Theoretical Studies on the Fluorescence Enhancement of Benzaldehydes by Intermolecular Hydrogen Bonding

Abstract

Density functional theory and its time-dependent extension are employed to investigate the intermolecular hydrogen-bonding- (Inter-HB-) induced fluorescence enhancement of benzaldehydes. The fluorescence quenching mechanism of benzaldehydes in chloroform is attributed to the low energy gap between the lowest ππ* and nπ* states at Franck–Condon (FC) point and conical interaction between ππ* and nπ* states. The Inter-HB between the aldehyde group of benzaldehydes and methanol can considerably increase the energy gap between ππ* and nπ* states at the FC point, and this type of hydrogen bond is strengthened in the excited ππ* state, thereby precluding the nonradiative transition from nπ* state and enhancing fluorescence. The Inter-HB between the hydroxy group of benzaldehydes and methanol is considerably stronger than that between the aldehyde group of benzaldehydes and methanol; which has a little effect on the energy gap of ππ* and nπ* states. Therefore, if Inter-HB exists between the hydroxy group of benzaldehydes and methanol, then it will play an important role in ground and excited ππ* states, and fluorescence enhancement by Inter-HB between the aldehyde group of benzaldehydes and methanol will be absent. Our proposed mechanism can explain the experimental observations and differs from that of “proximity effects”.

Unraveling the Mechanism for Tuning the Fluorescence of Fluorescein Derivatives: The Role of the Conical Intersection and nπ* State

“…9−12 Second, our recent theoretical study has demonstrated that TDDFT may underestimate the coupling between the ππ* and nπ* states and could not accurately predict the CI between the two states. 23 As reported in Table 1, the XMCQDPT2 computed vertical excitation energies (VEEs) of the six fluorescein derivatives in water were essentially identical and approximately 3.08 eV, which was larger than the experimentally measured absorption maxima (AM) of fluorescein in acidic media (437 nm, 2.84 eV), i.e., approximately 0.24 eV. 29 Due to neglecting the vibronic couplings and relaxation effects, the computed VEEs did not exactly correspond to the experimentally measured AM and were typically larger than AM by ca.…”

“…The energy of the nπ* state was larger than that of the ππ* state by only approximately 0.26 eV (see Figure b and Table ). The vibronic interaction between the close-lying ππ* and nπ* states can cause a significant fluorescence quenching of aromatic carbonyl compounds, and two different mechanisms can explain this phenomenon. − The first one is the so-called “proximity effect” which states that the nonradiative decay rate of the ππ* state will increase due to the vibronic coupling between the ππ* and nπ* states along the out-of-plane bending modes . The second one was proposed by us recently .…”

“…It is surprising why such a simple mechanism was overlooked in previous theoretical studies, − given that it has been observed for a number of nitrogen-heterocyclic and aromatic carbonyl compounds. − This instance may result from two reasons. First, our calculated results demonstrated that the contributions from double and triple excitations to the nπ* state were approximately 12% (see Table S1), indicating that the nπ* state of the neutral form of fluorescein derivatives possessed a remarkable multireference character and might not be described accurately with single-reference methods, i.e., time-dependent density functional theory (TDDFT), which has been adopted in previous theoretical studies. − Second, our recent theoretical study has demonstrated that TDDFT may underestimate the coupling between the ππ* and nπ* states and could not accurately predict the CI between the two states …”

“…The Inter-HB between carbonyl and alcoholic solvents or water will increase the energy of the nπ* state and lead to an enhancement of fluorescence. − However, once the molecule possesses carbonyl and hydroxyl groups simultaneously, the formed Inter-HB between solvents and hydroxyl was stronger than that of with carbonyl. The molecule preferred to form Inter-HB between hydroxyl group and solvents, and the fluorescence enhancement would not be observed.…”

Anomalous Intrinsic Fluorescence of HCl and NaOH Aqueous Solutions

“…3 Generally, the H-bond network facilitates internal conversion (IC) processes; 4,5 however, in some cases fluorescence enhancement is observed. 6,7 In the past decade, several studies reported nonconventional absorption due to charge transfer transitions 8 and anomalous photoluminescence induced by molecular aggregation or confinement of many common organic moieties. 9,10 Similarly, an anomalous fluorescence emission was observed in aqueous solutions of many peptides, proteins, and amyloid aggregates in the absence of any aromatic residue or π system.…”