Estimation of the solvent reorganization energy and the absolute energy of solvation of charge-transfer states from their emission spectra

Abstract

We report herein the solvent and temperature effects on the emission of the intermolecular exciplexes 1-cyanonaphthalene/triethylamine and 1-methylnaphthalene/triethylamine and the intramolecular exciplexes formed by the bichromophoric compounds diethyl-(3-naphthalen-1-yl-propyl)-amine (I), diethyl-(2-naphthalen-1-yl-ethyl)-amine (II), 3-[ethyl-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (III) and 3-[(2-cyano-ethyl)-(2-naphthalen-1-yl-ethyl)-amino]-propionitrile (IV). The results are interpreted within the formalism of the semiclassical Marcus theory for radiative back electron transfer (BET) reactions. We show that, following a few simple assumptions, reliable values of the Gibbs free energy changes (DeltaG(epsilon)(-et)) and the solvent reorganization energies (lambda(epsilon)(s)) associated to the BET reactions (and their corresponding enthalpic and entropic contributions) can be estimated directly from the emission of the CT states. We also show that for the 1-cyanonaphthalene/triethylamine exciplex, which exhibits emission in the vapour phase, the experimental calculation of the absolute energy of solvation of the CT state (DeltaG(epsilon)(s)) is also possible. The calculated DeltaG(epsilon)(-et) correlate quite satisfactorily with the corresponding values obtained from independent electrochemical and kinetics experiments. The temperature effects on DeltaG(epsilon)(-et), lambda(epsilon)(s) and DeltaG(epsilon)(s) are discussed qualitatively using different solvation models. The limitations of the present approach for the estimation of DeltaG(epsilon)(-et) and lambda(epsilon)(s) and its possible application to more complex A/D systems are also examined.

Impact of Temperature and Non-Gaussian Statistics on Electron Transfer in Donor–Bridge–Acceptor Molecules

“…This general result is a consequence of the liquid state of a polar molecular solvent, producing more structural fluctuations and distinct modes of thermal agitation ,, than allowed by continuum models better suited for modeling solids. This observation, now well supported by both experiment ,,, and numerical simulations, , calls for a critical re-examination of the early temperature-dependent kinetic data. , Those were mostly viewed as fully consistent with the Marcus picture, although some inconsistencies have been identified. In particular, the reorganization energy found from fitting the kinetics to the energy gap law for Miller’s set of donor–acceptor complexes fell significantly below the continuum Marcus equation .…”

Cucurbit[8]uril Mediated Donor–Acceptor Ternary Complexes: A Model System for Studying Charge-Transfer Interactions

“…Figure7. Calculated electrostatic potential (EP) at the B3LYP/6-31G* level of theory for (a) CB[5], (b) CB[6], (c) CB[7], and (d) CB[8] in the σ h plane and (left) in the σ v plane (right).…”

Intramolecular Photoinduced Charge Transfer and Recombination Dynamics in Vinylarene Terminated Organosilanes

“…Given that the charge-separated state is emissive, estimates for the total reorganization energy, λ, and the sum Δ G 0 + λ can be determined using the energies of peak emission (ℏω em ) and absorption (ℏω abs ) measured for each organosilane in DCM: ,, Δ G 0 + λ is directly related to the peak emission gap (i.e., −ℏω em = Δ G 0 + λ). The total reorganization energy was calculated as λ = 0.5­(ℏω abs – ℏω em ); in practice we used the peak energy from the fluorescence excitation spectrum for ℏω abs due to the overlap between the σ → π* absorption transition, which populates the emissive charge-separated state, and the acceptor π → π* absorption transition, which results in significantly weaker emission.…”

“…From these values one calculates a solvent reorganization energy, λ S , of ∼0.2 eV in DCM, which is consistent with λ S estimated independently from a Lippert–Mataga analysis of solvent-dependent emission Stokes’ shifts for the Si n s-E,C end-capped compounds (see additional discussion in Supporting Information). ,,, Given quantitative similarities across all structures, we used the average values for λ and λ I in our calculations of V ; we also estimated Δ G 0 for BET in each compound using this common value for λ (Table ). Importantly, the similarity in the reorganization energies for all structures reflects that the emitting state in each of these compounds is characteristically similar (i.e., regardless of acceptor identity, they can all be classified as a charge-separated state).…”

“…The total reorganization energy was calculated as λ = 0.5(ℏω abs − ℏω em ); in practice we used the peak energy from the fluorescence excitation spectrum for ℏω abs due to the overlap between the σ → π* absorption transition, which populates the emissive charge-separated state, and the acceptor π → π* absorption transition, which results in significantly weaker emission. We used the same method to determine the intramolecular reorganization energy, λ I , using emission and fluorescence excitation spectra collected for organosilanes in cyclohexane (CH); 42 the static and optical dielectric constants of CH are identical such that solvent reorganization energy is assumed to be negligible. Table 2 summarizes our values for −ℏω em and ℏω abs in DCM and CH as well as our estimates of ΔG 0 , λ, and λ I ; spectral data for Si2s-2E, Si2s-2C, Si4s-2E, and Si4s-2C used for these determinations are presented in the Suppporting Information (displayed in Figures S3−S6).…”