Unique dual fluorescence of sterically congested hexaalkyl benzenehexacarboxylates: Mechanism and application to viscosity probing

The static and dynamic fluorescence behavior of a series of hexaalkyl benzenehexacarboxylates (R₆BHC; R = methyl (Me), tert-butyl (tBu), (-)-menthyl (Men), (-)-bornyl (Bor), (-)-1-methylheptyl (MHp), neopentyl (neoPn), and 2-adamantyl (Ad)) was studied by steady-state and time-resolved fluorescence spectroscopy. Dual fluorescence from both the partially relaxed metastable Franck-Condon-like (FC′) and the fully relaxed (RX) state was observed for tBu₆BHC, Men₆BHC, Bor₆BHC, MHp₆BHC, neoPn₆BHC, and Ad₆BHC, whereas only single fluorescence from the RX state was observed for Me₆BHC. Picosecond time-resolved fluorescence spectroscopic measurements clearly demonstrated that the initially formed Franck-Condon (FC) state sequentially converts to the FC′ and then to RX state, with the relaxation hindered to such an extent that it shows variation with the steric bulk of the R groups. Thus, the fluorescence lifetimes (τ's) of FC′ and RX are critically dependent on the bulkiness of the R groups, varying from 17 to 130 ps and from 0.6 to 1.1 ns, respectively. The relative intensity of FC′ and RX fluorescence (/_RX//_FC′) was found to be dependent on the excitation wavelength, suggesting that the conformational relaxation from the FC′ to RX state can compete with the vibrational relaxation of the FC′ state. The temperature and pressure dependences were studied by steady-state fluorescence spectroscopy to give the activation energies of 1-3 kcal/mol for the FC′-to-RX relaxation of congested R₆BHCs, as well as the activation volumes of 2.0, -0.62, and 7.4 mL/mol for tBu₆BHC, Men₆BHC, and Bor₆BHC at room temperature. The fluorescence anisotropy (ρ), as a measure of molecular motion, was also determined to be in the ranges of 0.03-0.3 for FC′ and 0.003-0.01 for RX. The much larger ρ's for the FC′ fluorescence by a factor of 2-100 are attributed to the shorter τ's. The /_RX//_FC′ ratio was found to be insensisive to solvent polarity, but critically dependent on solvent viscosity, exhibiting an excellent linear relationship with the reciprocal viscosity. The potential use of these sterically congested R₆BHCs as microenvironmental viscosity probes is proposed.