Unique Dual Fluorescence of Sterically Congested Hexaalkyl Benzenehexacarboxylates:  Mechanism and Application to Viscosity Probing

Abstract

The static and dynamic fluorescence behavior of a series of hexaalkyl benzenehexacarboxylates (R(6)BHC; 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 tBu(6)BHC, Men(6)BHC, Bor(6)BHC, MHp(6)BHC, neoPn(6)BHC, and Ad(6)BHC, whereas only single fluorescence from the RX state was observed for Me(6)BHC. 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 (tau'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 (I(RX)/I(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 R(6)BHCs, as well as the activation volumes of 2.0, -0.62, and 7.4 mL/mol for tBu(6)BHC, Men(6)BHC, and Bor(6)BHC at room temperature. The fluorescence anisotropy (rho), 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 rho's for the FC' fluorescence by a factor of 2-100 are attributed to the shorter tau's. The I(RX)/I(F' ratio was found to be insensitive 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 R(6)BHCs as microenvironmental viscosity probes is proposed.

Diversified Excited‐State Relaxation Pathways of Donor–Linker–Acceptor Dyads Controlled by a Bent‐to‐Planar Motion of the Donor

“…On the basis of modulation to the energy level of acceptors and hence harnessing the complicated excited-state competition, effec-tive design of light harvesting systems based on DPAC as dual channel donor is thus achievable. Other types of fluorophores with conformation-dependent dual emissions [1,14] should also be the potential dual-channel donors for the light harvesting systems D-L-A dyads.…”

Tunable Dual Fluorescence of 3‐(2,2′‐Bipyridyl)‐Substituted Iminocoumarin

“…[2][3][4] Multiple models have been proposed to account for dual fluorescence in various structures. [5] The experimental verifications [6][7][8] and theoretical rendering [9,10] of the postulated excited-state occurrences have been challenging. [11] Therefore, the debate on the origin of dual fluorescence in both kinetic and structural aspects has been spirited, [12,13] which attracts attention to the fundamental issues in molecular photophysics, and consequently propels the progress in related fields.…”

New Fluorophore Design

“…Some key features obtained from such photophysical properties arise in the ability for selfcalibration of instrumental factors and more accurate cellular imaging where the variation of density/concentration of sensor molecules in the illuminated and observed volume is the limiting factor for quantitative assays. To date, 11 distinct mechanisms for dual fluorescence have been elucidated [23]. Among these mechanisms, twisted intramolecular charge transfer (TICT) remains one of the most well-studied and established photoprocesses to display two-color emission [24].…”