The UV-visible-near-IR absorption spectra, S-2 --> S-0 fluorescence quantum yields and S-2 fluorescence lifetimes of 1-fluoroazulene, 1,3-difluoroazulene, and several of their alkyl-substituted derivatives have been measured at room temperature in up to six solvents, benzene, dichloromethane, ethanol, acetonitrile, n-hexane, and perfluoro-n-hexane. The quantum yields (up to 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 singlet state of a polyatomic molecule with a closed-shell ground state. The nonradiative rate constants for the decay of the S-2 states of these molecules in these solvents and of azulene, 1,3-dichloroazulene and 1,3-dibromoazulene, determined previously, have been analyzed in terms of the weak coupling case of radiationless transition theory. The data show that the nonradiative rate constants for the S-2 states of azulene, 1-fluoroazulene, and 1,3-difluoroazulene in the nonpolar solvents follow the log-linear relationship 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 solvatochromism or fluorine substitution. Substitution by alkyl groups increases the nonradiative decay rates by increasing the effective number of coupled states while the electronic coupling matrix element remains constant. Substitution 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 multiplicative effect. Substitution by chlorine or bromine increases the S-2 decay rates by enhancing the rate of intersystem crossing to the triplet manifold. The rate enhancement is semiquantitatively modeled by considering the effects of spin-orbit coupling of the halogen atoms.