Control of the photophysical properties of polyatomic molecules by substitution and solvation: The second excited singlet state of azulene

The UV-visible-near-IR absorption spectra, S-2 --> S-0 fluorescence quantum yields and S-2 fluorescence lifetimes of 1-fluoroazulene, 1,3-difluoroazul ene, and several of their alkyl-substituted derivatives have been measured at room temperature in up to six solvents, benzene, dichloromethane, ethano l, acetonitrile, n-hexane, and perfluoro-n-hexane. The quantum yields (up t o 0.2) and lifetimes (up to 9.5 ns) of the S-2 state of 1,3-difluoroazulene are exceptionally large-the largest ever reported for an upper excited sin glet state of a polyatomic molecule with a closed-shell ground state. The n onradiative rate constants for the decay of the S-2 states of these molecul es in these solvents and of azulene, 1,3-dichloroazulene and 1,3-dibromoazu lene, determined previously, have been analyzed in terms of the weak coupli ng case of radiationless transition theory. The data show that the nonradia tive rate constants for the S-2 states of azulene, 1-fluoroazulene, and 1,3 -difluoroazulene in the nonpolar solvents follow the log-linear relationshi p expected of the energy gap law, provided that S-2-S-1 internal conversion is assumed to dominate the decay mechanism. The same linear correlation is obtained, irrespective of whether Delta E(S-2-S-1) is varied by solvatochr omism or fluorine substitution. Substitution by alkyl groups increases the nonradiative decay rates by increasing the effective number of coupled stat es while the electronic coupling matrix element remains constant. Substitut ion at the 6-position by an isopropyl group increases the rate constant by a constant factor of 2.9; however, multiple substitution does not have a mu ltiplicative effect. Substitution by chlorine or bromine increases the S-2 decay rates by enhancing the rate of intersystem crossing to the triplet ma nifold. The rate enhancement is semiquantitatively modeled by considering t he effects of spin-orbit coupling of the halogen atoms.