Environmental Photochemistry of Nitro-PAHs: Direct Observation of Ultrafast Intersystem Crossing in 1-Nitropyrene

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2008-07, Vol.112 (28), p.6313-6319
• Main Authors: Crespo-Hernández, Carlos E, Burdzinski, Gotard, Arce, Rafael
• Format: Article
• Language: English
• Subjects: A: Kinetics, Spectroscopy; Environmental Pollutants - chemistry; Molecular Conformation; Photochemistry; Polycyclic Aromatic Hydrocarbons - chemistry; Pyrenes - chemistry; Solvents; Spectrum Analysis
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp803847q

Femtosecond broadband transient absorption experiments of 1-nitropyrene, a nitro-polycyclic aromatic hydrocarbon of environmental concern are presented in cyclohexane and hexane solutions. The transient absorption spectra show the presence of three species that are assigned to the Franck−Condon excited lowest singlet (S1) state, the structurally relaxed S1 state, and the lowest excited triplet state. The spectral changes at early times are interpreted in terms of conformational dynamics; primarily due to an ultrafast rotation of the nitro group in the S1 state. This excited state relaxation is followed by intersystem crossing with a time constant of 7 ps. CIS/6-31G(d,p) calculations predict planarization of the nitro-aromatic torsional angle as the major nuclear relaxation coordinate, from 32.8° at the HF/6-31G(d,p) level of theory in the ground state (27.46° at B3LYP/6-31++G(d,p)) to 0.07° in the S1 state. Vertical excitation energies at the TDDFT/6-31++G(d,p) and TDDFT/IEFPCM/6-31++G(d,p) levels of theory predict a small energy gap (