solvent 
 polarity strongly affects the mechanism of photoinduced electron transfer 
 ( et ) reactions . in polar solvents , the process usually involves complete 
 et from the electron donor to the acceptor , thereby forming spin - correlated 
 radical ion pairs ( rips ) . in nonpolar solvents , the formation of excited - state 
 charge transfer complexes , so - called exciplexes , is often observed . expectedly , in moderately polar solvents , both quenching reactions 
 may contribute simultaneously and radical ions can result from direct 
 et as well as dissociation of the exciplex . due to similar spectral footprints of ion pairs and exciplexes , the magnetic field effect ( mfe ) on the exciplex emission provides 
 a versatile tool for the study of this intricate dynamics . the approach 
 has been applied to homogeneous solvents as well as under conditions 
 of preferential solvation , which often occur in binary solvent mixtures 
 of solvents of vastly different polarity . the term  preferential solvation  subsumes nonspecific 
 and specific interactions of solutes with specific components of binary 
 solvent mixtures . it often induces microheterogeneity in the solvent , 
 which is characterized by the spatially nonuniform distribution of 
 the solvent components in the vicinity of polar or ionic solutes . these microclusters of polar solvent molecules have a pronounced effect 
 on the mutual diffusion and reactivity of radical ion pairs and , thus , 
 the mfes they elicit . so far , microheterogeneous solvation has only 
 been studied by steady - state mfe measurements . this severely limits the insights in the 
 radical pair dynamics in these systems , because only time - resolved 
 mfe measurements allow one to discriminate the initial reaction products . the mfe relevant to this study results from the radical 
 pair mechanism . in short , the overall singlet and triplet 
 states of the radical pair are coherently interconverted by the hyperfine 
 interactions ( hfis ) of the magnetic nuclei of the radicals . an external magnetic field will remove the degeneracy of the three 
 electronic triplet sublevels ( t0 and t ) of the spin - correlated pair ( in the absence of significant exchange 
 or electron  electron dipolar coupling ) . when the energy separation 
 between these states exceeds the size of the mixing interactions , 
 t can not mix with the singlet state s. in this way , 
 the external magnetic field reduces the probability of intersystem 
 crossing in the radical pair and , thus , alters the relative concentrations 
 of singlet and triplet reaction products ( see scheme 1a ) . the hyperfine - induced spin mixing 
 between s and t0 and t can only efficiently 
 proceed when the electron exchange interaction , which energetically 
 splits the s from the t - states , is smaller in magnitude than the hyperfine 
 interactions . owing to the fact that the electron exchange interaction 
 depends exponentially on the distance between the radical ions , this 
 implies that only diffusively separated radical ions can undergo s / t - conversion . eventually , the singlet rip concentration can be detected through 
 the emission of the exciplex , which is produced by singlet rip cage 
 recombination ( see scheme 1b ) . in the absence of an external magnetic field ( b0 = 0 ) , the singlet ( s ) and all three triplet ( t0, ) states are interconverted by the radical hyperfine interactions 
 ( hfi ) . in the presence of a sufficiently large magnetic field b0 , the t states can not mix 
 with the s state . the 
 red and orange arrows refer to the radiative processes of the exciplex 
 and the locally excited fluorophore emissions , respectively . as a consequence of this mechanism , any factor influencing 
 the rip molecular dynamics may affect mfe features such as the magnitude 
 of the mfe observed under saturating field conditions and the b1/2 value , which gives the field intensity delivering 
 half the saturation mfe . in this way , the exciplex system can serve 
 as a magneto - fluorescent probe , sensitive to the local dielectric 
 heterogeneities of binary solvents . indeed , using magnetic - field - affected - reaction - yield 
 ( mary ) line broadening measurements , the b1/2 values were found to vary strongly with the solvent composition 
 in microheterogeneous solvents . solvent mixtures composed 
 of toluene ( to ) and dimethyl sulfoxide ( dmso ) have been studied in 
 detail . studies of the magnetic isotope effect revealed 
 no direct transfer of spin density on solvent molecules . instead , the observed effects were entirely 
 due to the radical pair dynamics . in general , the rp lifetime is reduced 
 in the microheterogeneous to / dmso environment due to an enhanced cage - effect 
 facilitating rp reencounters . as the dmso concentration and thus the 
 relative macroscopic permittivity , s , is increased , 
 the lifetime of the rip increases due to a smaller depth of the dielectric 
 traps . this is a consequence of the rips evading swift recombination 
 by their diffusive escape from the trap . accordingly , the extrapolated b1/2 values in the limit of zero donor concentration , b1/2(0 ) , are larger than expected for long - lived 
 rps but decrease with increasing macroscopic dielectric constant , 
 in contrast to what is seen in homogeneous solvents . nath and co - workers have recently suggested that two populations 
 of exciplexes exist for the pyrene / n , n - dimethylaniline donor  acceptor system in comparably polar 
 thf / dmf and benzene / acetonitrile solvent mixtures . the authors argue that the effect is attributed to ( incomplete ) 
 relaxation of the solvent shell surrounding the contact ion pair . the aforementioned steady - state approaches usually do not take the 
 exciplex kinetics into account . the mechanism of fluorescence quenching 
 by electron transfer is , however , ambiguous ; i.e. , the initial quenching 
 products are a rip via distant et ( pathway 2b of scheme 1b ) or the exciplex ( pathway 
 2a ) . several studies have illustrated the use of time - resolved mfes 
 of the exciplex to identify the initial quenching products in homogeneous 
 solvents and solvent mixtures . in this work , we describe 
 the solvent polarity dependence of the time - resolved mfe in homogeneous 
 and micro - heterogeneous binary solvent mixtures as measured by time - correlated 
 single photon counting ( tcspc ) . the well - studied donor n , n - dimethylaniline ( quencher)/9,10-dimethylanthracene 
 ( fluorophore ) serves as a magnetosensitive probe ( see scheme 1c ) . our study gives detailed 
 insights into the peculiar rp dynamics in microheterogeneous solvents . 
 throughout this work , we use the terms  preferential solvation  
 and  microheterogeneous medium  interchangeably , although , 
 strictly speaking , the former refers to specific and nonspecific solute solvent 
 interactions , while the latter signifies the nonuniform solvent structure , 
 which the former induce . in the present context n , n - dimethylaniline 
 ( dma , aldrich 99.5% ) was distilled under reduced pressure and subsequently 
 handled under an argon atmosphere . the concentration of the fluorophore 
 was constant at 1.5  10 m , while that of 
 the quencher was 0.06 m. solutions were prepared in septa - sealed fused 
 silica cuvettes . all solutions were purged with solvent - saturated 
 nitrogen gas for 15 min to remove dissolved oxygen before adding the 
 quencher through the septum using hamilton syringes . a series of solvent 
 mixtures of propyl acetate ( pa , aldrich 99.5% , distilled under reduced 
 pressure , s = 6 ; this and all subsequently reported 
 values refer to a temperature of 295 k ) and butyronitrile ( bn , fluka 
 99% , distilled under reduced pressure , s = 24.7 ) 
 and of toluene ( to , fluka 99% , distilled under reduced pressure , s = 2.38 ) and dimethyl sulfoxide ( dmso , aldrich 99.9% , used 
 as received , s = 46.4 ) with widely varying dielectric 
 constants , s , were prepared . the dielectric constants 
 of these mixtures are given by eq 1 and eq 2 for the pa / bn and to / dmso systems , respectively ( at 295 k):12here , wpa is the mass fraction of pa and xdmso is the mole fraction of dmso . the homogeneous mixtures 
 of pa / bn allow systematic variation of the solvent dielectric constants 
 within a range of 624.7 ( at 295 k ) while keeping the viscosity 
 (  ) and the refractive index ( n ) almost constant . the pekar factor ,  = ( 1/  
 1/s )  ( 1/n  
 1/s ) , which governs the outer - sphere electron transfer 
 reorganization energy , increases only moderately with increasing permittivity . for the microheterogeneous mixtures of to / dmso , 
 the solvent dielectric constant was varied from 4.3 to 15.5 . the viscosity 
 and the bulk pekar factor  of to / dmso mixtures slightly increase 
 with increasing dmso concentration . we 
 are not aware of a microheterogeneous solvent mixture for which this 
 could be avoided . using 
 the experimental setup described in ref ( 32 ) , the exciplex emission was recorded at 550 nm 
 under continuous excitation of the sample at 374 nm . for each sample , 
 the fluorescence intensities were acquired three times under conditions 
 of zero and saturating magnetic field exposition for 60 s each . field - on 
 and field - off measurements were recorded in an alternating fashion . the excitation slit width was 2 nm , the emission slit width 6 nm , 
 and the spectrometer time constant 1 s. measurements were conducted 
 at 295 k. the three repetitions were analyzed independently , and the 
 experimental errors were obtained according to the method described 
 in ref ( 32 ) . the absolute 
 mfe , ss , was evaluated from3here , i(em , bsat ) and i(em , b0 = 0 ) are the mean intensities at 
 em in a saturated magnetic field ( bsat = 62 mt ) and in the absence of an additional magnetic 
 field , respectively . if(em , b0 ) is the residual emission 
 of the locally excited fluorophore at em in the 
 absence of quencher . ic and i0 are the intensities of prompt emission of the fluorophore 
 in the presence and absence of the quencher , respectively , which have 
 been obtained from the decomposition of the fluorescence spectra . time - resolved data of the mfes on the exciplex 
 were obtained by the tcspc technique . in order to investigate the 
 effect of an external magnetic field on the exciplex emission ( figure 1 ) , a saturating magnetic 
 field ( the fluorophore 
 is excited at 374 nm by a laser diode ( picoquant , ldh - p - c-405 ) , and 
 a 550 nm long - pass filter in front of the detector ensured that only 
 the exciplex luminescence was detected . a detailed description of 
 the experimental setup is given in refs ( 1 ) and ( 2 ) . time - resolved exciplex emission ( upper panel ) of the 9,10-dimethylanthracene 
 ( 1.5  10 m)/n , n - dimethylaniline ( 0.06 m ) system in a to / dmso mixture at s = 7.3 in the absence ( gray ) and presence ( blue ) of an external 
 magnetic field ( 62 mt ) monitored with a long - pass filter ( lp550 ) after 
 pulsed excitation at 374 nm . in the presence of an external magnetic 
 field , the delayed emission of the exciplex is enhanced . the time 
 evolution of the magnetic field effect i(t ) ( lower panel ) was obtained by taking the difference of 
 the exciplex emission time traces in the absence and presence of an 
 external magnetic field . the red solid line is calculated from the 
 model , eq 6 . in the lower 
 panel , t = 0 ns coincides with the pulse excitation ; 
 in the upper panel , the emission traces are shifted by approximately 
 15 ns to longer times . the raw data of a typical time - resolved mfe measurement are 
 shown in figure 1 . the time traces rise with a time constant of approximately 2 ns , almost 
 independent of the magnetic field intensity . the decay kinetics of 
 the exciplex emission includes the dissociation into free ions and 
 recombination giving rise to delayed exciplex emission . the time - resolved 
 mfe ( tr - mfe ) is given as the difference in the exciplex emission intensity , 
 i(t ) , in the absence and 
 presence of the external , biasing magnetic field:4 in order to determine the mfe , the amplitudes within 
 the first nanosecond after the excitation pulse were matched . we integrated 
 the time traces so obtained to determine the steady - state mfe of the 
 exciplex , tr , from the time - resolved mfe measurements:5scheme 1b shows the pertinent reaction pathways for fluorescence quenching 
 by electron transfer . the abscissa can be expressed as the projection 
 of a two - dimensional reaction coordinate comprising the distance between 
 the fluorophore and the quencher and the outer - sphere electron transfer 
 reaction coordinate . an increase in the intensity of the exciplex emission 
 ( 6 ) is due to the enhancement of the population of the singlet rip , 
 which can reform the exciplex via ( 4 ) . the singlet rip can be indirectly 
 generated via exciplex dissociation ( 3 ) , e.g. , following a quenching 
 reaction yielding the exciplex ( 2a ) , or directly , via a remote electron 
 transfer reaction ( 2b ) . note that the et reactions preserve the overall 
 spin of the reactant pair ; i.e. , both the exciplex and the rip are 
 formed as singlet entities . the external magnetic field gives rise 
 to an increase in the singlet rip probability and hence the exciplex 
 via pathway 4 . as a consequence of the exciplex dissociation here 
 being a slow process , the mfes generated by the direct et route ( 2b ) 
 will be observed on a faster time scale than those formed by the exciplex 
 route ( 2a ) . in this way , time - resolved mfes of the exciplex emission 
 can distinguish the reaction channels populating the magneto - sensitive 
 rp state ( 2b versus 2a ) . to simulate the experimental data , 
 we used the model introduced in ref ( 2 ) . in this model , the probability that the radical pair 
 system exists as an exciplex , e(t , b0 ) , is given by6where i is the 
 probability that the rip is initially generated ( pathway 2b in scheme 1b ) , kd denotes the rate constant of exciplex dissociation , ri is the distance of rip formation via distant 
 et , and r(t , b0|ri ) is the probability that a 
 rip formed at distance ri at t = 0 has recombined by time t. re refers to the contact distance of the fluorophore and 
 quencher , at which the transition of the rip to the exciplex ( or contact 
 ion pair ) occurs . e = 1  i is the initial exciplex probability ( pathway 2a in scheme 1b ) , and e is the 
 intrinsic exciplex lifetime ( without dissociation ) . the contributions 
 to the exciplex probability in eq 6 can be described as follows : the first term is the 
 probability that the exciplex is formed initially , while the second 
 term accounts for the probability that the initially generated rip 
 reforms an exciplex at t. the third term describes 
 the probability that the exciplex dissociated at time  is reformed 
 at t , and the last term models the depopulation of 
 the exciplex by dissociation ( kd ) and 
 radiative / nonradiative decay with the rate constant . r(t , b0|ri ) is field dependent and has been calculated in the low - viscosity 
 limit as described in detail in refs ( 1 ) and ( 2 ) . the radical pair was assumed to diffuse in a potential 
 of mean force approximately accounting for the microheterogeneous 
 environment . this potential was calculated following an approach based 
 on the d  e0 theorem 
 as outlined in refs ( 18 ) and ( 20 ) . note that 
 more elaborate schemes of treating the diffusion - influenced reversible 
 exciplex kinetics have been derived on the basis of the integral encounter 
 theory . while also comprehensively applicable to highly viscous mixtures , 
 this approach is difficult to employ for radical pairs with a multitude 
 of magnetic nuclei ( as is the case here ) and beyond the scope of the 
 current presentation . figure 2 illustrates the microheterogeneous medium structure to clarify 
 typical length scales and the extent of the local enrichment of the 
 polar component as predicted on the basis of the continuum model from 
 ref ( 20 ) . illustration 
 of the structure of the microheterogeneous solvent surrounding a radical 
 ion pair ( contact distance : 6.8  ) as predicted by the continuum 
 solvation model described in ref ( 18 ) . ( a ) local mole fraction of dmso in dmso / toluene 
 mixtures of bulk concentration xdmso = 
 0.2 ( left ; s = 7.3 ) and 0.4 ( right ; s = 13 ) . ( b ) deviation of the local dielectric constant from 
 the macroscopic dielectric constant for the two solvent mixtures from 
 part a. the plots show contours through a plane containing the two 
 radical centers . figure 3 illustrates the dependence of the mfe of the dmant / dma system 
 on the solvent dielectric constant , s , of the microheterogeneous 
 to / dmso mixture as determined from steady - state ( ss ) and time - resolved ( tr ) measurements ( upper panel ) . the lower panel gives a comparison of the s dependence 
 for the binary solvent mixtures of pa / bn ( homogeneous ) and to / dmso 
 ( microheterogeneous ) . this observation suggests that in 
 both types of experiments the same photochemical reactions are probed . in fact , in view of the low light intensities and low concentrations 
 employed in these experiments , bulk processes such as f - pair reactions 
 and processes involving fluorophore triplets ( triplet  triplet 
 and triplet  doublet pair reactions ) are not expected to contribute 
 to the observed mfes here . instead , the mfes on the exciplex result 
 exclusively from the effect of an external magnetic field on the s / t 
 mixing in the geminate radical pair . the onset of the mfe is found 
 at s = 6 ( with  = 0.1% ) and s = 4.3 ( with  = 1.6% ) in the pa / bn and to / dmso mixture , respectively . 
 in the homogeneous pa / bn mixtures , the mfe is small for s < 10 but rises sharply for s  
 13 to attain its maximum value around s  
 18 ( 11% ) . for larger solvent polarities , it decreases slightly as 
 a larger population of the radical pairs dissociates indefinitely 
 without reencountering ( within the coherence time of the radical pair ) . 
 in the microheterogeneous to / dmso mixtures , the onset of the mfe occurs 
 at a lower bulk permittivity and the maximal value ( 14.5% ) is already 
 observed at s = 8.3 . in essence , the mfe appears 
 to report a more polar environment than would be expected on the basis 
 of the bulk permittivity . these peculiarities in the mfes can be explained 
 on the basis of a dielectric enrichment of the polar component ( dmso ) 
 in the vicinity of the magneto - sensitive rip . as a consequence , the 
 effective dielectric constant , eff , of the enriched 
 solvation shell this specific solvation increases the reencounter 
 probability of the geminate rp without impeding the spin - conversion 
 by too tight binding , which in homogeneous solvents of low polarity 
 is seen to hamper the mfe by confining the radical pairs to configurations 
 with large exchange coupling . as a consequence , larger mfes are possible 
 in the microheterogeneous environments as compared to homogeneous 
 solutions . solvent permittivity dependence of the magnetic field effect of 
 the 9,10-dimethylanthracene / n , n - dimethylaniline 
 exciplex determined from steady - state ( blue filled triangles ) and 
 time - resolved mfe ( red filled stars with barely visible error bars 
 reflecting the statistical error of the registration process ) measurements 
 in solvent mixtures of to / dmso ( upper panel ) . the lower panel compares 
 the mfe in the homogeneous and microheterogeneous solvent mixtures . for various dielectric constants , the time - resolved mfes are shown in figure 4 together with least - squares fits applying 
 the model given by eq 6 . the maximum of the tr - mfes occurs in the range from 10 to 75 ns 
 after photoexcitation , with larger values occurring at lower dielectric 
 constants . for all samples , the mfes reached the noise level of the 
 experiment within 400 ns after excitation . experimental time - dependent 
 magnetic field effects ( i ) at different dielectric 
 constants , s , in pa / bn ( top panel ) and to / dmso ( middle 
 panel ) solvent mixtures for the system 9,10-dimethylanthracene / n , n - dimethylaniline . the bottom panel compares 
 the tr - mfes of the exciplex in pa / bn and to / dmso mixtures of the same 
 dielectric constant ( s = 13 ) . the exciplex lifetime , e , is one of the central parameters characterizing the time 
 evolution of the mfe . we have determined e from 
 the initial decay of the tcspc traces of the exciplex , for which the 
 rp recombination is negligible . figure 5 shows that the apparent exciplex lifetime decreases 
 with increasing bulk dielectric constant , s , in 
 both binary solvent mixtures . furthermore , as expected in view of 
 the above discussion , the exciplex lifetimes are considerably smaller 
 in the to / dmso mixtures as compared to the iso - bulk dielectric pa / bn 
 mixtures . note also that the exciplex is an excited charge - transfer 
 complex with correspondingly large dipole moment . as a consequence , 
 its formation is expected to enhance the diffusive enrichment of polar 
 solvent molecules in its surrounding . in this way , the exciplex can 
 act as a catalyst to its own dissociation ( pathway 3 in scheme 1b ) . as the exciplex lifetime 
 is considerably larger than the characteristic time of solvation , 
 these dynamic aspects can however not be resolved here . permittivity 
 dependence of the exciplex lifetimes of the 9,10-dimethylanthracene / n , n - dimethylaniline exciplex in pa / bn ( red 
 circles ) and to / dmso mixtures ( blue squares ) . the exciplex kinetics are also characterized by the association 
 constant ka = ka / kd ( top panel in figure 6 ) and the exciplex dissociation quantum yield 
 d = kde ( middle panel in figure 6 ) . d was estimated from the dependence of 
 e on dielectric constants assuming that the intrinsic , 
 radiative and nonradiative decay rates are independent of solvent 
 composition . ka has a strong effect on 
 the shape , i.e. , the temporal evolution , of the time - resolved mfes 
 and can thus be extracted from the experimental data by least - squares 
 fitting . following this approach as detailed in ref ( 2 ) , ka and d were obtained as a function of s . in general , d increases with s , likely because polar environments reduce the exciplex stabilization 
 free energy by increasing the reorganization energy . this effect is indeed found and much more pronounced in the microheterogeneous 
 environment as compared to the homogeneous one . for to / dmso , the exciplex 
 dissociation quantum yield approaches d = 1 for 
 a bulk dielectric constant as low as s = 8 , while 
 for pa / bn mixtures this is only the case at s  
 22 . dependence of the association constant , ka , the dissociation quantum yield of the exciplex , d , and the initial rip formation , i , on the dielectric 
 constants , s , of homogeneous ( pa / bn ) and microheterogeneous 
 ( to / dmso ) solvent mixtures . the filled circles and squares with error 
 bars have been determined from the experimental data by modeling the 
 tr - mfe . likewise , the association constant ka is expected to decrease with polarity , as is clearly 
 evidenced in figure 6 for the homogeneous pa / bn solutions . while ka in general also decreases with increasing polarity for to / dmso , 
 it is nearly constant in the range from s = 8 to 
 12 . this surprising behavior suggests that ka increases in this region in parallel to kd such that ka is approximately 
 constant . this can likely be attributed to an increased association 
 rate brought about by the microheterogeneous solvation impeding the 
 separation of the radical ions with respect to the homogeneous environment . just as ka , the probability of the 
 initial rip formation , i , governs the time evolution 
 of mfes of the exciplex and was extracted from the experimental data 
 by least - squares fitting . the extracted i values are also shown in figure 6 as a function of the dielectric constant . the data reveal that , for the dmant / dma system with moderate free 
 energy of charge separation ( get = 0.28 ev ) , the direct exciplex formation 
 ( pathway 2a in scheme 1b ) contributes at all dielectric constants in homogeneous and microheterogeneous 
 environments . furthermore , the probability of distant et quenching 
 increases with increasing solvent polarity . note that the requirement 
 to detect the exciplex luminescence limits the accessible s range to the indicated range ; beyond the respective limits , 
 the exciplex emission is insufficient for the accurate determination 
 of i . in both solvent mixtures , for to / dmso , this 
 is the case for s  9 , and for pa / bn , for 
 s  20 , in agreement with the general bearing . distant electron transfer quenching is more favored in polar solutions , 
 where it leads to a better solvation of the reaction products ( rips ) . this holds true despite a minor increase in the reorganization energy 
 resulting from the increasing polarity . in the low - viscous solutions 
 studied here , the diffusive approach of the reactants is fast enough 
 compared to the intrinsic electron transfer rate to always form a 
 contact complex ( exciplex ) at the contact distance . as a consequence , i is smaller than unity , even for the most polar and hardly 
 emissive solutions . in 
 this work , we have employed time - resolved mfe measurements to study 
 the exciplex and rip dynamics of the dmant / dma system in homogeneous 
 and microheterogeneous solvent mixtures of various bulk permittivities . we use a model accounting for the reversible exciplex dissociation 
 to identify the peculiarities resulting from microheterogeneous solvent 
 environments in solvent mixtures of to and dmso . the exciplex dynamics 
 and the initial quenching products depend strongly on the solvent 
 properties . in microheterogeneous solvents , specific solvation of 
 the exciplex and rip gives rise to an environment that is significantly 
 more polar than that observed in homogeneous solvents of comparable 
 bulk permittivity . for low permittivities , this brings about a larger 
 dissociation quantum yield of the exciplex , a lower exciplex lifetime , 
 and a larger probability that the system undergoes distant electron 
 transfer quenching . specific solvation furthermore impedes the radical 
 separation such that , in the permittivity region from 7 to 15 , the 
 association constant decays only slowly with the solvent polarity 
 in the to / dmso mixtures . in both environments , the probability of 
 the initial formation of rips is always less than unity ; i.e. , the 
 exciplex formation contributes at all s . in microheterogeneous 
 solution , the local concentration of dmso in the solvation shell around 
 the exciplexes and rips plays an important role in the exciplex kinetics 
 and the fluorescence quenching mechanism . together with earlier works , this study demonstrates that time - resolved mfe studies have the potential 
 to provide detailed insights into the reaction dynamics of rips and 
 exciplexes . since the reaction dynamics of the geminate radical pair 
 are very sensitive to small changes in the local dielectric environment , 
 the technique is here recognized as a versatile tool in revealing 
 and characterizing the phenomenon of specific solvation in solvent 
 mixtures .