|Abstract:||The nonraddiative dececy route involving trans $rightarrow$ cis photo-isomerization from the S$_1$ ($pi$$pi$*) state has been investigated for several trans-cinnamate derivatives, which are known as sunscreen reagents. We examined two types of substitution effects._x000d_
One is structural isomer such as ortho-, meta-, and para-hydroxy-methylcinnmate (o-, m-, p-HMC). The S$_1$ lifetime of p-HMC is less than 8 ps at zero-point level, and it undergoes rapid S$_1$ $rightarrow$ $^1$n$pi$* $rightarrow$ T$_1$ decay via multiple conical intersections. Finally, the trans $rightarrow$ cis isomerization proceeds in the T$_1$ state. On the other hand, both o- and m-HMC show very slow decay. Their S$_1$ lifetimes are in the order of 100 ps even at the excess energy of 2000-3000 wn. _x000d_
The other is the effect of the complexity of ester group in para-subsitituted species, such as para-methoxy-methyl, -ethyl and -2ethylhexyl cinnamate (p-MMC, p-MEC, p-M2EHC). p-MMC and p-MEC show sharp S$_0$ $rightarrow$ S$_1$ ($pi$$pi$*) vibronic bands, while p-M2EHC shows only broad structureless feature even under the jet-cooled condition. In addition, we found that the S$_0$ $rightarrow$ $^1$n$pi$* absorption appears at 1000 wn below the S$_0$ $rightarrow$ S$_1$ ($pi$$pi$*) transition in p-MEC and p-M2EHC, but not in p-MMC. Thus, the complexity of the ester group is very important for the appearance of the $^1$n$pi$* state.