Conformationally restricted 2,5-diphenyloxazoles: failure of the electromer model for anomalous amplified spontaneous emission spikes

The amplified spontaneous emission (ASE) that develops on pulsed excitation of concentrated solutions of 2,5-diphenyloxazole (PPO) and related molecules is a nonlinear optical response that is considered anomalous. It consists of dual laser spikes whose relative intensity depends on laser excitation intensity: At the highest laser powers, the dominant spike corresponds to the less intense vibronic band in the normal fluorescence spectrum. This behaviour was attributed to electromers, a class of excited state conformers distinguished by having either the 2- or the 5-phenyl moiety coplanar with the oxazole ring and the other phenyl moiety twisted out-of-plane. Here, it is reported that under the same excitation conditions (266 nm laser excitation) the same ASE response is exhibited when nonplanarity at the 5 position is sterically favoured by 4-alkyl substitution (methyl or tert-butyl, MePPO and t-BuPPO, respectively). Although the X-ray structure of t-BuPPO shows the 5-phenyl group twisted 72° relative to the oxazole plane, t-BuPPO develops two ASE spikes and, on raising laser excitation intensity, the spike corresponding to the weaker vibronic band gains in intensity relative to the spike corresponding to the Franck–Condon (F–C) maximum, mirroring the behaviour of PPO. Accordingly, the electromer model for anomalous ASE spikes must be abandoned. Interestingly, imposing coplanarity of the 5-phenyl group in the rigid phenylindenooxazole (PIO) analogue leads to normal ASE behaviour. A single ASE spike develops that corresponds to the F–C maximum in the fluorescence spectrum of PIO. The different responses of these molecules may reflect different vibronic relaxation rates in the ground state manifold that control the ability of the molecules to maintain inverted populations with respect to ASE active transitions. Another possibility is that changes in relative spike intensity are due to weak differential transient absorption of the two ASE spikes. In the course of this work, a new method for the synthesis of 2,4,5-trisubstituted oxazoles was developed which may be of general use.