Excited-state dynamics of 1-piperidino- or 1-pyrrolidino-anthraquinone forformation of N-ylide and reduced compounds

By means of subpicosecond laser photolysis of the title compound at room te mperature, time-dependent shift of an absorption band has been observed at short delay times which is ascribed to solvent reorganization around the lo west excited singlet state of the solute molecule with an intramolecular ch arge-transfer character. In comparison with the results obtained by nanosec ond laser photolysis and steady-state photolysis at 77 K, it is concluded t hat the lowest excited singlet state undergoes rapid intramolecular proton transfer forming an N-ylide compound. For formation of the N-ylide compound from the lowest excited singlet state of 1-piperidino-anthraquinone, it is considered that there exists an intermediate which can be ascribed to tran sfer of the equatorial proton to the carbonyl oxygen anion. At room tempera ture, no formation of N-ylide compound as a stable compound can be observed due to its rapid reversion to 1-piperidino- or 1-pyrrolidino-anthraquinone . Steady-state photolysis in ethanol at room temperature gives rise to form ation of a reduced compound. Yield of the reduced compound is obtained by 4 05 or greater than or equal to 510 nm photolysis, however, is negligibly sm all compared with that obtained by 313 or 366 nm photolysis. Hence, it is p roposed that intersystem crossing from a higher excited singlet state (prob ably the second excited singlet state) to a higher excited triplet state (p robably the third excited triplet state) followed by internal conversion to the lowest excited tripler (T-1) state plays an important role for the pho toreduction, i.e., the reduced compound may be formed from the T-1 state, a lthough the possibility of additional photoreduction originating from the s econd excited triplet state cannot be ruled out.