Relaxed S1 Research Articles

THE PHOTOPHYSICS OF p‐AMINOBENZOIC ACID

Abstract— The lowest‐lying allowed UV transition in p‐aminobenzoic acid (PABA) is assigned Γ→1La 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 S0 and S1, 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 S1) was studied in several solvents. Nonradiative decay may be due to intersystem crossing, which would be fast enough to compete with thermally activated intramolecular NH2 twisting. The phosphorescence spectrum and lifetime obtained in an EPA glass at 77 K are reported, and the triplet energy of PABA is estimated.

Mechanistic investigations in the photochemistry of acyclic conjugated dienes. Significance of the [1,5] sigmatropic shifts of hydrogen

The determination of initial quantum yields for product formation in the direct photolysis of the three interconvertible dienes, 4-methylpenta-1,3-diene and cis- and trans-2-methylpenta-1,3-diene, leads to revision of the mechanism formerly proposed by Crowley and brings more insight into the properties of the excited dienes in solution. Thus it is shown that cis-2-methylpenta-1,3-diene largely assumes an s-cis ground state conformation and that its S1 state makes efficient use of both the [1,5] hydrogen shift and the 1,4-cyclomerization co-ordinates for decaying to the ground state; the two other dienes undergo unambiguously, though with a low quantum yield, an abnormal [1,5] shift of hydrogen which can only originate from a trans configuration and hence supports the doubly twisted (bicyclobutane-like) structure which we have previously assigned to at least one of the possible vibrationally relaxed S1 states of acyclic 1,3-dienes.

Triplet states of biacetyl and energy transfer as revealed by opto-acoustic spectroscopy

Using the technique of opto-acoustic spectroscopy, it is first demonstrated that radiationless decay from S1 of biacetyl when excited at wavelengths longer than 4430 Å involves a very slow (knr ∼ 103 sec−1) T1→S0 step, whereas beyond this wavelength a second fast channel opens: T2→S0. Addition of oxygen makes the apparent T1→S0 relaxation very fast. In mixtures of biacetyl and benzene, monitoring the slow component of the heat released from the biacetyl T1 state while irradiating at wavelengths absorbed by benzene shows that energy transfer from benzene directly to the Tn manifold of biacetyl occurs from the onset of 1B2u absorption to 2440 Å rather than throughout the band. In pyridine-biacetyl mixtures, the lowest n→π* state of pyridine transfers to the triplet manifold of biacetyl, but the lowest π→π* state does not.