Exciplex mechanism of fluorescence quenching in polar media

Abstract

The formation of exciplexes (non-emitting or poorly emitting) is suggested as one of the causes for deviations of experimental data on fluorescence quenching in polar solvents from the classical model of excited-state electron transfer yielding radical ion pairs. Several evidences for the formation of such exciplexes were found for fluorescence quenching of aromatic compounds by weak electron donors and acceptors. For cyano-substituted anthracenes exciplex emission can be observed in the presence of quenchers even in polar solvents. In other systems, indirect evidences of exciplex formation were observed: nonlinear dependence of the inverse value of excited pyrene lifetime oa the concentration of the quencher; very small and, in some cases, even negative experimental activation energies of pyrene fluorescence quenching, which are much less than activation energies, calculated from the experimental values of the quenching rate constants etc. The proposed model explains the difference between theoretical and experimental dependencies of log kQ vs. Gibbs energy of electron transfer AGeT and other experimental features known for fluorescence quenching by electron donors and acceptors. This model states that the exciplex is in equilibrium with the encounter complex and apparent quenching rate constants are controlled by two main factors-the lifetime of the exciplex and the enthalpy of its formation. Experimentally observed dependence of apparent quenching rate constant on AGET is caused by the dependence of the eXciplex formation enthalpy on AGET, which is quite different from the dependence of electron transfer activation energy on AGET predicted Joy the theoretical models. Simulations of the dependencies of log kQ vs. AGEv according to the exciplex formation model confirms its agreement with the experimental data. Electronic structure of the exciplex involved maylbe close to contact radical-ion pair only at AGEv < 0, when the rate of quenching is limited mainly by the diffusion, but for AGET > 0, the structure of the exciplex should be much less polar.

Effect of Diffusion on Photo-Induced Excited-State Energy Transfer between Fluorescent Semiconducting Molecules: Tris-(8-hydroxyquinoline) Aluminum and 6,13-Bis (Triisopropylsilylethynyl) Pentacene

“…In low viscosity solvents, where diffusion plays a significant role, the energy transfer obeys the Stern–Volmer kinetics, resulting in a single exponential fluorescence decay of the donor. Exciplexes are transient donor–acceptor complexes involving the donor excited state and the ground state of the acceptor and are characterized by long radiative decay times and broad red-shifted fluorescence spectra concerning the isolated molecule. , …”

Transient Exciplex Formation Electron Transfer Mechanism

“…Probably this mechanism dominates at ΔG ET < −λ for ET between uncharged molecules [32]. Transient exciplex (M ∓z Q ±z ) formation mechanism (bottom line of Scheme 1) was discussed by several groups of authors [10][11][12][13][14][15][16][17] to explain some experimentally observed deviations from Marcus and Weller relationships between k Q and ΔG ET . Here k F , k IC , k ISC , and k R are rate constants of fluorescence, internal conversion, intersystem crossing, and dissociation of the exciplex into radical ions, respectively; z is a degree of charge transfer.…”

“…Both ΔG Ex * and a degree of charge transfer in the exciplex, z, depend on ΔG ET * and medium polarity [34][35][36][37][38][39][40][41]. This approach was developed in [10,11,15,16] by simultaneous study of kinetics of an excited state quenching and an exciplex formation and decay. Some earlier approach to the transient exciplex formation [42] considered constant z in the exciplex independent of ΔG ET which provided erroneous dependence of ΔG Ex * on ΔG ET * .…”

“…However, it must take into account that actually these experimental dependences are cut off by the horizontal limit near log(1/k Q − 1/k 1 ) = − (10)(11) and have sigmoid shape since one has to use some overestimated values for k 1 to prevent too large scattering and negative values of (1/k Q − 1/k 1 ), arising due to some variations k Diff and k 1 for different compounds (log k 1 ≈ 10.1-10.3).…”

“…In this case an intermediate with partial charge transfer (A −z D +z , 0 < z < 1) is formed, and the reaction mechanism is no longer that of a single elementary step through a transition state. In excited-state ET reactions these transients were observed experimentally and are well known as exciplexes [10][11][12][13][14][15][16][17]. Strong electronic coupling can even cause some decrease of the diffusion barrier for formation of contact pair from loose pair of reactants.…”