Charge-transfer and isomerization reactions of trans-4-(N-arylamino)stilbenes

Abstract

We studied the excited-state dynamics of trans-4-(N-arylamino)stilbenes with aryl = phenyl (p1H), 4-methoxyphenyl (p1OM), or 4-cyanophenyl (p1CN) in solvents of varied polarity and viscosity by using femtosecond transient absorption and time-correlated single photon counting techniques. In nonpolar solvents the decay is triexponential, in which the rapid component corresponds to vibrational cooling combined with solvation, the intermediate temporal component 41-120 ps to trans-cis isomerization, and the long one ∼1 ns to fluorescence decay of the S state. The S state has a delocalized geometry and charge-transfer characteristics, corresponding to a planar intramolecular charge transfer (PICT) state. In polar solvents, an excited-state absorption band appears near 520 and 480 nm for p1OM and p1CN, respectively but not for p1H. This band has a rise lifetime of 4.3/7.5, 16.3/9.4, and 29.5/16 ps for p1CN/p1OM in acetonitrile (ACN), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), respectively and matches the decay of the 600 nm PICT band. This band is thus assigned to the absorption of a singlet twisted intramolecular charge transfer state (TICT). The conversion rate decreases as the solvent viscosity is increased and is consistent with a large structural variation amplitude. Theoretical calculations using density functional theory (DFT), method PEB0, were employed to obtain the optimized structures and energies of those states. The PICT state possesses delocalized π electrons along the molecule. The TICT for p1CN is formed by twisting about the aminostilbene-benzonitrile C-N bond by ∼90°, but it is about the stilbene-aniline C-N bond for p1OM. We observed faster conversion rates for p1CN in alcoholic solvents, in which the lifetimes for both the PICT and TICT states are shortened to 20-99 ps and 120-660 ps, respectively, as a result of solvent-solute H-bonding interactions. In p1OM, the TICT state has an elongated C[double bond, length as m-dash]C bond in the stilbene moiety, which might facilitate the trans-cis isomerization reaction and thus account for the relatively short lifetime of 58-420 ps in polar solvents.

Theoretical study on the reaction mechanism of the thermal cis–trans isomerization of fluorine-substituted azobenzene derivatives

“…[1][2][3][4][5] Because of their unique photochromic characteristics, 6 they have been widely used in molecular data storage, 7-9 photopharmacology, 10 optochemical genetics, 11 materials science, 12 and biomolecules 13,14 among other areas. Over the past few decades, most research has been focused on azobenzenes, 15 spiropyrans, 16 diarylethenes, 17 and stilbenes 18 which are all characterized by photoisomerization. Among these molecules, azobenzene, which is based on the diatomic structure N]N, has unique structural characteristics and can be used for optical control under different conditions.…”

Influence of charge transfer on the isomerisation of stilbene derivatives for application in cancer therapy

“…Functionalised charge transfer stilbenes, like 4-dimethylamino-4'-cyanostilbene (DCS), have been studied with experimental 5 and theoretical methods for more than forty years, but there is no general consent concerning the existence and nature of the TICT state. The dual fluorescence phenomenon of DCS can be partially explained by the existence of a TICT state formed by rotation around the C-C anilino bond [ [31][32][33] or the C-N amino bond [ [34][35][36][37], but no evidence for the existence of a TICT state and/or of dual fluorescence has been found in several studies [38][39][40]. Excited state re-absorption of fluorescence has been proposed as a potential alternative to the formation of a TICT state [ 41 ].…”

Isomerization and Properties of Isomers of Carbocyanine Dyes

“…Photoisomerization (followed by back isomerization of a photoisomer) is one of the main properties of polymethine dyes; at present, it is being studied in detail with the use of modern micro-, nano-, pico-and femtosecond techniques. While trans-cis (photo)isomerization of various compounds is widely used in molecular photoswitches, over the past decades, most works have been focused mainly on azobenzene derivatives [63], spiropyrans [64], diarylethenes [65], and stilbenes [66], and, until recently, this property of cyanine dyes has not been applied in practice. The use of cyanine photoisomerization in research is mainly for probing microviscosity of microheterogeneous media [67,68].…”