Excited-state Double Proton Transfer Research Articles

Effects of Chalcogen Atoms on Excited-State Double-Proton Transfer Behavior for 3,6-bis(4,5-Dihydroxyoxazo-2-yl)benzene-1,2-diol Derivatives: A Computational Investigation.

The impact of the chalcogen atomic electronegativity (O, S, and Se atoms) of new organic molecules on excited-state dynamical reactions is self-evident. Inspired by this kind of distinguished photochemical characteristic, in this work, we performed a computational investigation of chalcogen-substituted 3,6-bis(4,5-dihydroxyoxazo-2-yl)benzene-1,2-diol (BDYBD) derivatives (i.e., BDYBD-O, BDYBD-S, and BDYBD-Se). In this paper, we pay close attention to characteristic BDYBD derivatives that contain intramolecular double hydrogen bonds (O1-H2···N3 and O4-H5···N6). The main goal of this study was to explore how changes in atomic electronegativity affect the way hydrogen bonds interact and how excited molecules affect transfer protons. We go into further detail in the main text of the paper. By fixing our attention to geometrical variations and infrared (IR) vibrational spectra between the S0 and S1 states, exploring hydrogen bonding behaviors using the core-valence bifurcation (CVB) index, and simulating hydrogen bonding energy (EHB) via the atom in molecule (AIM) method, we clarified the photo-induced strengthened dual hydrogen bonding interactions that facilitate the excited-state dual-proton transfer (ESDPT) behavior of BDYBD derivatives. The reorganization of charge stemming from photoexcitation further verifies the tendencies of ESDPT reactions. We relied on constructing potential energy surfaces (PESs) by adopting a restrictive optimization approach, and herein, we finally clarify the gradual ESDPT mechanism of BDYBD derivatives. Particularly, we confirm that the variation in chalcogen atomic electronegativity has a regulatory effect on the ESDPT behavior of BDYBD derivatives; that is, the lower the atomic electronegativity, the more favorable it is for the gradual ESDPT reaction.

Regulating and controlling the stepwise ESDPT channel of BP(OH)2DCEt2 using the strategy of solvent polarity and external electric field.

Excited state double proton transfer (ESDPT) has attracted great scientific interest because of its excellent luminescent properties. However, the complex process of ESDPT has plagued theoretical and experimental scientists for a long time and has become a hot issue. In this work, the ESDPT process of 2,2'-bipyridine-3,3'-diol-5,5'-dicarboxylic acid ethyl ester (BP(OH)2DCEt2) is systematically studied and the regulation of the ESDPT process is further realized. The potential energy curves indicate that BP(OH)2DCEt2 shows the characteristics of stepwise ESDPT in different polar solvents. The increase in solvent polarity will be beneficial to the stepwise ESDPT reaction. Regrettably, it is not possible to distinguish the specific stepwise transfer path of the BP(OH)2DCEt2 molecule due to the symmetry of the potential energy surface along the diagonal. On this basis, we proposed a method to control and regulate the stepwise ESDPT path using an external electric field. The results show that the increase of external electric field intensity is favorable to stepwise ESDPT. It is interesting to note that applying an external electric field in a specific direction will effectively distinguish stepwise ESDPT reaction paths. Therefore, this work not only helps to understand the mechanism of ESDPT, but also contributes to regulation and design of new luminescent materials with excellent luminescent properties.

Effect of external electric fields on the ESDPT process and photophysical properties of 1,8-dihydroxy-2-naphthaldehyde.

In this work, by capitalizing on the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, it has been systematically studied that the excited state double intramolecular proton transfer (ESDPT) process and the photophysical properties of 1,8-dihydroxy-2-naphthaldehyde (DHNA) are affected by the distinct external electric fields (EEFs). The obtained intramolecular hydrogen bond (IHB) parameters containing bond lengths and angles, as well as infrared (IR) vibrational spectra demonstrate that IHB strength changes in the distinct EEFs. Moreover, not only do the potential energy surfaces (PESs) indicate that the ESDPT process of DHNA is stepwise, but also increasing the positive EEF results in a decrease in the energy barrier accordingly, while vice versa. The absorption and fluorescence spectra also undergo a corresponding red or blue shift in the EEF; for instance, when the EEF changes from +10 × 10-4 a.u. to +20 × 10-4 a.u., the fluorescence peak undergoes a blue shift from 602 nm to 513 nm in the keto2 form. In a nutshell, the ESDPT process of DHNA can be influenced by the EEF, which will serve as a reference in regulating and controlling proton transfer that causes luminescence.

Synchronous or stepwise Mechanism? a theoretical study on the Excited-State double proton transfer properties of shikonin and acetylshikosin

The excited state double proton transfer (ESDPT) mechanism of shikonin (Shk) and its derivative acetylshikosin (AcShk) were studied by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The potential energy curves scanned along the coordinates of proton transfer indicate a preference for the ESDPT reaction to occur step by step. The AcShk molecule possesses an additional reaction pathway in comparison to the Shk molecule. Furthermore, efforts have been made to compute the absorption and fluorescence peak, which exhibits favorable conformity with the experimental findings of the system investigated. The fluorescence spectra in cyclohexane and acetonitrile solvents indicate that the solvent polarity affects the location of the ESDPT fluorescence peak in both Shk and AcShk systems. The fluorescence spectra concentrated in the green light region (504 nm ∼ 550 nm) are obtained, which has the potential to promote human health through disinfection and boosting the immune system.

Accelerating the concerted double proton transfer process of 2,2′-bipyridine-3,3′-diol-5,5′-dicarboxylate acid ethyl ester (BPDC) molecule by centrosymmetric dual intermolecular hydrogen bonds

This study has conducted a systematic investigation into the excited state double proton transfer (ESDPT) process of 2,2′-bipyridine-3,3′-diol-5,5′-dicarboxylate acid ethyl ester (BPDC) in cyclohexane (CYH) and water (H2O) by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The calculated geometrical parameters, reduced density gradient and infrared vibration spectra confirm that BPDC undergoes the ESDPT process in both CYH and H2O. In comparison to the CYH solvent, the intramolecular hydrogen bonds (IAHBs) are strengthened in the S0 state by the formation of centrosymmetric intermolecular hydrogen bonds (IEHBs) between H2O and the OH group of BPDC molecules. Furthermore, the constructed potential energy surfaces show that BPDC molecules undergo a concerted ESDPT process without potential barriers in both solvents. Time evolution analysis confirms that the ESDPT process of BPDC molecules in H2O solvent (19 fs) is faster than that in CYH solvent (62 fs). The results indicate that the formation of IEHBs causes zero disruption to the original I AHBs, but it can effectively promote the ESDPT process of BPDC molecules. Overall, this work serves as important theoretical guidance for the development of high-performance fluorescence materials.

Dynamics simulation of excited-state proton transfer reactions of 8-hydroxyquinoline with water clusters: A TD-DFT study

Detailed pictures of the excited-state proton transfer reactions of 8-hydroxyquinoline (8HQ) and its small clusters of water have been systematically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods at B3LYP/TZVP and TD-B3LYP/TZVP level of theory, respectively. Intramolecular hydrogen bond (intraHB) and intermolecular hydrogen bond (interHB) interactions between 8HQ and water cluster become stronger in the first excited state (S1), confirmed by hydrogen bond distances and topological analysis. In addition, TD-B3LYP dynamics simulations have been elucidated and revealed that excited-state intramolecular proton transfer (ESIntraPT) and multiply excited-state intermolecular proton transfer (ESInterPT) reactions of these complexes can take place in the ultrafast time scale of femtoseconds. Increasing the number of water molecules may give rise to a barrier energy that is anti-correlated to the probability of tautomer species from phototautomerization, especially in 8HQ-W1 and 8HQ-W1+1. Moreover, water molecules initiate multiple proton transfer occurring through two- and three-step ESInterPT namely excited-state double proton transfer (ESDPT) and excited-state triple proton transfer (ESTPT) in which the interHBs are formed by rearrangement of water molecules of 8HQ-W2. This circumstance takes a longer time (up to 470 fs) compared to that of ESIntraPT. Thus, the simulated results from this study could provide insight into ESIntraPT and ESInterPT of phototautomerization and the significant photodynamics which could not be found from the experimental aspects of 8HQ and its solvent clusters.

Exploring the excited state proton transfer mechanisms of flavonoid probe in three representative solvents

The role of intermolecular hydrogen bond is often ignored in the study of the solvent effect on the excited state proton transfer (ESPT) processes. The ESPT mechanisms of 2-(4-(dimethylamino)phenyl)-3‑hydroxy-6,7-dimethoxy-4H-chromen-4-one (HOF) in three representative solvents, which were toluene, dimethyl sulfoxide (DMSO) and ethanol (EtOH), were investigated through time-dependent density functional theory (TD-DFT) method, and the effect of intermolecular hydrogen bond on the ESPT was fully considered in this paper. The results show that the enhancement of intra- and inter-molecular hydrogen bonds is the key factor of proton transfer process. The change of electron density distribution is also more conducive to the ESPT processes. Due to the existence of intermolecular hydrogen bond in HOF-DMSO complex, the energy barrier of ESPT in DMSO is higher than that in toluene. In EtOH solvent, the excited state double proton transfer (ESDPT) reaction can occur between HOF and EtOH along two different reaction channels with the assistance of dual intermolecular hydrogen bonds. Although the ESPT modes in DMSO and EtOH solvents are different, the energy barriers are basically the same. The energy barriers of ESPT processes in two polar solvents increase obviously due to intermolecular hydrogen bond. Moreover, in polar solvents, a relatively large red-shift was observed in the fluorescence spectra.

Different competition mechanism between ESPT and TICT process regulated by protic and aprotic solvent in DHP

In this study, we adopt density functional theory and time-dependent density functional theory (DFT/TDDFT) methods to compare and analyze the disparate competing mechanisms of excited state proton transfer (ESPT) and twisted intramolecular charge transfer (TICT) process of 2-(4′-diethylamino-2′-hydroxyphenyl)-1H-imidazo-[4,5-b] pyridine (DHP) in protic and aprotic solvent. The results show that the DHP molecule undergoes excited state intramolecular proton transfer (ESIPT) in aprotic solvent ether. Differently, ESIPT is effectively inhibited in protic solvent methanol, whereas the coupling process of excited state double proton transfer (ESDPT) and TICT occurs. Combined with the analysis of potential energy surfaces, we clarify that DHP molecule in ether is more inclined to the channel of TICT → ESIPT → reverse TICT process to implement the ESIPT process. In DHP-MeOH complex, we confirm that the significant redshift phenomenon of fluorescence spectra observed in the experiment is attributed to the ESDPT induced TICT process. Our work offers a new avenue for switching ESPT and TICT competing mechanisms through protic and aprotic solvent, and considerably help the synthesis and development of highly efficient molecular photoswitch.

Recognition-Induced Enhanced Emission of Core-Fluorescent ESIPT-type Macrocycles.

Core-fluorescent cavitands based on 2-(2'-resorcinol)benzimidazole fluorophores (RBIs) merged with the resorcin[4]arene skeleton were designed and synthesized. The cavitands, due to the presence of intramolecular hydrogen bonds and increased acidity, show excited state intramolecular proton transfer (ESIPT) and readily undergo deprotonation to form dianionic cavitands, capable of strong binding to organic cations. The changes in fluorescence are induced by deprotonation and binding events and involve huge Stokes shifts (due to emission from anionic double keto tautomers) and cation-selective enhancement of emission originating from the restriction of intramolecular motion (RIR) upon recognition in the cavity. Ab initio calculations indicate that the macrocyclic scaffold stabilizes the ground state tautomeric forms of the fluorophores that are not observed for non-macrocyclic analogs. In the excited state, the emitting forms for both macrocyclic scaffolds and non-macrocyclic analogs are anionic double keto tautomers, which are the result of excited state intramolecular proton transfer (ESIPT) or excited state double proton transfer (ESDPT).

The invalidity of intermolecular proton transfer triggered twisted intramolecular charge transfer in excited state for 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine

AbstractThe excited‐state dynamics of the excited‐state proton transfer and intramolecular twisted charge transfer (TICT) reactions of a molecular photoswitch 2‐(4′‐diethylamino‐2′‐hydroxyphenyl)‐1H‐imidazo‐[4,5‐b]pyridine (DHP) in aprotic and alcoholic solvents have been theoretically investigated by using time‐dependent density functional theory. The excited‐state intramolecular proton transfer (ESIPT) reaction of DHP proceeding upon excitation in all the solvents has been confirmed, and the dual emission has been assigned to the enol and keto forms of DHP. However, for methanol and ethanol solvents within strong hydrogen‐bonded capacity, the intermolecular hydrogen bonds between DHP and methanol/ethanol would promote an excited‐state double proton transfer (ESDPT) along the hydrogen‐bonded bridge. Importantly, the previous proposed ESDPT‐triggered TICT mechanism of DHP in methanol and ethanol was not supported by our calculations. The twist motion would increase the total energy of the system for both the products of ESIPT and ESDPT. According to the calculations of the transition states, the ESDPT reaction occurs much easier in keto form generated by ESIPT. Therefore, a sequential ESIPT and ESDPT mechanism of DHP in methanol and ethanol has been reasonably proposed.

Excited state double proton transfer efficient probe: Theoretical investigation and sensing ability towards Pb2+ and Al3+ ions

Dual ESIPT efficient Schiff-base probe possessing multiple binding sites was synthesized which is capable of showing Aggregation Induced Emission (AIE) properties. Probe 1 behaves as a fluorescence “turn-on” chemosensor, showing interactions with Pb2+ and Al3+ ions in H2O-CH3CN solvent at pH≈7. The binding constant and detection limit for all the ions were calculated. The binding properties of probe 1 with all the ions were checked by Job's plot and the 1H NMR titration method. Further, the presence of dual intramolecular hydrogen bonding was confirmed theoretically by geometrical parameters and FTIR analysis. The potential energy curves (PECs) calculations for proton transfer coordinates verified that probe 1 shows a single ESIPT process. Thus with good selectivity and sensitivity and fast response time, a highly efficient platform for simultaneous detection of Pb2+ ions was generated using a simple molecule.

Excited-state double proton transfer of 1,8-dihydroxy-2-naphthaldehyde: A MS-CASPT2//CASSCF study

Excited-state double proton transfer (ESDPT) is a controversial issue which has long been plagued with theoretical and experimental communities. Herein, we took 1,8-dihydroxy-2-naphthaldehyde (DHNA) as a prototype and used combined complete active space self-consistent field (CASSCF) and multi-state complete active-space second-order perturbation (MS-CASPT2) methods to investigate ES-DPT and excited-state deactivation pathways of DHNA. Three different tautomer minima of S1-ENOL, S1-KETO-1, and S1-KETO-2 and two crucial conical intersections of S1S0-KETO-1 and S1S0-KETO-2 in.and between the S0 and S1 states were obtained. S1-KETO-1 and S1-KETO-2 should take responsibility for experimentally observing dual-emission bands. In addition, two-dimensional potential energy surfaces (2D-PESs) and linear interpolated internal coordinate paths connecting relevant structures were calculated at the MS-CASPT2//CASSCF level and confirmed a stepwise ESDPT mechanism. Specifically, the first proton transfer from S1-ENOL to S1-KETO-1 is barrierless, whereas the second one from S1-KETO-1 to S1-KETO-2 demands a barrier of ca. 6.0 kcal/mol. The linear interpolated internal coordinate path connecting S1-KETO-1 (S1-KETO-2) and S1S0-KETO-1 (S1S0-KETO-2) is uphill with a barrier of ca. 12.0 kcal/mol, which will trap DHNA in the S1 state while therefore enabling dual-emission bands. On the other hand, the S1/S0 conical intersections would also prompt the S1 system to decay to the S0 state, which could be to certain extent suppressed by locking the rotation of the C5−C8−C9−O10 dihedral angle. These mechanistic insights are not only helpful for understanding ESDPT but also useful for designing novel molecular materials with excellent photoluminescent performances.

Effect of solvent polarity on excited-state double proton transfer process of 1,5-dihydroxyanthraquinone

The excited-state double proton transfer (ES-DPT) properties of 1,5-dihydroxyanthraquinone (1,5-DHAQ) in various solvents were investigated using femtosecond transient absorption spectroscopy and the DFT/TDDFT method. The steady-state fluorescence spectra in toluene, tetrahydrofuran (THF) and acetonitrile (ACN) solvents presented that the solvent polarity has an effect on the position of the ESDPT fluorescence emission peak for the 1,5-DHAQ system. Transient absorption spectra show that the increasing polarity of the solvent accelerates the rate of excited state dynamics. Calculated potential energy curves analysis further verified the experimental results. The ESDPT barrier decreases gradually with the increase of solvent polarity from toluene, THF to ACN solvent. It is convinced that the increase of solvent polarity can promote the occurrence of the ESDPT dynamic processes for the 1,5-DHAQ system. This work clarifies the mechanism of the influence of solvent polarity on the ESDPT process of 1,5-DHAQ, which provides novel ideas for design and synthesis of new hydroxyanthraquinone derivatives.

Excited-state intramolecular double proton transfer mechanism associated with solvent polarity for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound

Excited-state intramolecular proton transfer (ESIPT) has been well investigated in recent years, whereas we investigated the system containing double hydrogen bonds. We explore the influence of solvent polarity on the proton transfer mechanism for 9,9-dimethyl-3,6-dihydroxy-2,7-bis(4,5-dihydro-4,4-dimethyl-2-oxazolyl)fluorene compound ((Oxa-OH)2). We consider three solvents with different polarities: cyclohexane, chloroform and acetonitrile. Various analytical methods indicate that hydrogen bonding is enhanced in the first excited state, which facilitates the proton transfer within the excited-state molecule. The frontier molecular orbital theory is employed to demonstrate that the system is subjected to photoexcitation to generate charge density redistribution. Three-dimensional potential energy surfaces are also constructed along the direction of hydrogen bond elongated, and their corresponding projection in the X-axis and Y-axis planes are plotted. The comparison of the potential barriers and the transition state structures of each proton transfer path reveals the excited-state intramolecular single proton transfer mechanism of the system. By comparing the potential barriers in different solvents, we also propose a mechanism that can modulate the excited-state proton transfer by changing the solvent polarity.

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH2) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S0 and S1 states were scanned with varying O–H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH2 can undergo the ESDPT process in the S1 state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.

Elaborating the excited-state double proton transfer mechanism and multiple fluorescent characteristics of 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole

Recently, Krishnamoorthy and coworkers reported a new type of proton transfer, which was labeled as ‘proton transfer triggered proton transfer’, in 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole (bis-HPTA). In this work, the excited-state double proton transfer (ESDPT) mechanism and multiple fluorescent characteristics of bis-HPTA were investigated. Upon photo-excitation, the intramolecular hydrogen bonding strength changed and the electron density of bis-HPTA redistributed. These changes will affect the proton transfer process. In S0 state, the proton transfer processes of bis-HPTA were prohibited on the stepwise and concerted pathways. After vertical excitation to the S1 state, the ESIPT-II process was more likely to occur than the ESIPT-I process, which was contrary to the conclusion that the ESIPT-II process is blocked and the ESIPT-II process takes place after the ESIPT-I process proposed by Krishnamoorthy and coworkers. When the K2 tautomer was formed through the ESIPT-II process, the second proton transfer process on the stepwise pathway was prohibited. On another stepwise pathway, after the ESIPT-I process (form the K1 tautomer), the second proton transfer process should overcome a higher potential barrier than the ESIPT-I process to form ESDPT tautomer. On the concerted pathway, the bis-HPTA can synchronous transfer double protons to form the ESDPT tautomer. The ESDPT tautomer was unstable and immediately converted to the K2 tautomer via a barrierless reverse proton transfer process. Thus, the fluorescent maximum at 465 nm from the ESDPT tautomer reported by Krishnamoorthy and coworkers was ascribed to the K2 tautomer. Most of the fluorophores show dual fluorescent properties, while the bis-HPTA undergoing ESDPT process exhibited three well-separated fluorescent peaks, corresponding to its normal form (438 nm), K1 tautomer (462 nm) and K2 tautomer (450 nm), respectively.

Double proton transfer in a polar nano-droplet: Phototautomerization of alloxazine in AOT/alkane reverse micelles containing water or glycerol

Alloxazine phototautomerization is believed to occur through an excited state double proton transfer (ESDPT) mechanism involving cyclic intermolecular H-bonded complexes between Alloxazine and hydroxylic solvents like water and alcohols. In AOT/alkane dispersions in the absence of any polar liquid, Alloxazine molecules reside inside the polar core of the AOT reverse micelle nanoparticles, where they involve in H-bonding with the anionic sulfonate head-groups of the AOT molecules, but are unable to generate the appropriate cyclic intermolecular H-bonded complexes conducive to ESDPT. However, tautomerization is switched on with addition of water and formation ofwater nano-droplet at the core of reverse micelle. Evidently, the Alloxazine⋅⋅⋅⋅AOT H-bonds are now replaced by Alloxazine⋅⋅⋅⋅Water H-bonds, promotingESDPT. On the other hand, Alloxazine phototautomerization is hindered in Glycerol, irrespective of whether the latter is in the bulk liquid state or in the form of a polar nano-droplet. This may be explained by steric considerations.

Excited-State Intramolecular Proton Transfer: A Short Introductory Review.

In this short review, we attempt to unfold various aspects of excited-state intramolecular proton transfer (ESIPT) from the studies that are available up to date. Since Weller’s discovery of ESIPT in salicylic acid (SA) and its derivative methyl salicylate (MS), numerous studies have emerged on the topic and it has become an attractive field of research because of its manifold applications. Here, we discuss some critical aspects of ESIPT and tautomerization from the mechanistic viewpoint. We address excitation wavelength dependence, anti-Kasha ESIPT, fast and slow ESIPT, reversibility and irreversibility of ESIPT, hydrogen bonding and geometrical factors, excited-state double proton transfer (ESDPT), concerted and stepwise ESDPT.

Phototautomerism of Alloxazine in Acetic acid – Water solvent systems

The photophysics of Alloxazine was studied in Acetic acid – Water solvent systems which included neutral water (pH = 7), pure acetic acid, and their binary mixtures. Alloxazine was found to exhibit tautomerization into Isoalloxazine over a time-scale which depended on the nature of the solvent. While the process was too fast in neutral water to be detected with a ~100 ps time-resolution, it was drastically inhibited on addition of small quantities of acetic acid, displaying a remarkably long time-constant ~700 ps. However, further addition of acetic acid gradually decreased the time-constant, until it finally reached ~280 ps in pure acetic acid. Quantum chemical calculations indicated that the tautomerization occurred via Excited-state Double Proton Transfer (ESDPT) mechanism in cyclic intermolecular H-bonded complexes of Alloxazine with the solvent molecule (water or acetic acid). This involved reorganization of the H-bonded structures in the S1 energy surface, causing effective transfer of the N(1) bound proton of Alloxazine to its N(10) position, producing the Isoalloxazine tautomer. The H-bond reorganization of the complexes were strongly coupled with the solvation dynamics of the solvent concerned, and this helped to explain the large divergence of the tautomerization time-constants in the different solvents and solvent mixtures used in this study.

DUAL EMISSION OF 2-AMINO-4-METHYLPYRIMIDINE: A THEORETICAL STUDY

DFT and TD-DFT calculations at the B3LYP/LANL2DZ level of theory are performed in order to explain the origins of dual fluorescence and uncover the relaxation pathways of 2-amino-4-methylpyrimidine (L). The computations are in good agreement with experimental data. A high-energy emission band in the photoluminescence spectrum of 2-amino-4-methylpyrimidine (λmax = 350 nm) is related to the fluorescence of monomer L. Comparison of calculated and experimental IR and absorption spectra reveals that in the solid state L molecules form supramolecular {N–H···N}n hydrogen bonded associates. The formation of such associates favors the excited state double proton transfer (ESDPT). Calculated 2D potential energy curves for the ground and first singlet excited states of dimeric associates L2 indicate the presence of two stable forms, L2-amino and L2-imino. The low-energy band in the emission spectrum of 2-amino-4-methylpyrimidine (λmax = 445 nm) is probably associated with the fluorescence of the L2-imino form. Both fluorescence bands have a mixed π–π*, n–π* character as is evidenced by frontier orbitals involved in S1–S0 transitions.