Thermochromism of intramolecular charge transfer emission bands. Probing the temperature dependence of Franck—Condon factors

The temperature dependence of the charge transfer emission band, Franck—Condon maximum has been determined for two donor—spacer—acceptor molecules in six solvents with dielectric constants between 4 and 10. The CT band maximum shifted to higher energies (27 cm−1 K−1 in Et2O, 14 cm−1 K−1 in ClCH2CH2Cl) with increasing temperature. Analysis of the thermochromism solvent dependence using a continuum model yielded values of the geometric factor, e2(1/rA + 1/rD −2/Rcc), of 22.9 and 28.5 kK for DSAs 1 and 2. Solvent-independent thermocheomism of ≈ 8 cm−1 K−1 was found in both DSA molecules. Possible sources of this thermochromism are discussed.