Spectroscopic, kinetic, and thermodynamic deuterium isotope effects in the hexamethylbenzene/tetracyanoethylene charge-transfer complex

The charge-transfer absorption and fluorescence spectra, resonance Raman spectra, and equilibrium constants and molar absorptivities of the complexes between tetracyanoethylene and hexamethylbenzene (h₁₈-HMB) or perdeuteriated hexamethylbenzene (d₁₈-HMB) are compared. The enthalpies and entropies of complexation and the absorptivities in CCl₄ solution are the same for the two isotopes to within an experimental uncertainty of about ±10%. The vibrations that carry significant intensity in the resonance Raman spectra are only slightly shifted by perdeuteriation of the HMB, suggesting that hydrogen motions are only weakly coupled to the charge-transfer transition. However, the fluorescence quantum yields in both CCl₄ and cyclohexane solvents indicate that perdeuteriation decreases the rate of nonradiative return electron transfer by a factor of about 1.6, implying more, significant participation of modes involving hydrogen motion. Perdeuteriation shifts the absorption spectra about 120 cm^-1 to the blue while having little effect on the fluorescence spectra. Complexes of HMB, durene, and p-xylene with tetracyanobenzene as acceptor similarly exhibit negligible isotope effects on the fluorescence band shapes but significant (factors of 1.3-2.1) effects on the fluorescence yields. Calculations on HMB/TCNE within the harmonic approximation are unable to reproduce the isotope effects on both the spectra and the kinetics with a common set of parameters. Anharmonicities of the CH (CD) stretches may play an important role as is thought to be the case in other radiationless transition processes.