The Electronic Origin of the Dual Fluorescence in Donor−Acceptor Substituted Benzene Derivatives

Abstract

The origin of the dual fluorescence of DMABN (dimethylaminobenzonitrile) and other benzene derivatives is explained by a charge transfer model based on the properties of the benzene anion radical. It is shown that, in general, three low-lying electronically excited states are expected for these molecules, two of which are of charge transfer (CT) character, whereas the third is a locally excited (LE) state. Dual fluorescence may arise from any two of these states, as each has a different geometry at which it attains a minimum. The Jahn-Teller induced distortion of the benzene anion radical ground state helps to classify the CT states as having quinoid (Q) and antiquinoid (AQ) forms. The intramolecular charge transfer (ICT) state is formed by the transfer of an electron from a covalently linked donor group to an anti-bonding orbital of the pi-electron system of benzene. The change in charge distribution of the molecule in the CT states leads to the most significant geometry change undergone by the molecule which is the distortion of the benzene ring to a Q or AQ structure. As the dipole moment is larger in the perpendicular geometry than in the planar one, this geometry is preferred in polar solvents, supporting the twisted intramolecular charge transfer (TICT) model. However, in many cases the planar conformation of CT excited states is lower in energy than that of the LE state, and dual fluorescence can be observed also from planar structures.

Entropic effects in excited state CT reactions

“…The twisting model is also supported by using CASSCF optimizations in the excited state and shows that the dipole moment of the CT state reaches a maximum for perpendicular geometries [9]. These calculations, moreover, identify a second coordinate active in reaching the conical intersection to the CT state: this is the Quinoid-Antiquinoid transition (the benzene aromatization coordinate) involving bond length changes within the benzene ring.…”

“…The latter model corresponds to a large amplitude rearrangement which should strongly depend on viscosity, former does not involve large amplitude changes and therefore the reactions kinetics should be little dependent on solvent viscosity. Recent theoretical advances in the understanding of DMABN favour a large amplitude twisting model [7][8][9]. Robb et al [7] insinuate in the title of their publication, that the reaction coordinate does not include the twist.…”

Ab Initio Multiple Spawning Photochemical Dynamics of DMABN Using GPUs

“…Theory has not been able to unambiguously determine the fluorescence mechanism, although many electronic structure methods have been employed to probe the critical points of the S 1 potential energy surface in Curchod, et al -DMABN -Page 3 both gas and condensed phases. Electronic structure methods used to examine the details of the emission process have included linear-response time-dependent density functional theory [12][13][14] (LR-TDDFT), single-reference wavefunction methods such as TDHF/CIS, 15 DFT/SCI, 16 CC2, 17 and ADC(2), 18 and multi-reference wavefunction methods such as CASSCF [19][20][21] and CASPT2. [22][23][24][25] Even though a large number of experimental and theoretical data have been generated on the emission properties of DMABN and its chemical derivatives, much less information is available on the early nonadiabatic, i.e., non-Born-Oppenheimer, relaxation pathway following DMABN photoexcitation.…”

An anti-quinoid structure in dual fluorescence of fluozazene molecule and solvent effect of intramolecular charge transfer

“…Lengthening the two central CC bonds (C7-C8 and C10-C11) of the benzene ring, the AQ geometry is obtained using the same active space. Cogan et al [47] reported the similar results in the case of the benzene anion radical, and both Q and AQ both have CT character. After inspecting, however, the Q structure is LE nature and the AQ one has ICT property in this work.…”