Femtosecond Fluorescence Research Articles

Enhancement of femtosecond laser-induced plasma fluorescence using a nanosecond laser

We demonstrate the enhancement of femtosecond (fs) laser-induced filaments in air and nitrogen flow fields using a nanosecond (ns) laser. With the ns laser being imposed on the filaments, the length and the emission intensity of the filaments were largely increased. Temporally resolved spectra of the enhanced filaments were obtained. The results show that the ns laser enhanced the short-lifetime fluorescence of nitrogen, which comes from the transition processes of N2 +(B2Σu + - X2Σg +), N2(B3Пg - A3Σu +) and N2(C3Пu - B3Пg). However, it had little effect on the long-lifetime chemiluminescence, which mainly comes from reactions such as N2(A3Σu +) + N2(A3Σu +) → N2(X1Σg +, v = 0) + N2(B3Пg). A possible explanation of this phenomenon is given, and this phenomenon might have potential applications in instantaneous one-dimensional measurements of various species in gas flow fields.

Solvent-induced symmetry-breaking charge transfer in an octupolar triphenylamine derivative resolved with transient fluorescence spectroscopy

The excited-state symmetry-breaking charge transfer (SBCT) dynamics in quadrupolar or octupolar molecules without clear infrared markers is usually hard to be tracked directly. In this work, on the basis of the evolution of instantaneous emission dipole moment obtained by femtosecond transient fluorescence spectroscopy, we presented a real-time characterization of the solvent-induced SBCT dynamics in an octupolar triphenylamine derivative. While the emission dipole moment of the octupolar trimer in weakly polar toluene changes little during the excited-state relaxation, it exhibits a fast reduction in a few picoseconds in strongly polar tetrahydrofuran. In comparison with the fluorescence dynamics of dipolar monomer, we deduced that the emitting state of the octupolar trimer in strongly polar solvent, which undergoes solvent-induced structural fluctuation, changes from exciton-coupled octupolar to excitation localized dipolar symmetry. In weakly polar solvent, the octupolar symmetry of the trimer is largely preserved during the solvation stabilization.

Spectroscopic investigation of high-pressure femtosecond two-photon laser-induced fluorescence of carbon monoxide up to 20 bar.

Extension of laser diagnostic methods to high pressure is critically important because most practical combustion systems operate at elevated pressures. However, increased collisional quenching, fluorescence trapping, and other spectroscopic complications make high-pressure laser diagnostics extremely challenging. As the pressure increases, collisional effects broaden and shift excitation spectral lines, reducing the excitation quantum efficiency of the laser-induced fluorescence (LIF) technique, in particular when using conventional narrowband, nanosecond (ns)-duration laser pulses. In this work, spectroscopic investigation of broadband, femtosecond two-photon LIF (fs-TPLIF) of carbon monoxide (CO) was performed in a high-pressure static gas cell, up to total pressures of 20 bar. The fluorescence emission spectrum broadened marginally at the highest pressure investigated and hence can be neglected in most cases. The subquadratic dependence of the CO fs-TPLIF signal on the laser fluence increased as the pressure is increased. Moreover, the CO fs-TPLIF signal decays more slowly with increasing pressure compared to the previously reported ns-TPLIF data. The signal drop when the total pressure is increased from 1 to 13 bar is approximately 30% in the fs excitation, as compared to approximately a 60% drop in the ns excitation in CO/N2/O2 mixtures. The pressure effects on the fluorescence signal were observed to be similar in the Ångström and third positive bands, suggesting that the third positive band could also be used for CO measurements with broadband fs laser pulses. Furthermore, experimentally investigated are the effect of pressure on the CO fluorescence signal in different quenching gases using fixed CO mole fractions, as well as number densities. Overall, the fs-TPLIF scheme is shown to be a promising diagnostics tool for CO detection in practical combustion systems at elevated pressures.

Direct observation of an intramolecular charge transfer state in epigenetic nucleobase N6-methyladenine.

N6-Methyladenine (6MeAde), the most abundant internal modification in mRNA, has proved to be an important epigenetic biomarker for gene regulation just like 5-methylcytosine in DNA. Recently, a unique UV-induced response of 6MeAde was reported, which makes it instructive and intriguing to reveal the excited state relaxation mechanism in this methylated adenine and its derivatives. In this work, we investigated 6MeAde and its ribose species N6-methyladenosine (6MeAdo) by using femtosecond time-resolved fluorescence up-conversion (FUC) and broadband transient absorption (TA) spectroscopy. Both 6MeAde and 6MeAdo exhibit a hundreds of femtoseconds lifetime, which originates from the efficient depletion of the ππ* (La) state. A several picoseconds lifetime is also observed and it should be attributed to the ππ* (Lb) state. Surprisingly, dual peak fluorescence emission is observed in 6MeAde and the long wavelength emission is ascribed to an intramolecular charge transfer (ICT) state. The lifetime of this ICT state is determined to be 107 ps. The kinetic isotope effect shows that the ICT state is closely associated with the solute-solvent H-bonding in aqueous solution. In 6MeAdo, the ICT state is apparently quenched and adenine-like excited state dynamics suggests that DNA/RNA containing such modification could still possess excellent photostability under UV irradiation. Our results contain an important insight for understanding excited state properties in epigenetic modified DNA/RNA.

Insights into the Femtosecond to Nanosecond Charge Carrier Kinetics in Perovskite Materials for Solar Cells

In this work, we analyze a few simple methods that permit getting insight into the charge dynamics in methylammonium lead iodide (MAPbI3) perovskites, on the basis of femtosecond transient absorption and fluorescence studies. We show that proper determination of the charge population decay kinetics requires broad spectral integration of the long-wavelength bleach signal (∼760 nm) assigned to the band-filling effect, which can be realized, e.g., by band integral analysis. On the contrary, the kinetics related to a shorter-wavelength bleach band (∼480 nm) behave differently: a single-wavelength decay is sufficient to probe charge population. Moreover, both the shorter-wavelength excitation and higher pump pulse intensities result in significantly longer charge cooling times, which might be important in efficient hot carrier extraction. The validity of our previously proposed easy approach for the elimination of higher-order recombination rates is also discussed and confirmed in terms of proper estimation of...

Fluorescence and Ultrafast Fluorescence Unveil the Formation, Folding Molecularity, and Excitation Dynamics of Homo-Oligomeric and Human Telomeric i-Motifs at Acidic and Neutral pH.

i-Motifs are tetraplex DNAs known to be stable at acidic pH. The structure of i-motifs is important in DNA nanotechnology; i-motif-forming sequences with consecutive cytosine (C) molecules are abundant throughout the human genome. There is, however, little information on the structure of C-rich DNAs under physiologically relevant neutral conditions. The electron dynamics of i-motifs, crucial to both biology and materials applications, also remains largely unexplored. In this work, we report a combined femtosecond and nanosecond broadband time-resolved fluorescence (TRF) and steady-state fluorescence investigation on homo-oligomer dC20 , a human telomeric sequence (HTS) 5'-dC3 (TA2 C3 )3 , and its analogue performed with different excitation at both acidic and neutral pH. Our study provides direct observation of intrinsic fluorescence and the first full probe of the real-time dynamics of the intrinsic fluorescence from i-motifs formed from varied sequences and pH conditions. The results obtained demonstrate concrete evidence for the existence at neutral pH of i-motifs from both dC20 and the HTS. It also identifies that, under neutral conditions, the i-motif from dC20 adopting the bimolecular folding structure is significantly more stable than the HTS i-motif featuring the unimolecular topology. Our femtosecond and nanosecond TRF study unveils excitation dynamics distinctive of the interdigitated architecture of i-motifs with the excited states involved exhibiting deactivation over a remarkably broad timescale through multiple channels involving proton-coupled electron transfer lasting tens of picoseconds, as signified by the solvent kinetic isotope effect, and structure-dependent charge recombination in the hundreds of picoseconds to tens of nanoseconds time regime.

Correlation among Hydrogen Bond, Excited-State Intramolecular Proton-Transfer Kinetics and Thermodynamics for −OH Type Proton-Donor Molecules

A series of new molecules bearing alkyl-substitutes on the parent molecule 1-hydroxy-11H-benzo[b]fluoren-11-one (HBF) has been designed and synthesized, which possesses an intramolecular hydrogen bond (H-bond) between −OH proton donor and carbonyl proton acceptor. All studied molecules present an equilibrium type of excited-state intramolecular proton transfer (ESIPT) at 298 K. The alkyl-substitutions at various positions subtly alter the intramolecular H-bond strength, which then fine-tune the excited-state equilibrium and hence thermodynamics between normal and tautomer species. These, in combination with finite rates of ESIPT resolved by femtosecond fluorescence upconversion techniques, lead to the establishment of an empirical relationship among H-bond strengths, ESIPT kinetics, and thermodynamics for HBF series of molecules, in which the stronger H-bond is, the faster and more exergonic ESIPT is. The intensity ratio for normal versus proton-transfer tautomer emission can be systematically tuned, demo...

Solvent and Structural Fluctuations Induced Symmetry-Breaking Charge Transfer in a Porphyrin Triad

This study aims to show that the symmetry-breaking charge transfer (SBCT) in a donor–acceptor–donor (DAD) porphyrin triad occurs via solvent and structural fluctuations measured by using femtosecond broadband fluorescence upconversion spectroscopy, which can directly trace the origin of the emitting state by monitoring its emission dipole moment as a function of time. While the emission dipole moment of the triad in the excited state remains nearly unchanged in nonpolar solvents such as cyclohexane and toluene, it is significantly reduced in polar solvents such as benzonitrile due to a change in the emitting state from quadrupolar (the exciton coupled state) to dipolar symmetry (the relaxed S1 state). The latter state is formed by the SBCT process of DAD via a combination of solvent and structural fluctuations.

Effect of Quantum Size on the Luminescent Properties of Quantum Dots Based on Cadmium Halcogenides

The quantum size effects of colloid water-organic media synthesized nanocomposites (CdSe–CdS core–shell) in toluene solution is studied. Femtosecond fluorescence up-conversion spectroscopy is used to establish that quantum dots are characterized by a biexponential decay of luminescence kinetics: the decay times are 1.8 and 26.8 ps for nanoparticles with average sizes of 2 nm and 4.5, and 68 ps for nanoparticles with an average size of 2.9 nm.

Solvent effect on multiple emission and ultrafast dynamics of higher excited states

We present ultrafast-depopulation-dynamics of higher-lying-excited-states using femtosecond fluorescence up-conversion techniques for two near-infrared (NIR) tricarbocyanine dyes (IR144 and IR140) in primary alcohols. With visible excitation wavelengths, such dyes show two distinct emission-bands with large peak wavelength difference: one at the visible region: S2 → S0, and the other at NIR region: S1 → S0. We show that exact band-positions, intensities, and fluorescence-decay-timescales (τ) depend strongly on viscosity and polarity of solvents. Interestingly, though the faster component of τ increased for IR144 with increasing viscosity and chain-length of alcohols, the reverse was seen for IR140, indicating the possible formation of ion-pair of IR140 with alcohols.

Unraveling the Detailed Mechanism of Excited-State Proton Transfer.

As one of the most fundamental processes, excited-state proton transfer (ESPT) plays a major role in both chemical and biological systems. In the past several decades, experimental and theoretical studies on ESPT systems have attracted considerable attention because of their tremendous potential in fluorescent probes, biological imaging, white-light-emitting materials, and organic optoelectronic materials. ESPT is related to fluorescence properties and usually occurs on an ultrafast time scale at or below 100 fs. Consequently, steady-state and femtosecond time-resolved absorption, fluorescence, and vibrational spectra have been used to explore the mechanism of ESPT. However, based on previous experimental studies, direct information, such as transition state geometries, energy barrier, and potential energy surface (PES) of the ESPT reaction, is difficult to obtain. These data are important for unravelling the detailed mechanism of ESPT reaction and can be obtained from state-of-the-art ab initio excited-state calculations. In recent years, an increasing number of experimental and theoretical studies on the detailed mechanism of ESPT systems have led to tremendous progress. This Account presents the recent advances in theoretical studies, mainly those from our group. We focus on the cases where the theoretical studies are of great importance and indispensable, such as resolving the debate on the stepwise and concerted mechanism of excited-state double proton transfer (ESDPT), revealing the sensing mechanism of ESPT chemosensors, illustrating the effect of intermolecular hydrogen bonding on the excited-state intramolecular proton transfer (ESIPT) reaction, investigating the fluorescence quenching mechanism of ESPT systems by twisting process, and determining the size of the solute·(solvent) n cluster for the solvent-assisted ESPT reaction. Through calculation of vertical excitation energies, optimization of excited-state geometries, and construction of PES of the ESPT reactions, we provide modifications to experimentally proposed mechanisms or completely new mechanism. Our proposed new and inspirational mechanisms based on theoretical studies can successfully explain the previous experimental results; some of the mechanisms have been further confirmed by experimental studies and provided guidance for researchers to design new ESPT chemosensors. Determination of the energy barrier from an accurate PES is the key to explore the ESPT mechanism with theoretical methods. This approach becomes complicated when the charge transfer state is involved for time-dependent density functional theory (TDDFT) method and optimally tuned range-separated TDDFT provides an alternative way. To unveil the driving force of ESPT reaction, the excited-state molecular dynamics combined with the intrinsic reaction coordinate calculations can be employed. These advanced approaches should be used for further studies on ESPT systems.

Femtosecond Fluorescence Upconversion Study of a Naphthalimide-Bithiophene-Triphenylamine Push-Pull Dye in Solution.

There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photoexcited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophene-naphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC) evolves toward a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher nonradiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in nonpolar solvents, hinting that the excited-state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.

Femtosecond Hydration Map of Intrinsically Disordered α-Synuclein

Protein hydration water plays a fundamentally important role in protein folding, binding, assembly, and function. Little is known about the hydration water in intrinsically disordered proteins that challenge the conventional sequence-structure-function paradigm. Here, by combining experiments and simulations, we show the existence of dynamical heterogeneity of hydration water in an intrinsically disordered presynaptic protein, namely α-synuclein, implicated in Parkinson’s disease. We took advantage of nonoccurrence of cysteine in the sequence and incorporated a number of cysteine residues at the N-terminal segment, the central amyloidogenic nonamyloid-β component (NAC) domain, and the C-terminal end of α-synuclein. We then labeled these cysteine variants using environment-sensitive thiol-active fluorophore and monitored the solvation dynamics using femtosecond time-resolved fluorescence. The site-specific femtosecond time-resolved experiments allowed us to construct the hydration map of α-synuclein. Our results show the presence of three dynamically distinct types of water: bulk, hydration, and confined water. The amyloidogenic NAC domain contains dynamically restrained water molecules that are strikingly different from the water molecules present in the other two domains. Atomistic molecular dynamics simulations revealed longer residence times for water molecules near the NAC domain and supported our experimental observations. Additionally, our simulations allowed us to decipher the molecular origin of the dynamical heterogeneity of water in α-synuclein. These simulations captured the quasi-bound water molecules within the NAC domain originating from a complex interplay between the local chain compaction and the sequence composition. Our findings from this synergistic experimental simulation approach suggest longer trapping of interfacial water molecules near the amyloidogenic hotspot that triggers the pathological conversion into amyloids via chain sequestration, chain desolvation, and entropic liberation of ordered water molecules.

Anion halogen bonding effect on solvatochromism and excited state dynamics of hemicyanine dye in chlorinated solvents

Anion halogen bonding effect on solvatochromism and excited state dynamics of donor-π-acceptor hemicyanine dye 1-ethyl-2-(4-(p-dimethylaminophenyl)-1,3-butadienyl)pyridinium perchlorate (LDS-698) in chlorinated solvents were explored using steady state absorption, fluorescence and femtosecond time-resolved fluorescence studies. The anomalous bathochromic shift in the intramolecular charge transfer (ICT) absorption maximum of LDS-698 in chlorinated solvents is due to the specific halogen bonding interaction between the perchlorate anion and chlorinated solvents, which affect the ion-pair interaction and ground state energy. The presence of halogen bonding decreases the ion-pair interaction and account for the polarizability correction term in the in the Kamlet-Abboud-Taft solvatochromic analysis. The ultrafast time-resolved fluorescence studies confirm the radiative relaxation from locally-excited (LE), ICT and twisted intramolecular charge transfer (TICT) state. The excitation wavelength dependent decay of LDS-698 indicates the direct population of ICT state in chlorinated solvents. In cationic polymethine dyes, the symmetry breaking induced by the ion-pair effect is reversed by the halogen bonding interaction of the anion with chlorinated solvents.

Exploring the Photodynamics of a New 2D-MOF Composite: Nile Red@Al-ITQ-HB.

In this work, we unravel the photodynamics of Nile Red (NR) interacting with Al–ITQ-HB nanostructure, a new layer-type metal–organic framework (MOF) with potential catalytic and photonic applications. Steady-state spectroscopy reveals the presence of NR monomers and aggregates when interacting with the MOF structure. Time-resolved experiments provide emission lifetimes of the interacting monomers, H- and J-type aggregates. We observed contributions from two monomer populations having different environments. One monomer species emits from the local-excited state and another from a photoproduced charge-separated state resulting from an ultrafast intramolecular charge transfer (ICT). Femtosecond fluorescence experiments reveal that the ICT process occurs in ∼1 ps. Fluorescence microscopy on single crystals and agglomerates of the composites shows a homogenous distribution of the dye lifetimes within the material. This study shows that the photobehavior of NR in Al–ITQ-HB MOF is dictated by its location within the material. The reported findings using a well-known polarity probe and a new two-dimensional MOF provide information on the microenvironment of this material, which may help for designing smart MOFs with potential applications in photonics and nanocatalysis.

Effect of Microheterogeneity of Different Aqueous Binary Mixtures on the Proton Transfer Dynamics of [2,2'-Bipyridyl]-3,3'-diol: A Femtosecond Fluorescence Upconversion Study.

In this article, we have investigated the excited-state intramolecular double proton transfer dynamics of [2,2′-bipyridyl]-3,3′-diol, BP(OH)2, in three alcohol–water binary mixtures, namely, ethanol (EtOH)–water, n-propanol (PrOH)–water, tert-butyl alcohol (TBA)–water, and dimethyl sulfoxide (DMSO)–water utilizing the femtosecond fluorescence upconversion technique. We have found that in alcohol–water binary mixtures the proton transfer (PT) pathway of BP(OH)2 is sequential and the anomalous slowdown in PT dynamics is observed in mole fraction (χ) ranges χEtOH = 0.04–0.07, χEtOH = 0.23–0.28, χPrOH = 0.17–0.30, χTBA = 0.12–0.21, and χTBA = 0.40–0.46. Our study sheds light on the involvement of water network in the PT dynamics. Reduction in water accessibility due to the involvement of water molecules in cluster formation results in hindered PT dynamics, and this retardation is more for the TBA–water binary mixture compared to that for the other two mixtures. Additionally, we have found two anomalous regions for the DMSO–water binary mixture in ranges χDMSO = 0.12–0.16 and χDMSO = 0.26–0.34. However, most interestingly, beyond χDMSO = 0.40, we do not find any growth component in the femtosecond fluorescence upconversion trace, which may be due to the change in the PT mechanism from a sequential water-mediated pathway to a concerted intramolecular pathway.

Two-photon absorption spectra of fluorescent isomorphic DNA base analogs.

Fluorescent DNA base analogs and intrinsic fluorophores are gaining importance for multiphoton microscopy and imaging, however, their quantitative nonlinear excitation properties have been poorly documented. Here we present the two-photon absorption (2PA) spectra of 2-aminopurine (2AP), 7-methyl guanosine (7MG), isoxanthopterin (IXP), 6-methyl isoxanthopterin (6MI), as well as L-tryptophan (L-trp) and 3-methylindole (3MI) in aqueous solution and some organic solvents measured in the wavelength range 550 - 810 nm using femtosecond two-photon excited fluorescence (2PEF) and nonlinear transmission (NLT) methods. The peak 2PA cross section values range from 0.1 GM (1 GM = 10-50 cm4 s photon-1) for 2AP to 2.0 GM for IXP and 7MG. Assuming typical excitation conditions for a scanning 2PEF microscope, we estimate a maximum image frame rate of ~175 frames per second (FPS).

Nonradiative dynamics determined by charge transfer induced hydrogen bonding: a combined femtosecond time-resolved fluorescence and density functional theoretical study of methyl dimethylaminobenzoate in water.

As a case study of the interplay and the consequence of the interplay between intramolecular charge transfer (ICT) and intermolecular hydrogen (H)-bonding, a combined femtosecond time-resolved fluorescence (fs-TRF) and density functional theoretical (DFT) and time-dependent DFT (TDDFT) study has been conducted on methyl dimethylaminobenzoate (MDMABA) largely in a water solvent. Direct observation of the broadband spectra, anisotropy, and kinetic decays of fs-TRF from photo-excited MDMABA revealed a rapid ICT reaction occurring with a time constant of ∼0.7 ps from an initial locally excited (LE) state identified to have the Laππ* character; this produced a weakly emissive ICT state featuring radiative rate constant decreased by more than two orders of magnitude. The fluorescence of the ICT state is strongly quenched exhibiting a decay time of ∼49.7 ps, unusually faster than the nanosecond range lifetime in a polar aprotic solvent when intersystem crossing (ISC) is the major deactivation channel. This, according to the study of the solvent kinetic isotope effect, is identified to originate from an instantly enhanced strong solute-solvent H-bonding induced by the ICT reaction which allows elimination of the ISC, and enables the nonradiative decay to proceed almost entirely through the otherwise inaccessible internal conversion from the ICT state. The enhancement of H-bonding is verified by the calculation which presents theoretical evidence for not only the binding site and binding energy of the H-bonding configuration but also the electronic and structural characterization, lending support to the twisted ICT (TICT) description of the photo-excited MDMABA. This study contributes a prominent example for the extraordinary ability of water and a decisive role of ICT promoted H-bonding in offering a highly effective molecular mechanism for rapid elimination of the electronic excitation energy. The results contain an important insight for the in-depth understanding of the excited state H-bonding dynamics, and also have significant implication for clarifying the "sunscreen controversy" of the DMABA type of UVB sunscreen molecule.

Substituent control of the ultrafast twisted intramolecular charge transfer rate in dimethylaminochalcone derivatives.

The effect of acceptor strength variation on ultrafast twisting relaxation dynamics was investigated by comparing the ultrafast relaxation dynamics in a series of dimethylaminochalcone (DMAC) derivatives. Employing femtosecond resolved transient absorption and fluorescence experiments, the twisted intramolecular charge transfer (TICT) relaxation rate is shown to vary from 2.0 picoseconds in a weak electron accepting system to 420 femtoseconds in a strong electron accepting system. The strength of the acceptor, empirically expressed as Hammett's constant, is shown to exhibit a linear free energy relationship (LFER) with the twisting rate. It is proposed that variation in the charge pulling capacity of the acceptor modifies the torsional barrier along the TICT coordinate in the S1 state, resulting in a tunable TICT relaxation rate, depending on the acceptor strength.

Photoprotection or photodamage: a direct observation of nonradiative dynamics from 2-ethylhexyl 4-dimethylaminobenzoate sunscreen agent.

Apart from being an analogue of the prototype for photoinduced intramolecular charge transfer (ICT), 2-ethylhexyl 4-dimethylaminobenzoate (EHDMABA) is also one of the earliest patented and most commonly used sunscreen components. There is, however, little documented information about the photophysics and factors affecting the photophysics of this molecule. Such information is of importance for both the understanding of the ICT reaction and assessing the underlying process of photoprotection, especially in view of the "sunscreen controversy" that has arisen from the contrasting in vivo vs. in vitro photobiological results on this and related UV filters. We report herein a femtosecond broadband time-resolved fluorescence (fs-TRF), complemented by transient absorption (fs-TA) to allow a full probe of the excited state cascades for EHDMABA and two of its derivatives in solvents of varied properties. The results provide direct evidence for a nearly solvent independent inner sphere ICT reaction occurring on the sub-picosecond time scale, and an ensuing solvent dictated deactivation of the ICT state. The ICT state in the aprotic solvent acetonitrile decayed solely through the intrinsic intersystem crossing (ISC) to produce a potentially harmful triplet excited state. In the protic solvent, the solvation and formation of ICT-induced solute-solvent hydrogen (H)-bonding opened the originally inaccessible internal conversion (IC) channel of the ICT state, leading to the rapid reformation of the ground state molecule with a unitary efficiency in the aqueous solution. This H-bonding-mediated IC restrained or eliminated the intrinsic ISC, providing a mechanism at the molecular level for the benign dissipation of the electronic excitation. The precise rate of IC was observed to vary with the alkoxy substituent and its efficiency was affected by the H-bonding capacity of the solvent. The findings of this work demonstrate the pivotal role of the microenvironment and the direct participation of solvent molecules through H-bonding in drastically altering the nonradiative dynamics and promoting or inhibiting photostability and photoprotection. This may assist in developing next-generation UV filters and help in improving formulation design for the optimal efficacy of sunscreen products. The pronounced H-bonding-induced fluorescence quenching and variation in the fluorescence wavelength imply that these molecules may also serve as a sensitive fluorescence probe for the H-bonding properties of the microenvironment.