Intramolecular charge transfer in aminobenzonitriles: Requirements for dual fluorescence

Abstract

Dual fluorescence and intramolecular charge transfer (ICT) are observed with aminobenzonitriles when two excited state levels (Sl and S2(CT) in DMABN) have an energy gap sufficiently small for vibronic coupling: a solvent-induced pseudo-Jan-Teller effect. It is argued that the N-inversion of the amino group acts as a promoting mode. These conclusions are based on a comparison of absorption spectra and photostationary and time-resolved fluorescence data. Dual fluorescence is also observed with MMD, in which the dimethylamino group is twisted towards a perpendicular configuration with respect to the phenyl ring.

Twisted intramolecular charge transfer states: Rotationally resolved fluorescence excitation spectra of 4,4′-dimethylaminobenzonitrile in a molecular beam

“…These values are not only larger than those of the ground state, but they are also larger than the values for the 2 , 4 , and 6 torsional levels, evidencing substantial displacements along other outof-plane coordinates. But ambiguities exist concerning the assignments of both pairs of these bands.…”

“…3 In the following years, other competing models of structural relaxation were proposed, including the socalled planar ICT ͑PICT͒ model advocated by Zachariasse and co-workers. 4,5 Despite the passage of time, the intense controversy generated by these proposals, and the extensive application of ICT molecules as fluorescence markers, sensors, and "switches" in materials science and in biology, very little is known about their electronic and geometric structures. Gas phase, fluorescence excitation spectra of DMABN in supersonic jets were first reported by Kobayashi et al, 6 Gibson et al, 7 and Bernstein and co-workers.…”

Spectral Signature of 2-[4-(Dimethylamino)styryl]-1-methylquinolinium Iodide: A Case of Negative Solvatochromism in Water

“…Different models like pseudo-JahnÀTeller interaction of excited states 3,4 and in-plane bending and rehybridization of acceptor 5,6 were proposed for the dual emission of DMABN, but the most accepted one is twisted intramolecular charge transfer (TICT), where a π/2 twist of the donor moiety with respect to the benzene plane is considered to stabilize the ICT state. 7À9 As these types of ICT systems are very sensitive to solvent polarity and viscosity, much research 10,11 is being done to understand and systematize the ICT systems.…”

Picosecond Transient Absorption Spectra and the Twisted Intramolecular Charge Transfer Phenomenon

“…In their very stimulating papers [6,7,29] Zachariasse, Warman et al suppose that the main factor determining the dual fluorescence ability in DMABN and in related compounds is the proximity of two close-laying 1π , π* states. In all compounds of the studied stuctural families which exhibit the dual fluorescence, there are, indeed, two neighbouring electronic states (or even three, as in DMAA the saturated calomel electrode (SCE).…”

“…Now, the most generally accepted hypothesis [2] is the one of the "twisted" intramolecular charge transfer state (TICT) according to which the D and Α moieties, limited in their stuctural degrees of flexibility, twist to orthogonality of their π-electronic systems [3,4]. Among the recently supported principally opposing hypotheses there are those assigning the dual fluorescence of DMABN and structurally related compounds either to the formation of exciplexes with the solvent [5], or to the pseudo Jahn-Teller distortion [6,7]. Marginally: both these views mutually oppose.…”