Dynamics of the Formation of a Charge Transfer State in 1,2-Bis(9-anthryl)acetylene in Polar Solvents: Symmetry Reduction with the Participation of an Intramolecular Torsional Coordinate

Abstract

We have studied 1,2-bis(9-anthryl)acetylene as a model compound for the characterization of the process of solvent-mediated symmetry reduction in an excited state. Thanks to the acetylenic bridge that joins the two anthracenic moieties, this system maintains minimal steric hindrance between the end chromophores in comparison with the classic 9,9'-bianthryl model compound. The acetylenic bridge also allows for significant electronic coupling across the molecule, which permits a redistribution of electron density after light absorption. Femtosecond resolved fluorescence measurements were used to determine the spectral evolution in acetonitrile and cyclohexane solutions. We observed that, for 1,2-bis(9-anthryl)acetylene, the formation of a charge transfer state occurs in a clear bimodal fashion with well separated time scales. Specifically, the evolution of the emission spectrum involves a first solvent-response mediated subpicosecond stage where the fluorescence changes from that typical of nonpolar solvents (locally excited) to an intermediate, partial charge transfer state. The second stage of the evolution into a full charge transfer state occurs with a much longer time constant of 37.3 ps. Since in this system the steric hindrance is minimized, this molecule can undergo much larger amplitude motions for the torsion between the two anthracenic moieties associated with the charge redistribution in comparison with the typical model compound 9,9'-bianthryl. Clearly, the larger range of motions of 1,2-bis(9-anthryl)acetylene gives the opportunity to study the electron transfer process with a good separation of the time scales for the formation of a partial charge transfer state, determined by the speed of solvent response, and the intramolecular changes associated with the formation of the fully equilibrated charge transfer state.

Rational design of near‐infrared absorbing organic dyes: Controlling the HOMO–LUMO gap using quantitative molecular orbital theory

“…With the aim to push the absorption of our model into the NIR region, we increased the π‐conjugated core from benzene to anthracene units. Spectra from time‐resolved fluorescence experiments on 1,2‐Bis(9‐anthryl)acetylene show a broad CT‐like band in polar or semipolar solvents . This suggests a large change in dipole moment upon excitation which we felt would be magnified by introducing appropriate X‐ and Y‐substituents on and/or including a heteroatom in the aromatic moieties.…”

Presence of Two Emissive Minima in the Lowest Excited State of a Push–Pull Cationic Dye Unequivocally Proved by Femtosecond Up-Conversion Spectroscopy and Vibronic Quantum-Mechanical Computations

“…It is clear that right after the formation of the fluorescence signal, at 0.5 ps, an apparent isoemissive point is detected in the TRANES at 594 nm, which is retained during the whole decay process. Instead of a simplistic model based on mere solvation, 19,20 this finding promotes the idea of a two-state deactivation [12][13][14]49 where the first emitting state is completely depopulated within the solvation process (3 ps, cf. concentration profile, Supporting Information, section SI.2, Figure SI The Journal of Physical Chemistry B Article of DASPMI: 49 its TRANES evolution showed two well separated emission bands, whose contributions had previously emerged as a clear enlargement of the stationary fluorescence spectrum in highly polar solvents.…”

Concomitant conformational dimorphism in 1,2-bis(9-anthryl)acetylene

“…23 This dependence on method arises because of the important role of intramolecular dispersion, as well as electronic conjugation and steric effects in giving conformational maxima for both planar and perpendicular aromatic rings. The energy difference is very dependent on the theoretical method used: PBE0/6-31GIJd,p) functional gives a slightly larger energy barrier than previously reported B3LYP/6-311+GIJd,p) results.…”