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

Abstract

Despite intensive research, the role of the H-bonding environment on ultrafast PET remains illusive. For example, coumarin 153 (C153) undergoes ultrafast photoinduced electron transfer (PET) in electron-donating solvents, in both aniline (AN) and N,N-dimethylaniline (DMA), despite their very different H-bonding abilities. Thus, donor-acceptor (AN-C153) H-bonding may have only a minor role in PET (Yoshihara and co-workers, J. Phys. Chem. A, 1998, 102, 3089). However, donor-acceptor H-bonding may be somehow less effective in the neat H-bonding environment but could become dominant in the presence of an inert solvent (Phys. Chem. Chem. Phys., 2014, 16, 6159). We successfully applied and tested the proposal here. The nature of PET modulation of C153 in the presence of a passive component cyclohexane is found to be very different for aniline and DMA. Upon addition of cyclohexane to DMA, the PET process gradually becomes retarded but in the case of AN, the PET rate was indeed found to be accelerated at some intermediate composition (mole fraction of aniline, XAN∼ 0.74) compared to that of neat aniline. It is intuitive that cyclohexane may replace some of the donors (AN or DMA) from the vicinity of the acceptor and, thus, should disfavour PET. However, in the hydrogen bonding environment using molecular dynamics simulation, for the first time, we show that the average number of aniline molecules orienting their N-H group in the proximity of the C=O group of C153 is actually higher at the intermediate mole fraction (0.74) of aniline in a mixture rather than in neat aniline. This small but finite excess of C153-AN H-bonding already present in the ground state may possibly account for the anomalous effect. The TD-DFT calculations presented here showed that the intermolecular H-bonding between C153 and AN strengthens from 21.1 kJ mol(-1) in the ground state to 33.0 kJ mol(-1) in the excited state and, consequently, H-bonding may assist PET according to the Zhao and Han model. Thus, we not only justified both the theoretical prediction (efficient H-bond assisted PET within the C153-AN pair) and experimental observation (minor H-bond assisted PET in neat solvent) but also established our previous hypothesis that an inert co-solvent can enhance the effect of H-bonding from molecular insights.

Elucidating the H-Bonding Environment of Coumarin 102 in a Phenol–Cyclohexane Mixture by Molecular Dynamics Simulation: Implications for H-Bond-Guided Photoinduced Electron Transfer

“…This agreement between experiment and theory affirms the theoretical predication that H-bonding should favor PET within the C153–aniline complex. However, the AN–cyclohexane system was somewhat restricted by the immiscibility of aniline in cyclohexane over the mole fraction range of 0.14–0.73 . Hence, we could not find the exact mole fraction which corresponds to the highest quenching.…”

“…However, the AN− cyclohexane system was somewhat restricted by the immiscibility of aniline in cyclohexane over the mole fraction range of 0.14−0.73. 26 Hence, we could not find the exact mole fraction which corresponds to the highest quenching. The C102 dissolved in the phenol−cyclohexane binary mixture is a better system since there is no solubility restriction; the phenol− cyclohexane binary mixture is miscible in all proportions.…”

“…26 The MD simulation showed that the probability of having C153−AN H-bonding is at most the same mole fraction where the PET rate was found to be fastest from the experiment. 26 This agreement between experiment and theory affirms the theoretical predication that H-bonding should favor PET within the C153−aniline complex. However, the AN− cyclohexane system was somewhat restricted by the immiscibility of aniline in cyclohexane over the mole fraction range of 0.14−0.73.…”

“…MD simulation was recently successfully applied to correlate the H-bonding environment with the observed ultrafast PET for coumarin 153 (C153) in the cyclohexane–aniline (AN) mixture . The MD simulation showed that the probability of having C153–AN H-bonding is at most the same mole fraction where the PET rate was found to be fastest from the experiment .…”

Disentangling Time Scales of Vibrational Cooling, Solvation, and Hydrogen Bond Reorganization Dynamics Using Ultrafast Transient Infrared Spectroscopy of Formylperylene

“…Solution-phase chemical reactivity relies on solute–solvent interaction at the molecular level. , Nonspecific dipolar interaction and specific interaction such as hydrogen bonding (H-bond) often play crucial role in the kinetics and outcome of a chemical reaction. Ubiquitous nature of hydrogen bonding in physical, chemical, and biological systems has attracted intense spectroscopic investigation for past several decades. − Hydrogen bonding interaction significantly contributes to chemical stability and often plays decisive role in the structure and function of many biological and chemical systems, e.g., electron transfer dynamics, charge separation and stabilization, excited state deactivation, chemical and biological catalysis, and so on. − In the solution phase, hydrogen bonding interaction between a solute and the surrounding environment is known to be dynamic in nature. − With the advent of femtosecond laser spectroscopic techniques, it has been possible to interrogate the dynamical features of hydrogen bonding interactions with great details to reveal how hydrogen bond making and breaking processes control the energy flow in a chemical system and how it does affect the chemical reactivity. − Femtosecond-resolved fluorescence and transient absorption (TA) techniques have popularly been used to address the intermolecular hydrogen bonding dynamics by watching transient spectral evolution measuring energetic stabilization of photoexcited chromophores capable of engaging themselves with hydrogen-bond-donating solvents. However, these methods extract the dynamic H-bond reorganization event by measuring the temporal shift of transient emission or absorption spectra and do not provide molecular-level insight into this fascinating interaction.…”