Internal Conversion with 3,5-Dimethyl-4-(methylamino)benzonitrile in Alkane Solvents

Abstract

With 3,5-dimethyl-4-(methylamino)benzonitrile (MHD) in n-hexane and n-hexadecane at 25°C, relatively small values are obtained for the fluorescence quantum yield Φf (0.006) and the lifetime τ (0.103 ns). From Φf and the yield of intersystem crossing ΦISC (0.024), it follows that MHD in these solvents undergoes efficient internal conversion with a yield Φ IC of 0.96 and an IC rate constant k IC of 9 ϫ 10 9 s Ϫ1 . An IC activation energy of 12 kJ/ mol is obtained from the measurement of τ as a function of temperature in n-hexane. The amino group of MHD in the electronic ground state S0 is twisted relative to the benzonitrile moiety, as deduced from quantum chemical calculations supported by experiments. It is assumed that the IC reaction is a consequence of the planarisation of the amino group upon excitation to the relaxed S 1 state and passes through a conical intersection. In support of this mechanism, efficient IC is not observed with the planar derivative 4-amino-3,5-dimethylbenzonitrile (HHD), in contrast to the strongly twisted 3,5-dimethyl-4-(dimethylamino)benzonitrile (MMD).

Decay times of 4-(dimethylamino)benzonitrile in acetonitrile and conclusions on entropy of activation

“…This comes from the condition that e max is proportional to cos 2 h [19]. A planar structure for DMABN in the ground state is also found by several computations [6,9,20].…”

“…ICT reaction is questionable anyway, as the molecular structure of the S 1 (LE) state (twist angle, amino pyramidality, bond lengths) and not that of the ground state S 0 is significant for the ICT reaction. In addition, twist angles up to around 30 o will be of little photophysical consequence as compared to a planar structure, because of the fact that the electronic coupling between the amino and benzonitrile moieties is governed by cos 2 h [19].…”

Dual fluorescence and intramolecular charge transfer with crystalline 4-(diisopropylamino)benzonitrile

“…The¯uorescence spectrum of crystalline MABN at 25°C (Fig. 1a) consists of an emission band with a shape similar to that measured in solution [16]. Its spectral position is red-shifted by approximately 1300 cm À1 as compared with that in diethyl ether [16].…”

“…DMABN does not show appreciable dual¯uorescence in the crystalline state at room temperature [11], nor at this temperature in weakly polar solvents such as toluene [6] and diethyl ether [14], having eective polarities similar to those prevailing in crystalline aromatic amines. The relatively small twist of the amino group occurring in the singlet excited state of crystalline DMABN [13] can therefore not be attributed to an ICT reaction resulting in a twisted intramolecular charge transfer (TICT) state [15], for which a full twist over 90°and electronic decoupling (proportional to cos 2 h [16]) of the amino and benzonitrile moieties is required [4].…”

Influence of the alkyl chain length on the excited‐state properties of 4‐dialkyl‐benzonitriles. A theoretical DFT/MRCI study

“…The results are obtained by the recently developed DFT/MRCI method 12, a combination of density functional theory with a multireference configuration interaction ansatz. The details of this method can be found elsewhere 12, but we like to point out that a combination of DFT with CI already has proven to yield reliable results of CASPT2 quality in the case of donor–acceptor substituted molecules 13–17. DFT ground‐state optimized geometries are presented along with excitation energies and excited‐state properties for the twisted and planarized systems.…”