Band-shape analysis of the charge-transfer fluorescence in barrelene-based electron donor-acceptor compounds

In this paper we analyze the band shapes of the stationary fluorescence spectra for the radiative charge transfer recombination D⁺-A^- → D-A + hv in a series of barrelene-based organic bridged donor (D)-acceptor (A) molecules D-A, exploring solvent polarity and temperature effects on the band shape. Free energy changes for charge recombination, medium reorganization energies, and intramolecular reorganization energies (for high- and medium-frequency vibrational modes) were evaluated from the band maximum energies and the full widths at half-maximum and from the fits of standard Franck-Condon factors to the entire fluorescence band shape. Utilization of a single high-frequency intramolecular model (ℏω_i = 1300 cm^-1) in conjunction with solvent modes provided a satisfactory fit of the band shape; however, such an analysis results in unphysically large medium reorganization energies for nonpolar solvents (0.30-0.45 eV) and their large temperature dependence. The inclusion of medium-frequency (ℏω ≃ 500 cm^-1) intramolecular modes in the band-shape analysis is important for the proper distinction between solvent and intramolecular reorganization.