Possibility of ring opening and closure with pyrylium ions under laser excitation

Abstract

When substituted at position(s) 2 and/or 6 by a p-anisyl group, the pyrylium ion exhibits a valley in the middle of its spectral response in a laser cavity. A similar phenomenon is observed with 4,5,8,9-tetrahydrodibenzo[c,h]xanthylium ion when substituted at positions 3 and/or 11 by a methoxy group. This feature is not observed in the fluorescence spectrum of the compounds of the two series. Triplet–triplet absorption, singlet–singlet absorption and emission from two methoxy-substituted conformers interconverting by rotation of the methoxy groups, does not appear to be responsible for the formation of the valley in the laser effect. The accumulation of a degradation product in the solution could also be excluded. The hypothesis put forward to explain the anomalous lasing response is that of a very short-lived transient intermediate, formed with a low quantum yield and having its absorption maximum at the position of the valley. This transient could have a structure similar to the keto-enol derivative formed by ring opening of the xanthylium ion under basic conditions.