Solvent dependence of effective S1 torsional potentials in 9,9′-bianthryl and 9-phenylanthracene

(Oligo)aromatic species with one or two conjugated Si[double bond, length as m-dash]Si bonds: near-IR emission of anthracenyl-bridged tetrasiladiene

A series of aryl disilenes Tip₂Si[double bond, length as m-dash]Si(Tip)Ar (2a-c) and para-arylene bridged tetrasiladienes, Tip₂Si[double bond, length as m-dash]Si(Tip)-LU-Si(Tip)[double bond, length as m-dash]SiTip₂ (3a-d) are synthesized by the transfer of the Tip₂Si[double bond, length as m-dash]SiTip unit to aryl halides and dihalides by nucleophilic disilenides Tip₂Si[double bond, length as m-dash]SiTipLi (Tip = 2,4,6-iPr₃C₆H₂, Ar = aryl substituent, LU = para-arylene linking unit). The scope of the nucleophilic Si[double bond, length as m-dash]Si transfer reaction is demonstrated to also include substrates of considerable steric bulk such as mesityl or duryl halides Ar-X (Ar = Mes = 2,4,6-Me₃C₆H₂; Ar = Dur = 2,3,5,6-Me₄C₆H, X = Br or I). Bridged tetrasiladienes Tip₂Si[double bond, length as m-dash]Si(Tip)-LU-Si(Tip)[double bond, length as m-dash]SiTip₂ with more extended linking units surprisingly exhibit fluorescence at room temperature, albeit weak. DFT calculations suggest that partial charge transfer character of the excited state is a possible explanation.

QM/MM Study of the Role of the Solvent in the Formation of the Charge Separated Excited State in 9,9‘-Bianthryl

In this paper the role of the solvent in the formation of the charge-separated excited state of 9,9'-bianthryl (BA) is examined by means of mixed molecular mechanical/quantum mechanical (QM/MM) calculations. It is shown that in weakly polar solvents a relaxed excited state is formed with an interunit angle that is significantly smaller than 90 degrees . This relaxed excited state has a considerable dipole moment even in weakly polar solvents; for benzene and dioxane dipole moments of ca. 6 D were calculated, which is close to experimental data. These dipoles are induced by the solvent in the highly polarizable relaxed excited state of BA, and the dipole relaxation time is governed by solvent reorganizations. In polar solvent the charge separation is driven to completion by the stronger dipoles in the solvent and a fully charged separated excited state is formed with an interunit angle of 90 degrees.

Symmetry breaking in the relaxed S(1) excited state of bianthryl derivatives in weakly polar solvents

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.