Anomalous modulation of photoinduced electron transfer of coumarin 102 in aniline–dimethylaniline mixture: dominant role of hydrogen bonding

Abstract

In a previous study, we reported a striking observation that photoinduced electron transfer (PET) from aniline (AN) to photoexcited coumarin 102 (C102) can be accelerated by adding an inert component (cyclohexane or toluene) to the neat electron donor solvent AN (Phys. Chem. Chem. Phys., 2014, 16, 6159-6166). The H-bond linking the electron donor (D, AN) and the acceptor (A, C102) was proposed to dictate the PET process. To account for the unusual variation of quenching pattern with AN mole fraction, two possible reasons were cited - (1) the D-A (AN-C102) H-bonding may be modulated due to change in polarity of the medium or (2) the additional D-D (AN-AN) H-bonding may restrain the D-A H-bonding to adjust optimally for the PET. Here, we investigate the PET of C102 in an AN-dimethylaniline (DMA) mixture to negate the polarity variation. Since, both AN and DMA have similar polarities, the polarity of the mixture should remain invariant at all compositions. Nevertheless, we found that the fluorescence quantum yield and lifetime of C102 in the mixtures follows a similar unusual trend as observed earlier in the AN-toluene or AN-cyclohexane mixtures; it first decreases up to a particular mole fraction (XD) of the H-bond donor AN and, thereafter, increases on further enrichment of the donor. The observed PET modulation may be rationalized by considering efficient PET in the 1 : 1 H-bonded C102-AN complex but less efficient PET in higher order C102-(AN)n≥2 complexes, where additional D-D (AN-AN) H-bonding may influence the key C102-AN H-bonding and thus inhibit the PET process.

Modulation of ultrafast photoinduced electron transfer in H-bonding environment: PET from aniline to coumarin 153 in the presence of an inert co-solvent cyclohexane

“…The breakdown of the D-D hydrogen bonding network may assist the key D-A H-bond to exert its favorable influence on PET. 18 A similar observation was also noted for a C102-phenol system. 5 However, it was just a speculation to rationalize the experimental observations.…”

“…To resolve this inconsistency, we recently conjectured that the effect of H-bonding required to enhance PET may be suppressed in neat AN possibly because of the competitive nature of the donor-acceptor (D-A) and donor-donor (D-D) H-bonding (Scheme 1). 17,18 Note that the intermolecular H-bonding of the type N-HÁ Á ÁN has been reported for aniline solutions 19 and aniline clusters. 20 However, the competitive H-bonding situation may break down drastically in the presence of an inert component.…”

“…Apparently, a third component that cannot participate either in H-bonding or in electron transfer should merely replace some of the donor molecules from the vicinity of the acceptor (Scheme 1) and thus may retard the PET event. 17,18 It was reported by Castner et al that chlorobenzene (a non-interacting component) retards PET of coumarin acceptors (C151 or C152) dissolved in neat DMA. 21 Very recently, Letrun et al reported that the number of DMA around coumarin varies in mixed solvent.…”

“…22 However, we observed that in the presence of a non-interacting component (cyclohexane or toluene) the PET of C102 dissolved in AN gets accelerated at an intermediate mole fraction in the mixture compared to neat aniline. 17,18 The D-D H-bonding, which may possibly resist the D-A (AN-C102) H-bonding to attain an optimum conformation for PET in neat H-bonding solvent, could be disrupted by the inert component. The breakdown of the D-D hydrogen bonding network may assist the key D-A H-bond to exert its favorable influence on PET.…”

Reduced fluorescence quenching of coumarin 102 at higher phenol mole fractions in cyclohexane–phenol and anisole–phenol solvent mixtures: role of competitive hydrogen bonding

“…Very recently, we have also observed such unusual uorescence quenching modulation of C102 in similar polarity aniline-DMA mixture. 39 To check the effect of polarity only and to exclude H-bonding possibility, we have considered another solvent mixturecyclohexane-anisole. Here quantum yield of C102 decreases linearly with increase in the mole fraction of anisole.…”