Reconsideration on hydrogen bond strengthening or cleavage of photoexcited coumarin 102 in aqueous solvent: A DFT/TDDFT study

Theoretical insights into excited state behaviors of D3HF derivatives via altering atomic electronegativity of chalcogen

Inspired by the distinguished photochemical characteristics of new organic molecule containing the chalcogenide substitution that could be potentially applied across various disciplines, in this work, the effects of atomic electronegativity of chalcogen (O, S and Se) on hydrogen bond interactions and proton transfer (PT) reaction. We present the characteristic 2,8-diphenyl-3,7-dihydroxy-4H,6H-pyrano[3,2-g]-chromene-4,6-dione (D3HF), which is based on 3-hydroxyflavone (3HF) and contains intramolecular double hydrogen bonds that is the main objective of this study to explore in detail the influence of the change of atomic electronegativity on the dual hydrogen bond interaction and the excited state proton transfer (ESPT) behavior by photoexcitation. By comparing the structural changes and infrared (IR) vibrational spectra of the D3HF derivatives (D3HF-O, D3HF-S and D3HF-Se) fluorophores in S0 and S1 states, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that the hydrogen bond is strengthened in S1 state, which is favorable for the occurrence of ESPT reactions. The charge recombination behavior of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy surfaces (PESs) based on restrictive optimization, we finally clarify the excited state single PT mechanism for D3HF derivatives. Specially, we confirm change of atomic electronegativity has a regulatory effect on the ESIPT behavior of D3HF and its derivatives, that is, the lower the atomic electronegativity is more conducive to the ESIPT reaction.

Unveiling the effects of atomic electronegativity on ESIPT behaviors for FQ-OH system: A theoretical study

In this work, we mainly focus on exploring the effects of atomic electronegativity on excited state intramolecular proton transfer (ESIPT) behavior for novel FQ-OH derivatives theoretically. Combining analyses of geometrical changes, infrared (IR) spectral variations, and bonding energies via band critical point (BCP) parameters, we clarify the excited state hydrogen bonding strength is enhancing with decrease of atomic electronegativity. In addition, photo-induced charge reorganization and different energy gap of momentous frontier molecular orbitals (MOs) further reflect intramolecular charge transfer (ICT) promotes ESIPT reaction. Low atomic electronegativity reveals excited state high kinetic dynamics and chemical activities. Via constructing potential energy curves (PECs) and searching transition state (TS), we clarify atomic electronegativity dependent ESIPT behavior for FQ-OH. Particularly, the modification of atomic electronegativity also plays critical roles in regulating UV-Vis spectra. This work not only uncovering detailed ESIPT mechanism for FQ-OH, but also presents a novel regulated mechanism via atomic electronegativity.

Effects of azole rings with different chalcogen atoms on ESIPT behavior for benzochalcogenazolyl-substituted hydroxyfluorenes

ESIPT behavior has attracted a lot of eyes of researchers in recent years because of its unique optical properties. Due to its large Stokes shift and double emission fluorescence, white light can be generated in the fluorophore based on the excited state intramolecular proton transfer (ESIPT) principle. The excited state proton transfer behavior of hydroxylated benzoxazole (BO-OH), benzothiazole (BS-OH) and benzoselenazole (BSe-OH) have been investigated in heptane, chloroform and DMF solvents. By comparing the infrared vibration spectra and the variation of bond parameters from the S₀ to S₁ states, and analyzing the frontier molecular orbitals, the influence of hydrogen bond dynamics, the solvent polarity, charge redistribution and the effects of different proton acceptors on proton transfer were observed. The only structural difference among the three substituted hydroxyfluorenes is the heteroatom in the azole ring (oxygen, sulfur and selenium, respectively). We have scanned the potential energy curve of the ESIPT process, and compared the potential barrier, it is found that the heavier chalcogen atoms are more favorable for proton transfer. At the same time, the potential application of changing heteroatoms in the azole ring by walking down the chalcogenic group in crystal luminescence color regulation is also discussed.

Coumarin 102 excitation in aqueous media: contributions of vibronic coupling and hydration

For the first time, vibronic coupling was considered when analyzing the excitation of coumarin C102.

The influence of intermolecular hydrogen bonds on single fluorescence mechanism of 1-hydroxy-11H-benzo [b]fluoren-11-one and 10-hydroxy-11H-benzo [b]fluoren-11-one

Solvent effects usually have an essential effect on excited-state intramolecular proton transfer (ESIPT) processes and fluorescence mechanism. This contribution presents new insights into a newly synthesized compound, namely, 10-hydroxy-11H-benzo [b]fluoren-11-one (10-HHBF), and its analogue 1-hydroxy-11H-benzo [b]fluoren-11-one (1-HHBF), which exhibit single-fluorescence properties in protic solvents (methanol, MeOH), using time-dependent density functional theory (TDDFT). The results established four schemes, namely, MeOH-1, MeOH-2, MeOH-3, and MeOH-4, for 1-HHBF and 10-HHBF in MeOH. Absorption and emission spectra showed that the 1-HHBF and 10-HHBF at the conformation MeOH-2, MeOH-3 and MeOH-4 were closer to the experimental values than those at the MeOH-1. Energy barriers indicate the possibility of the ESIPT and ESPT process in 1-HHBF and 10-HHBF under the four schemes. Moreover, reverse PT processes were easy to occur at the conformations of MeOH-2, MeOH-3, and MeOH-4 in the S₁ state. Given the single-fluorescence properties of 1-HHBF and 10-HHBF in the experiment, the conformation MeOH-1 was excluded. Therefore, our contribution proved that MeOH-2, MeOH-3, and MeOH-4 might exist in single fluorescence, and the hydrogen bond at the MeOH-2 position plays a decisive role, indicating the intermolecular hydrogen bonding interaction on the acceptor atom will have a more significant impact on the fluorescence properties of the substance.

Determinant of ESIPT Mechanism by the Structure Designed for Symmetrical and Unsymmetrical Molecules

In this work, we present that different structures lead to different excited state properties based on the investigations of the systems with symmetrical and unsymmetrical properties. Our work shows that the symmetrical and unsymmetrical compounds with a modified structure play key roles in regulating the excited state intramolecular proton transfer (PT) (ESIPT) process. For N,N'-bis(salicylidene)-p-phenyle-nediamine (BSP) and N,N'-disalicylidene-1,6-pyrenediamine (BSD), when they are excited, the torsion angels result in the rematch of the spectra. By analyzing the potential energy surfaces (PESs) of the torsion angle and PT process, we conclude that the size of the π stack largely affects the molecular properties in the excited states. For 2'-hydroxychalcone derivatives, which have important applications in biotransformation reactions, investigating the molecules of M1 and M2 could promote innovation in bioengineering. The results of molecular electrostatic potential (MEP) and the real space intramolecular interactions show the state and position of the hydrogen bond (HB) in both S₀ and S₁ states. The corresponding PT PESs show that the ESIPT reaction is much easier for the M1 system due to the lack of the side chain hydroxyl compared with the M2 system. This work is not only consistent with experimental results and explains its mechanism but also presents that symmetric and nonsymmetric structures modify their own potential regulation and controlling effects for ESIPT behaviors.

Excited state hydrogen bond and proton transfer mechanism for (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine: A theoretical investigation

A novel fluorescence molecule (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine (HMPM) has been explored theoretically in this present work. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigate the excited state hydrogen bonding behaviors and excite state intramolecular proton transfer (ESIPT) process for HMPM molecule. Via simulating the reduced density gradient (RDG) versus sign(λ₂)ρ, we firstly verify the double intramolecular hydrogen bonds (O1H2⋯N3 and O4H5⋯N6) for HMPM system. Comparing with the changes about these two hydrogen bonds (i.e., bond distances, bond angles and infrared (IR) vibrational spectra), we find that they should be enhanced in the first excited state upon the photo-excitation. The shortened hydrogen bonding distance of H2⋯N3 and H5⋯N6 provide the possibility for ESIPT reaction. Given the photo-excitation process, we confirm the charge redistribution around the hydrogen bonding moieties plays an important role as a driving force for the ESIPT process. Further, via constructing S₀-state and S₁-state potential energy surfaces (PESs), we confirm the excited state double proton transfer (ESDPT) is excludable since the high optimized energy and high potential energy barrier. While the low potential barrier for excited state single proton transfer path results in the ultrafast ESIPT reaction, which explains why the initial HMPM fluorescence peak cannot be detected in previous experimental phenomenon. This work not only clarifies the excited state dynamical behavior for HMPM system, but also explains previous experimental phenomenon and attributions about steady state spectra. We hope this work can facilitate novel applications based on the novel HMPM system in future.

Excited state hydrogen atom transfer in micro-solvated dicoumarol: A TDDFT/EFP1 study

Ground (S₀) and excited (S₁) state properties of dicoumarol (DC) are investigated by applying density functional theory (DFT) and time dependent DFT (TDDFT) interfacing with the effective fragment potential (EFP) method of solvation. Benzene and pyrone rings of the each 4-hydroxy coumarin (4HC) moiety are in a plane and these planes are twisted by 180° with respect to each other. Two intra-molecular hydrogen bonds (HB) CO⋯HO exist between the carbonyl (CO) and hydroxyl (OH) groups of different 4HC moieties (4HC-1 and 4HC-2). DC(H₂O)₃ complex is formed using the original EFP model (EFP1). Four inter-molecular HBs are established by the carbonyl and hydroxyl oxygen atoms of 4HC-1 and 4HC-2 moieties; two HBs with two solvent molecules on one side of the complex and other two HBs with one solvent molecule at the other side. In S₁ state, the hydrogen atomtransfer takes place only from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2. The natural charge analysis and the modification of HBs manifest the intra-molecular charge transfer (ICT) from one 4HC moiety to another. Theoretical and experimental studies of the absorption spectra, and the theoretical study of potential energy curves of OH bonds at both S₀ and S₁ states affirm the hydrogen atom transfer from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2 moiety.

New Insights on Hydrogen-Bond-Induced Fluorescence Quenching Mechanism of C102-Phenol Complex via Proton Coupled Electron Transfer

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.

The mechanism of ratiometric fluoride sensing and the ESIPT process for 2,6-dibenzothiazolylphenol and its derivative

In this work, we explore the excited state intramolecular proton transfer (ESIPT) process and the relevant fluoride-sensing mechanism of two novel chemical systems, 2,6-dibenzothiazolylphenol (26DB) and bis-2,6-dibenzothiazolylphenol (Bis-26DB).

A theoretical investigation on excited-state single or double proton transfer process for aloesaponarin I

The excited-state proton transfer (ESPT) dynamical behavior of aloesaponarin I (ASI) was studied using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Our calculated vertical excitation energies based on TDDFT reproduced the experimental absorption and fluorescence spectra well [Nagaoka et al. J. Phys. Chem. B, 117, 4347 (2013)]. Two intramolecular hydrogen bonds were confirmed to be strengthened in the S1 state, which makes ESPT possible. Herein, the ESPT process is more likely to happen, along with one hydrogen bond (O1–H2⋯O3). Qualitative analyses about charge distribution further demonstrate that the ESPT process could occur because of the intramolecular charge transfer. Our constructed potential energy surfaces of both S0 and S1 states show that a single proton transfer reactive is more reasonable along with the intramolecular hydrogen bond (O1–H2⋯O3) rather than O4–H5⋯O6 in the S1 stated potential energy surface. Then, ASI-SPT* decays to the ground state with a 640 nm fluorescence; subsequently, the ASI-SPT form shows that reverse ground state single-proton transfer back to the ASI structure occurs. Particularly, dependent on relatively accurate potential energy barriers among these excited-state stable structures, we confirmed the excited-state single proton transfer process rather than using the controversial nodal plane model.

Exploring the excited state behavior for 2-(phenyl)imidazo[4,5-c]pyridine in methanol solvent

In this present work, we theoretically investigate the excited state mechanism for the 2-(phenyl)imidazo[4,5-c]pyridine (PIP-C) molecule combined with methanol (MeOH) solvent molecules. Three MeOH molecules should be connected with PIP-C forming stable PIP-C-MeOH complex in the S₀ state. Upon the photo-excitation, the hydrogen bonded wires are strengthened in the S₁ state. Particularly the deprotonation process of PIP-C facilitates the excited state intermolecular proton transfer (ESIPT) process. In our work, we do verify that the ESIPT reaction should occur due to the low potential energy barrier 8.785 kcal/mol in the S₁ state. While the intersection of potential energy curves of S₀ and S₁ states result in the nonradiation transition from S₁ to S₀ state, which successfully explain why the emission peak of the proton-transfer PIP-C-MeOH-PT form could not be reported in previous experiment. As a whole, this work not only put forward a new excited state mechanism for PIP-C system, but also compensates for the defects about mechanism in previous experiment.

A mechanistic study on Decontamination of Methyl Orange Dyes from Aqueous Phase by Mesoporous Pulp Waste and Polyaniline

The dispersion-corrected density functional theory (DFT-D3) is used to investigate the mechanism of mesoporous pulp waste (MPW) and polyaniline (PANI) adsorptive removal methyl orange (MO) dye from their aqueous solutions. The results are absolutely reliable because of the sufficiently accurate method although such big systems are studied. It is demonstrated that hydrogen bond and Van Der Waals interactions play a significant role in MO adsorption by MPW and PANI. For MO adsorption by MPW, hydrogen bond and Van Der Waals interactions are both weakened in S₁ state. In contrast, hydrogen bond and Van Der Waals interactions between PANI and MO are both enhanced in S₁ state. The thermodynamic parameters such as enthalpy and free energy change reveal that the MO adsorption by MPW and PANI are spontaneous and exothermic. The adsorption of MO on MPW is less favorable in S₁ state and the adsorption of MO on PANI is more favorable in S₁ state. Therefore, the photoexcitation should be controlled during the MO adsorption by MPW and applied for MO adsorption by PANI.

The role of hydrogen bonding in the fluorescence quenching of 2,6-bis((E)-2-(benzoxazol-2-yl)vinyl)naphthalene (BBVN) in methanol

The excited state hydrogen bonding dynamics of BBVN in hydrogen donating methanol solvent was explored at the TD-BMK/cc-pVDZ level of theory with accounting for the bulk environment effects at the polarizable continuum model (PCM). The heteroatoms of the BBVN laser dye form hydrogen bonds with four methanol molecules. In the formed BBVN-(MeOH)₄ complex, the A-type hydrogen bond (N…HO), of an average strength of 25kJmol^-1, is twofold stronger than the B-type (O…HO) one. Upon photon absorption, the total HB binding energy increases from 78.5kJmol^-1 in the ground state to 82.6kJmol^-1 in the first singlet (S₁) excited state. In consequence of the hydrogen bonding interaction, the absorption band maximum of the BBVN-(MeOH)₄ complex, which was anticipated at 398nm (exp. 397), is redshifted by 5nm relative to that of the free dye in methanol. The spectral shift of the stretching vibrational mode for the hydrogen bonded hydroxyl groups (with a maximum shift of 285cm^-1) from that of the free methanol indicated the elevated strengthening of hydrogen bonds in the excited state. The vibrational modes associated with hydrogen bonding provide effective accepting modes for the dissipation of the excitation energy, thus, decreasing the fluorescence quantum yield of BBVN in alcohols as compared to that in the polar aprotic solvents. Since there is no sign of photochemistry or phosphorescence, it seems reasonable in view of the outcomes of this study to assign the major decay process of the excited singlet (S₁) of BBVN in alcohols to vibronically induced internal conversion (IC) facilitated by hydrogen bonding.

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

Recently, we have experimentally demonstrated that the fluorescence intensity of coumarin 102 (C102) modulates anomalously upon hydrogen bonding to phenol in a nonpolar solvent: cyclohexane. The fluorescence intensity is first quenched gradually up to a particular mole fraction (X_PH ≈ 0.013) but thereafter increases with further increases in the phenol mole fraction. These studies speculate about the importance of C102-phenol H-bonding to induce photoinduced electron transfer (PET) and propose a competition between the C102-phenol and phenol-phenol H-bonding to account for the anomalous fluorescence modulation. In this work, we investigate the exact H-bonding environment around the acceptor C102 at various compositions by molecular dynamics simulation and correlate the H-bonding environment to the observed fluorescence quenching. In addition to the 1:1 C102-phenol complex, 1:2 C102-(phenol)₂ complexes with two different types of geometries were also found. Furthermore, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were carried out to understand the H-bonding in these complexes in the ground state and in the excited state and their possible contribution to the observed fluorescence quenching.

Exploring excited state properties of 7-hydroxy and 7-methoxy 4-methycoumarin: a combined time-dependent density functional theory/effective fragment potential study

Hydrogen bond dynamics, C–OH bond contracting, O–H bond stretching and O–H⋯O HB strengthening reveal the ESHT in 4MU at the S₁state.

New insights into the solvent-assisted excited-state double proton transfer of 2-(1H-pyrazol-5-yl)pyridine with alcoholic partners: a TDDFT investigation

Excited-state double proton transfer (ESDPT) in the hydrogen-bonded 2-(1H-pyrazol-5-yl)pyridine with propyl alcoholic partner (PPP) was theoretically investigated by time-dependent density functional theory (TDDFT) method. Great changes have taken place for the calculated geometric structures, the electron density features and vibrational spectrum of PPP system in S0 and S1 state. Our results have demonstrated that ESDPT reaction happens within the system upon photoexcitation. We also found that the ESDPT process is facilitated by the electronically excited state intermolecular hydrogen bond strengthening. Particularly, after the photoexcitation from HOMO(π) to the LUMO(π(∗)), the rearrangement of electronic density distribution of frontier molecular orbitals (MOs) on pyridine and the pyrazol moieties exhibits a very important positive factor for the ESDPT. Furthermore, by the investigation of the stretching vibrations of NH and OH groups, the infrared (IR) spectroscopic results provide us not only a theoretical evidence of ESDPT, but also a considerable clue to characterize the nature of intermolecular reaction. In addition, efforts have also been devoted towards calculating the absorption peak, which shows good consistency with the experimental result of the studied system.

A TDDFT/EFP1 study on hydrogen bonding dynamics of coumarin 151 in water

Change in energy of hydrogen bonds (HBs) upon excitation, plays an important role on the spectra of chemical and biological molecules. Effective fragment potential (EFP) method of explicit water molecules embedded in polarizable continuum medium (PCM) is used for the solvation of 7-Amino-4-(trifluoromethyl)coumarin (C151). Time dependent density functional theory (TDDFT) calculations combined with EFP/PCM had been carried out to study the electronic structure and the exited state properties of C151 with five water molecules (C151-(H2O)5 complex). S0 state and S1 state geometries were optimized using DFT/TDDFT with PBE0 functional combined with cc-pVDZ basis set, the transition energies are computed with same basis set and functional. Change in HB energy is calculated using the procedure proposed by T. Nagata et al. to calculate solute-solvent interaction energy in Nagata et al. (2011). Upon photoexcitation of C151-(H2O)5 complex, A type (N⋯H-O) HB is weakened with decrease of energy by 4.37 kJ/mol, whereas B and C type (C=O⋯H-O and N-H⋯O) HBs are strengthened with increase of 5.62 and 10.21 kJ/mol energy, respectively. This study again confirmed that the intermolecular hydrogen bonds between C151 chromophore and aqueous solvents are strengthened, not cleaved upon electronic excitation.

Different mechanisms of ultrafast excited state deactivation of coumarin 500 in dioxane and methanol solvents: experimental and theoretical study

The effect of hydrogen bonding on the intermolecular photoinduced ICT and TICT processes for coumarin 500 has been demonstrated, and a reliable mechanism has been revealed to explain the unusual behavior of C500 in dioxane and methanol.

Time-dependent density functional theory study on the excited-state intramolecular proton transfer in salicylaldehyde

Time-dependent density functional theory method was performed to investigate the excited state intramolecular hydrogen bond dynamics of salicylaldehyde (SA). The geometric structures and IR spectra in the ground state S0 state and the excited state S1 state of SA are calculated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods, respectively. In addition, the absorption and fluorescence peaks are also calculated using TDDFT methods. It is noted that the calculated large Stokes shift is in good agreement with the experimental results. Furthermore, our results have demonstrated that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which are distinct monitored by the formation and disappearance of the characteristic peaks of IR spectra involved in the formation of hydrogen bonds in different states and in the potential energy curves. We find that the hydrogen bonded quasi-aromatic chelating ring in the excited state becomes smaller which can facilitate the ESIPT process. The results presented here suggest that the ESIPT process of the SA molecule in the excited state can be attributed to the electronegativity change of O1 induced by excitation.

Solvation and protonation of coumarin 102 in aqueous media: a fluorescence spectroscopic and theoretical study

The ground- and excited-state protonation of Coumarin 102 (C102), a fluorescent probe applied frequently in heterogeneous systems with an aqueous phase, has been studied in aqueous solutions by spectroscopic experiments and theoretical calculations. For the dissociation constant of the protonated form in the ground state, pKa = 1.61 was obtained from the absorption spectra; for the excited-state dissociation constant, pKa* = 2.19 was obtained from the fluorescence spectra. These values were closely reproduced by theoretical calculations via a thermodynamic cycle (the value of pKa* also by calculations via the Förster cycle) using an implicit–explicit solvation model (polarized continuum model + addition of a solvent molecule). The theoretical calculations indicated that (i) in the ground state, C102 occurs primarily as a hydrogen-bonded water complex, with the oxo group as the binding site, (ii) this hydrogen bond becomes stronger upon excitation, and (iii) in the ground state, the amino nitrogen atom is the protonation site, and in the excited state, the carboxy oxygen atom is the protonation site. A comprehensive analysis of fluorescence decay data yielded the values kpr = 3.27 × 10(10) M(–1) s(–1) for the rate constant of the excited-state protonation and kdpr = 2.78 × 10(8) s(–1) for the rate constant of the reverse process (kpr and kdpr were treated as independent parameters). This, considering the relatively long fluorescence lifetimes of neutral C102 (6.02 ns) and its protonated form (3.06 ns) in aqueous media, means that a quasi-equilibrium state of excited-state proton transfer is reached in strongly acidic solutions.

Ground-state and excited-state multiple proton transfer via a hydrogen-bonded water wire for 3-hydroxypyridine

The multiple proton transfer reactions of 3-hydroxypyridine-(H₂O)₃ have been demonstrated, and a perfect proton transfer cycle has been revealed in the ground and excited states.

Fluorescence of tryptophan in aqueous solution

In this work, the absorption and emission spectra of Tryptophan (Trp) in aqueous solution were studied. Moreover, a hydrogen-bonded zwitterionic Trp(H2O)9 model was proposed and its ground-state and excited-state properties were investigated using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, respectively. All spectroscopic data in our experiments can be well explained by the hydrogen bond strengthening in the excited state of the model complex. The delocalization of electron density between indole moiety and neighboring H2O molecules in fluorescent state was proposed to be facilitated by the strengthened hydrogen-bond chain, and thus resulting in the large red-shift fluorescence of Trp in aqueous solution.