Cage effects on conformational preference and photophysics in the host-guest complex of benzenediols with 18-Crown-6

The conformational preference and modification of photophysics of benzenediols, namely hydroquinone (HQ), resorcinol (RE) and catechol (CA), upon host-guest complex formation with 18-Crown-6 (18C6) have been investigated, under supersonically jet-cooled conditions. Laser induced fluorescence (LIF) and UV-UV hole-burning spectra indicate the presence of two conformers for HQ and RE and one conformer for CA. On the other hand, the number of isomers is reduced to one in the 18C6 center dot HQ and 18C6 center dot RE complexes, while the 18C6 center dot CA complex has three stable isomers. The IR spectra of the OH stretching vibration reveal that the two OH groups are H-bonded in 18C6 center dot CA and 18C6 center dot RE. In 18C6 center dot RE, RE adopts the highest energy conformation in the bare form. In 18C6 center dot HQ, the H-bonding of one OH group affects the orientation of the other OH group. The complex formation changes the photophysics of the S-1 state of the benzenediols in a different manner. In our previous work, we reported a remarkable S1 lifetime elongation in 18C6 center dot CA complexes; the S-1 lifetime of CA is elongated more than 1000 times longer (8 ps -> 10.3 ns) in 18C6 center dot CA (F. Morishima et al., J. Phys. Chem. B, 2015, 119, 2557-2565), which we called the "cage effect''. In 18C6 center dot RE, the increase of S-1 lifetime is moderate: 4.0 ns (monomer) -> 10.5 ns (complex). On the other hand, the S-1 lifetime of HQ is shortened in 18C6 center dot HQ: 2.6 ns (monomer) -> 0.54 ns (complex). Density functional theory (DFT) calculations suggest that these behaviors are related to the S-1 ((1)pi pi*)-(1)ps* energy gap, the character of the S-2 state and the symmetry of benzenediol. These experimental results clearly show the potential ability of 18C6 to control the conformation and modification of the electronic structure of guest species.