Photophysical properties of Pt(ii) complexes based on the benzoquinoline (bzq) ligand with OLED implication: a theoretical study.

In this study, we investigate photophysical properties of eight inorganic Pt(ii) complexes containing the bzq (benzoquinoline) ligand for OLED applications using high-level density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. We explore the radiative and non-radiative relaxation constants (k r, k nr), spin-orbit coupling (SOC) matrix elements, and spectral properties. To ensure compatibility between the host and guest compounds, we determine the HOMO and LUMO energy levels, as well as the triplet excitation energies of the selected systems, and evaluate their efficiency for OLED devices. Our findings indicate that all systems, except for 2a and 2b, exhibit a small S1-T1 energetic gap (ΔE ≤ 0.60 eV) and promising SOC matrix elements (25-93 cm-1), leading to a significant intersystem crossing (ISC) process. These complexes also show promising radiative relaxation rates (k r = ∼10-4 s-1) and high phosphorescent quantum yields (Φ > 30%). Thus, our results confirm that six out of the eight selected Pt(ii) complexes are promising candidates for use in the emitting layer (EML) of OLED devices as efficient green emitters.

Theoretical Insights into the Ultrafast Deactivation Mechanism and Photostability of a Natural Sunscreen System: Mycosporine Glycine.

In this work, different levels of quantum computational models such as MP2, ADC(2), CASSCF/CASPT2, and DFT/TD-DFT have been employed to investigate the photophysics and photostability of a mycosporine system, mycosporine glycine (MyG). First of all, a molecular mechanics approach based on the Monte Carlo conformational search has been employed to investigate the possible geometry structures of MyG. Then, comprehensive studies on the electronic excited states and deactivation mechanism have been conducted on the most stable conformer. The first optically bright electronic transition responsible for the UV absorption of MyG has been assigned as the S2 (1ππ*) owing to the large oscillator strength (0.450). The first excited electronic state (S1) has been assigned as an optically dark (1nπ*) state. From the nonadiabatic dynamics simulation model, we propose that the initial population in the S2 (1ππ*) state transfers to the S1 state in under 100 fs, through an S2/S1 conical intersection (CI). The barrierless S1 potential energy curves then drive the excited system to the S1/S0 CI. This latter CI provides a significant route for ultrafast deactivation of the system to the ground state via internal conversion.

DFT/TD-DFT study of electronic and phosphorescent properties in cycloplatinated complexes: implications for OLEDs.

High level density functional and time-dependent density functional (DFT, TD-DFT) theoretical methods have been employed to investigate the photophysical properties of 5 inorganic compounds resulting from Pt(ii) and ppy (2-phenyl-pyridine) ligands. This study is intended to provide insight into the capability of the selected systems to be used in OLED devices. In addition to an exploration of their ground and excited state geometry and electronic structures, the electronic transitions responsible for their absorption and spectra, as well as other photophysical properties, have been analyzed. To this end, their charge transfer parameters, the triplet exciton generation, phosphorescence quantum yield, and radiative decay rates have been studied. Overall, the results confirm that the selected systems are promising candidates to be used in OLED devices. Moreover, the results of this study assist in understanding the photophysical properties of Pt(ii) complexes with ppy ligands.

Excited state deactivation mechanisms of protonated adenine: a theoretical study.

The quantum chemical computational method and Born-Oppenheimer (BO) dynamics simulation were employed to investigate the non-radiative relaxation mechanism of protonated 9H- and 7H-adenine (AH+). We located three conical intersections (CIs) between the first 1ππ* excite state and the S0 ground state potential energy surfaces for the two most stable protonated isomers of adenine. It was predicted that the barrier-free potential energy profile along the out-of-plane deformation coordinates of the six-member ring plays the most prominent role in the deactivation of the excited AH+ from 1ππ* to the ground state via ultrafast internal conversions. This ring deformation was predicted to provide a common deactivation pathway in protonated DNA/RNA bases, describing their high level of photostability, and corresponding neutral homologues.

Insights into the effect of distal histidine and water hydrogen bonding on NO ligation to ferrous and ferric heme: a DFT study.

The effect of distal histidine on ligation of NO to ferrous and ferric-heme, has been investigated with the high-level density functional theoretical (DFT) method. It has been predicted that the distal histidine significantly stabilizes the interaction of NO ferrous-heme (by -2.70 kcal mol-1). Also, water hydrogen bonding is quite effective in strengthening the Fe-NO bond in ferrous heme. In contrast in ferric heme, due to the large distance between the H2O and O(NO) and lack of hydrogen bonding, the distal histidine exhibits only a slight effect on the binding of NO to the ferric analogue. Concerning the bond nature of FeII-NO and FeIII-NO in heme, a QTAIM analysis predicts a partially covalent and ionic bond nature in both systems.

Theoretical insights on the excited-state-deactivation mechanisms of protonated thymine and cytosine.

Ab initio and surface-hopping nonadiabatic dynamics simulation methods were employed to investigate relaxation mechanisms in protonated thymine (TH+) and cytosine (CH+). A few conical intersections were located between 1ππ* and S0 states for each system with the CASSCF (8,8) theoretical model and relevant contributions to the deactivation mechanism of titled systems were addressed by the determination of potential energy profiles at the CASPT2 (12,10) theoretical level. It was revealed that the relaxation of the 1ππ* state of the most stable conformer of both systems to the ground state is mostly governed by the accessible S1/S0 conical intersection resulting from the barrier-free out-of-plane deformation. Interestingly, it was exhibited that the ring puckering coordinate driven from the C6 position of the heterocycle ring in TH+ and CH+ plays the most prominent role in the deactivation mechanism of considered systems. Our ab initio results are also supported by excited-state nonadiabatic dynamics simulations based on ADC(2), describing the ultrashort S1 lifetime of TH+/CH+ by analyzing trajectories leading excited systems to the ground. It was confirmed that the excited-state population mostly relaxes to the ground via the ring puckering coordinate from the C6 moiety. Overall, the theoretical results of this study shed light on the deactivation mechanism of protonated DNA bases.

Excited State Deactivation Mechanism in Protonated Uracil: New Insights from Theoretical Studies.

We have conducted here a theoretical exploration, discussing the distinct excited state lifetimes reported experimentally for the two lowest lying protonated isomers of uracil. In this regard, the first-principal computational levels as well as the nonadiabatic surface hopping dynamics have been employed. It has been revealed that relaxation of the 1ππ* state of enol-enol form (EE+) to the ground is barrier-free via out-of-plane coordinates, resulting in an ultrashort S1 lifetime of this species. For the second most stable isomer (EK+), however, a significant barrier predicted in the CASPT2 S1 potential energy profile along the twisting coordinate has been proposed to explain the relevant long lifetime reported experimentally.

DFT study of α-Keggin, lacunary Keggin, and ironII-VI substituted Keggin polyoxometalates: the effect of oxidation state and axial ligand on geometry, electronic structures and oxygen transfer.

Herein, the geometry, electronic structure, Fe-ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii-vi) and H2O/OH-/O2- ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PW11O39(FeVO)]4-, possesses a high reactivity for oxygen transfer.

Water binding to FeIII hemes studied in a cooled ion trap: characterization of a strong 'weak' ligand.

The interaction of a water molecule with ferric heme-iron protoporphyrin ([PP FeIII]+) has been investigated in the gas phase in an ion trap and studied theoretically by density functional theory. It is found that the interaction of water with ferric heme leads to a stable [PP-FeIII-H2O]+ complex in the intermediate spin state (S = 3/2), in the same state as its unligated [PP-FeIII]+ homologue, without spin crossing during water attachment. Using the Van't Hoff equation, the reaction enthalpy for the formation of a Fe-OH2 bond has been determined for [PP-FeIII-H2O]+ and [PP-FeIII-(H2O)2]+. The corrected binding energy for a single Fe-H2O bond is -12.2 ± 0.6 kcal mol-1, while DFT calculations at the OPBE level yield -11.7 kcal mol-1. The binding energy of the second ligation yielding a six coordinated FeIII atom is decreased with a bond energy of -9 ± 0.9 kcal mol-1, well reproduced by calculations as -7.1 kcal mol-1. However, calculations reveal features of a weaker bond type, such as a rather long Fe-O bond with 2.28 Å for the [PP-FeIII-H2O]+ complex and the absence of a spin change by complexation. Thus despite a strong bond with H2O, the FeIII atom does not show, through theoretical modelling, a strong acceptor character in its half filled 3dz2 orbital. It is also observed that the binding properties of H2O to hemes seem strikingly specific to ferric heme and we have shown, experimentally and theoretically, that the affinity of H2O for protonated heme [H PP-Fe]+, an intermediate between FeIII and FeII, is strongly reduced compared to that for ferric heme.

The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory.

The binding energy of CO, O2 and NO to isolated ferric heme, [FeIIIP]+, was studied in the presence and absence of a σ donor (N-methylimidazole and histidine) as the trans axial ligand. This study combines the experimental determination of binding enthalpies by equilibrium measurements in a low temperature ion trap using the van't Hoff equation and high level DFT calculations. It was found that the presence of N-methylimidazole as the axial ligand on the [FeIIIP]+ porphyrin dramatically weakens the [FeIIIP-ligand]+ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the FeIII(3d) orbitals. This trans σ donor effect is characteristic of ligation to iron in hemes in both ferrous and ferric redox forms; however, to date, this has not been observed for ferric heme.

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).

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.

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.

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.

Excited-state intramolecular proton transfer and photoswitching in hydroxyphenyl-imidazopyridine derivatives: A theoretical study.

The MP2/CC2 and CASSCF theoretical approaches have been employed to determine the excited state proton transfer and photophysical nature of the four organic compounds, having the main frame of hydroxyphenyl-imidzaopyridine (HPIP). The nitrogen insertion effect, in addition to amine (-NH2) substitution has been investigated extensively by following the transition energies and deactivation pathways of resulted HPIP derivatives. It has been predicted that the excited state intramolecular proton transfer with or without small barrier is the most important feature of these compounds. Also, for all of the considered HPIP derivatives, a conical intersection (CI) between ground and the S1 excited state has been predicted. The strong non-adiabatic coupling in the CI (S1/S0), drives the system back to the ground state in which the proton may either return to the phenoxy unit and thus close the photocycle, or the system can continue the twisting motion that results in formation of a γ-photochromic species. This latter species can be responsible for photochromism of HPIP derivative systems.

Excited State Proton Transfer and Deactivation Mechanism of 2-(4'-Amino-2'-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine and Its Analogues: A Theoretical Study.

In the present study, the results of comprehensive theoretical exploration on the nonradiative relaxation of three (hydroxyphenyl)imidazole-based organic compounds (abbreviated AHP, HPIP, and HPBI) in the gas phase are presented. Having small structural differences, the selected systems have commonalities in the excited state intramolecular proton transfer (ESIPT) process. The ground and S1 excited state potential energy profiles of titled systems have been determined on the basis of the RI-MP2 and RI-CC2 methods, and the effect of small structural distinctions on their photophysical characters will be extensively addressed. Although, in the presence of solvent, high fluorescence quantum yield is another characteristic of AHP and HPIP, owing to accessible conical intersections between the S1/S0 state potential energy profiles of both systems, nonradiative relaxation can be proposed as the most important feature of these two systems in the gas phase. These conical intersections are responsible for ultrafast deactivation of excited systems via internal conversions to the ground state. The nonradiative deactivation mechanism determined in this work deals with the remarkable photostability of the AHP and HPIP molecules.

A theoretical exploration on electronically excited states of protonated furan and thiophene.

The geometry, electronic structures and potential energy profiles of protonated furan and thiophene have been extensively investigated, using the RI-MP2 and RI-CC2 methods. According to RI-CC2 calculated results, the adiabatic S1((1)ππ*)-S0 transition energies of protonated furan and thiophene, have been predicted to be 4.41 eV and 3.70 eV respectively. Thus, protonation is accompanied by a large red shift effect on the first (1)ππ* transition of the title systems (ΔE > 1.5 eV). The significant spectral-movements, predicted based on the calculated results of this work, indicate an essential effect of protonation on the geometry, electronic structures and optical characters of the five membered heterocyclic systems. In addition, it has been found that excitation of protonated furan and thiophene, with sufficient excess energy above the band origin of S1((1)ππ*)-S0 transition, is accompanied by the S-C or O-C bond breaking. This mechanism is mostly governed by a dissociative (1)πσ* PE profile in both protonated systems.

Electronically Excited States of Neutral, Protonated α-Naphthol and Their Water Clusters: A Theoretical Study.

The RI-MP2 and RI-CC2 methods have been employed to determine the potential energy profiles of neutral and protonated α-naphthol, in their individual forms and microhydrated with 1 and 3 water molecules, at different electronic states. According to calculated results, it has been predicted that dynamics of nonradiative processes in protonated α-naphthol is essentially different from that of its neutral homologue. In protonated α-naphthol, the calculations reveal that (1)σπ* state, is the most important photophysical state, having a bound nature with a broad potential curve along the OH coordinate of isolated system, while it is dissociative in monohydrated homologue. In neutral system, similar to phenol, the (1)πσ* state, plays the fundamental relaxation role along the O-H stretching coordinate. Moreover, microhydration strongly affects the photophysical properties of α-naphthol, mostly by alteration of the (1)ππ* PE profile, from a bound state in an isolated analogue to a dissociative state in hydrated systems. Furthermore, it has been found that three water molecules are necessary for ground state proton transfer between protonated α-naphthol and water; with a small barrier; (ΔE< 0.1 eV).

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).

Excited state deactivation pathways of neutral/protonated anisole and p-fluoroanisole: a theoretical study.

The potential energy profiles of neutral and protonated anisole and p-fluoroanisole at different electronic states have been investigated extensively by the RI-MP2 and RI-CC2 methods. The calculations reveal that the relaxation dynamics in protonated anisole and p-fluoroanisole are essentially different from those of the neutral analogues. In neutral anisole/p-fluoroanisole, the (1)πσ* state plays a vital relaxation role along the O-CH3 coordinate, yielding the CH3 radical. For both of these molecules, the calculations indicate conical intersections (CIs) between the ground and excited state potential energy (PE) curves, hindered by a small barrier, and providing non-adiabatic gates for radiation-less deactivation to the ground state. Nevertheless, for the protonated cases, besides the prefulvenic deformation of the benzene ring, it has been predicted that the lowest (1)(σ,n)π* state along the C-O-C bond angle plays an important role in photochemistry and the relaxation dynamics. The S1, S0 PE profiles of protonated anisole along with the former reaction coordinate (out-of-plane deformation) show a barrierless relaxation pathway, which can be responsible for the ultrafast deactivation of excited systems to the ground state via the low-lying S1/S0 conical intersection. Moreover, the later reaction coordinate in protonated species (C-O-C angle from 120°-180°) is consequently accompanied with the bond cleavage of C-OCH3 at the (1)(σ,n)π* state, hindered by a barrier of ∼0.51 eV, and can be responsible for the relaxation of excited systems with significant excess energy (hν≥ 5 eV). Furthermore, according to the RI-CC2 calculated results, different effects on the S1-S0 electronic transition energy of anisole and p-fluoroanisole upon protonation have been predicted. The first electronic transitions of anisole and p-fluoroanisole shift by ∼0.3 and 1.3 eV to the red respectively due to protonation.