Ab Initio Study on Ultrafast Excited-State Decay of Allopurinol Keto-N9H Tautomer from Gas Phase to Aqueous Solution

The excited-state decay of the biologically relevant allopurinol keto-N9H tautomer populated at the optically bright S₁(¹<i>ππ</i>*) state in the gas phase and in aqueous solution has been explored theoretically. In solution, the hybrid quantum-mechanical/molecular-mechanical simulations were performed, where the QM region (keto-N9H) was treated at the ab initio SA-CASSCF level, while the MM region (water) was described by the TIP3P model. Here we find that there exist four parallel relaxation pathways in the gas phase, but only two of them occur in aqueous solution. In addition, an ultrafast S₁ → S₀ internal conversion is found in vacuum, with an estimated excited-state lifetime of 104.7 fs, much faster than that in water (242.8 fs), showing reasonable agreement with the available experimental finding in aqueous solution (τ < 200 fs). Calculations indicate that the presence of water solvent plays an important role in the excited-state dynamics of DNA base, showing the pronounced environmental effects on its decay pathways and excited-state lifetimes

<i>Ab initio</i> insights on photophysics of 9-methylhypoxanthine

<p>In this work, the low-lying electronic singlet states of 9-methylhypoxanthine (9MHPX) were explored by the complete active space self-consistent-field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations, and the conical intersections between the optically bright excited S₁ state and ground S₀ state were optimised by the two-root state-averaged SA-2-CASSCF approach. These studies indicate that four slightly different kinds of S₁/S₀ conical intersections are identified computationally for 9MHPX, corresponding to four main internal conversion pathways, respectively, all of which are found to show the comparable timescales according to dynamics simulations. At the CASPT2 level, four bright <i>ππ</i>* transitions of 9MHPX are calculated to locate at 4.47, 5.35, 5.97 and 6.30 eV, respectively, responsible for the available experimental absorption peaks of 9MHPX in the vapour phase (4.41, 5.19, 6.05 and 6.42 eV). Though one relatively weak <i>ππ</i>* transition computed at 5.69 eV is not observed in the vapour phase, it is in accordance with the circular dichroism measurement of another hypoxanthine derivative deoxyinosine 5'-phosphate near 5.51 eV.</p

Theoretical insights into excited-state intramolecular proton transfer in 1,8-dihydroxydibenzo[<i>a,h</i>]phenazine

<p>1,8-Dihydroxydibenzo[<i>a,h</i>]phenazine (DHBP) is a new synthetic compound possessing two intramolecular hydrogen bonds; however, it has been found to exhibit the excited-state intramolecular single proton transfer (ESSPT) behaviour, in recent experiment. To explain the phenomenon reasonably, two combined methods of CASSCF/CASPT2 and DFT/TD-DFT have been employed to investigate the structural and spectral properties of its three tautomers, corresponding to the non-proton-transferred (E), the single-proton-transferred (SK) and the double-proton-transferred (DK) forms. These studies suggest that the E form is the global minimum in the S₀ state, while the SK form is the most stable in the S₁ state, both of which are responsible for the experimental absorption peak at 2.54 eV and emission band at 1.64 eV, respectively. Because of the relatively high energy barrier, the DK form will play no important role in the fluorescence emission of DHBP. The present results lend a good support to the experimental finding of single proton transfer (SPT).</p

Computational studies on amino-type excited-state intramolecular proton transfer and subsequent <i>cis</i>–<i>trans</i> isomerisation reactions of three 2-(2'-aminophenyl)benzothiazole derivatives

<p>Excited-state intramolecular proton transfer (ESIPT) reactions of three amino-type 2-(2'-aminophenyl)benzothiazole (PBT-NH₂) derivatives, that is, 2-(2'-methylaminophenyl)benzothiazole (PBT-NHMe), 2-(2'-acetylaminophenyl)benzothiazole (PBT-NHAc) and 2-(2'-tosylaminophenyl)benzothiazole (PBT-NHTs), have been explored by the time-dependent density functional theory (TD-DFT) method with the B3LYP density functional. In addition, their absorption and fluorescence spectra were also simulated at the same theoretical level. The present studies reveal that the energy barriers of the first singlet excited state of the three titled compounds along the ESIPT reactions are predicted at 0.39, 0.30 and 0.12 eV, respectively, suggesting that the inclusion of a strong electron-withdrawing tosyl group can remarkably facilitate the occurrence of the ESIPT reaction, while the involvement of an electron-donating methyl group has no effect on the ESIPT process of the amino-type hydrogen-bonding system. Following the ESIPT, both PBT-NHAc and PBT-NHTs molecules can also undergo the <i>cis</i>–<i>trans</i> isomerisation reactions along the C₂–C₃ bond between benzothiazole and phenyl moieties, in which the energy barriers of the <i>trans</i>-tautomer → <i>cis</i>-tautomer isomerisations in both ground states are calculated at 0.33 and 0.27 eV, respectively. This implies that there may exist a long-lived <i>trans</i>-tautomer species in the ground states for PBT-NHAc and PBT-NHTs, as observed in the spectroscopic experiments of PBT-NHTs.</p

Sequential Construction Strategy for Rational Design of Luminescent Iridacycles

A convenient and general strategy has been developed to synthesize stable iridapolycycles <b>5</b>–<b>8</b>. Reaction of arylacetylenes with iridium-hydride complex [IrH­(CO)­Cl­(PPh₃)₃]­BF₄ via nucleophilic addition, oxidative decarbonylation, and C–H bond activation results in the formation of a series of iridacyclopentadiene derivatives, including benzo-iridacyclopentadiene <b>5</b>, naphtho-iridacyclopentadiene <b>6</b>, pyreno-iridacyclopentadiene <b>7</b>, and thieno-iridacyclopentadiene <b>8</b>. These iridapolycycles display high thermal and air stability yet can be further functionalized via facile ligand substitution reactions. As an example, complex <b>5</b> was used as a metallosynthon to react with 2,2′-dipyridyl to give intensely luminescent Ir­(III) complex <b>9</b> bearing one C^∧C and one N^∧N ligands. Density functional theory (DFT) calculations reveal that the lowest unoccupied molecular orbitals (LUMOs) of iridapolycycles <b>5</b>–<b>8</b> are located on the phosphonium groups while the highest occupied molecular orbitals (HOMOs) are mainly located on the metal-embedded C^∧C frameworks. Our method offers a sequential construction strategy for constructing luminescent iridacycles, which potentially allows facile tuning of the photoluminescence properties by modulating the C^∧C and N^∧N moieties independently

Sequential Construction Strategy for Rational Design of Luminescent Iridacycles

A convenient and general strategy has been developed to synthesize stable iridapolycycles <b>5</b>–<b>8</b>. Reaction of arylacetylenes with iridium-hydride complex [IrH­(CO)­Cl­(PPh₃)₃]­BF₄ via nucleophilic addition, oxidative decarbonylation, and C–H bond activation results in the formation of a series of iridacyclopentadiene derivatives, including benzo-iridacyclopentadiene <b>5</b>, naphtho-iridacyclopentadiene <b>6</b>, pyreno-iridacyclopentadiene <b>7</b>, and thieno-iridacyclopentadiene <b>8</b>. These iridapolycycles display high thermal and air stability yet can be further functionalized via facile ligand substitution reactions. As an example, complex <b>5</b> was used as a metallosynthon to react with 2,2′-dipyridyl to give intensely luminescent Ir­(III) complex <b>9</b> bearing one C^∧C and one N^∧N ligands. Density functional theory (DFT) calculations reveal that the lowest unoccupied molecular orbitals (LUMOs) of iridapolycycles <b>5</b>–<b>8</b> are located on the phosphonium groups while the highest occupied molecular orbitals (HOMOs) are mainly located on the metal-embedded C^∧C frameworks. Our method offers a sequential construction strategy for constructing luminescent iridacycles, which potentially allows facile tuning of the photoluminescence properties by modulating the C^∧C and N^∧N moieties independently