Effects of Preferential Solvation Revealed by Time-Resolved Magnetic Field Effects.

External magnetic fields can impact recombination yields of photoinduced electron transfer reactions by affecting the spin dynamics in transient, spin-correlated radical pair intermediates. For exciplex-forming donor-acceptor systems, this magnetic field effect (MFE) can be investigated sensitively by studying the delayed recombination fluorescence. Here, we investigate the effect of preferential solvation in microheterogeneous solvent mixtures on the radical pair dynamics of the system 9,10-dimethylanthracene (fluorophore)/N,N-dimethylaniline (quencher) by means of time-resolved magnetic field effect (TR-MFE) measurements, wherein the exciplex emission is recorded in the absence and the presence of an external magnetic field using time-correlated single photon counting (TCSPC). In microheterogeneous environments, the MFE of the exciplex emission occurs on a faster time scale than in iso-dielectric homogeneous solvents. In addition, the local polarity reported by the exciplex is enhanced compared to homogeneous solvent mixtures of the same macroscopic permittivity. Detailed analyses of the TR-MFE reveal that the quenching reaction directly yielding the radical ion pair is favored in microheterogeneous environments. This is in stark contrast to homogeneous media, for which the MFE predominantly involves direct formation of the exciplex, its subsequent dissociation to the magneto-sensitive radical pair, and re-encounters. These observations provide evidence for polar microdomains and enhanced caging, which are shown to have a significant impact on the reaction dynamics in microheterogeneous binary solvents.

Introduction

Solvent polarity strongly affects the mechanism of phopan class="Chemical">toinduced electron transfer (ET) reactions. In polar solvents, the process usually involves complete ET from the electron n>n class="Species">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.[1−8] Due to similar spectral footprints of ion pairs and exciplexes, it is generally difficult to elaborate which channel dominates.[9−12] 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.[13−18] 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.[16,19,20] 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.[1,2]

The pan class="Chemical">MFE relevant n>n class="Chemical">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.[21−25] 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± cannot 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 a).[26−28] 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 b).[19,20,29−33]

Scheme 1

(a) Illustration of the Origin of the MFE Detected via the Exciplex Emission; (b) Important Reaction Pathways of the Transient Species Involved in the MFE of the Exciplex; (c) Chemical Structures of the Fluorophore and the Donor

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 cannot 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.

In the absence of an external magnetic field (B0 = 0), the pan class="Chemical">singlet (S) and all three tpan class="Gene">riplet (T0,±) states are interconverted by the radical hyperfine interactions (HFI). In the presence of a sufficiently large magnetic field B0, the T± states cannot mix with the S state.

The red and orange arrows refer pan class="Chemical">to the radiative processes of the exciplex and the locally excited fluorophore emissions, respectively.

As a consequence of this mechanism, any facpan class="Chemical">tor influencing the n>n class="Gene">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.[19,20] Solvent mixtures composed of toluene (TO) and dimethyl sulfoxide (DMSO) have been studied in detail.[15,34] Studies of the magnetic isotope effect revealed no direct transfer of spin density on solvent molecules.[20] 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, εs, 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.[20] 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.[35,36] 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 inpan class="Chemical">to account. The mechanism of fluorescence quenching by electron transfer is, however, ambiguous; i.e., the initial quenching products are a n>n class="Gene">RIP via distant ET (pathway 2B of Scheme b) 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.[1,2] 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–acceptor system N,N-dimethylaniline (quencher)/9,10-dimethylanthracene (fluorophore) serves as a magnetosensitive probe (see Scheme c).[1,2,19,20,31,32] 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, this semantic distinction is immaterial.

Experimental Section

Sample and Solvent Preparation

pan class="Chemical">9,10-Dimethylanthracene (n>n class="Chemical">DMAnt, Aldrich 99%) was used as received. 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 × 10–5 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, εs = 6; this and all subsequently reported values refer to a temperature of 295 K) and butyronitrile (BN, Fluka 99%, distilled under reduced pressure, εs = 24.7) and of toluene (TO, Fluka 99%, distilled under reduced pressure, εs = 2.38) and dimethyl sulfoxide (DMSO, Aldrich 99.9%, used as received, εs = 46.4) with widely varying dielectric constants, εs, were prepared. The dielectric constants of these mixtures are given by eq and eq for the PA/BN and TO/DMSO systems, respectively (at 295 K):[1,2,19,20]Here, 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 6–24.7 (at 295 K) while keeping the viscosity (η) and the refractive index (n) almost constant. The Pekar factor, γ = (1/ε∞ – 1/εs) ≈ (1/n2 – 1/εs), which governs the outer-sphere electron transfer reorganization energy, increases only moderately with increasing permittivity.[1,2,20,37,38] 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 γ of TO/DMSO mixtures slightly increase with increasing DMSO concentration.[20] We are not aware of a microheterogeneous solvent mixture for which this could be avoided.

Steady-State MFE Measurements

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 pan class="Chemical">to the method described in ref (32). The absolute n>n class="Chemical">MFE, χSS, was evaluated fromHere, I̅(λem, Bsat) and I̅(λem, B0 = 0) are the mean intensities at λem in a saturated magnetic field (Bsat = 62 mT) and in the absence of an additional magnetic field, respectively. I̅F(λem, B0) is the residual emission of the locally excited fluorophore at λem 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. (λem) is the mean background intensity.

Time-Resolved MFE Measurements and Modeling

Time-resolved data of the pan class="Chemical">MFEs on the exciplex were obtained by the TCSPC technique. In order n>n class="Chemical">to investigate the effect of an external magnetic field on the exciplex emission (Figure ), a saturating magnetic field (B0 = 62 mT) was applied. 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).

Figure 1

Time-resolved exciplex emission (upper panel) of the 9,10-dimethylanthracene (1.5 × 10–5 M)/N,N-dimethylaniline (0.06 M) system in a TO/DMSO mixture at εs = 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 ΔI(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 . 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.

Time-resolved exciplex emission (upper panel) of the pan class="Chemical">9,10-dimethylanthracene (1.5 × 10–5 M)/n>n class="Chemical">N,N-dimethylaniline (0.06 M) system in a TO/DMSO mixture at εs = 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 ΔI(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 . 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 pan class="Chemical">MFE measurement are shown in Figure . 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 inn>n class="Chemical">to 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, ΔI(t), in the absence and presence of the external, biasing magnetic field: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, χTR, from the time-resolved MFE measurements:Scheme b 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.[1,2] The ordinate corresponds to the free energy. 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).

pan class="Chemical">To simulate the experimental data, we used the model introduced in ref (2). In this model, both initial quenching products (exciplex or n>n class="Gene">RIP) are possible. The probability that the radical pair system exists as an exciplex, ρE(t, B0), is given bywhere ϕI is the probability that the RIP is initially generated (pathway 2B in Scheme b), 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. ϕE = 1 – ϕI is the initial exciplex probability (pathway 2A in Scheme b), and τE′ is the intrinsic exciplex lifetime (without dissociation). The contributions to the exciplex probability in eq 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 τ 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.[39−42] 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 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).

Figure 2

Illustration of the structure of the microheterogeneous solvent surrounding a radical ion pair (contact distance: 6.8 Å) 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; εs = 7.3) and 0.4 (right; εs = 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. The parameter values as given in ref (20) have been used.

Illustration of the structure of the microheterogeneous solvent surrounding a radical ion pair (contact distance: 6.8 Å) as predicted by the continuum solvation model described in ref (18). (a) Local mole fraction of pan class="Chemical">DMSO in n>n class="Chemical">DMSO/toluene mixtures of bulk concentration xDMSO = 0.2 (left; εs = 7.3) and 0.4 (right; εs = 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. The parameter values as given in ref (20) have been used.

Results and Discussion

Figure illustrates the dependence of the pan class="Chemical">MFE of the n>n class="Chemical">DMAnt/DMA system on the solvent dielectric constant, εs, of the microheterogeneous TO/DMSO mixture as determined from steady-state (χSS) and time-resolved (χTR) measurements (upper panel). The lower panel gives a comparison of the εs dependence for the binary solvent mixtures of PA/BN (homogeneous) and TO/DMSO (microheterogeneous). Steady-state and time-resolved measurements agree within experimental error. 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 εs = 6 (with χ = 0.1%) and εs = 4.3 (with χ = 1.6%) in the PA/BN and TO/DMSO mixture, respectively. In the homogeneous PA/BN mixtures, the MFE is small for εs < 10 but rises sharply for εs ∼ 13 to attain its maximum value around εs ∼ 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 εs = 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, εeff, of the enriched solvation shell is increased over the bulk dielectric constant, εs.[19,20] 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.

Figure 3

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.

Solvent permittivity dependence of the magnetic field effect of the pan class="Chemical">9,10-dimethylanthracene/n>n class="Chemical">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 pan class="Chemical">MFEs are shown in Figure together with least-squares fits applying the model given by eq . 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.

Figure 4

Experimental time-dependent magnetic field effects (ΔI) at different dielectric constants, εs, 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 (εs = 13). The gray and solid colored lines refer to experiment and simulation, respectively.

Experimental time-dependent magnetic field effects (ΔI) at different dielectric constants, εs, in PA/pan class="Chemical">BN (n>n class="Chemical">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 (εs = 13). The gray and solid colored lines refer to experiment and simulation, respectively.

The exciplex lifetime, τE, is one of the central parameters characterizing the time evolution of the pan class="Chemical">MFE. We have determined τE from the initial decay of the TCSPC traces of the exciplex, for which the RP recombination is negligible. Figure shows that the appan>rent exciplex lifetime decreases with increasing bulk dielectric constant, εs, in both binary solvent mixtures. Furthermore, as expected in view of the above discussion, the exciplex lifetimes are considerably smaller in the n>n class="Chemical">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 b). As the exciplex lifetime is considerably larger than the characteristic time of solvation, these dynamic aspects can however not be resolved here.

Figure 5

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 solid lines have been added to guide the eye.

Permittivity dependence of the exciplex lifetimes of the pan class="Chemical">9,10-dimethylanthracene/n>n class="Chemical">N,N-dimethylaniline exciplex in PA/BN (red circles) and TO/DMSO mixtures (blue squares). The solid lines have been added to guide the eye.

The exciplex kinetics are also characterized by the association constant Ka = ka/kd (pan class="Chemical">top panel in Figure ) and the exciplex dissociation quantum yield ϕd = kdτE (middle panel in Figure ). ϕd was estimated from the dependence of τE 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 pan class="Chemical">MFEs and can thus be extracted from the experimental data by least-squares fitting.[1,2] Following this approach as detailed in ref (2), Ka and ϕd were obtained as a function of εs. In general, ϕd increases with εs, likely because polar environments reduce the exciplex stabilization free energy by increasing the reorganization energy.[7,32] 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 ϕd = 1 for a bulk dielectric constant as low as εs = 8, while for PA/BN mixtures this is only the case at εs ∼ 22.

Figure 6

Dependence of the association constant, Ka, the dissociation quantum yield of the exciplex, ϕd, and the initial RIP formation, ϕI, on the dielectric constants, εs, 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. The solid lines have been added to guide the eye.

Dependence of the association constant, Ka, the dissociation quantum yield of the exciplex, ϕd, and the initial pan class="Gene">RIP formation, ϕI, on the dielectric constants, εs, of homogeneous (PA/n>n class="Chemical">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. The solid lines have been added to guide the eye.

Likewise, the association constant Ka is expected pan class="Chemical">to decrease with polarity, as is clearly evidenced in Figure for the homogeneous PA/n>n class="Chemical">BN solutions. While Ka in general also decreases with increasing polarity for TO/DMSO, it is nearly constant in the range from εs = 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 pan class="Gene">RIP formation, ϕI, governs the time evolution of pan class="Chemical">MFEs of the exciplex and was extracted from the experimental data by least-squares fitting.[1,2] The extracted ϕI values are also shown in Figure as a function of the dielectric constant. The data reveal that, for the DMAnt/DMA system with moderate free energy of charge separation (ΔGet = −0.28 eV),[43,44] the direct exciplex formation (pathway 2A in Scheme b) 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 εs range to the indicated range; beyond the respective limits, the exciplex emission is insufficient for the accurate determination of ΔI. In both solvent mixtures, ϕI levels off at ϕI = 0.55. For TO/DMSO, this is the case for εs ≥ 9, and for PA/BN, for εs ≥ 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, ϕI is smaller than unity, even for the most polar and hardly emissive solutions.

Conclusions

In this work, we have employed time-resolved pan class="Chemical">MFE measurements n>n class="Chemical">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 εs. 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.