Source of anomalous fluorescence from solutions of 4-N,N-dimethylaminobenzonitrile in polar solvents

Abstract

The interactions of the Grst excited singlet state of 4-N,N-dimethylaminobenzonitrile (DMABN) and of 3,5-N,N-tetramethyl-4-aminobenzonitrile (3,5-TMABN) with polar molecules have been studied. Mixtures of cyclohexane and a small amount of polar liquid have been used as solvents. The anomalous fluorescence of DMABN originates from a 1 : 1 complex between excited solute and a polar solvent molecule. The fluorescence of 3,5-TMABN in polar solvents arises only from exciplexes. A model is presented which describes the binding in the exciplexes in terms of a localized interaction between lone pair electrons of the polar solvent molecule with lone pair electrons of the dimethylamho group of the excited solute. The solvent-induced wavelength shift of any particular exciplex fluorescence of the two compounds can be correlated with the dielectric properties of the medium. If the conformation of the amino group is fused as in 1-methyl-5-cyano-indoline (l-M-5-CI) and in l-methyl-6-cyano-l,2,3,4-tetrahydroquinohe (1-M-6-CTHQ) no formation of exciplexes with polar solvents is observed. The formation of exciplexes does not only lead to a new fluorescence, but also enhances the intersystem crossing in the two compounds, DMABN and 3,5-TMABN. There is no enhancement of the triplet quantum yield in polar solvents in the case of 1-M-6-CTHQ. The quantum yield of the normal fluorescence of both DMABN and 3,5-TMABN in polar solvents depends on the wavelength of excitation. The quantum yield of exciplex emission does not show this behaviour. These two observations are explained by taking the existence of weakly bound complexes of ground state soIutes as well as the high rate of exciplex formation into account.Solutions of dimethylaminobenzonitrile (DMABN ; I) in polar solvents are known to exhibit two fluorescence bands. At the short wavelength side a band appears around 340 nm. Depending on the solvent used, a second band is observed in the region between 420 and 520nm. In non-polar solvents only the short wavelength fluorescence is emitted.After the discovery of this phenomenon,l a variety of theories have been offered for the explanation. A number of discussions are based on reversal in the ordering of the excited levels Lb and La when solvent reorientation is completed after excitation in polar solutions. Using the solvent shift of the long wavelength fluorescence together with the shift of the first absorption band, the dipole moment of the La state is estimated to be 76.7 C m.I However, within the picture of reordered levels the adopted procedure for determining the excited state dipole moment is inapplicable. The objection concerns the fact that the position of the first absorption band depends on Apb, whereas the location of the long wavelength fluorescence should be a function of Apa.6 The quantities Ap, and Apb are defined as Ap, = p(La)-pg and Apb = p(&) -u,, where p(La), [p(&.,) and pg are the dipole moments, respectively, in the La, Lb and ground state.Other mechanisms have been proposed for the dual fluorescence of DMABN, ...

Time‐resolved study of the photochemical reaction of 4‐dimethylaminobenzonitrile with carbon tetrachloride

“…Visser and Varma also suggested the existence of CCl 4 and its decomposition products CCl 3 and Cl in their low-temperature (193 K) ESR measurements. 7 This means that, although ground-state DMABN is inert to IMET reactions with CCl 4 , as indicated by its ground-state absorption and resonance Raman spectrum, the B state can be a good electron donor. The B state of DMABN has been suggested to be a planar conjugated state, which is similar with the ground-state but with small extent of ICT to the cyano group indicated by ¾40 cm 1 wavenumber downshift from the ground to B state (Table 1).…”

“…6 Among many solvents used in DMABN studies, carbon tetrachloride (CCl 4 ) induces unique photochemical behaviour 7 -9 manifested by rapid fluorescence quenching in both the solution and gas phases. Stern-Volmer analysis carried out by Visser and Varma in 1980 revealed that the quenching is diffusion controlled. 7 Although the data indicated a quenching mechanism involving intermolecular electron transfer upon the *Correspondence to: C. Ma, Department of Chemistry, Imperial College, London SW7 2AY, UK.…”

“…Stern-Volmer analysis carried out by Visser and Varma in 1980 revealed that the quenching is diffusion controlled. 7 Although the data indicated a quenching mechanism involving intermolecular electron transfer upon the *Correspondence to: C. Ma, Department of Chemistry, Imperial College, London SW7 2AY, UK. Email: c.ma@ic.ac.uk Contract/grant sponsor: EPSRC; Contract/grant number: GR/K20989.…”

Dynamic NMR and Twisted Intramolecular Charge Transfer Excited States

“…The previous publications have presented influential theoretical [8] and experimental proofs for the existence of twisted intramolecular charge transfer (TICT) excited state [9] [10] responsible for the long wavelength emission. However, an exciplex [11] has been suggested for this anomalous emission by other investigators.…”

Ground and excited state dipole moments of planar vs. twisted p-N,N-(dimethylamino)benzonitrile systems: maximum charge transfer for minimum overlap

“…However, controversy still exists regarding the role of solvent molecules in creating the TICT state (4,5 ) . For ex- 'Author to whom correspondence may be addressed at the Department of Chemistry, University of Saskatchewan, Saskatoon, Sask., Canada S7N OW0 ample, it has been proposed that local coupling of solvent molecules, with a particular local configuration, and the reactant (DMABN) is essential for the twisting of the C-NMe, bond (4e, 5 ) .…”