SUB-PICOSECOND INTERSYSTEM CROSSING AND VIBRATIONAL COOLING IN THE TRIPLET MANIFOLD OF 1-NITRONAPHTHALENE
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Date
2009
Authors
Reichardt, Christian
Vogt, R. Aaron
Crespo-Hernandez, Carlos E.
Journal Title
Journal ISSN
Volume Title
Publisher
Ohio State University
Abstract
The electronic energy relaxation of 1-nitronaphthalene (1NN) was studied in different solvents using broadband transient absorption spectroscopy with femtosecond time resolution. UV excitation of 1NN populates an unrelaxed S$_1$($\pi\pi$*) state, which decays by conformational relaxation (primarily twisting of the NO$_2$ group) with a time constant of ~100 fs. The twisting of the NO$_2$ group and formation of a structurally relaxed singlet state opens up a doorway for ultrafast intersystem crossing (ISC) to a high-energy receiver triplet state T$_\mathrm{n}$(n$\pi$*), which then undergoes internal conversion to form a vibrationally excited T$_1$($\pi \pi$*) state. Quantum chemical calculations that include solvent effects support the experimental observations. Our results show that an essentially barrierless path connects the initial S$_1$ state to the receiver T$_\mathrm{n}$ state, which enables the observation of vibrational energy transfer and its dependence on the surrounding solvent. According to this kinetic model, which was first proposed by Crespo-Hernandez et$\,$al. for 1-nitropyrene[1], the S$_1$($\pi \pi$) electronic energy decays rapidly and irreversibly to dark triplet states, explaining why small nitro-polycyclic aromatic compounds are typically considered to be nonfluorescent.\\ {\textbf {Reference}} [1] C.$\,$E. Crespo-Hernandez, G. Burdzinski, R. Arce, {\textit {J. Phys. Chem. A.}}, {\textbf{2008}}, 112,6313
Description
Author Institution: Center for Chemical Dynamics, Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106