Picosecond time-resolved fluorescence spectroscopy has been used for studying the intramolecular dynamics of trans-stilbene (TS) derivatives. Nonradiative decay rate constants (k(nr)) of jet-cooled 4-methoxy-trans-stilbene (MS), 4,4’-dimethoxy-trans-stilbene (DMS), 2-phenylindene (PI), 4,4’-dihydroxy-trans-stilbene (DHS), and 1-(trans-beta-styryl)cyclohexene (SCH) have been obtained from time-resolved fluorescence measurements with wide-band detection from photoselected vibrational states in the S-1 manifold. Time- and frequency-resolved fluorescence have been measured to characterize intramolecular vibrational energy redistribution (IVR) in the first three of these molecules. Statistical nonadiabatic RRKM theory is applied to model the energy dependence of k(nr) and extract the reaction barriers. Dispersed fluorescence is measured and analyzed in order to determine the substitution effect on vibrational frequencies most significant to the RRKM calculations. While k(nr) is almost invariant to pam substitution of a methoxy group, with similar isomerization thresholds for MS and TS (E(0)(MS) similar to E(0)(TS)), a dramatic slowing of the rate is observed in both disubstituted derivatives, and reaction barriers of similar to 1800 and 1700 cm(-1) are derived for DMS and DHS, respectively. The altered conjugation of the system in SCH produces a large blue shift of the 0(0)(0) and results in a lowering of the isomerization barrier (E(0)(SCH) < 740 cm(-1)). The twisting around the ethylenic double bond (C-e-C-e) is prevented upon intramolecular bridging in PI, but another nonradiative channel, attributed to twisting around the C-e-C-phi single bond, is modeled by RRKM theory with E(0)(PI) similar to 1770 cm(-1). From the electronic structure effects on both E(0) and the shift of the S-1 transition, we elucidate the nature of the transition state, as zwitterionic, and we correlate substitution effects with the HOMO/LUMO (frontier orbital) description. The results are relevant to studies in polar solutions and to the dimensionality of the reaction coordinate in the dynamics of barrier crossing.