Experimental and theoretical study of excited-state structure and relaxation processes of betaine-30 and of pyridinium model compounds.

The long-wavelength absorption band of 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl) phenolate (betaine-30, B30) in ethanol and 1-chlorobutane shifts to the blue by cooling in the temperature range of 294-128 K. In addition, B30 shows fluorescence in both solvents at 77 K, which is absent at room temperature. The study of the ground and excited state of B30 and its model compound 4-(pyridinium-1-yl)-phenolate PyPo by DFT/TDDFT calculations indicates that for the perpendicular conformation, the ground state possesses a charge-separated closed-shell hole pair (hp) electronic configuration, and the S(1) state corresponds to a biradicaloid electronic structure (dd) with a small dipole moment caused by an unpaired electron on each of the orthogonal fragments. Following the absorption process, there is a driving force for geometrical relaxation within the S(1) state toward an orthogonal arrangement of the phenolate and the pyridinium ring. In this final S(1) equilibrium geometry, the energy gap between the excited and the ground state is strongly reduced and causes very efficient radiationless deactivation of the S(1) state at room temperature. At 77 K, the viscous barrier hindering large-amplitude motion enables the appearance of the fluorescence from the partially or nonrelaxed excited Franck-Condon structure. By variation of the donor and acceptor strength of the two moieties, the energy gap for perpendicular systems can be tuned, allowing, in principle, the switching between the two cases hp < dd and hp > dd. This enables a new access to the efficient construction of mnemonic systems and NLO dyes.