Solvent and temperature dependence of the lowest excited singlet state lifetime of all-t r a n s-7′,7′-dicyano-7′-apo-β-carotene

Abstract

The lifetime of the lowest excited singlet state of a highly polarizable polyene, all-trans-7′,7′-dicyano-7′-apo-β-carotene, 1, was measured as a function of solvent dielectric constant at 295 K and temperature from 7.4–295 K. The polyene was excited in separate experiments with 610 nm, 2 ps, and 450 nm, 4 ps laser pulses. The broad 400–650 nm absorption band of the polyene bleaches immediately upon excitation, while a strong absorption centered between 680–705 nm appears. Both the recovery of the ground state absorption band and the disappearance of the 680–705 nm excited state absorption band exhibit the same single exponential kinetics that depend on both solvent dielectric constant and on temperature. As the dielectric constant of the solvent increases from 1.9 to 36, the lifetime of the lowest excited singlet state of 1 decreases from 11.7 ps to 1.9 ps. As the temperature is lowered the lifetime of the lowest excited singlet state of 1 approaches that observed in low dielectric constant media. The behavior of 1 is ascribed to a large change in dipole moment between the ground and lowest excited singlet state. Interaction of the excited state dipole with solvent dipoles results in enhanced charge transfer character within the excited state, which in turn enhances nonradiative decay of the excited state. The excited state dipole moments of the ground, first, and second excited states of both 1 and β-8′-apocarotenal, 2 were calculated using the AM1 molecular orbital method. The calculations predict that 1 has a 5.1 Debye difference in dipole moment between the ground and first excited state.