Fluorescence Quenching of Hydrogen-Bonded Coumarin 102-Phenol Complex: Effect of Excited-State Hydrogen Bonding Strength

Abstract

The fate of intermolecular hydrogen bond (H-bond) upon electronic excitation of a H-bonded complex has been debated in literature. For a model H-bonded complex, coumarin 102 (C102)-phenol in a noninteracting solvent ethylene tetrachloride, time-resolved infrared spectroscopy experiment of Nibbering and coworkers suggests that the H-bond between the C102 and phenol ruptures upon electronic excitation (C. Chudoba et al. J. Phys. Chem. A1999, 103, 5625-5628). On the contrary, Zhao and Han have demonstrated for the first time that the intermolecular hydrogen bond is significantly strengthened, while not disrupted, in the electronically excited states of the hydrogen-bonded complexes upon electronic excitation using the time-dependent density functional theory method (G. J. Zhao and K. L. Han J. Phys. Chem. A2007, 111, 2469-2474). The two excited-state hydrogen bonding dynamics mechanisms have widely different predictions of the emission or electronic relaxation of the excited H-bonded complex. The excited-state hydrogen-bond strengthening mechanism proposed by Zhao and Han anticipates a stronger intermolecular interaction, while the H-bond breaking mechanism speculates no interaction between C102 and phenol. The speculation has been tested here on the same system (H-bonded C102-phenol complex) in another noninteracting solvent cyclohexane. We found a strong quenching of the C102 emission in the H-bonded complex. Selectively excited (λex = 405 nm) H-bonded complex relaxes on a fast time scale of 400-600 ps and may be attributed to the conversion of the locally excited (LE) state to a nonfluorescent charge transfer (CT) state assisted by the strong excited-state H-bond formation. A minor component (∼10%) of 2.5 to 1.8 ns is ascribed to the LE complex without a H-bond. The findings are in accordance with the new fluorescence quenching mechanism that the excited-state intermolecular hydrogen bond strengthening facilitates CT from phenol to coumarin in the excited state (G. J. Zhao et al. J. Phys. Chem. B2007, 111, 8940-8945). Fluorescence quenching was absent for anisole, where H-bond formation is not possible and was more pronounced for p-Cl-phenol, where even stronger H-bonding is expected.

Effects of X-shaped reduction-sensitive amphiphilic block copolymer on drug delivery

“…This might be caused by the hydrogen bound formed between water molecules and coumarin 6, just as the quenching process between coumarin 102 and phenol molecules. 33 The emission peak of coumarin 6-loaded nanomicelles, which shifted to 492.3 nm (16 nm blue shift in Figure 4B), demonstrated that coumarin 6 was located in an apolar microenvironment. 34 No fluorescence quenching was displayed by the formulations, and polymers exhibited no fluorescence.…”

A Molecular Lock with Hydrogen Sulfate as “Key” and Fluoride as “Hand”: Computing Based Insights on the Functioning Mechanism

“…From the potential energy scans (Figure ), one can easily follow the potential energy changes with the movement of proton towards the pyridinic nitrogen atom of NHM. Further a curve has been fitted based upon the Morse Equation and we have found the equilibrium ‐N−H distance, bond dissociation energy parameters. The equilibrium bond distance almost matches with the calculated distances from optimized geometries.…”

Computational study on the ion interaction of ellipticine: A theoretical approach toward selecting the appropriate anion

“…At each step, energy of this point calculated using DFT and TD-DFT method for S 0 and S 1 states respectively. Two very important parameters regarding hydrogen bonding, the equilibrium hydrogen bond distances (r 0 ) and the equilibrium hydrogen bond energy (D e ) can be calculated from the fitting of the curves with Morse equation [29] for each of the electronic states.…”