Photoinduced electron transfer to N-alkoxypyridiniums, which leads to N–O bond cleavage and alkoxyl radical formation, is highly chain amplified in the presence of a pyridine base such as lutidine. Density functional theory calculations support a mechanism in which the alkoxyl radicals react with lutidine via proton-coupled electron transfer (PCET) to produce lutidinyl radicals (BH•). A strong electron donor, BH• is proposed to reduce another alkoxypyridinium cation, leading to chain amplification, with quantum yields approaching 200. Kinetic data and calculations support the formation of a second, stronger reducing agent: a hydrogen-bonded complex between BH• and another base molecule (BH•···B). Global fitting of the quantum yield data for the reactions of four pyridinium salts (4-phenyl and 4-cyano with N-methoxy and N-ethoxy substituents) led to a consistent set of kinetic parameters. The chain nature of the reaction allowed rate constants to be determined from steady-state kinetics and independently determined chain-termination rate constants. The rate constant of the reaction of CH3O• with lutidine to form BH•, k1, is ∼6 × 106 M–1 s–1; that of CH3CH2O• is ∼9 times larger. Reaction of CD3O• showed a deuterium isotope effect of ∼6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases k1 by a factor of ∼400.

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.

Compared to linear acenes, only a few examples of peri-condensed organic semiconductors are known. Here, we report synthesis and charge transport properties of peri-condensed heteropyrenes in organic field effect transistors (OFETs). We show that 1,6-dioxapyrene derivatives are easily accessible and stable semiconductors that form well-ordered polycrystalline films, which results in moderately large hole mobility (ca. 0.25 cm2 V−1 s−1) in OFET devices.

In organic thin film transistors (OTFT), the morphology and microstructure of an organic thin film has a strong impact on the charge carrier mobility and device characteristics. To have well-defined and predictable thin film morphology, it is necessary to adapt the basic structure of semiconducting molecules in a way that results in an optimum crystalline packing motif. Here we introduce a new molecular design feature for organic semiconductors that provides the optimized crystalline packing and thin film morphology that is essential for efficient charge-carrier transport. Thus, cyclohexyl end groups in naphthalene diimide assist in directing intermolecular stacking leading to a dramatic improvement in field effect mobility. Accordingly, OTFT devices prepared with vapor deposited N,N′-bis(cyclohexyl) naphthalene-1,4,5,8-bis(dicarboximide) (1) regularly exhibit field effect mobility near 6 cm2/(V s), which is one of the highest carrier mobilities reported for either n- or p-type organic semiconducting thin films.