Kingsley Salisbury

The relaxation of electronically excited styrenes: a ‘twisted’ excited state

Gas-phase studies of a series of styrene derivatives have provided evidence for relaxation from initially excited 1La states to ‘twisted’ singlet states which have anomalously long lifetimes. By examining derivatives with different constraints for rotation about the excited π-bond it has been demonstrated that it is a change in excited state geometry involving some rotation which controls the efficiency of population of the long-lived singlet states. Comments on a recent analysis of the solution phase photophysics of cyclic styrene derivatives are made.

Substituent effects in the di-π-methane rearrangement of 1,1-dicyano-2-methyl-3-phenylpropene

1,1-Dicyano-2-methyl-3-phenylpropene and ring-substituted derivatives on photolysis undergo a di-π-methane rearrangement to give the corresponding cyclopropane derivatives. The effect of ring substitution on the quantum yields of these singlet state reactions can be analysed in terms of the balance between diradical and charge-transfer character on initial bonding. If the extent of charge transfer is large, efficient non-radiative relaxation of the ‘zwitterion’ to the ground state olefin takes place, resulting in low quantum yields for the di-π-methane rearrangement.

Quenching of the 3B1u→1A1g emission of benzene in low temperature matrices at 10 K

The effects of concentration and environment on the triplet state emission of benzene (3B1u→1A1g) have been investigated in low temperature gas matrices at 10 K. For benzene + argon matrices a Monte Carlo calculation of the mean separation between the interacting centres of benzene molecules at the critical concentration has been shown to be 6.5 ± 0.5 A. The phosphorescence lifetime in a variety of matrices at a concentration beyond the self-quenching limit decreased in the order CH4 > N2 > Ar Kr > Xe in accordance with the heavy atom effect which has been noted previously. The fluorescence lifetime of benzene (1B2u→1A1g) has been measured for the first time in an argon matrix and found to be 45 ± 5 ns for matrix ratios benzene:argon in the range 1 : 20 to 1 : 1000.

Singlet–triplet absorption spectra of some aromatic molecules in gas matrices at 10 K

Singlet–triplet transitions of some aromatic compounds have been investigated in oxygen matrices at 10 K. The technique developed in this study circumvents experimental difficulties of the classical high pressure technique of oxygen perturbation. It also overcomes the problem of estimating an accurate profile for the contact charge–transfer spectrum in the subtraction procedure by its actual measurement during the experiment. Vibrational progressions are not always lost for the studied aromatic compounds at the temperature employed and the data obtained for toluene at low temperatures may be used to add further support to recent interpretations of non-radiative rate constant data. Xenon matrix experiments indicated that the “heavy atom” effect of the gas is insufficient to produce perturbed singlet–triplet spectra at 10 K.

The wavelength- and solvent-dependent photochemistry of 1,1-dicyano-2-methyl-4-phenylbut-1-ene; reaction from two excited states1

1,1-Dicyano-2-methyl-4-phenylbut-1-ene (4), on u.v. irradiation in solution, yields two products, 3,3-dicyano-2-methyl-4-phenylbut-1-ene (5) and 1-dicyanomethyl-1-methylindane (6). The photoreaction is dependent both on solvent polarity and on the excitation wavelength. A long-wavelength band in the absorption spectrum of (4), not present in the spectra of its isolated chromophores, is attributed to an intramolecular charge-transfer interaction. The unusual dependence on solvent and wavelength is explained in terms of a reaction mechanism in which (5) is formed from a delocalised excited state, with a staggered conformation, and (6) from a charge-transfer excited state, with an eclipsed conformation.

Photochemical reactions of an αβ-unsaturated nitro compound

(E)-2-Nitro-1-(9-phenanthryl)prop-1-ene on irradiation in solution undergoes geometric isomerisation to the Z-isomer (ϕE→Z 0.50), and also yields small amounts of phenanthrene-9-carbaldehyde. No other products are detected at low conversions to the Z-isomer. The Z-isomer behaves differently and yields 2-methylphenanthro-[9,10-b]furan and phenanthrene-9-carbaldehyde, the former being formed from an excited singlet state of the Z-isomer and the latter from an excited triplet state. Flash photolysis and low temperature n.m.r. studies demonstate the presence of a photochromic transient formed from the Z-isomer and the probable structure of this transient is suggested.

Time-resolved fluorescence excitation spectroscopy of styrenes

The time-dependent photophysical processes occurring in low pressures of styrene and trans-1-phenylprop-1-ene vapours have been investigated as a function of excitation wavelength using synchrotron radiation from the Orsay Storage Ring (ACO). Earlier observations made with an Argon Ion laser have been confirmed and it has been found that the fluorescence lifetimes of the observed dual emissions show a strong dependence on excitation wavelength.Quantum yield measurements have enabled trends in the sum of the nonradiative processes from S1 and S2 to be followed for the two styrenes studied. The results have been interpreted in terms of the population of a twisted excited state on excitation to S2 and the presence of very fast competing non-radiative process(es) when high vibrational levels of S2 are populated.

The triplet state of styrenes

The energy levels of the S1,S2 and T1 states of a range of cyclic and acyclic phenylalkenes have been determined and the structure–excited state energy level relationship examined. The S2(Franck–Condon maximum)–T1(absorption onset) energy gap is a constant for all the systems studied (189.5 ± 4 kJ mol–1). except the severely twisted cis-t-butylstyrene, and thus allows the determination of the triplet energy of simple phenylalkenes merely by obtaining the S0–Sn spectra. An extensive search for phosphorescence (on direct irradiation and triplet sensitisation) even at 10 K in a nitrogen matrix was unsuccessful and this may be interpreted in terms of a non-radiative relaxation requiring little molecular distortion.

The photochemistry of 1-phenylpropenes

An examination of the photochemical and photophysical processes resulting from excitation in the α and A bands of cis- and trans-1-phenylpropene in the gas phase has been carried out. Measurement of quantum yields of fluorescence, fluorescence lifetimes, and quantum yields of isomerization on excitation in the α bands has revealed that the value for the sum of the non-radiative processes in this first excited singlet state (ΣKNR) is the same for both isomers. A consideration of the potential energy diagrams for the ground state and first two excited singlet and triplet states strongly suggests that geometric isomerization is solely a triplet process for excitation in the α bands. This in turn implies that the major contribution to ΣKNR for each isomer is the intersystem crossing rate constant. In contrast to observations made on solutions of monophenylalkenes, excitation in the A bands of the phenylpropenes does not lead to efficient geometric isomerization. Polymerization is the major detectable process.

The photochemistry of 1-cyano-2-methyl-3-phenylpropone and ring substituted derivatives

The E- and Z-isormers of 1-cyano-2-methyl-3-phenylpropane and ring-substituted derivatives of the E-isomer, are shown to fluoresce and undergo Z–E-isomerization and di-π-methane rearrangement on irradiation. The rate constants for these processes, obtained from fluorescence lifetimes and quantum yields, can be analysed in terms of an initial interaction between the two π systems, facilitated by partial charge-transfer, followed by two diverging pathways, one, towards a diradicaloid species leading to cyclopropane formation, the other towards a ‘zwitterion’ is species and relaxation to the ground state and resulting in singlet state Z–E- isomerization.