Of the many known examples of exciplexes, those formed from bimolecular encounter between a cationic, excited state electron acceptor and a neutral donor in fluid media have not been previously reported. We now show that emissive exciplexes formed from excited N-methyl isoquinolinium cation (NMiQ+) with alkyl benzene donors are readily detected in acetonitrile. These cationic exciplexes result in a charge shift (A+* + D → A•D•+) with no net change in charge, which differs fundamentally from the charge-generation of conventional exciplex formation (A* + D → A•–D•+). We find that cationic and conventional exciplexes show similar trends, e.g., bathochromic shifts and decreases in fluorescence quantum yields with decreasing oxidation potentials of the donors. In the presented examples of NMiQ+ exciplexes, the fluorescence quantum yield decreases by a factor of 30 and the radiative rate constant by 6.6 as the fractional CT character of the exciplex increases from ∼0.79 to ∼0.95. Interestingly, the electronic coupling matrix elements for the NMiQ+ exciplexes, derived from a correlation of the radiative rate constants with the average emission frequencies, are similar to those of related conventional exciplexes, in spite of the absence of Coulombic stabilization in the cationic exciplexes.

As we reported recently, endergonic to mildly exergonic electron transfer between neutral aromatics (benzenes and biphenyls) and their radical cations in acetonitrile follows a Sandros–Boltzmann (SB) dependency on the reaction free energy (ΔG); i.e., the rate constant is proportional to 1/[1 + exp(ΔG/RT)]. We now report deviations from this dependency when one reactant is sterically crowded: 1,4-di-tert-butylbenzene (C1), 1,3,5-tri-tert-butylbenzene (C2), or hexaethylbenzene (C3). Obvious deviation from SB behavior is observed with C1. Stronger deviation is observed with the more crowded C2 and C3, where steric hindrance increases the interplanar separation at contact by ∼1 Å, significantly decreasing the π orbital overlap. Consequently, electron transfer (ket) within the contact pair becomes slower than diffusional separation (k–d), causing deviation from the SB dependency, especially near ΔG = 0. Fitting the data to a standard electron-transfer theory gives small matrix elements (∼5–7 meV) and reasonable reorganization energies. A small systematic difference between reactions of C3 with benzenes vs biphenyls is rationalized in terms of small differences in the electron-transfer parameters that are consistent with previous data. The influence of solvent viscosity on the competition between ket and k–d was investigated by comparing reactions in acetonitrile and propylene carbonate.

Rate constants (k) for exergonic and endergonic electron-transfer reactions of equilibrating radical cations (A•+ + B ⇌ A + B•+) in acetonitrile could be fit well by a simple Sandros−Boltzmann (SB) function of the reaction free energy (ΔG) having a plateau with a limiting rate constant klim in the exergonic region, followed, near the thermoneutral point, by a steep drop in log k vs ΔG with a slope of 1/RT. Similar behavior was observed for another charge shift reaction, the electron-transfer quenching of excited pyrylium cations (P+*) by neutral donors (P+* + D → P• + D•+). In this case, SB dependence was observed when the logarithm of the quenching constant (log kq) was plotted vs ΔG + s, where the shift term, s, equals +0.08 eV and ΔG is the free energy change for the net reaction (Eredox − Eexcit). The shift term is attributed to partial desolvation of the radical cation in the product encounter pair (P•/D•+), which raises its free energy relative to the free species. Remarkably, electron-transfer quenching of neutral reactants (A* + D → A•− + D•+) using excited cyanoaromatic acceptors and aromatic hydrocarbon donors was also found to follow an SB dependence of log kq on ΔG, with a positive s, +0.06 eV. This positive shift contrasts with the long-accepted prediction of a negative value, −0.06 eV, for the free energy of an A•−/D•+ encounter pair relative to the free radical ions. That prediction incorporated only a Coulombic stabilization of the A•−/D•+ encounter pair relative to the free radical ions. In contrast, the results presented here show that the positive value of s indicates a decrease in solvent stabilization of the A•−/D•+ encounter pair, which outweighs Coulombic stabilization in acetonitrile. These quenching reactions are proposed to proceed via rapidly interconverting encounter pairs with an exciplex as intermediate, A*/D ⇌ exciplex ⇌ A•−/D•+. Weak exciplex fluorescence was observed in each case. For several reactions in the endergonic region, rate constants for the reversible formation and decay of the exciplexes were determined using time-correlated single-photon counting. The quenching constants derived from the transient kinetics agreed well with those from the conventional Stern−Volmer plots. For excited-state electron-transfer processes, caution is required in correlating quenching constants vs reaction free energies when ΔG exceeds ∼+0.1 eV. Beyond this point, additional exciplex deactivation pathways—fluorescence, intersystem crossing, and nonradiative decay—are likely to dominate, resulting in a change in mechanism.

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A•–/D•+ for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, kq, to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm–Weller formula rests on an incorrect mechanism. We have remeasured kq for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and kq, including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros–Boltzmann (SB) equation kq = klim/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A•–/D•+). The quantity klim represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A•–/D•+ vs A•– + D•+). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of −0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A•–/D•+ relative to the free A•– + D•+, which opposes the Coulombic stabilization of A•–/D•+. The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A•– + D•+). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of kq on ΔG.

Green light excitation of platinum octaethylporphyrin (PtOEP) in poly(methyl methacrylate) (PMMA) films containing 9,10-diphenylanthracene (DPA) yielded emission of blue light under ambient conditions. This energy upconversion process was easily observed using low-power monochromatic excitation. The blue light emission arose from fluorescence of the lowest excited singlet state of DPA formed by spin-allowed annihilation of two DPA triplets excited by the transfer of the triplet energy from PtOEP to DPA. The upconversion emission intensity was proportional to the square of the excitation intensity, and an upconversion quantum efficiency of ∼0.0002 was measured at an excitation intensity of only 0.9 mW cm−2. High concentrations of molecularly doped emitter facilitated triplet energy migration in rigid PMMA. Upconverted fluorescence was also observed from other emitters and with other sensitizers, including blue-to-UV upconversion in a PMMA film containing the non-heavy metal sensitizer 2-methoxythioxanthone and the emitter 2,5-diphenyloxazole.

Nanosecond transient absorption methods were used to determine accurate oxidation potentials (Eox) in acetonitrile for benzene and a number of its alkyl-substituted derivatives. Eox values were obtained from a combination of equilibrium electron-transfer measurements and electron-transfer kinetics of radical cations produced from pairs of benzene and biphenyl derivatives, with one member of the pair acting as a reference. Using a redox-ladder approach, thermodynamic oxidation potentials were determined for 21 benzene and biphenyl derivatives. Of particular interest, Eox values of 2.48 ± 0.03 and 2.26 ± 0.02 V vs SCE were obtained for benzene and toluene, respectively. Because of a significant increase in solvent stabilization of the radical cations with decreasing alkyl substitution, the difference between ionization and oxidation potentials of benzene is ∼0.5 eV larger than that of hexamethylbenzene. Oxidation potentials of the biphenyl derivatives show an excellent correlation with substituent σ+ values, which allows Eox predictions for other biphenyl derivatives. Significant dimer radical cation formation was observed in several cases and equilibrium constants for dimerization were determined. Methodologies are described for determining accurate electron-transfer equilibrium constants even when dimer radical cations are formed. Additional equilibrium measurements in trifluoroacetic acid, methylene chloride, and ethyl acetate demonstrated that solvation differences can substantially alter and even reverse relative Eox values.

With the judicious selection of triplet energy donor (D) and acceptor (A) pairs, a laser flash photolysis procedure has provided a sensitive method for the study of triplet energy transfer in rigid polymer films. By monitoring changes in triplet−triplet (T−T) absorptions the kinetics of triplet energy transfer were evaluated at short time scales, and overall energy-transfer quantum yields were also obtained. Combinations of xanthone- or thioxanthone-type donors and polyphenyl acceptors were particularly suited to these measurements because the former have high intersystem-crossing quantum yields and the latter have very high extinction coefficients for T−T absorption. For exothermic transfer most of the energy transfer that occurred within the lifetime of triplet D (3D) took place in less than a few microseconds after 3D formation in poly(methyl methacrylate), and triplet A yields were limited largely by the number of A molecules in near contact with 3D. The kinetics of triplet energy transfer were modeled using a modified Perrin-type statistical arrangement of D/A separations with allowance for excluded volume in combination with a Dexter-type formula for the distance-dependent exchange energy-transfer rate constant. Experimental observations were best explained by constraining D/A separations to reflect the dimensions of intervening molecules of the medium. Rate constants, k0, for exothermic energy transfer from 3D to A molecules in physical contact are approximately 1011 s−1 and very similar to triplet energy-transfer rate constants determined from solution encounters. Energy-transfer rate constants, k(r), fall off as approximately exp(−2r/ 0.85), where r is the separation distance between D and A centers in angstroms. Exchange energy transfer is not restricted to 3D and A in physical contact, but at ≤0.4 M A at least 85% of the energy transfer arises from interaction of 3D with a single nearest-neighbor A molecule. The modified Perrin model was also applied to quantum yields of quenching in rigid media. Comparison to the simple Perrin model for quenching shows that the latter may be adequate as long as molecular volumes are accommodated in the Perrin expression. Under these conditions the critical radius, rc, corresponds to the 3D/A separation at which the effective rate constant for energy transfer equals the inverse of the 3D lifetime.