Excited-state intramolecular proton transfer: photoswitching in salicylidene methylamine derivatives

Concerning for the solvent-polarity-dependent conformational equilibrium and ESIPT mechanism in Pz3HC system: A novel insight

In this report, we propose a new insight into the interaction between the solvent-polarity-dependent conformational equilibrium and excited state intramolecular proton transfer (ESIPT) behavior of Pz3HC system in four different polar solvents (polarity order: ACN > THF > TOL > CYC). Using quantum chemistry method, we first announce a coexistence mechanism between Pz3HC-1 and Pz3HC-3 in the ground state in four solvents based on the Boltzmann distribution. In particular, Pz3HC-1 is the principal configuration in non-polar solvent, but Pz3HC-3 is the principal configuration in polar solvent. In addition, the simulated fluorescence spectra interprets the negative solvatochromism effect of Pz3HC-1 and Pz3HC-3 in four solvents. The evidence from intramolecular hydrogen bonding (IHB) parameters and electronic perspective collectively confirms the light-induced IHB enhancement and intramolecular charge transfer (ICT) properties in Pz3HC-1 and Pz3HC-3, which raises the likelihood of the ESIPT process. Combining the calculation of potential energy curve (PEC) and intrinsic reaction coordinate (IRC), we demonstrate that the ESIPT ease of Pz3HC-1 in different polar solvents obeys the order of CYC > TOL > THF > ACN, while the order of ESIPT ease in Pz3HC-3 is opposite. Notably, the ESIPT process of Pz3HC-3 in CYC solvent is accompanied by the twisted intramolecular charge transfer (TICT) process. In addition, we also reveal that the enol* and keto* fluorescence peaks of Pz3HC-3 in CYC solvent are quenched by ISC and TICT process, respectively. Our work not only provides a satisfactory explanation of the novel dynamics mechanism for Pz3HC system, but also brings light to the design and application of new sensing molecules in the future.

Amination and Protonation Facilitated Novel Isoxazole Derivative for Highly Efficient Electron and Hole Separation

It is of great importance to understand the intrinsic relationship between phototautomerization and photoelectric properties for the exploration of novel organic materials. Here, in order to chemically control the protonation process, the aminated isoxazole derivative (2,2'-(isoxazolo[5,4-d]isoxazole-3,6-diyl)dibenzenaminium, DP-DA-DPIxz) with -N═ as the proton acceptor was designed to achieve the twisted intramolecular charge transfer (TICT) state which was triggered by an excited-state intramolecular proton transfer (ESIPT) process. This kind of protonation enhanced the intramolecular hydrogen bonding, conjugative effect, and steric hindrance effects, ensuring a barrierless spontaneous TICT process. Through the intramolecular proton transfer, the configuration torsion and conjugation dissociation of the DP-DA-DPIxz molecule was favored, which led to efficient charge separation and remarkable variations in light-emitting properties. We hope the present investigation will provide a new approach to design novel optoelectronic organic materials and shine light on the understanding of the charge transfer and separation process in molecular science.

Wavelength-dependent photochemistry of a salicylimine derivative studied with cryogenic and ultrafast spectroscopy approaches

Salicylimines are versatile compounds in which an excited-state intramolecular proton transfer and torsional motions may set in upon photoexcitation. Here, we study N-(α-phenylethyl)salicylimine (PESA) to elucidate how the photochemical reaction pathways depend on the excitation wavelength and to what extent the relative photoproduct distribution can be steered towards a desired species. DFT structure and potential energy calculations disclose that the most stable ground-state conformer is an enol species and that the photodynamics may proceed differently depending on the excited state that is reached. With matrix isolation infrared spectroscopy, the predominance of the enol conformer of PESA is confirmed. Illumination of the cryogenic sample with different wavelengths shifts the ratio of enol and keto products, and by sequential irradiation a selective re- and depopulation is possible. Femtosecond transient absorption spectroscopy further reveals that also at room temperature, the outcome of the photoreaction depends on excitation wavelength, and in combination with the calculations, it can be rationalized that the decisive step occurs within the first hundred femtoseconds. Since the ultrafast dynamics mostly match those of similar salicylimines, our findings might also apply to those systems and provide additional insight into their reported sensitivity on excitation energy.

Long-Range Proton Transfer in 7-Hydroxy-Quinoline-Based Azomethine Dyes: A Hidden Reason for the Low Efficiency

In the tautomeric Schiff bases, derived from 7-hydroxyquinoline, two competitive channels are possible upon excitation of the enol tautomer, namely proton transfer (PT) through intramolecular hydrogen bonding to the corresponding keto form and trans-cis isomerization around the azomethine double bond. The former leads to switching, based on twist-assisted excited state intramolecular PT, where the long-range proton transfer can occur as a targeted process. The latter, determined by the flexibility of the crane part, reduces the efficiency of the main targeted process. In previously studied molecular switches based on the 7-hydroxyquinoline skeleton, only the intramolecular PT photo-process undergoing from the excited enol form towards the keto tautomer, which is in most cases barrierless, has been discussed. Therefore, in the current study, the ground state PT properties and isomerization of (E)-8-((phenylimino)methyl)quinolin-7-ol and (E)-8-(((pentafluorophenyl)imino)methyl)quinolin-7-ol are investigated in depth using the MP2 methodology, while the excited state energy profiles are calculated with the ADC(2) method. The obtained results are discussed in light of the existing experimental data.

Spectroscopic Tracking of Salicylideneaniline Photocolored Crystals: An Attempt to Quantify Polymorph-Dependent Features toward Precise Structure-Function Correlation Analysis

Three polymorphs of salicylideneaniline (SA) were prepared, and their photochromic behavior was examined using a recently developed single-crystal microscopic UV-vis spectroscopy approach. This system enabled us to acquire absorption data during the bleaching process as a function of temperature and visible light intensity. First, we demonstrated that, in contrast to the generally accepted assumption, the bleaching curves were notably influenced by the degree of photosaturation at the initial stage. By modifying our kinetic model to include the term representing the initial degree of photosaturation, we successfully obtained the kinetic parameters intrinsic to each crystal structure. Second, we further analyzed the kinetic parameters to show that the bleaching process was accelerated by visible light irradiation to a significantly higher degree than by thermal relaxation. The two bleaching-prompting effects were quantitatively compared between two photochromic polymorphs, α₁ and α₂; the long life of the photoproduct in α₂ was attributed to efficient self-shielding from visible light irradiation enabled by its structural features. These results prompted us to reexamine the simple dualistic photochemical and thermal classification of photochromic systems and will provide a foundation for the precise structure-function analysis of crystalline materials, including SAs.

Solvent-Triggered Long-Range Proton Transport in 7-Hydroxyquinoline Using a Sulfonamide Transporter Group

The ability of long-range proton transport by substitution of 7-hydroxyquinoline at the eighth position with sulfonamide and sulfonylhydrazone rotor units to act as a crane-arm has been studied. Different proton transport pathways triggered by different stimuli have been established depending on the structure of the crane-arms. Solvent-driven proton switching from OH to the quinoline nitrogen (N_quin) site, facilitated by a sulfonamide transporter group in polar protic and aprotic solvents, has been confirmed by optical (absorption and fluorescence) and NMR spectroscopies as well as by single-crystal X-ray structure analysis. Photoinduced long-range proton transport to the N_quin site upon 340 nm UV light irradiation has been estimated in sulfonylhydrazone, which is not sensitive to solvent-driven switching. Both compounds have exhibited acid-triggered switching by trifluoroacetic acid due to the formation of a stable six-membered intramolecular hydrogen bonding interaction between the protonated N_quin and crane-arm. The structures of acid-switched form were confirmed by NMR spectroscopy and single-crystal X-ray structure analysis. The behavior of the compounds suggests a big step forward in the advanced proton pump-switching architecture because they cover three distinct driving forces in the switching process: solvent, light, and acid.

Simple Protocol for Capturing Both Linear-Response and State-Specific Effects in Excited-State Calculations with Continuum Solvation Models

We present an effective computational protocol (cLR²) to describe both solvatochromism and fluorosolvatochromism. This protocol, which couples the polarizable continuum model to time-dependent density functional theory, simultaneously accounts for both linear-response and state-specific solvation effects. A series of test cases, including solvatochromic and fluorosolvatochromic compounds and excited-state intramolecular proton transfers, are used to highlight that cLR² is especially beneficial for modeling bright excitations possessing a significant charge-transfer character, as well as cases in which an accurate balance between states of various polarities should be restored.

Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases

The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV-Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (K_T in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol^-1 of 4 and 0.63 kcal mol^-1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Full-Dimensional Photodynamics of Bistable Proton Transfer Switches

Excited-state intramolecular proton transfers (ESIPT) are one of the fastest reactions in chemistry (<100 fs) which – among other features like high photostability – makes them an important reaction class for molecular switches. ESIPTs can be coupled with double bond rotation/isomerization, so that molecules can act as “molecular cranes”, facilitating long-range proton transfer. A versatile model system is 7-hydroxy-4-methylquinoline-8-carbaldehyde (HMQCA): it features two proton-accepting sites, two stable ground-state isomers and should allow for easy derivatization. There is also experimental and theoretical reference data available, however, only for static properties, e.g. ground-state IR spectra or potential energy surface scans. In this contribution we show the results of full-dimensional surface-hopping molecular dynamics (MD) of HMQCA after photo-excitation, employing semiempirical quantum mechanics coupled to floating-occupation configuration interaction. The results support the potential of HMQCA as prototype system for directed proton transport by ESIPT.

Femtosecond real-time probing of the excited-state intramolecular proton transfer reaction in methyl salicylate

The excited-state intramolecular proton transfer (ESIPT) process and subsequent electronic relaxation dynamics in methyl salicylate have been investigated using femtosecond time-resolved ion yield spectroscopy combined with time-resolved photoelectron imaging. Excitation with a tunable pump pulse populates the keto tautomer in the first excited electronic state S₁(ππ^*). As a hydrogen atom transfers from the phenolic group to the carbonyl group within 100 fs, the molecular geometry changes gradually, leading to a variation in the electronic photoionization channel. By virtue of the accidental resonance with some intermediate Rydberg states, the time-dependent photoelectron spectra provide a direct mapping of the ESIPT reaction from the initially populated keto tautomer to the proton-transferred enol tautomer. Subsequently, the population around the enol configuration undergoes intramolecular vibrational redistribution on a subpicosecond time scale, followed by internal conversion to the ground state with a wavelength-dependent lifetime in the picosecond range. Furthermore, the excitation energies of several Rydberg states in methyl salicylate are determined experimentally.

Photophysics of proton transfer in hydrazides: a combined theoretical and experimental analysis towards OLED device application

Effect of conjugation to support ESIPT with impossible double proton transfer in structurally favored species.

Photophysics of Protonated and Microhydrated 2-Aminobenzaldehyde: Theoretical Insights into Photoswitchability of Protonated Systems

The photoswitchability of a protonated aromatic compound (2-aminobenzaldehyde, 2ABZH⁺) in its individual and microhydrated states has been addressed based on the RI-MP2/RI-CC2 theoretical methods. Our calculated results give insight into the ultrafast nonradiative deactivation mechanism of the 2ABZH⁺, driven by a conical intersection between the S₁/ S₀ potential energy surfaces. Also, it has been predicted that protonation accompanies a significant blue shift effect on the first ¹ππ* excited state of 2ABZ by 0.87 eV (at least 50 nm).

The photoinduced isomerization mechanism of the 2-(1-(methylimino)methyl)-6-chlorophenol (SMAC): Nonadiabatic surface hopping dynamics simulations

The photophysical properties of the Schiff base family are crucial for their applications such as molecular switches and molecular memories. However, it was found that the photophysical behavior is not uniform for all Schiff base molecules, which shows a significant substituent dependent property. In this article, we studied the photoisomerization mechanism of one Schiff base chlorosubstituted derivative 2-(1-(methylimino)methyl)-6-chlorophenol by employing geometrical optimization, energy profiles scanning, and on-the-fly dynamical simulations. Three types of minimum energy conical intersections were located on the S₁/S₀ crossing seam, with two characterized by twisting motion of the C=N bond and one featured with the excited state intramolecular proton transfer process and then twisting motion around the C=C bond [excited-state intramolecular proton transfer process (ESIPT)-then-twisting]. By a combination of the dynamics simulation results with the energy profiles scanned along with the ESIPT coordinate, it was found that the photophysical property of the targeted molecule is different from that of most Schiff base members, which prefer to decay by a twisting motion around the C=N bridge bond rather than the ESIPT-then-twisting channel. The minor ESIPT channel is probably governed by a tunneling mechanism. The proposed deactivation mechanism can provide a reasonable explanation for the observations in the experiment and would provide fundamental indications for further design of new and efficient photochromic products.

Substituent Effect in the First Excited Singlet State of Monosubstituted Benzenes

sEDA, pEDA, and cSAR descriptors of the substituent effect were determined for >30 monosubstituted benzenes in the first excited singlet S₁ state at the LC-ωB97XD/aug-cc-pVTZ level. It was found that in the S₁ state, the σ- and π-valence electrons are a bit less and a bit more affected, respectively, than in the S₀ state, but basically, the effect in both states remains the same. In the S₀ and S₁ states, the d(C-X) distances to the substituent's first atom and the ring perimeter correlate with the sEDA and pEDA in the appropriate states, respectively. The energies and the gap of the frontier orbitals in the two states are linearly correlated and for the HOMO(S₁), LUMO(S₁), and HOMO(S₁)-LUMO(S₁) gap correlate also with the pEDA(S₁) and cSAR(S₁) descriptors. In all studied correlations, three similar groups of substituents can be distinguished, for which correlations (i) are very good, (ii) deviate slightly, and (iii) deviate significantly. Comparison of the shape of the HOMO(S₀) and HOMO(S₁) orbitals shows that for case (i) HOMO orbitals exhibit almost perfect antisymmetry against the benzene plane, for case (ii) the antisymmetry of HOMO in one of the states is either perturbed or changed, and for case (iii) one HOMO state has σ-character.

Photochromism of 2-(2-Hydroxyphenyl) Benzothiazole (HBT) and Its Derivatives: A Theoretical Study

Hydroxyphenyl benzothiazole (HBT), is a well-known organic system based on its special characteristic of the excited state hydrogen transfer (ESHT) following photoexcitation. However, the capability of this system regarding photochromism and photoswitching has not been addressed yet. In this study, we have investigated this issue by the aim of the MP2, CC2, ADC(2), and CASSCF theoretical methods. Also, we have considered several electron withdrawing groups and investigated their effects on the photophysical characteristics and spectroscopic properties of the enol and keto tautomers of the titled system. It has been predicted that the main HBT and its considered substitutions fulfill the essential characteristics required for photochromism. Also, substitution is an effective idea for tuning the photophysical nature of HBT and its similar systems. Our theoretical results verify that different substitutions alter the UV absorption of HBT systems from 330 to 351 nm and also the corresponding absorption wavelength of the γ-forms of 526-545 nm.

Excited-State Proton Transfer in Thiazolo-[4, 5-d]thiazo Heterocyclic Systems and the Geometry Alterations' Effect on Photophysical Characters: A Theoretical Study

Thiazolo-[4,5- d]-thiazo-frame (tztz) compounds are important heteroaromatic organic systems, which recently became a subject of several studies in the field of organic electronics and organic photovoltaics. The most important physical nature of these systems is reported to be an equilibrium between enol and keto forms following excited-state proton transfer. This process originates from a flat trend of the S₁ PE (potential energy) profile along the proton transfer coordinate. In the present work, we determined and interpreted the excited-state proton transfer and photophysical nature of these systems extensively by means of the MP2/CC2 and CASSCF theoretical approaches. Also, the effects of amine (-NH₂) and cyano (-CN) substitutions were taken into account comprehensively by considering the transition energies and proton transfer pathways of the resulting tztz derivatives. It has been predicted that the physical nature of the excited-state intramolecular proton transfer, as the main character of these systems, is being affected significantly by substitutions. For all of the considered tztz derivatives, a conical intersection (CI) between ground and the S₁ excited state was predicted. This CI makes the considered species capable to be responsible for photochromism and photoswitching as well.

Single-crystal UV-vis spectroscopic examination of a striking odd-even effect on structure and chromic behaviour of salicylidene alkylamines

We have found that a series of N-(5-bromosalicylidene) alkylamines exhibited distinct chromic behaviour depending on the parity of their alkyl chain length. A group with an even number of carbon atoms in the alkyl chain showed photochromism, while another group with odd number showed thermochromism.

An Ab initio study on the photophysics of tris(salicylideneaniline)

Sequential excited-state proton transfers result in multiple band fluorescence spectrum.

Protonated serotonin: Geometry, electronic structures and photophysical properties

The geometry and electronic structures of protonated serotonin have been investigated by the aim of MP2 and CC2 methods. The relative stabilities, transition energies and geometry of sixteen different protonated isomers of serotonin have been presented. It has been predicted that protonation does not exhibit essential alteration on the S₁←S₀ electronic transition energy of serotonin. Instead, more complicated photophysical nature in respect to its neutral analogue is suggested for protonated system owing to radiative and non-radiative deactivation pathways. In addition to hydrogen detachment (HD), hydrogen/proton transfer (H/PT) processes from ammonium to indole ring along the NH⁺⋯π hydrogen bond have been predicted as the most important photophysical consequences of SERH⁺ at S₁ excited state. The PT processes is suggested to be responsible for fluorescence of SERH⁺ while the HD driving coordinate is proposed for elucidation of its nonradiative deactivation mechanism.

Toward Exploring Novel Organic Materials: MP4-DFT Properties of 4-Amino-3-Iminoindene

Tautomerism links with many applications and remains an attracting feature in exploring novel systems. In this regard, properties of indene-based HNCCCN segments have not received any considerable attention. In this computational organic chemistry study, first, to calculate the proton transfer energy barrier at a reasonable cost, the study identified an accurate forth order Møller–Plesset perturbation theory-density functional theory (MP4-DFT) protocol equivalent to the outstanding pioneering benchmark calculations. The calculations illustrate that the two tautomers of the 4-amino-3-iminoindene nucleus are separated by a considerable energy barrier while featuring different molecular orbital characteristics; frontier orbital distribution, λmax, and energies, which are known basic requirements in molecular switching and logic circuit applications. The N-H/BH₂ substitution was found to have significant influence on the electronic structure of the skeleton. Similarities in the two tautomers and the boron derivative to properties of known molecular materials have been found.

Theoretical insight into the excited-state intramolecular proton transfer mechanisms of three amino-type hydrogen-bonding molecules

Excited-state intramolecular proton transfer (ESIPT) dynamics of the amino-type hydrogen-bonding compound 2-(2'-aminophenyl)benzothiazole (PBT-NH₂) as well as its two derivatives 2-(5'-cyano-2'-aminophenyl)benzothiazole (CN-PBT-NH₂) and 2-(5'-cyano-2'-tosylaminophenyl)benzothiazole (CN-PBT-NHTs) were studied by the time-dependent density functional theory (TD-DFT) approach with the B3LYP density functional, and their absorption and emission spectra were also explored at the same level of theory. A good agreement is observed between the theoretical simulations and experimental spectra, indicating that the present calculations are reasonably reliable. In addition, it is also found that the energy barriers of the first excited singlet state of the three targeted molecules along the ESIPT reaction are computed to be 0.38, 0.34 and 0.12eV, respectively, showing the trend of gradual decrease, which implies that the introduction of the electron-withdrawing cyano or tosyl group can facilitate the occurrence of the ESIPT reaction of these amino-type H-bonding systems. Following the ESIPT, both CN-PBT-NH₂ and CN-PBT-NHTs dye molecules can undergo the cis-trans isomerization reactions in the ground-state and excited-state potential energy curves along the C2-C3 bond between benzothiazole and phenyl moieties, where the energy barriers of the trans-tautomer→cis-tautomer isomerizations in the ground states are calculated to be 0.83 and 0.34eV, respectively. According to our calculations, it is plausible that there may exist the long-lived trans-tautomer species in the ground states of CN-PBT-NH₂ and CN-PBT-NHTs.

Tailoring the Schiff base photoswitching – a non-adiabatic molecular dynamics study of substituent effect on excited state proton transfer

Dynamics reveals how to design chemical substitutions to control excited-state proton transfer efficiency.

Photophysical properties of betaxanthins: miraxanthin V – insight into the excited-state deactivation mechanism from experiment and computations

Fast radiationless S₁ → S₀ transition in photo-excited betaxanthins is due to conical intersection seam between S₁ and S₀ surfaces.

Tuning ESIPT fluorophores into dual emitters

Using first-principle approaches, we show how ESIPT can be controlled by fine-tuning of substituents, hence leading to new dual emitters.

Dyes undergoing excited-state intramolecular proton transfer (ESIPT) are known to present large Stokes shifts as a result of the important geometrical reorganisation following photon absorption. When the ESIPT process is not quantitative, one can obtain dual emitters characterised by two distinct fluorescence bands, observed due to emissions from both the canonical and ESIPT isomers. However, dual emission generally requires to maintain a very specific balance, as the relative excited-state free energies of the two tautomers have to be within a narrow window to observe the phenomenon. Consequently, simple chemical intuition is insufficient to optimise dual emission. In the present contribution, we investigate, with the help of quantum-mechanical tools and more precisely, time-dependent density functional theory (TD-DFT) and algebraic diagrammatic construction (ADC), a wide panel of possible ESIPT/dual emitters with various substituents. The selected protocol is first shown to be very robust on a series of structures with known experimental behaviour, and next is applied to novel derivatives with various substituents located at different positions. This work encompasses the largest chemical library of potential ESIPT compounds studied to date. We pinpoint the most promising combinations for building dual emitters, highlight unexpected combination effects and rationalise the impact of the different auxochromes.

Contacts for organic switches with carbon-nanotube leads

We focus on two classes of organic switches operating due to the photo- or field-induced proton transfer (PT) process. By means of first-principles simulations, we search for the atomic contacts that strengthen diversity of the two swapped current-voltage (I-V) characteristics between two tautomers. We emphasize that the low-resistive contacts do not necessarily possess good switching properties. Very often, the higher-current flow makes it more difficult to distinguish between the logic states. Instead, the more resistive contacts multiply a current gear to a larger extent. The low- and high-bias work regimes set additional conditions, which are fulfilled by different contacts: (i) in the very low-voltage regime, the direct connections to the nanotubes perform better than the popular sulfur contacts, and (ii) in the higher-voltage regime, the best are the peroxide (-O-O-) contacts. Additionally, we find that the switching-bias value is not an inherent property of the conducting molecule, but it strongly depends on the chosen contacts.

Excited-state deactivation mechanisms of protonated and neutral phenylalanine: a theoretical study

Minimum energy paths (MEPs) of protonated phenylalanine (PheH⁺) at the electronic ground and S₁ (¹ππ*) excited states along the C_α–C_β bond stretching coordinate, following proton transfer to the aromatic chromophore.

Photophysics and photochemistry of cis- and trans-hydroquinone, catechol and their ammonia clusters: a theoretical study

Excited state hydrogen transfer in hydroquinone- and catechol-ammonia clusters has been extensively investigated by high level ab initio methods. The potential energy profiles of the title systems at different electronic states have been determined at the MP2/CC2 levels of theory. It has been predicted that double hydrogen transfer (DHT) takes place as the main consequence of photoexcited tetra-ammoniated systems. Consequently, the DHT processes lead the excited systems to the (1)πσ*-S0 conical intersections, which is responsible for the ultrafast non-radiative relaxation of UV-excited clusters to their ground states. Moreover, according to our calculated results, the single hydrogen detachment or hydrogen transfer process essentially governs the relaxation dynamics of smaller sized clustered systems (mono- and di-ammoniated).