The exothermic nonradiative relaxation processes from photoexcited nitrobenzene (NB) are studied by the picosecond time-resolved transient grating method. The decay rate constants and energy of the excited NB are determined. The lifetime of the lowest excited singlet state is found to be very short (≤10 ps) and a surprisingly short lifetime of the lowest excited triplet state (∼480 ps) is detected. From quantitative measurements of the thermal energies released from the excited states of NB, the lowest excited triplet state is determined to be located at (22−27) × 103 cm-1. The triplet formation is very efficient and its quantum yield is found to be grater than 0.80. From the temperature and solvent dependence of the triplet lifetime, it is suggested that motions of the nitro group may cause the surprisingly rapid relaxation from the lowest excited triplet state to the ground state. The photophysical properties and excited states of monosubstituted nitrobenzenes are also discussed.

Optimized geometries and energies in the ground (S0) and excited states of nitrobenzene were calculated using the CAS-SCF method. The optimized geometries in the S1, S2, T1, T2, and T3 states are very different from that in the S0 state. Most significantly, the nitro group is largely bent out of the phenyl plane in the excited states. The potential curves along the nitro rotation coordinate around the C−N bond and the out-of-plane bending mode of the nitro group in the S0, S1, T1, and T2 states were calculated in relation to the excited states dynamics. It is found that the potential curves along the bending mode in the excited states are very flat compared with that in the S0 state. The mechanisms for the fast relaxation from the S1 and T1 states are discussed based on the ab initio results.

The lifetime of the lowest excited singlet (S1) state of nitrobenzene in ethanol was measured using a picosecond time-resolved transient grating method. The lifetime was found to be 6 ps and the rate constant of the intersystem crossing relaxation process from the S1 state was determined to be 1.3–1.6×1011 s−1. The relaxation mechanism from the S1 state is also discussed.

A simple method to measure a rate of the translational temperature increase after the nonradiative transition with a few ps time resolution is presented. This method uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensitive to rates and the relative amount of the thermal energy from the nonradiative transition. Using this method, the thermalization rate after the photoexcitation of trans-azobenzene to the S2(ππ*) state is measured. The acoustic peak shift indicates that the temperature of the solvents rises within less than ∼3 ps after the nonradiative transition of the S2→S1 process. The thermalization after the S1→S0 transition completes faster than the decay of the S1 state (16 ps in acetonitrile and 18 ps in ethanol). The fast energy transfer from the solute to the solvent is discussed.

The synthesis, characterization, and photophysical properties of a series of supramolecular triads consisting of a tin(IV) porphyrin with axial ligands of ortho‐, meta‐, and para‐hydroxyphenyl naphthalenediimides are presented. For the meta‐ and para‐hydroxyphenyl derivatives, efficient and solvent‐viscosity‐dependent quenching of porphyrin fluorescence is observed. Experimental and theoretical studies demonstrate that, in these compounds, photoinduced electron transfer from the phenolate to the porphyrin is modulated by large amplitude rotational motions of the naphthalenediimide. These compounds are novel examples of fluorescent molecular rotors, and their potential use as environmental probes of local viscosity and temperature are discussed.