Nonradiative Relaxation Processes and Electronically Excited States of Nitrobenzene Studied by Picosecond Time-Resolved Transient Grating Method

Abstract

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.

Ultrafast Dynamics of o-Nitrophenol: An Experimental and Theoretical Study

“…Compared to the literature values, the obtained values of τ 1,solv in our study confirm the upper limit estimated by Takezaki et al in benzene (≤50 ps). 13 Subpicosecond time constants for ISC have been observed for a large number of nitrated polycyclic aromatic compounds. 67−69 In such systems, fast ISC was considered to be the key radiationless process facilitated by a very low energy gap between the first excited singlet and an upper triplet state analogously to the findings for o-NP in the present study.…”

“…Neither fluorescence nor phosphorescence has been reported to date for o-NP, which indicates ultrafast radiationless processes. Picosecond time-resolved transient absorption experiments of o-NP in benzene, conducted by Takezaki et al, 13 point to relaxation into the first excited triplet state, T 1 , after excitation in the UV. The underlying ISC process, however, remained unresolved due to limited time resolution and could only be estimated to occur on a time scale ≤50 ps.…”

Effect of electric fields on the reaction rates in shock initiating and detonating solid explosives

“…An efficient chemical process is the excitation of various molecular species from the ground electronic state to the first (or higher) excited electronic state. This excitation has been shown experimentally [12] and theoretically [13] to be followed within picoseconds by a nonradiative transition back to the ground electronic state accompanied by excitation of that state's vibrational levels. The highly vibrational excited molecules undergo more frequent reactive collisions than those in lower vibrational states [14], thus increasing the chemical energy release rate and reducing the overall reaction zone length of the detonation wave [14] or lower pressure chemical processes [15].…”

Photoacoustic viscoelasticity imaging dedicated to mechanical characterization of biological tissues

“…Since the relaxation time for nonradiative transition is about 10 À11 s, 65 it then can be neglected in our measurement. Although the system delay is di±cult to measure, it can maintain stably and keep constant in the same system.…”