Fast internal conversion in 1-(dimethylamino)naphthalene: Effects of methoxy substitution on the naphthalene ring

Radiationless transitions of 1-(dimethylamino)naphthalene (1DMAN) and its 4-, 5-, 6-, and 7-methoxy derivatives having a twisted dimethylamino group in the Franck-Condon excited state are investigated by means of picosecond time-resolved fluorescence measurements. The fast internal conversion (IC), that has previously been observed for 1DMAN, is also found for its 5- and 6-methoxy derivatives, while the IC rate decreases substantially in the 4- and 7-methoxy derivatives, showing that the fast IC is related to the proximity of the S1 and S2 states and also to the electronic structures of the S1 and S2 states.