Journal of Physical Chemistry A, Vol.115, No.34, 9354-9364, 2011
Role of Conformational Isomerism in Solvent-Mediated Charge Transfer in Chiral (S) 1,2,3,4-Tetrahydro-3-isoquinoline Methanol (THIQM): Condensed-Phase to Jet-Cooled Spectroscopic Studies
Intramolecular charge-transfer reaction in chiral (S) 1,2,3,4-tetrahydro3-isoquinoline methanol (THIQM) has been investigated in the condensed phase and in jet-cooled conditions by means of laser-induced fluorescence, dispersed emission, resonance-enhanced two-photon ionization, and IR-UV double resonance experiments, as well as quantum chemical calculations. In the condensed phase, THIQM only shows local emission in nonpolar and protic solvents and dual emission in aprotic polar solvents, where the solvent-polarity dependent Stokes shifted emission is ascribed to a state involving charge transfer from the nitrogen lone pair to the benzene pi-cloud. Ab initio calculations reveal two low-energy conformers, which are observed in jet-cooled conditions. In the most stable conformer, THIQM(1), the CH(2)OH substituent acts as a hydrogen bond donor to the nitrogen lone pair in the equatorial position, while the second most stable conformer, THIQM(II), corresponds to the opposite NH center dot center dot center dot O hydrogen bond, with the nitrogen lone pair in the axial position. The two low-energy jet-cooled conformers of THIQM evidenced from the laser-induced fluorescence and dispersed emission spectra only show structured local emission. Complexes with usual solvents reproduce the condensed phase properties. The jet-cooled complex with aprotic polar, solvent acetonitrile shows both local emission and charge transfer emission as observed in solution. The jet-cooled hydrate mainly shows local emission due to the unavailability of the nitrogen lone pair through intermolecular hydrogen bonding.