The photoinduced hydrogen atom transfer and trans-cis isomerization of the C=C double bond in 1-(1-hydroxy-2-naphthyl)-3-(1-naphthyl)-2-propen-1-one and related compounds studied using nanosecond time-resolved infrared spectroscopy

The mechanism and intermediates of photoinduced hydrogen atom transfer and photoisomerization in 1-(1-hydroxy-2-naphthyl)-3-(1-naphthyl)-2-propen-1-one (1) and 1-(1-methoxy-2-naphthyl)-3-(1-naphthyl)-2-propen-1-one (2) have been investigated using nanosecond time-resolved infrared spectroscopy. For compound 1, a bleach signal was observed at 1575 cm-1 just after the laser excitation and recovered within 2 ms. The transient absorption at 1500 cm -1 decayed with two components of 1 μs and 2 ms, which are due to the triplet state and the ground state tautomer, respectively. On the basis of these results, the potential energy surface of hydrogen atom transfer in compound 1 was deciphered. Upon laser excitation of compound 2, the transient absorption signals at 1528 cm-1 and 1392 cm-1 assigned to the triplet state were observed in addition to the bleach signals at 1664 cm-1 and 1600 cm-1. The bleach signal at 1600 cm -1 remained as a permanent bleaching after the fast recovery with the 1.2 μs time constant. This result indicates that compound 2 underwent trans-to-cis isomerization upon laser excitation. By utilizing the data obtained in stationary and transient IR spectroscopy, we have directly determined the quantum yield of trans-to-cis isomerization to be 0.23 ± 0.01 from the difference in the IR signals.