, recorded a few picoseconds aer photoexcitation, with those of their parent chromophores, 58 no signicant differences are observed in either of the two solvents. In addition, no clear spectral signatures indicating electron transfer could be discerned.
The short time constant of photoinduced electron transfer observed previously for PDI 2sg -TEMPO in tetrahydrofuran 33 is in contrast to the absence of any PDI anion signatures in the fsTA spectra of both PDI 0sg -TEMPO and PDI 4sg -TEMPO. In search for an explanation, we calculated the free energies (driving forces for electron transfer, ÀDG 0 ) of the hypothetical charge separated states of the three different PDI-TEMPO compounds (including PDI 2sg -TEMPO) and considered also the reorganization energies in our detailed discussion in the ESI. † It is found that the driving forces for charge separation are higher in 2-methyltetrahydrofuran by about 0.5 eV as compared to toluene, but this larger driving force is roughly compensated by the larger outer sphere reorganization energy so that the electron transfer reaction could theoretically proceed almost barrierless ÀDG 0 x l in both solvents.
When considering all land DG 0 -dependent terms in the expression of the Marcus rate equation for photoinduced electron transfer 65,66 it seems that electron transfer should be fast and equally feasible for all three considered PDI chromophores (PDI 0sg , PDI 2sg , and PDI 4sg ). Although these types of calculations necessarily rely on many assumptions and estimated values, it is puzzling why electron transfer does not seem to occur for any of the PDI-TEMPO compounds investigated here.
Two different explanations for this apparent inconsistency could be invoked. (i) The rate constant for electron transfer further depends on the square of the coupling matrix element jH AB j 2 (see eqn S12 in the ESI †), describing the coupling between the electron donor and acceptor states. 67,68 The value of H AB is difficult to estimate reliably but could be responsible for a considerably smaller rate constant than would be expected based on our considerations of the reorganization energies and driving forces. Considering the short distance between electron donor and acceptor, there is no obvious reason to assume that H AB should be particularly small in our PDI-TEMPO systems. However, the saturated carbon atoms of the TEMPO radical will exponentially reduce the coupling between the nitroxide group and PDI. In addition, an inspection of the HOMO and LUMO orbitals of PDI 0sg and PDI 4sg , as shown in the ESI in Fig. S16, † reveals that a nodal plane runs through the axis connecting the two imide nitrogen atoms in PDI where the TEMPO radical is attached, potentially signicantly reducing jH AB j. However, this argument cannot explain the observations made previously for PDI 2sg -TEMPO in tetrahydrofuran, 33 since jH AB j should be affected in a similar way.
(ii) An alternative explanation for the absence of any clearly identiable PDI radical anion signatures in the spectra of both PDI 0sg -TEMPO and PDI 4sg -TEMPO could be that electron transfer does indeed occur for all PDI-TEMPO systems, as predicted, with a rate constant of $k EISC but is directly followed by fast charge recombination (CR) with k CR $ 2-3 Â k EISC to the chromophore triplet state, so-called "inverted" kinetics. Based on the spectroscopic S 1 state energies, the driving forces for charge recombination (i.e. energies of the charge-transfer, CT, states) computed from experimental redox potentials, and the calculated vertical triplet energies obtained from TD-DFT calculations (see the ESI †), the charge recombination to the chromophore triplet state should be exergonic. For PDI 0sg -TEMPO, for instance, these energies amount to E S1 ¼ 2.33 eV, E CT ¼ 1.80 eV, and E T1 ¼ 1.45 eV. Triplet formation via charge recombination thus seems to be a viable option which will be discussed in more detail further below.
Apart from electron transfer, another possible excited state decay mechanism to be considered here is excitation energy transfer (EET). In this context, two different mechanisms are frequently invoked, referred to as Förster-type and Dexter-type excitation energy transfer. [69][70][71] Both mechanisms require spectral overlap of the uorescence spectrum of the energy donor (PDI) and the absorption spectrum of the energy acceptor (TEMPO). While Dexter-type energy transfer is based on an exchange mechanism, Förster-type energy transfer relies on a dipolar mechanism. The rate constants for Dexter energy transfer are difficult to predict, but the Förster excitation energy transfer rate constants can be calculated in a straightforward manner, assuming the validity of the point-dipole approximation. 69 Although the calculated rate constants are likely not accurate because the center-to-center distance between chromophore and radical is short compared to their individual molecular sizes, a good estimate of the feasibility and importance of excitation energy transfer can be gained. The calculation is shown in the ESI † and reveals that the contribution from excitation energy transfer can become signicant, even though the TEMPO radical absorbs only weakly in the visible range. Assuming a collinear orientation of the transition dipole moments (k 2 ¼ 4), the results further show that energy transfer could still be competitive for PDI 0sg -TEMPO with an approximated Förster time constant of 190 ps, while the energy transfer quenching efficiency signicantly drops for PDI 4sg , associated with a Förster time constant of 1.5 ns. The latter is a consequence of the red-shied uorescence spectrum of PDI 4sg , signicantly reducing the spectral overlap of the TEMPO absorption and PDI emission as illustrated in Fig. S3. † In a previous study, 37 the occurrence of EET could be conrmed by the observation of the excited state absorption signatures and corresponding ground state bleach of the radical in fsTA experiments. Unfortunately, this is not possible in the present case, since the excited state absorption of the TEMPO radical, potentially buried underneath the intense PDI signatures, is too weak to be detected. However, the Förster time constants calculated for the two PDI-TEMPO compounds strongly suggest that excitation energy transfer cannot be neglected for PDI 0sg -TEMPO: it might be a major excited state deactivation channel and could account for the much smaller S 1 state deactivation time constant and lower uorescence quantum yield as compared to PDI 4sg -TEMPO.
Finally, apart from ET and EET, enhanced internal conversion (EIC) certainly is a viable deactivation pathway for all of the investigated PDI-TEMPO systems. It can be assumed that the yield of EIC is given as

Transient EPR spectroscopy
Transient EPR spectroscopy was employed to characterize the magnetic properties of the photogenerated spin states and the coupling strengths between chromophore triplet and radical in the three PDI-TEMPO compounds. Fig. 5 shows a comparison of the eld-swept echo-detected EPR spectra recorded for PDI 0sg -TEMPO and PDI 4sg -TEMPO in frozen toluene solution at the Q-band aer light irradiation at 535 nm. PDI 4sg -TEMPO was previously characterized by pulse EPR spectroscopy 36 and its transient EPR spectrum could be assigned to the trip-quartet state of PDI 4sg -TEMPO by transient nutation experiments. The rather unusual, complex central feature was shown to arise from an additional splitting of the quartet sublevels by the hyperne interaction of the 14 N nucleus of the TEMPO moiety. When including the nitrogen hyperne coupling in a simulation of the EPR spectrum using a spin Hamiltonian approach and assuming the magnetic interaction parameters (g tensor, hyperne coupling tensor) expected for a quartet state, the spectral shape could be qualitatively reproduced, conrming the assignment of the transient spectrum to a pure quartet state. Since the spectral features of the transient EPR spectra Fig. 5 Comparison of the field-swept echo-detected quartet state EPR spectra recorded for PDI 0sg -TEMPO and PDI 4sg -TEMPO in frozen toluene solution at 80 K together with a visualization of the spin densities of the corresponding trip-quartet states (iso value 0.001) as predicted by DFT calculations. The spectra were recorded at the Qband (34 GHz) after light irradiation at 535 nm (0.5 mJ, 50 Hz) and the dark state signal of the TEMPO radical was subtracted.
shown in Fig. 5 are very similar, it can be assumed that tripquartet states are also formed in PDI 0sg -TEMPO. The interaction between the PDI excited triplet state and the TEMPO radical thus falls within the strong coupling regime, indicating that the exchange interaction between the two spin centers is larger than any other magnetic interactions in the system. When analyzing the shape of the trip-quartet spectra recorded for PDI 0sg -TEMPO and PDI 4sg -TEMPO, one notices that the outer wings of both EPR spectra (multiplet polarization) show the same sequence of absorptive (a) and emissive (e) transitions: on going from low to high eld, an aeeaae multiplet polarization pattern is observed in both cases. This spin polarization pattern is rather unusual since it cannot be obtained by "normal" (i.e., spin-orbit coupling induced) intersystem crossing, suggesting a triplet state formation mechanism involving more than one step. In the case of triplet states, the aeeaae polarization pattern is indicative for a selective population of the high-eld triplet jT 0 i state for each canonical orientation, 72 and was previously observed for triplet states resulting from singlet ssion but also for triplets formed by radical ion pair recombination. 2,[73][74][75][76] In the case of quartet states, an aeeaae multiplet polarization pattern indicates a selective population of the high-eld quartet levels with m S ¼ AE which could be consistent with the hypothesis that electron transfer occurs in these PDI-TEMPO systems and is followed by rapid charge recombination to the chromophore triplet state. We propose the following mechanism: (i) electron transfer occurs from 2 TEMPOc to 1 *PDI leaving 2 PDIc À and the diamagnetic 1 TEMPO + ; (ii) charge recombination yields 3 PDI and 2 TEMPOc in an overall tripdoublet state as a result of the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism; 77 (iii) due to the exchange interaction J TR between chromophore triplet and radical, the trip-doublet and trip-quartet states are split in energy; (iv) the trip-quartet state is populated from the tripdoublet state via a spin-exchange mechanism 78,79 and spin conservation will lead to an overpopulation of the quartet sublevels with m S ¼ AE Compared to the light-induced EPR spectrum of PDI 4sg -TEMPO, the width of the spectrum of PDI 0sg -TEMPO is markedly increased due to an increased localization of the tripquartet state wavefunction in the absence of the four side groups on the PDI core. This difference in the spin density distribution is also conrmed by DFT calculations as shown in the ESI † and graphically illustrated in Fig. 5, where it can be seen that the oxygen atom of the phenoxy linker still carries signicant spin density. The spectral widths amount to 78 mT and 58 mT for PDI 0sg -TEMPO and PDI 4sg -TEMPO, respectively. Since the spectral width of a quartet state is given as 4 jD Q j, this corresponds to experimental zero-eld splitting jD Q j values of 545 MHz and 405 MHz.
From the D value of the trip-quartet state, the interaction strength between chromophore triplet and radical (D TR ) can, in principle, be calculated from the relation For a PDI without any core substituents, the triplet jDj value has been determined before and amounts to 45.5 mT, i.e., 1275 MHz, 81 which would result in jD TR j ¼ 3 jD Q j À jD T j ¼ 360 MHz. From a DFT model of the structures (see the ESI †), a centerto-center distance between PDI and TEMPO of $0.99 nm was obtained. However, the effective coupling distance between the two spin centers is likely to be considerably shorter than the center-to-center distance, due to a signicant delocalization of both the radical and triplet spin densities.
A calculation, based on a simple point-dipole model, of the coupling between triplet and radical (D TR ) yields D TR ¼ À80 MHz for a distance of r TR ¼ 0.99 nm, according to where m 0 , g e , b e and h are the vacuum permeability, the electronic g value, the Bohr magneton and the Planck constant, respectively, and r TR is the distance between triplet and radical. A value of À360 MHz, as estimated above, would thus correspond to an effective coupling distance of 0.60 nm in PDI 0sg -TEMPO. For a PDI chromophore with two phenoxy side groups on the PDI core, PDI 2sg , an experimental triplet jD T j value of 40 mT was determined. 82 Based on this and the knowledge of the value for PDI 0sg , we estimate the experimental jD T j value for PDI 4sg to be roughly 35 mT, i.e., 981 MHz. This consideration would yield D TR ¼ À234 MHz, corresponding to r TR ¼ 0.69 nm. A slightly stronger effective coupling can thus be assumed for PDI 0sg -TEMPO.
Regarding the PDI compound with two nitroxide radicals, PDI 4sg -(TEMPO) 2 , we would expect an overall quintet spin state to be formed upon photoexcitation in the case of strong coupling between the two radicals and the PDI triplet state. However, before interpreting the excited state behavior of the molecule, we rst needed to verify the absence of signicant through-bond (exchange) coupling between the two nitroxides in the ground state. To this end, a double electron-electron resonance experiment was carried out in frozen toluene solution at 80 K. The experimental details are given in the ESI † and the data are shown in Fig. S15. † A model-free analysis of the obtained dipolar evolution trace based on Tikhonov regularization using the program DeerAnalysis 83,84 yielded a narrow distance distribution centered at 2.08 nm. This value is in very good agreement with the distance that would be expected based on a DFT model of the structure (2.10 nm, see the ESI †), con-rming the structural model and indicating that any signicant contribution of exchange coupling can likely be excluded. Any weak exchange coupling between the two nitroxides, of the order of the dipolar coupling, would alter the detected modulation frequency and corresponding Pake pattern of the trace. [85][86][87][88] The fact that the obtained distance distribution is relatively narrow further points towards a high rigidity of the molecular structure.
To explore whether overall quintet spin states are formed aer photoexcitation of PDI 4sg -(TEMPO) 2  As can be seen from Fig. 6, the EPR spectrum of PDI 4sg -(TEMPO) 2 measured aer photoexcitation, has a close resemblance with that of PDI 4sg -TEMPO. Since the quintet state EPR spectrum is expected to have a reduced spectral width and smaller nitrogen hyperne coupling constants leading to a narrower feature in the center of the spectrum, this observation suggests that most of the observed spectral features can be attributed to the trip-quartet state, rather than the trip-quintet state. In particular, the prominent central feature can clearly be attributed to a quartet state as revealed by the transient nutation frequency map shown in Fig. S14. † Although quartet state features seem to dominate the appearance of the transient EPR spectrum of PDI 4sg -(TEMPO) 2 , small differences in the spectral shape are observed between PDI 4sg -TEMPO and PDI 4sg -(TEMPO) 2 that are likely due to quintet formation. To conrm the formation of quintet states, we focused on the magnetic eld position showing the largest spectral difference and compared the transient nutation data to those obtained at a eld position where only quartet state signals are expected (outer wings).
According to u mS;mSþ1 transition of the quartet state and the jAE 1i 4 jAE 2i transition of the quintet state, so a frequency of ffiffiffi 3 p u 0 would be expected for a quartet state, while a quintet state would be characterized by a frequency of 2u 0 . These frequencies are also indicated in the frequency plot in Fig. 6. The good agreement between the measured and expected frequencies conrms quintet state formation in PDI 4sg -(TEMPO) 2 . However, the quintet formation yield appears to be relatively low for reasons that could not be revealed with the available methods. This observation will require further investigation in the future. Assuming that the sample is pure, as suggested by thin-layer chromatography and quantitative EPR experiments, it might be that only certain conformations of the molecule (i.e., mutual orientations of the TEMPO moieties) lead to quintet formation. Alternatively, although the presence of two TEMPO radicals per PDI unit is consistent with quantitative EPR as shown in Fig. S13, † a small amount of sample degradation cannot be excluded and might also be responsible for the residual quartet signal observed here for PDI 4sg -(TEMPO) 2 , keeping in mind that the signal intensities of spin polarized species are not proportional to concentration.

Conclusions
In conclusion, the excited state kinetics and yields as well as the magnetic properties of three PDI-TEMPO compounds have been investigated by optical and transient EPR spectroscopies. The TEMPO radical was directly attached to the imide position of the PDI in all cases, but the structures differ with respect to the number of p-tert-butylphenoxy side groups (none vs. four) and the number of TEMPO substituents (one vs. two). We could show that the PDI excited triplet state, difficult to access by other methods, can be efficiently sensitized aer photoexcitation of the compounds in the presence of the TEMPO radical. The triplet yields were determined by femtosecond UV-vis transient absorption spectroscopy, while transient EPR experiments revealed strong coupling between the PDI excited triplet state and the TEMPO radical, leading to the formation of tripquartet (or trip-quintet) states.
The presence of the side groups on PDI induces a core twist and enhances exciton delocalization, reected by a red-shi in the UV-vis absorption spectrum and the narrowing of the transient EPR spectrum of the trip-quartet state of PDI 4sg -TEMPO compared to PDI 0sg -TEMPO. Interestingly, the excited state reaction kinetics were found to be inuenced considerably by this core substitution: triplet formation is about four times slower in PDI 4sg -TEMPO as compared to PDI 0sg -TEMPO. However, the triplet yield was shown to be almost identical. When the experiments were performed in 2-methyltetrahydrofuran solution instead of toluene, the S 1 state deactivation rate constants were found to be smaller and the triplet yields are considerably reduced.
Calculations of the electron transfer rate based on driving forces and reorganization energies suggest that electron transfer should be fast and equally feasible for all investigated PDI-TEMPO compounds both in toluene and 2-methyltetrahydrofuran. Nevertheless, no PDI anion signatures were observed in the fsTA spectra, which led us to propose that electron transfer in these systems is followed by rapid charge recombination to the chromophore triplet state. The latter hypothesis is also supported by the unusual polarization pattern observed in the transient EPR spectra of the corresponding trip-quartet states, clearly suggesting a multi-step process. This new triplet formation mechanism proposed here, referred to as "sequential EISC", will need to be further explored and experimentally conrmed in future studies by investigating different tripletradical systems.
Calculations of the rate constants of Förster-type excitation energy transfer suggest that energy transfer is competitive with EISC for PDI 0sg -TEMPO, while such a contribution can virtually be excluded for PDI 4sg -TEMPO. Energy transfer as an additional excited state deactivation channel in PDI 0sg -TEMPO might account for the faster S 1 state deactivation and much lower uorescence quantum yield of PDI 0sg -TEMPO as compared to PDI 4sg -TEMPO.
The presence of a second TEMPO substituent, attached to the second imide position of the PDI chromophore, was shown to enhance the deactivation of the excited singlet state by a factor of about two, accompanied by an increase of the triplet yield and considerable decrease of the uorescence quantum yield in toluene. DEER experiments showed that the exchange coupling between the two radicals in PDI 4sg -(TEMPO) 2 is close to zero in the ground state. However, upon photoexcitation, a trip-quintet state is formed as revealed by transient nutation experiments in frozen toluene solution.
The present study shows that faster triplet formation in photoexcited chromophore-radical compounds does not guarantee a higher triplet yield, since competing deactivation processes, like excitation energy transfer or electron transfer, if they can occur, are oen faster. As the choice of easily functionalizable stable radicals is limited, most effort to suppress these unwanted deactivation processes will need to be directed at the identication of suitable chromophores. Electron transfer can be suppressed, to a large extent, by tuning the redox potentials of the chromophore by core substitution. Core substitution, especially with electron donating groups, can also result in a considerable red-shi of the UV-vis absorption spectrum of the chromophore and may reveal as an important strategy to limit the yield of excitation energy transfer by reducing the spectral overlap between chromophore emission and radical absorption. This and a previous study 37 suggest that excitation energy transfer is oen the main factor limiting the yield of EISC in strongly-coupled photogenerated multi-spin systems. Consequently, regarding the design of novel chromophore-radical systems with high triplet yields, we believe that it will be essential to reect on strategies to limit EET as a possible deactivation pathway. Apart from core substitution with ethers or amines, another strategy could be to add the linker in a position that is perpendicular to the orientation of the transition dipole moment of the chromophore. For PDI chromophores, this would imply connecting the linker to the core of the PDI, rather than the imide position.
Our results further indicate that fast electron transfer may also be benecial for triplet formation in chromophore-radical compounds if charge recombination is fast and an energetically low-lying triplet state of the chromophore is available. This strategy termed "sequential EISC" will be further explored by us in the near future.
In general, we believe that modular chromophore-radical compounds are ideal model compounds for the study of spindependent light-induced processes. As demonstrated here, they provide access to excited states with high spin multiplicities, like triplet and quintet states, which are key intermediates in photophysical processes for solar energy conversion, such as singlet ssion or triplet-triplet annihilation photon upconversion. Future studies will reveal whether the study of the properties of these states, enabled by EISC, can provide new insight into the mechanisms underlying these processes.

Data availability
The data supporting the ndings of this study are available within the article and in the ESI. †

Conflicts of interest
There are no conicts to declare.