An Experimental Test of C−N Bond Twisting in the TICT State: Syn−Anti Photoisomerization in 2-(N-Methyl-N-isopropylamino)-5-cyanopyridine

Abstract

A previously untested essential consequence of the "TICT" (twisted internal charge transfer) interpretation of the nature of the "anomalous" excited state of p-dimethylaminobenzonitrile (1) and related compounds has now been verified: at -90 degreesC, the dual fluorescence of a desymmetrized analogue, 2-(N-methyl-N-isopropylamino)-5-cyanopyridine (2), in methanol (MeOH) is accompanied by syn-anti isomerization around the C-N bond, whereas its ordinary fluorescence in tetrahydrofuran is not.

Charge‐Transfer Interactions in 4‐Donor 4′‐Acceptor Substituted 1,1‐Diphenylethenes

“…[39,40] Convincing evidence for the former hypothesis has recently been put forward. [41] Irrespective of the exact nature of the CT state, the fact that 1D2 populates this state clearly shows that photoind-uced electron transport occurs. If a cavity radius ρ of 4.2 Å is used, [42] for the polar excited state a dipole moment of 13.9 D is obtained upon application of Equations (2) and (3).…”

Direct Observation of a Hydrogen‐Bonded Charge‐Transfer State of 4‐Dimethylaminobenzonitrile in Methanol by Time‐Resolved IR Spectroscopy

“…In nonpolar solvents, a single fluorescence band is observed from a locally excited (LE) state. In polar solvents, the initially populated LE state reacts further to produce a stable intramolecular charge-transfer (ICT) state, which gives rise to a second fluorescence band that overlaps with, but is abnormally red-shifted from, the LE emission.[1] Results of experiments using aprotic solvents are well described by models in which polarity is the only solvent property that affects the charge transfer reaction activation energy and the relative stabilization of the ICT and LE states.[2] Whilst much work continues to concentrate on determining the structures of the LE and ICT states, [3][4][5][6][7] the precise nature of the difference between the properties of the excited state in protic and aprotic solvents is little understood. For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret.…”

Molecule matters

“…However, each model has its drawback and none of these models can be considered seriously as a substitute for the TICT model. This short account on DMABN will be incomplete if it does not refer to a work, which certified that the TICT model passed a critical test [9]. 10 is a DMABN analogue that exhibits dual fluorescence.…”