Nature of the lowest triplet states of 4′-substituted N-phenylphenothiazine derivatives

Abstract

The nature of the lowest triplet state of the donor–acceptor N-phenylphenothiazine derivatives has been studied by means of phosphorescence and EPR spectroscopy in various low temperature glasses. The triplet excitation of N-phenylphenothiazine and N-(p-anisyl)-phenothiazine is localised within the phenothiazine subunit. On the contrary, the 77 K phosphorescence of the molecules containing an electron acceptor group (i.e. –CN, –COCH₃ or –COC₆H₅ at the 4′ position) is similar to that for benzonitrile, acetophenone or benzophenone, respectively, although the energy levels of their T₁ states are higher than that of phenothiazine. With increasing temperature, however, their phosphorescence becomes similar to that characteristic for phenothiazine. This finding has been explained in terms of the excited-state intramolecular energy transfer accompanied by the planarisation of the phenothiazine kernel. The results of crystallographic investigations support this hypothesis.