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THE PHOTOPHYSICS OF p‐AMINOBENZOIC ACID
Author(s) -
Halpern Arthur M.,
Ramachandran B. R.
Publication year - 1995
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1995.tb08717.x
Subject(s) - chemistry , intersystem crossing , methylcyclohexane , fluorescence , phosphorescence , photochemistry , absorption spectroscopy , absorption (acoustics) , reaction rate constant , intramolecular force , transition dipole moment , quantum efficiency , oscillator strength , dipole , analytical chemistry (journal) , spectral line , excited state , atomic physics , singlet state , stereochemistry , materials science , kinetics , organic chemistry , toluene , physics , quantum mechanics , astronomy , composite material , optoelectronics
— The lowest‐lying allowed UV transition in p ‐aminobenzoic acid (PABA) is assigned Γ→ 1 L a based on quantitative absorption and fluorescence studies, as well as semiempirical PM3 multielec‐tron configuration interaction calculations. The oscillator strengths, fluorescence quantum efficiencies and lifetimes are reported for PABA in several polar, nonpolar, protic and aprotic solvents (aerated) at 296 K. Reasonable agreement is found between the observed radiative rate constant and that calculated from the absorption and fluorescence spectra. Shifts in the absorption and fluorescence spectra in aprotic solvents are analyzed in terms of the Onsager reaction field model; results are consistent with an increase in dipole moment of ca 4 D between the relaxed S 0 and S 1 , states. No evidence is found for the emission from the amino‐twisted form of PABA in all solvents studied although calculations show that the amino‐twisted S, state is highly polar, but higher in energy by ca 35 kJ/mol ( in vacuo ). The fluorescence efficiency is excitation wavelength independent in both methylcyclohexane and water. The temperature dependence of the nonradiative rate constant (from S 1 ) was studied in several solvents. Nonradiative decay may be due to intersystem crossing, which would be fast enough to compete with thermally activated intramolecular NH 2 twisting. The phosphorescence spectrum and lifetime obtained in an EPA glass at 77 K are reported, and the triplet energy of PABA is estimated.