Symmetry Breaking in the Relaxed S₁ Excited State of Bianthryl Derivatives in Weakly Polar Solvents

Abstract

The flash-photolysis time-resolved microwave conductivity technique (FP-TRMC) has been used to investigate the nature of the relaxed S(1) state of 9,9'-bianthryl (AA), 10-cyano-9,9'-bianthryl (CAA), and 10,10'-dicyano-9,9'-bianthryl (CAAC). Changes in both the real, Deltaepsilon' (dielectric constant), and imaginary, Deltaepsilon' ' (dielectric loss), components of the complex permittivity have been measured. The dielectric loss transients conclusively demonstrate the dipolar nature of S(1) for all three compounds in the pseudopolar solvents benzene and 1,4-dioxane, and even in the nonpolar solvents n-hexane and cyclohexane. The required symmetry breaking is considered to result from density and structural fluctuations in the solvent environment. The dipole relaxation times for AA (CAAC) are ca. 2 ps for the alkanes and 7.9 (5.3) and 14 (14) ps for benzene and dioxane, respectively. The time scale of dipole relaxation for the symmetrical compounds is much shorter than that for rotational diffusion and is attributed to intramolecular, flip-flop dipole reversal via a neutral excitonic state. The dipole moment of the transient dipolar state is estimated to be ca. 8 D, that is much lower than the value of ca. 20 D determined from the solvatochromic shifts in the fluorescence in intermediate to highly polar solvents which corresponds to close to complete charge separation. For the asymmetric compound, CAA, a dipole moment close to 20 D is found in all solvents, including n-hexane. Dipole relaxation in this case occurs on a time scale of several hundred picoseconds and is controlled mainly by diffusional rotation of the molecules. The mechanism and kinetics of formation of the dipolar excited states are discussed in the light of these results.

Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering

“…We expect that this stage of the process is where solvent polarisability becomes significant. In 9,9′-bianthryl, it has been shown theoretically43 and experimentally4445 that lowering of the inter-unit angle increases the polarisability of S 1 , and we anticipate similar behaviour in DP-TIPS. The excess polarisability in relaxed S 1 causes a rapid rearrangement of solvent molecules which is more pronounced with increased solvent polarisability.…”

Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc Dipyrrins

“…This state most likely has CT character and is formed via a symmetry-breaking mechanism similar to that which occurs in other bichromophoric systems. 23,33,62−64 Further support for the hypothesis that the observed photophysical properties of zDIP1–zDIP4 are associated with SBCT is the weaker dependence on solvent polarity for the heteroleptic complexes zDIP2′ and zDIP3′, where SBCT is impossible.…”

Excited state dynamics of 9,9′-bianthryl in room temperature ionic liquids as revealed by picosecond time-resolved fluorescence study

“…The 0-0 transition is estimated to be around 25400 cm -1 from the structured absorption spectrum, which is not very different from that observed in polar conventional solvents such as acetonitrile (25650 cm -1 ). 16 The steady-state fluorescence spectra of BA in ILs are characterized by a broad emission, which is indicative of the charge transfer nature of the band, as is the case with the ordinary polar solvents. [13][14][15] One can also observe some structure near the onset of the broad emission (~ 410 nm), where the LE emission (as evident from the emission spectrum of BA in hexane, shown in figure 1) is expected.…”

“…12 BA is a well-known system whose excited state behaviour and dynamical processes have been studied extensively in a range of conventional solvents. [13][14][15][16][17][18][19][20][21] The interest in BA is mainly due to the fact that this pre-twisted molecule with mutually perpendicular anthracene rings in the ground state undergoes symmetry breaking upon excitation and its flourescence spectra show distinct charge transfer (CT) character, especially in polar solvents. [13][14][15] In nonpolar solvents the first excited state (S 1 ) is predominantly a locally excited (LE) state with very little or no CT character, 14-16 whereas, in highly polar solvents a CT state, which is formed rapidly from the LE state, is the emitting state.…”