Synthesis and spectroscopic properties of carotenoid bis-phenylhydrazone astaxanthin: Extending conjugation to a C=N group

We report on synthesis and detailed spectroscopic characterization of bis-phenylhydrazone astaxanthin (BPH-Asx), a derivative of astaxanthin (Asx), in which the conjugated carbonyl group of Asx is replaced by a conjugated C=N bond. BPH-Asx was successfully synthesized and characterized using various spectroscopic techniques, revealing subtle changes in absorption spectra and significant alterations in excited-state dynamics compared to Asx. The results reveal a shortened S1 lifetime, 1.4 ps for BPH-Asx compared to 5 ps for Asx, indicating a significant impact on its excited-state dynamics. Since no polarity-induced effect was observed for BPH-Asx, the changes induced by the conjugated C=N group are due to prolongation of effective conjugation. Moreover, the identification of a distinctive S* signal with a 3 ps lifetime in BPH-Asx underscores the relation between effective conjugation and presence of the S* signal that is not detected in Asx.