How the Way a Naphthalimide Unit is Implemented Affects the Photophysical and -catalytic Properties of Cu(I) Photosensitizers

Driven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two heteroleptic Cu(I) complexes CuNIphen (NIphen = 5-(1,8-naphthalimide)-1,10-phenanthroline) and Cubiipo (biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthroline-16-one) were prepared and compared with the novel unsubstituted reference compound Cuphen (phen = 1,10-phenanthroline). Beside a comprehensive structural characterization, including two-dimensional nuclear magnetic resonance spectroscopy and X-ray analysis, a combination of electrochemistry, steady-state and time-resolved spectroscopy was used to determine the electrochemical and photophysical properties in detail. The nature of the excited states was further examined by (time-dependent) density functional theory (TD-DFT) calculations. It was found that CuNIphen exhibits a greatly enhanced absorption in the visible and a strong dependency of the excited state lifetimes on the chosen solvent. For example, the lifetime of CuNIphen extends from 0.37 µs in CH2Cl2 to 19.24 µs in MeCN, while it decreases from 128.39 to 2.6 µs in Cubiipo. Furthermore, CuNIphen has an exceptional photostability, allowing for an efficient and repetitive production of singlet oxygen with quantum yields of about 32%.


1
Experimental Details NMR spectroscopy. Nuclear magnetic resonance (NMR) spectra were measured by the analytical service of the Institute of Inorganic and Analytical Chemistry at the Technische Universität Braunschweig at 298 K with a Bruker Avance IIIHD 500 spectrometer operating at frequencies of 500 MHz ( 1 H), 126 MHz ( 13 C) and 203 MHz ( 31 P). The spectra were then processed using the TopSpin software (version 4.1.1). All spectra are referenced against the deuterated solvent as internal standard.
Coupling constants are represented as absolute values in Hz. Characterization of the NMR signal splittings is denoted using the following abbreviations: s = singlet, d = doublet, t = triplet, dd = doublet of doublets, td =triplet of doublets and m = multiplet. Quintet splitting is described as quintet.
Mass spectrometry. Mass spectrometric (MS) measurements were performed by the analytical service of the Institute of Organic Chemistry at the Technische Universität Braunschweig. High resolution mass spectra were measured using electrospray ionization (ESI) on an LTQ-Orbitrap Velos orbitrap mass analyser from ThermoFisher Scientific. Samples were dissolved in methanol spiked with 0.1 mg/mL tetradecyltrimethylammonium bromide. MS values are given as m/z.
X-ray analysis. Data acquisition for CuNIphen: A suitable crystal was fixed on a Teflon loop mounted on the goniometer head of a Bruker Kappa APEXII dual source diffractometer. X-ray intensities were generated by a Cu-Kα Incoatec Microfocus source (wavelength λ = 1.54178 Å) and detected on a APEXII CCD-detector. Data collection, cell refinement and data reduction were established by the Bruker APEXII Software Suite. The structure of CuNIphen was solved by Direct Methods and refined with the full-matrix least squares method using the SHELXL-97 program package.
Data acquisition for Cuphen: A single crystal of suitable quality for X-ray crystallography was mounted on a Hampton loop and placed in the cold (110 K) nitrogen gas stream on the diffractometer. The intensity data were collected on an Oxford Diffraction Xcalibur EOS instrument using graphite monochromated MoK radiation. The reflections were indexed, integrated and absorption corrections were applied as implemented in the CrysAlisPro software package. 1 The structures were solved employing the program SHELXT and refined anisotropically for all non-hydrogen atoms by full-matrix least squares on all F 2 using SHELXL software. 2 During refinement and analysis of the crystallographic data the programs Mercury, PLATON, and OLEX 2 were used 3 , as well as SHELXL-97 program package.
CCDC 2165289 (CuNIphen) and CCDC 2165279 (Cuphen)  Photostability tests. UV/vis absorption spectra were acquired with an Avantes AvaSpec-ULS2048CL spectrophotometer. A 150 W xenon lamp (LOT-QuantumDesign GmbH, LSE140/160.25C) was used as light source and 0.5 OD filter was introduced for the measurements. The measurements were carried out under oxygen free conditions by using degassed acetonitrile. Under aerated conditions by using dichloromethane a 360 nm long pass filter was implemented. The samples were prepared using the same method as for the absorption and emission measurements applying sealed quartz glass cuvettes with a pathlength of 10 mm.
Nanosecond transient absorption spectroscopy. Excitation pulses were generated using a Qswitched pulsed Nd:YAG laser (Q-smart 450mJ, Quantel laser) with an output centered at 355 nm (approx. 6 ns pulse duration, repetition rate of 10 Hz). The pulses were passed through a laser line filter (CWL = 355 ± 2 nm, FWHM = 10 ± 2 nm) to ensure that the samples were only excited by 355 nm light. The power of the pump beam was about 3 mJ per pulse at the sample. The stability of the sample was verified by means of UV/Vis spectra before and after each measurement. The spectrometer used was a LP980-K spectrometer from Edinburgh Instruments, where the pump and probe beams spatially overlapped at the sample position in a perpendicular beam setup. The probe lamp was operated in flash mode (150 W ozone-free xenon arc lamp, 30 A). After passing the sample the probe light was recorded using a photo multiplier tube (Hamamatsu R928P). A standard fused silica cuvette with a layer thickness of 10 mm and a sample OD of approximately 0.3 at the pump wavelength was used in this setup. The compounds, NIphen, CuNIPhen, Cuphen and Cubiipo, were dissolved in acetonitrile (Carl Roth, ROTISOLV, UV/IR grade solvents) and in dichloromethane (Fischer Chemical, HPLC grade solvents) under inert conditions and under aerated conditions. Emission lifetime. Measurements were performed using a Q-switched pulsed Nd:YAG laser (Q-smart 450 mJ, Quantel laser) with pulse durations of approx. 6 ns at a repetition rate of 10 Hz. As excitation pulses the Nd:YAG output centered at 355 nm were used. Afterwards, the excitation light additionally passed a laser line filter (CWL = 355 ± 2 nm, FWHM = 10 ± 2 nm) to exclude excitation by light with wavelengths of lower order which can be present due to harmonic generation. The power of the pump beam was about 1.0 mJ per pulse at the sample and a sample OD of approximately 0.1 at the pump wavelength was used. The emission lifetime of the samples was measured at their respective emission maxima. The emitted light was recorded using a photo multiplier tube (Hamamatsu R928P) of the LP980 spectrometer (Edinburgh Instruments).

Singlet oxygen measurement.
For evaluation of the singlet oxygen quantum yield, the phosphorescence of 1 O2 at approximately 1276 nm was detected with a Horiba Jobin-Yvon FluoroMax Plus-C automated benchtop spectrofluorometer equipped with a 150 W Xe arc excitation lamp, a R13456 photomultiplier tube detector (190-930 nm), a liquid-nitrogen cooled DSS-IGA020L InGaAs photodiode detector (800-1550 nm) and Czerny-Turner monochromators with NIR grating blazed at 1000 nm. Absorption spectroscopy was measured with a JASCO Spectrometer V-770 and was done before and after each singlet oxygen measurement. Thus, stability of the respective complex on the timescale of the measurements was ensured. For each sample, emission spectra were recorded upon excitation at 387 nm. As a reference the whole procedure was repeated for the known standard phenalenone. The detected singlet oxygen emissions were corrected to 0 intensity at 1350 nm. After baseline correction for each measurement, the area below the signal was integrated. The respective singlet oxygen quantum yield φ( 1 O2) was calculated and referenced against the literature reported value for phenalenone of 0.98±0.02. 5 The following equation was used: In this equation, Φx is the quantum yield and the absorbance of the respective substance (reference (R) phenalenone or compound (C)) and the integral of the singlet oxygen emission. This was done for all three different optical densities and the final quantum yield was calculated as the average. 5,6 The continuous 1 O2 production measurement was carried out in a 10 mm quartz cuvette with only halffilled deuterated dichloromethane solution making sure that there is enough air/O2 inside. Between every record of the characteristic emission of 1 O2 the covered cuvette was vigorously shaken three times and the absorption spectra of the sample were obtained to ensure its photostability.

DFT calculations.
Quantum chemical calculations at the density functional theory (DFT) level were performed utilizing the ORCA program package (Version 5.0.0). 7 Geometry optimizations of the electronic ground state were conducted using the BP86 8 exchange-correlation functional for preoptimization. The B3LYP 9 hybrid functional was used for final optimization and for TD-DFT calculations. Dispersion effects were accounted for using the D3 correction by S. Grimme including the Becke-Johnson (BJ) damping. 10 The Karlsruhe's valence triple-zeta polarization functions basis sets (def2-TZVP) were applied. 11 Solvation effects were treated with the conductor-like polarizable continuum model, CPCM. 12 Optimized geometries were verified as minima on the potential energy surface by frequency calculations (analytical, B3LYP-D3(BJ)/def2-TZVP, CPCM). Visualizations of the B3LYP molecular orbitals and of the electron difference density plots were evoked using the Chemcraft software package (Version 1.8). 13

Synthetic Details
Reagents and Chemicals. All chemicals were purchased from commercial suppliers (i.e. Sigma-Aldrich, Carl-Roth and ABCR) and used as received. Acetonitrile (MeCN) and dichloromethane (CH2Cl2) were distilled over calcium hydride under argon atmosphere and stored under argon.

Synthesis of the complexes Cuphen and CuNIphen
The complexes were synthesized following a literature known protocol 15 : Into a schlenk-tube equipped with a magnetic stir bar were added tetrakis(acetonitrile)copper(I) hexafluorophosphate (74.5 mg, 0.2 mmol, 1 eq.) and (9,9-dimethyl-9H-xanthene-4,5-diyl)bis (diphenylphosphane) (115.7 mg, 0.2 mmol, 1 eq.). The vessel was attached to a reflux cooler and the whole apparatus was put under vacuum and refilled with argon three times. 20 ml of dry and degassed dichloromethane were added and the solution was refluxed for 14 hours.
The solution was then cooled to 0 °C and a solution of the respective phenanthroline-based ligand (36.0 mg phen or 80.7 mg NIphen, 0.2 mmol, 1 eq.) in 20 ml of dry and degassed dichloromethane was added drop wise very slowly (for Cuphen using a syringe pump at ca. 13 ml/h). After 30 min of stirring at 0°C the bright yellow solution was refluxed for four hours and then cooled to room temperature.
The complex was slowly precipitated by carefully adding n-hexane until the point where precipitation initiated and then stored at -20 °C overnight. The bright yellow crystalline precipitate was filtered off and washed with n-hexane and cold diethyl ether. CuNIphen was again dissolved in dichloromathane and reprecipitated with n-hexane twice to increase its purity. The solids were finally dried in vacuo.

X-ray Analysis
Single crystals in suitable quality could be obtained by preparing a concentrated solution of the respective complex in dichloromethane and over layering with a thin film of ethanol and finally with a layer of n-heptane. Crystal growth at room temperature was completed in 4-7 days.