Influence of helicity on the excited-state as well as chiroptical properties of two sets of regiohelical coumarins that are differentiated by "inward" and "outward" disposition of the pyran-2-one ring has been investigated. A subtle difference in the helicities manifests in divergent excited-state properties and significant differences in the dipole moments. The latter permit heretofore unprecedented regiodifferentiation in the O-H⋅⋅⋅O hydrogen-bond assisted electron-transfer quenching by phenols. Furthermore, the enantiopure hexahelical coumarins exhibit strong Cotton effects and lend themselves to a very high differentiation in the specific rotations and anisotropic dissymmetry factors. The specific rotation observed for 6-in turns out being the highest of the values reported for all hexahelicenes reported so far.

Two regioisomeric pairs of heptahelical mono- and biscoumarins that are differentiated by “inward” and “outward” disposition of the pyran-2-one moiety have been synthesized and investigated to understand the influence of helicity on excited-state and chiroptical properties. A slight variation in the helicities is found to manifest in contrasting excited-state properties of coumarin-annelated heptahelicenes; in addition to the intramolecular charge transfer, structural relaxation in the excited state is shown from theoretical calculations to cause decrease in the fluorescence quantum yield for a system with higher helicity. The optically pure enantiomers of heptahelical coumarins exhibit helicity-dependent chiroptical properties, namely, specific rotations, molar ellipticities, Cotton effects, and anisotropic dissymmetry factors. Theoretical calculations point to factors that are not readily explicable.

The H-bonded coumarin 102 (C102)-phenol complex has been a model system usually used to understand the influence of H-bonding on photophysical processes. Zhao and Han first showed that significant H-bond strengthening occurs in the excited state and proposed the possibility of fluorescence quenching in the complex via internal conversion from a locally excited (LE) state to a low-lying charge transfer (CT) state. Later, we experimentally confirmed fluorescence quenching of C102-phenol complex in a nonpolar solvent (cyclohexane). However, we also found that the existence of the low-lying CT state is ambiguous. Here, we proposed an alternative mechanism for the fluorescence quenching in the H-bonded complex. For this, we evaluate the excited state potential energy surface considering complete H atom-transfer from phenol to C102 along the H-bonding coordinate. Surprisingly, we observed two distinct minima separated by a low-energy barrier. One minimum corresponds to the complex with shortening of H-bond consistent with that of Zhao and Han. On the other hand, the second minimum, which has even lower energy than the first minimum, is likely to be arising from the proton-coupled electron transfer (PCET) process. The nature of the lowest excited state alters from LE to CT type at the second minimum, which may account for the fluorescence quenching phenomena in the system.

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.

It is often difficult to assign the nature of an excited-state process unambiguously based on a limited number of experimental evidence. The methylbipyridine/phenol complex is a classic example, where experimental observations support a proton-coupled electron transfer (PCET) or a photo-induced electron transfer (PET) process. Here, we implemented time-dependent density functional theory calculation to elucidate the nature of the process. We found that PCET is possible only when mediated by a H-bond between methylbipyridine and phenol. However, a conventional PET can occur through π–π stacking interaction between the donor and the acceptor. Thus, the photophysical process in the complex is indeed governed by competition of H-bonding versus π–π interaction. Our calculations including the solvent model based on density (SMD) suggest that π–π stacking is more favorable than H-bonding, and hence, conventional PET is a more favorable excited-state process for the methylbipyridine/methoxyphenol complex than PCET.