many studies have been 
 devoted to organic semiconductors largely 
 because of their significant application potential in optoelectronic 
 devices that results from their useful intrinsic properties combined 
 with low manufacturing costs .
strategies to improve the efficiency 
 of organic light - emitting devices ( oleds ) and organic 
 solar cells ( osc ) have in particular been the focus 
 of many recent reports .
exciton diffusion is one of the determining 
 factors of oled efficiency roll - off at high brightness ; it affects the device efficiency of oscs and 
 also influences the bimolecular recombination losses and thereby lasing 
 threshold in organic lasers .
therefore , 
 the understanding and quantification of exciton diffusion is fundamental 
 for further improving organic device performance .
organic -conjugated 
 small molecules can self - assemble on 
 a template surface to form crystalline , needle - shaped nanofibers .
such nanofibers can be prepared from a single material or , as recent reports have demonstrated , as 
 multilayered nanofibers composed of p6p and 6 t consisting of a relatively thick ( 0.1 m ) p6p nanofiber 
 template onto which a periodic multilayer of 6 t and p6p was deposited .
furthermore , it has been demonstrated that such 
 a growth procedure can be applied to a broad class of molecules .
the initial p6p template layer was deposited 
 on a muscovite mica substrate resulting in the formation of mutually 
 parallel p6p nanofibers , which were aligned along the mirror symmetry 
 directions of the mica surface .
the well - defined growth 
 direction and morphology observed for p6p on mica ( in contrast to 
 organic nanofibers made from other materials or on other substrates ) makes this system a useful template for the subsequent periodic 
 deposition of 6 t and p6p layers .
the photoluminescence 
 ( pl ) emission spectrum of the p6p/6 t multilayered nanofiber samples 
 can be tuned by varying the thickness of either the p6p or the 6 t 
 layers . if ultrathin ( monolayer thickness ) 6 t layers 
 are used , it results in a homogeneous coverage of the p6p template 
 fibers and a homogeneous green pl emission , as illustrated in figure 1a .
( a ) schematic representation of p6p and 
 6 t nanofibers grown on 
 muscovite mica and fluorescence microscope image ( exc = 365 nm ) of sample 1 .
( b ) cie chromaticity diagram and ( c ) photoluminescence 
 spectra obtained at different temperatures illuminating the sample 
 with 3.8 ev photon energy , which directly excites fluorescence from 
 the p6p layers , while the 6 t layers emit light because of sensitization 
 from the excited p6p .
( d ) photoluminescence spectra of the 6 t monolayer 
 directly excited with 2.5 ev photon energy , which is below the optical 
 band gap of p6p .
both single - layered ( only 
 p6p ) and multilayered p6p/6 t nanofibers 
 show unique characteristics ( for example , polarized light emission ) due to their crystallinity and the specific orientation of the 
 emitting dipoles along the molecules long axes .
furthermore , 
 the p6p nanofibers have a high pl quantum yield ; their pl spectrum shows a well - defined vibronic progression ; they can function as optical ( both photonic and plasmonic ) 
 waveguides ; and amplified spontaneous emission ( ase ) and laser action 
 can be achieved under femtosecond optical excitation .
random lasing caused by light scattering from imperfections in the 
 nanofiber morphology ( which provides the optical feedback ) has been 
 observed in the deep blue for p6p nanofibers and also in multilayered p6p/6 t heteroepitaxial nanofibers , in which 
 the wavelength of the nanofiber s lasing was tuned from deep 
 blue to red - orange .
lasing in the monomolecular 
 regime for single - layered p6p nanofibers was achieved at a temperature 
 of 80 k , whereas the 6 t ( in the multilayered 
 p6p/6 t nanofibers ) monomolecular lasing was achieved at room temperature , 
 which is because the exciton diffusion and thus the bimolecular recombination 
 in the p6p/6 t heteroepitaxial nanofibers is much less effective in 
 the 6 t phases than in p6p .
in addition , 
 the large spectral overlap between the donor emission and acceptor 
 absorption enables efficient resonance - energy transfer ( ret ) between 
 the two materials . a comparable material class is 
 electrospun nanofibers from blend polymer materials that brings advantages 
 such as mechanical flexibility , low fabrication costs , and ease of 
 material doping .
however , the p6p/6 t 
 nanofibers form a monolithic , highly oriented heteroepitaxial system 
 yielding highly polarized emission
. the planar geometry can also bring 
 additional advantages , e.g. , realization of device structures via 
 lamination with the possibility of integrating high power density 
 leds for indirect electrical pumping .
because of the unique properties of the p6p/6 t multilayered 
 nanofibers , 
 we have chosen this system for detailed investigations of the exciton 
 dynamics and resonance - energy donor - to - acceptor transfer processes 
 that determine the resulting optical properties . in this work , we 
 study the efficiency of the exciton diffusion and energy - transfer 
 processes in the multilayered nanofibers via low - temperature pl spectroscopy 
 and quantify the contribution of the different recombination processes 
 via time - resolved pl measurements .
theoretical calculations using 
 density functional theory ( dft ) and its time - dependent approach ( tddft ) 
 were also carried out in order to obtain equilibrium energies , geometries , 
 and frequencies for both ground ( s0 ) and first excited 
 ( s1 ) states of p6p and 6 t .
these properties allow us to 
 understand better the mechanisms behind the absorption and emission 
 processes involved in the isolated molecules and to build a qualitative 
 model to understand the energy transfer in the p6p/6 t complex .
multilayered p6p/6 t heteroepitaxial 
 nanofibers were prepared on muscovite mica via hot wall epitaxy ( hwe ) 
 under high - vacuum conditions .
two samples were investigated 
 via low - temperature pl and time - resolved studies .
after the deposition of 
 the p6p template ( nominal thickness of 84 nm ) , a sequence of 50 cycles 
 of 6 t monolayers ( 0.26 nm ) intercalated with p6p ( 1.51 
 nm ) were deposited .
sample 2 was deposited at a substrate temperature 
 of 120 c . after the deposition of the p6p template , a sequence 
 of 10 cycles of 6 t monolayers ( 0.37 nm ) intercalated with 
 p6p ( 16.3 nm )
the pl spectra 
 at low temperature were obtained under vacuum conditions by controlling 
 the sample temperature with a helium cryo - cooler and recording the 
 luminescence spectra with a spectrometer ( acton research co. spectrapro-150 ) 
 coupled to a cooled ccd array ( princeton instrument pixis 100 ) while 
 exciting the sample with either a he  cd laser emitting at 325 
 nm ( 3.8 ev photon energy ) or an ar - ion laser ( 161 lgs , lg laser tech ) 
 emitting at 488 nm ( 2.5 ev photon energy ) .
samples were kept in a coldfinger 
 cryostat ( janis research co. 
 st-500 ) fed with liquid n2 for measurements in the 80300 
 k temperature range and photoexcited by the frequency - doubled pulses 
 ( 100 fs duration , 375 nm wavelength ) of a passively mode - locked 
 ti - sapphire laser ( spectra physics tsunami ) running at 82 mhz focused 
 to a spot of 100 m in diameter .
optical absorption 
 by the samples , strongly polarization dependent , was maximized using 
 a half - wave plate placed in the laser beam .
the photoluminescence 
 emission was collected in the transmission geometry by a lens system 
 and acquired by a streak camera ( hamamatsu c5680 ) working synchronously 
 with the exciting laser pulses coupled to a grating spectrometer ( acton 
 research co. spectrapro 2300i ) .
geometries , frontier 
 orbitals , and 
 total energies of the ground state ( s0 ) and first excited 
 state ( s1 ) of p6p and 6 t were obtained using dft .
we make 
 use of two different dft functionals : ( i ) the m06 - 2x functional , which has proven to properly describe the spectroscopic properties 
 of these type of systems , and ( ii ) the cam - b3yp 
 functional , which has a good track record in the description of the 
 electronic processes in organic molecules .
the 6 - 31g(d ) basis set was used for all calculations presented here . 

this basis set was chosen because of the relative large size of the 
 systems under investigation in addition to the successful reproduction 
 of the absorption and emission data of short oligomers .
by photoexcitation 
 of the p6p molecules with a photon energy above 
 the 6 t absorption spectrum , a spectrum that contains emission from 
 both p6p and 6 t can be observed .
the p6p emission 
 is the directly excited pl , whereas the 6 t emission results from sensitization 
 of the 6 t monolayer , via frster resonance - energy transfer 
 ( fret ) from the excited p6p molecules due to the overlap of the emission 
 spectrum of the p6p and the absorption spectrum of the 6 t material .
time - resolved measurements of 
 the 6 t emission have revealed that two distinct exciton populations 
 within the p6p layers are involved in the sensitization process .
those 
 generated within a distance corresponding to the frster radius 
 ( 3.6 nm in p6p ) from the p6p/6 t interface 
 are transferred on a very short time - scale via ret giving rise to 
 prompt 6 t emission , while the p6p excitons generated further away 
 but within the diffusion range from the p6p/6 t interface result in 
 a delayed 6 t emission via diffusion - assisted ret . to further understand 
 the contributions of the short ( frster s ) and long 
 ( diffusion - mediated ) transfer , we investigated two samples with different 
 number of layers and layer thickness using pl spectroscopy and ultrafast 
 time - resolved measurements of the luminescence dynamics and their 
 temperature dependence .
sample 1 consists of a p6p template with a 
 thickness of 84 nm on top of which 50 cycles of 0.26 nm 6 t 
 and 1.51 nm p6p were deposited , while sample 2 was prepared 
 by a sequence of 10 cycles of monolayer - thick 6 t ( 0.37 nm ) 
 intercalated with much thicker p6p layers ( 16.3 nm ) .
figure 1b shows the 1931 commission internationale 
 de leclairage ( cie ) chromaticity diagram of the pl spectra 
 from sample 1 acquired at different temperatures in vacuum and under 
 illumination with ultraviolet ( uv ) light ( 3.8 ev photon energy ) . a 
 change of sample temperature from 280 to 6 k causes the emission color 
 to gradually change from green ( cie coordinates , x = 0.39 , y = 0.54 at 280 k ) to a more white / blue 
 color ( cie coordinates , x = 0.31 , y = 0.34 at 6 k ) .
these changes are also reflected in the pl spectra 
 shown in figure 1c , in which both p6p and 6 t 
 emission can be observed simultaneously .
the illumination with uv 
 directly excites fluorescence from the p6p layers because of the radiative 
 decay of singlet excitons from the vibrational 
 ground state of the first excited electronic state to various vibrational 
 levels of the electronic ground state . at room temperature ,
the characteristic 
 peaks from p6p are positioned at 3.09 ev ( 00 ) , 2.93 ev ( 01 ) , 
 2.77 ev ( 02 ) , and 2.61 ev ( 03 ) , while the peaks originating from the 6 t monolayers are 
 at 2.35 ev ( 00) , 2.18 ev ( 01) , and 
 2.01 ev ( 02). figure 1c shows that the p6p emission intensity 
 increases by an order of magnitude when cooling the substrate from 
 280 to 6 k. this indicates that the nonradiative processes ( related 
 to exciton diffusion to quenching sites and to the p6p/6 t interface ) 
 are suppressed at low temperature . these aspects will be discussed 
 in detail in the section modeling of the pl temperature 
 dependence . in figure 1c , one can also 
 observe that the green emission intensity from the sensitized 6 t monolayers 
 increases when cooling from 280 to 80 k and then decreases again upon 
 further cooling to 6 k. a weak temperature dependence is also observed 
 by directly exciting the 6 t monolayer with 2.5 ev photon energy , which 
 is below the optical gap of p6p and therefore excites 
 only the 6 t , as shown in figure 1d , where the spectra from the intrinsic decay process 
 of the 6 t excitons are collected at different temperatures .
the weak 
 temperature dependence of the directly excited pl from the 6 t monolayer , 
 consisting of thermal ( homogeneous ) broadening not involving significant 
 intensity redistribution among the vibronic peaks , suggests that the 
 nonradiative processes are not significant , i.e. , that the 6 t has 
 intrinsic high quantum yield , confirming the appropriateness of 6 t 
 as the acceptor material for this system .
we attribute this huge temperature 
 dependence difference between p6p and 6 t to the fact that crystalline 
 p6p layers are h aggregates while the ultrathin 6 t layers have a radically 
 different aggregation state ( most presumably weakly bound j aggregates ) .
the intensities of the spectrally integrated p6p and 6 t emissions 
 are plotted in figure 2a .
the pl intensity 
 of the p6p emission monotonically increases with decreasing temperature , 
 and a qualitatively similar behavior is observed for the directly 
 excited 6 t ; however , the directly excited 6 t exhibits a much weaker 
 temperature dependence .
compared to the p6p and the directly excited 
 6 t emission , the pl intensity of the sensitized 6 t follows a different 
 trend , in which the intensity first increases with decreasing temperature , 
 but then reaches a maximum and decreases again .
we interpret this 
 as being due to two processes with different temperature dependencies 
 in the 6 t emission process : the sensitization process ( which depends 
 on the exciton diffusion and energy transfer ) and the actual intrinsic 
 decay of the 6 t excitons .
we propose that the temperature dependence 
 of the pl from the sensitized 6 t can therefore be described as a product 
 of a function f(t ) , which describes 
 the temperature dependence of the sensitization process , and a function 
 pl,6 t that describes the intrinsic radiative decay 
 yield , which is proportional to the temperature dependence of the 
 directly excited 6 t emission:1 ( a ) full dots : cw luminescence intensity 
 of p6p ( blue ) and 6 t ( green ) 
 photoexcited at 3.8 ev ; 6 t ( red ) luminescence intensity under direct 
 excitation at 2.5 ev .
blue continuous line is the fitting curve of 
 the p6p luminescence intensity data ( see text for details ) .
hollow 
 dots : steady - state p6p and 6 t emission intensities reconstructed from 
 transient photoluminescence data .
( b ) ratio between emission intensities 
 of the sensitized and directly excited 6 t measured under 3.8 and 2.5 
 ev pumping , respectively . an elementary rate equation model for the p6p and 6 t excited - state 
 populations
predicts that2where sens is the 6 t emission 
 sensitization quantum yield and w6 t and wp6p are the pump absorption rates for direct 
 6 t and p6p excitation , respectively . the rate equation model for the 
 exciton densities
it is clear that if the p6p and 6 t optical absorption 
 does not depend on temperature , the function f(t ) gives the temperature dependence for the 6 t emission 
 sensitization process ( sens ) .
the temperature dependence 
 of the normalized ratio of the intensities of the sensitized and directly 
 excited 6 t ( function f(t ) ) is shown 
 in figure 2b .
this ratio decreases when decreasing 
 the temperature from 280 to 6 k presumably because of less efficient 
 exciton diffusion and therefore less efficient energy transfer from 
 p6p to the 6 t at lower temperatures . to better understand
the fundamental 
 processes involved in the photoemission of p6p molecules and subsequent 
 absorption of 6 t , a dft investigation of the ground state ( s0 ) and first excited state ( s1 ) of p6p and 6 t was carried 
 out .
as the first step , we discuss the ground state ( s0 ) equilibrium geometries .
the optimized ground state of isolated 
 p6p and 6 t shows the typical alternating twists of the rings .
( the 
 definition of the dihedral angles and their values are presented in 
 figure s1 and table s1 in the supporting information ) .
the average dihedral angle between the rings computed with m06 - 2x 
 [ cam - b3lyp ] was found to be 37 [ 38 ] for p6p and 20 
 [ 22 ] for 6 t . upon excitation , as expected , both molecules 
 tend to assume a more planar structure .
the excited state of 6 t becomes 
 planar with both functionals , whereas for p6p , the average dihedral 
 angle reduces to 22 [ 21 ] when the m06 - 2x [ cam - b3lyp ] was 
 used .
figure 3a shows the ground - state 
 ( s0 ) highest occupied molecular orbital ( homo ) and lowest 
 unoccupied 
 molecular orbitals ( lumo ) wavefuction for both p6p and 6 t , whereas 
 figure 3b shows the frontier orbital energies 
 of the two oligomers , computed using cam - b3lyp . in solid state ,
both 
 p6p and 6 t are observed to be linear and planar molecules , in both ground and 
 excited states .
thus , to better compare the experimental results with 
 our calculations , we also optimized p6p and 6 t subjected to the constraint 
 of being planar and evaluated the impact of this constraint in the 
 s0  s1 transition energy , which is also 
 included in figure 3b .
the impact of planarization 
 on p6p is larger than that on 6 t , as expected from the larger deviation 
 from planarity presented by the former oligomer .
a comparison between 
 the homo  lumo gap of the two planar structures shows that p6p 
 has still a larger gap compared to that of 6 t , but the difference 
 between the two has reduced from 1.39 ev in the twisted form to 0.46 
 ev in the planar ( solid - state - like ) form .
the results for m06 - 2x shows 
 a similar trend ( see figure s2 in the supporting 
 information ) ; however , the mismatches between the gap of p6p 
 and 6 t were computed to be 1.37 and 0.837 ev , respectively .
( a ) 6 t ( top ) 
 and p6p ( bottom ) ground - state homo ( left ) and lumo 
 ( right ) wave functions .
( b ) cam - b3lyp ground state ( s0 ) 
 frontier orbital energies and homo  lumo gap for p6p ( left ) 
 and 6 t ( right ) .
both fully optimized ( twisted ) and optimized under 
 the planarity constraint ( planar ) are presented .
this small difference ( on the order of tenths of electronvolts ) 
 in the homo 
t already suggests that p6p can absorb a higher - energy 
 photon and transfer this energy to 6 t , as was observed for other thiophene - based 
 systems . to further investigate 
 the mechanisms behind the energy transfer 
 between the two molecules in the ground state ( s0 ) and 
 first excited state ( s1 ) , the corresponding potential energy 
 surfaces ( pess ) were analyzed .
these 
 energies are ( i ) the optimized ground - state energy ( es0s0 ) , ( ii ) the first excited - state 
 energy with the ground - state optimized geometry ( es1s0 ) , ( iii ) the ground - state energy with the optimized 
 s1 geometry ( es0s1 ) , and 
 ( iv ) the optimized first excited - state energy ( es1s1 ) .
these four energies allow us to compare not 
 only the vertical energy gaps but also the amount of energy that can 
 be lost in geometrical relaxation after a vertical transition .
figure 4 shows these energies for 6 t and 
 p6p calculated using cam - b3lyp ( see results for m06 - 2x in figure s3 
 in the supporting information ) .
the optical 
 gap is represented by the vertical transition between the ground state 
 and the first excited state , i.e. , the energy difference ( es1s0  es0s0 ) .
the two energy 
 differences , es1s0  es1s1 and es0s1  es0s0 are the excited - state 
 and the ground - state relaxation energies , respectively .
here it is 
 important to highlight that both p6p and 6 t assume a quinoid - like 
 ( and thus planar ) conformation in the first excited state . as a result , the relaxation energy associated 
 with the geometry relaxation after a vertical transition from the 
 ground state to the excited state ( s0  s1 ) is larger than the related energy associated with the inverse process 
 ( i.e. , the s1  s0 relaxation ) . figure 4 allows for direct 
 comparison between the experimental results reported here for the 
 vertical transition from the vibrational ground state of the first 
 excited electronic state ( es1s1 ) to the ground state ( es0s1 ) and our theoretical calculations .
the 
 experimental value of 3.09 ev for p6p [ 2.35 ev for 6 t ] compares well 
 with the corresponding theoretical values of 3.37 ev [ 2.40 ev ] obtained 
 with cam - b3lyp and 3.37 ev [ 2.42 ev ] obtained with m06 - 2x , respectively .
s0 and s1 potential energy levels for ( left ) 
 p6p and ( right ) 6 t using cam - b3lyp .
( center ) p6p s1  
 s0 transition energy from its lowest vibronic state ( red ) 
 together with the 6 t s0  s1 transition 
 from its lowest vibronic state ( black ) . to have a resonance - energy transfer between the emission 
 of p6p 
 and the absorption of 6 t
, one must have a match between the energy 
 emitted by p6p and the energy absorbed by 6 t . using cam - b3lyp [ m06 - 2x ]
, 
 we computed these energies , i.e. , the energy required for a transition 
 from the s1 lowest vibrational state to s0 for 
 p6p and the energy required for a transition from the s0 lowest vibrational state to the s1 state for 6 t , and 
 obtained 3.374 and 3.151 ev [ 3.374 and 3.123 ev ] , respectively .
the 
 difference of only 0.2 ev ( for both functionals ) validates our assumption 
 that a resonance - energy transfer is happening . to further characterize 
 this energy transfer , a simulation of the 
 vibronic resolved absorption spectrum of 6 t together with the counterpart 
 emission spectrum of p6p
a direct calculation 
 of the complete absorption and emission spectra 
 using gaussian09 was attempted , as was done before for fused oligothiophenes . however , the overlap between the two vibrational 
 ground states of s0 and s1 is negligibly small , 
 thus suggesting that this approach is not appropriate to treat p6p 
 and 6 t molecules .
is attributed to the considerable changes 
 in geometry when going from the ground to the excited states in these 
 systems ( the planarization mentioned before ) .
these changes restrict 
 the validity of the franck  condon approach used by gaussian09 
 to generate vibronically resolved spectra .
a detailed study 
 of the potential energy surfaces of the ground 
 and excited states of 6 t and p6p and subsequent generation of the 
 vibrationally resolved spectra is beyond the scope of this manuscript 
 and will be presented elsewhere .
to further 
 elucidate the exciton dynamics and explain the temperature dependence 
 of the pl spectra ( figure 1c ) we have used 
 temperature - dependent , ultrafast time - resolved pl spectroscopy that 
 can provide quantitative data on the exciton diffusion and energy - transfer 
 processes .
the excitons , which are generated in p6p within a distance 
 from the p6p/6 t interface corresponding to the frster radius , 
 are transferred via direct ret on a very short time scale ( faster 
 than 20 ps , the time resolution of our experimental setup ) .
on the 
 other hand , p6p excitations generated further away from the interface , 
 but still in the exciton diffusion length region , are transferred 
 in a two - step process : ( i ) excitons first diffuse to the interface 
 and ( ii ) excitons are transferred via ret ( diffusion - assisted ret ) . 

the results shown here for the time - resolved measurements were 
 obtained for sample 2 , which has thicker p6p layers compared to sample 
 1 .
samples with p6p layer thicknesses larger than the frster 
 radius allow the observation of both direct and diffusion - assisted 
 ret of p6p excitations to 6 t . on the other hand ,
time - resolved data 
 are difficult to analyze in samples with very thin p6p layers ( thickness 
 less than the frster radius ) because in this case a very large 
 fraction of the p6p excitons are involved in 6 t sensitization only 
 through direct ret ( mostly unresolved in the p6p decay dynamics ) , 
 whereas diffusion - assisted ret is practically absent .
figure 5 shows the time - resolved , temperature - dependent 
 measurements obtained from 300 k down to 80 k. the luminescence decay 
 was obtained using a streak camera that provides both temporally and 
 spectrally resolved data .
this allows separate analysis of the temporal 
 behavior of the two parts of the spectra corresponding to p6p emission 
 and 6 t emission .
figure 5a shows the time - resolved 
 p6p emission intensity obtained by spectral integration over the 400480 
 nm wavelength interval . in relaxation time approximation , the p6p 
 dynamics
is fitted with a biexponential decay curve as3where the fast component ( a1 , 1 ) represents the p6p exciton population 
 involved in diffusion - assisted ret ( generated within a distance from 
 the p6p/6 t interface corresponding to the diffusion length ) . the slow 
 component ( a2 , 2 ) represents 
 the intrinsic decay in p6p and is attributed to the p6p excitons that 
 do not reach the p6p/6 t interface and therefore are unable to contribute 
 to the 6 t sensitization .
the fast decaying pl component associated 
 with the p6p excitons generated within the frster distance 
 from the p6p/6 t interfaces is not visible because of the limited temporal 
 resolution of the experiment , which is supposed to attenuate its amplitude 
 to values not detectable on top of the ( a1 , 1 ) and ( a2 , 2 ) decaying components .
analysis of the fitting parameters 
 can provide information on the temperature dependence of these processes . 

the temperature dependence of the decay time constants ( 1 and 2 ) and the prompt pl emission intensity , 
 which is estimated as the sum of the amplitudes of the two exponentials 
 ( a1 + a2 ) , 
 is plotted in the inset of figure 5b . the time 
 constant 1 is almost an order of magnitude smaller 
 than 2 , which suggests that diffusion - mediated ret 
 is a highly efficient process in our all - organic heterostructures . 
 because the two time constants ( 1 and 2 ) display the same temperature dependence
, we also infer that 
 not only p6p - to-6 t long - range ret but also all other nonradiative 
 p6p exciton recombination processes are mediated by exciton diffusion . 

moreover , the initial p6p emission intensity exhibits very little 
 variation in the 80300 k temperature range , demonstrating 
 that p6p optical absorption at the uv pump wavelength is temperature - insensitive . 

figure 5b shows the sensitized 6 t emission 
 intensity spectrally integrated over the wavelength interval between 
 510 and 650 nm .
the 6 t dynamics is fitted as4where the single exponential decay ( ap , d ) describes the prompt 
 emission from 6 t excitons generated through direct ret ( from p6p excitons 
 originating within the frster radius ) .
the second , delayed 
 term stems from p6p excitons that diffuse to transfer energy , where 
 the 6 t dynamics are fitted with an exponential activation ( act , activation time ) and decay curve ( aact , act , d ) depicting 
 the evolution of the 6 t excitons generated through diffusion - assisted 
 ret .
the inset in figure 5b also shows that 
 the 6 t activation time ( act ) data nicely overlap 
 with the fast p6p decay ( 1 ) , further confirming 
 that 6 t delayed emission is activated by p6p excitations through ret . 

we note that diffusion - assisted ret has been treated in relaxation - time 
 approximation with a single time constant . from a theoretical point 
 of view , this represents a simplification of the exciton diffusion 
 kinetics ; however , it is consistent with the experimental data .
the 
 donor exponential decay component with the shorter decay time perfectly 
 matches the evolution of the acceptor delayed emission , which is very 
 well fitted using a single exponential rise function ( inset of figure 5b ) .
moreover , a detailed account of the kinetics 
 of diffusion - assisted ret and related 
 data analysis is beyond the scope of 
 this paper .
time - resolved pl obtained from sample 2 at different temperatures 
 for ( a ) the p6p emission intensity integrated over the interval 400480 
 nm and ( b ) the 6 t emission intensity integrated in the range of 510650 
 nm .
red continuous lines are best fits based on a biexponential decay 
 function ( a ) and an exponential activation  decay function ( b ) 
 as described in the text .
the inset of panel ( b ) shows the temperature 
 dependence of the p6p decay time constants , 1 and 
 2 ( black and red dots , respectively ) , of the 6 t 
 activation time constant ( hollow green dots ) and of the initial p6p 
 luminescence intensity , ipl(t = 0 ) ) ( blue squares ) ( see text for details ) .
the conclusion 
 from the time - resolved results in the inset in figure 5b enables us to model the temperature dependence of the pl 
 spectra ( figure 1c ) .
the fluorescence intensity 
 ( ip6p ) is proportional to the quantum 
 yield ( pl ) , which can be described by5where r is the radiative 
 decay time constant and nr is the ( total ) nonradiative 
 decay time constant . as proposed by wen 
 et al .
we will also assume that the exciton 
 diffusion ( and thus nonradiative p6p exciton recombination ) can be 
 described as a temperature - activated process6where diff is a 
 characteristic 
 time for exciton diffusion in p6p and ediff is the activation energy , k is boltzmann s 
 constant and t is the temperature . from this
we get 
 the expression for the temperature dependence of the pl intensity:7from which we 
 can find the activation energy 
 for the exciton diffusion process by fitting the p6p spectrally integrated 
 pl intensities versus reciprocal temperature as shown in figure 2a together with a fit to eq 7 .
the measured pl intensity of the p6p emission fits well to eq 7 , from which we can extract ediff 19 mev .
the small value of activation energy for 
 exciton diffusion makes p6p very suitable as donor material in a multilayered 
 organic epitaxial system . in crystalline rubrene ,
an activation energy 
 of 30 mev has been observed , while 
 significantly higher activation energies have been observed in disordered 
 films , e.g. , around 69 mev in an anthradithiophene derivative , i.e. , higher than the room - temperature thermal 
 energy .
steady - state pl intensities measured under continuous 
 wave ( cw ) optical excitation can be estimated on the basis of the 
 fit parameters of the excited - state dynamics retrieved from transient 
 pl measurements .
the amplitude a1 is proportional 
 to the population of photogenerated p6p excitons contributing to 6 t 
 sensitization through diffusion - assisted ret , whereas the amplitude a2 scales as the population of those p6p excitons 
 that recombine before reaching a p6p/6 t interface .
the exciton population 
 involved in direct ret within the frster distance from an 
 interface can in turn be estimated to be proportional to a1(ap / aact ) .
the temperature dependence of the steady - state intensities 
 reached under cw excitation can thus be calculated as8910where sens , diff = 2/(sens , diff + 2 ) is the 
 diffusion - assisted 6 t sensitization yield ( 1/1 = 
 1/sens , diff + 1/2 ) . in eq 10
it is further assumed that the absorbed pump power 
 under 6 t direct excitation , w6 t , is temperature - independent . 

they nicely agree with the measured steady - state 
 intensities in cw excitation experiments , confirming the overall consistency 
 of the experimental data and the validity of the interpretation model 
 of the pl intensity dynamics .
note that because the pl intensity with 
 direct 6 t excitation actually scales as d , the above 
 assumption of 6 t photoabsorption as a temperature - insensitive quantity 
 turns out to be valid , at least in the 80300 k temperature 
 range .
it can be concluded that the function f(t ) introduced above represents the correct temperature trend 
 of the 6 t emission sensitization efficiency .
time - resolved pl measurements also enable 
 the extraction of the quantum yield of the energy - transfer process 
 from p6p to 6 t .
the total 6 t energy sensitization quantum yield ( sens , u ) that includes the contributions from both the direct 
 and diffusion - assisted ret processes can now be found as the following 
 weighted average11where the contribution from the diffusion - assisted 
 ret is the product of its weight ( a1 ) 
 and its efficiency ( sens , diff ) .
the efficiency of 
 direct ret ( with weight a1(ap / aact ) ) is supposed to be 
 unity ( as it is an ultrafast process ) .
finally , the p6p exciton population 
 that does not reach the interface ( with weight a2 ) has zero probability of generating an exciton in 6 t (  
 = 0 ) .
the value sens , u can be considered an upper 
 bound on the efficiency of 6 t emission sensitization through ret . 

the lower bound , sens,1 , can be found by including 
 only the diffusion - assisted ret term12 in this approximation , the single exponential 
 decay component ( ap,d ) of the 6 t dynamics is traced back to direct photoexcitation of 
 6 t and direct ret processes are fully neglected .
upper and lower 
 bounds for 6 t sensitization quantum yield are displayed 
 in figure 6 , where the function f(t ) , extracted from cw measurements , is also shown 
 with suitable normalization to match the lowest bound for sens .
the sensitization efficiency depends on the p6p layer 
 thickness . for samples with very thin p6p layers , whose thickness 
 is comparable to or smaller than the exciton diffusion length in p6p , 
 sens , u is almost unity because all excitons are 
 generated close to the interface and the sensitization process is 
 therefore highly efficient , while for samples with thicker p6p layers , 
 more p6p excitons decay without being transferred to 6 t . the good 
 agreement between the temperature trend of sens retrieved 
 from the analysis of transient pl data and the one extracted from 
 cw pl measurements
further corroborates our understanding of the function f(t ) as the temperature dependence of the 
 efficiency of 6 t sensitization through the ret processes .
6 t emission 
 sensitization quantum yields estimated on the basis 
 of transient photoluminescence data versus reciprocal temperature ; 
 blue and red symbols refer to sample 1 and 2 , respectively .
black full 
 dots : ratio between cw emission intensities of the sensitized and 
 directly excited 6 t ( function f(t ) ) .
we 
 have probed the energy - transfer and exciton recombination characteristics 
 in multilayered epitaxial p6p/6 t nanofibers by temperature - dependent 
 pl spectroscopy using either 3.8 or 2.5 ev photons for stimulating 
 the p6p or 6 t layers , respectively . while the p6p quantum yield decreases 
 by an order of magnitude between 6 and 300 k , the emission of the 
 6 t monolayers exhibits much weaker temperature dependence , which was 
 also observed under direct excitation of the 6 t monolayers .
to further 
 elucidate the exciton dynamics , we have used temperature - dependent , 
 ultrafast time - resolved pl spectroscopy to provide quantitative data 
 on the exciton diffusion and energy - transfer processes . on the basis 
 of this
, we modeled the pl temperature dependence , which enabled us 
 to extract the activation energy for exciton diffusion in crystalline 
 p6p , which is only about 19 mev .
the strong temperature dependence 
 of the p6p pl emission was found to be due to a temperature - activated 
 exciton diffusion process that results in a significant rise in pl 
 quantum yield with decreasing temperature ( an order of magnitude from 
 300 to 6 k ) .
the 6 t monolayers have a much weaker temperature dependence 
 ( 40% increase from 300 to 6 k ) , presumably because of a different 
 aggregation state .
however , the sensitization process ( diffusion - assisted 
 ret from p6p ) has a temperature dependence that counteracts the increase 
 in intrinsic quantum yield at lower temperatures , giving rise to a 
 6 t pl emission that initially increases by a few percent upon cooling 
 from room temperature ; however , the emission then starts to drop below 
 160 k , reaching around 0.8 of its room - temperature value at 
 6 k. theoretical calculations in the framework of density functional 
 theory were used to characterize the potential energy surfaces of 
 the ground ( s0 ) and first excited ( s1 ) states 
 of p6p and 6 t . by a comparison between the energy gap related to the 
 emission of p6p and the absorption of 6 t from their lowest vibronic 
 state
, we determined a mismatch of 0.2 ev of the order of the vibrational 
 relaxation energy of the molecules , resulting in a high probability 
 for resonance - energy transfer between the two molecules .
this improved 
 understanding of the exciton diffusion and resonance - energy - transfer 
 processes in organic epitaxial heterostructures can aid in the development 
 of new organic nanomaterials with desired optical properties .
 multilayered epitaxial nanofibers 
 are exemplary model systems for 
 the study of exciton dynamics and lasing in organic materials because 
 of their well - defined morphology , high luminescence efficiencies , 
 and color tunability . 
 we use temperature - dependent continuous wave 
 and picosecond photoluminescence ( pl ) spectroscopy to quantify exciton 
 diffusion and resonance - energy transfer ( ret ) processes in multilayered 
 nanofibers consisting of alternating layers of para - hexaphenyl ( p6p ) 
 and -sexithiophene ( 6 t ) serving as exciton donor and acceptor 
 material , respectively . the high probability for ret processes 
 is 
 confirmed by quantum chemical calculations . 
 the activation energy 
 for exciton diffusion in p6p is determined to be as low as 19 mev , 
 proving p6p epitaxial layers also as a very suitable donor material 
 system . 
 the small activation energy for exciton diffusion of the p6p 
 donor material , the inferred high p6p - to-6 t resonance - energy - transfer 
 efficiency , and the observed weak pl temperature dependence of the 
 6 t acceptor material together result in an exceptionally high optical 
 emission performance of this all - organic material system , thus making 
 it well suited , for example , for organic light - emitting devices . 
Introduction
Experimental
Section
Results and Discussion
Conclusion