Xugeng Guo

Electronic spectra of the linear polyyne cations HC2nH+ (n=2–8): An ab initio study

The B3LYP, CAM-B3LYP, and RCCSD(T) calculations have been used to determine the ground-state geometries of the linear polyyne cations HC(2n)H(+) (n=2-8). The CASSCF method has also been used to optimize the ground and first excited states. The present results indicate that these linear cations generally have an acetylenic structure H-C identical withC-C identical withCcdots, three dots, centeredC identical withC-H(+) with the ground state of X (2)Pi(g) for even-numbered n or X (2)Pi(u) for odd-numbered n. Moreover, the bond length alternation of HC(2n)H(+) is less pronounced than the corresponding one of the neutral polyyne chains HC(2n)H. The CASPT2 approach has been employed to estimate the vertical excitation energies for the dipole-allowed (Acdots, three dots, centeredD) (2)Pi(u/g)<--X (2)Pi(g/u) transitions in HC(2n)H(+) (n=2-8) clusters. The predicted A (2)Pi(u/g)<--X (2)Pi(g/u) transition energies in the gas phase are 2.62, 2.14, 1.81, 1.52, 1.35, 1.22, and 1.10 eV, respectively, in excellent agreement with the corresponding observed values of 2.45, 2.07, 1.75, 1.52, 1.35, 1.20, and 1.08 eV. The present calculations show that the absorption wavelengths for the A (2)Pi(u/g)<--X (2)Pi(g/u) transitions exhibit notably linear size dependence, as shown in previous experimental studies, quite different from the nonlinear lambda-n relationship for origin bands in HC(2n)H.

A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer (ESIPT) fluorophore of BTImP and its protonated form (BTImP+) were explored using density functional theory/time-dependent density functional theory (DFT/TD-DFT) computational methods with a B3LYP hybrid functional and the 6-31+G(d,p) basis set. In addition, the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G(d,p) level of theory. Our results reveal that both BTImP and BTImP+ can undergo an ultrafast ESIPT reaction, giving rise to the single fluorescence emission with different fluorescence colors, which are nicely consistent with the experimental findings. Calculations also show that following the ultrafast ESIPT, BTImP and BTImP+ can experience the distinctly different cis-trans isomerization processes. The intersystem crossing between the first excited singlet S1 state and triplet T1 state is found to play an important role in the photoisomerization process of BTImP+. In addition, the energy barrier of the trans-keto→cis-keto isomerization in the ground state of BTImP+ is calculated to be 10.49kcalmol-1, which implies that there may exist a long-lived trans-keto species in the ground state for BTImP+.

Tunable excited-state intramolecular proton transfer reactions with NH or OH as a proton donor: A theoretical investigation

Excited-state intramolecular proton transfer (ESIPT) reactions occurring in the S1 state for five molecules, which possess five/six-membered ring intramolecular NH···N or OH···N hydrogen bonds bearing quinoline or 2-phenylpyridine moiety, have been described in detail by the time-dependent density functional theory (TD-DFT) approach using the B3LYP hybrid functional. For the five molecules, the constrained potential energy profiles along the ESIPT reactions show that proton transfer is barrierless in molecules possessing six-membered ring intramolecular H-bonds, which is smoother than that with certain barriers in five-membered ring H-bonding systems. For the latter, chemical modification by a more strong acid group can lower the ESIPT barrier significantly, which harnesses the ESIPT reaction from a difficult type to a fast one. The energy barrier of the ESIPT reaction depends on the intensity of the intramolecular H-bond, which can be measured with the topological descriptors by topology analysis of the bond critical point (BCP) of the intramolecular H-bond. It is found that when the value of electron density ρ(r) at BCP is bigger than 0.025a.u., the corresponding molecule might go through an ultrafast and barrierless ESIPT process, which opens a new scenario to explore the ESIPT reactions.

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

ABSTRACT 1,8-Dihydroxydibenzo[a,h]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).

The excited-state intramolecular proton transfer in N H-type dye molecules with a seven-membered-ring intramolecular hydrogen bond: A theoretical insight

Excited-state intramolecular proton transfer (ESIPT) reactions of a series of N(R)H⋯N-type seven-membered-ring hydrogen-bonding compounds were explored by employing density functional theory/time-dependent density functional theory calculations with the PBE0 functional. Our results indicate that the absorption and emission spectra predicted theoretically match very well the experimental findings. Additionally, as the electron-withdrawing strength of R increases, the intramolecular H-bond of the NS1 form gradually enhances, and the forward energy barrier along the ESIPT reaction gradually decreases. For compound 4, its ESIPT reaction is found to be a barrierless process due to the involvement of a strong electron-withdrawing COCF3 group. It is therefore a reasonable presumption that the ESIPT efficiency of these N(R)H⋯N-type seven-membered-ring H-bonding systems can be improved when a strong electron-withdrawing group in R is introduced.

Theoretical studies on electroluminescent mechanism of a series of thermally activated delayed fluorescence emitters possessing asymmetric-triazine-cored triads

A series of thermally activated delayed fluorescence (TADF) emitters using asymmetric-triazine-cored triad as the electron-accepting unit have been investigated theoretically. Based on two experimentally reported TADF molecules (TPXZ-as-TAZ and oDPXZ-as-TAZ), two new molecules (mDPXZ-as-TAZ and pDPXZ-as-TAZ) have been designed to explore the isomeric effect on their TADF properties. The present results reveal that the absorption and emission spectra calculated by the time-dependent density functional theory (TD-DFT) method at the M06-2X level are match well the available experimental findings, and mDPXZ-as-TAZ and pDPXZ-as-TAZ are found to exhibit the same yellow emission as their analogue oDPXZ-as-TAZ. In addition, the rates of reverse intersystem crossing of mDPXZ-as-TAZ and pDPXZ-as-TAZ estimated by the semiclassical Marcus theory are 2.51 × 106 and 4.57 × 106 s-1, respectively, one order of magnitude larger than that of oDPXZ-as-TAZ (1.27 × 105 s-1), which suggests that our newly designed two molecules mDPXZ-as-TAZ and pDPXZ-as-TAZ can be also considered as potential yellow-light TADF emitters.

Theoretical insights into the 1D-charge transport properties in a series of hexaazatrinaphthylene-based discotic molecules

Discotic liquid crystal (DLC) materials have attracted considerable attention mainly due to their high charge carrier mobilities in quasi‐one‐dimensional columns. In this article, five hexaazatrinaphthylene‐based DLC molecules were investigated theoretically, and their frontier molecular orbital energy levels, crystal structures, and electron/hole drift mobilities were calculated by combination of density functional theory (DFT) and semiclassical Marcus charge transfer theory. The systems studied in this work include three experimentally reported molecules (1, 2, and 3) and two theoretically designed molecules (4 and 5). Compared with the 1–3 compounds, 4 and 5 have three more extended benzene rings in the π‐conjugated core. The present results show that the orders of the frontier molecular orbital energy levels and electron drift mobilities agree very well with the experiment. For 4 and 5, the electron/hole reorganization energies are lower than those of compounds 1–3. Furthermore, the calculated electron/hole transfer integral of 5 is the largest among all the five systems, leading to the highest electron and hole mobilities. In addition, the hydrophobicity and solubility were also evaluated by DFT, indicating that compound 5 has good hydrophobicity and good solubility in trichloromethane. As a result, it is expected that compound 5 can be a potential charge transport material in electronic and optoelectronic devices.

Ultrafast excited-state deactivation of 9-methylhypoxanthine in aqueous solution: A QM/MM MD study

Photoinduced ultrafast non-adiabatic decay of 9-methylhypoxanthine (9MHPX) in aqueous solution was investigated by ab initio surface-hopping dynamics calculations using a combined quantum mechanical/molecular mechanical approach. The absorption spectra of 9MHPX in aqueous solution were also explored by the hybrid cluster-continuum model at the level of time-dependent density functional theory along with the polarizable continuum model (PCM). The static electronic-structure calculations indicate that the absorption spectra of 9MHPX simulated by TD-B3LYP/PCM and TD-X3LYP/PCM can reproduce very well the experimental findings, with the accuracy of about 0.20 eV. According to dynamics simulations, irradiation of 9MHPX populates the bright excited singlet S1 state, which may undergo an ultrafast non-radiative deactivation to the S0 state. The lifetime of the S1 state of 9MHPX in aqueous solution is predicted to be 115.6 fs, slightly longer than that in the gas phase (88.8 fs), suggesting that the solventwater has no significant influence on the excited-state lifetime of 9MHPX. Such a behavior in 9MHPX is distinctly different from its parent hypoxanthine keto-N9H tautomer in which the excited-state lifetime of the latter in watersolution was remarkably enhanced as compared to the gas phase. The significant difference of the photodynamical behaviors between 9MHPX and keto-N9H can be ascribed to their different hydrogen bond environment in aqueous solution.

Theoretical design of azaacene-based non-fullerene electron transport material used in inverted perovskite solar cells

ABSTRACT Inverted perovskite solar cells (PSCs) have attracted much attention due to their low-temperature and solution-based process. Electron transport layers are important components in inverted PSCs. Non-fullerene n-type organic small molecules seem to be more attractive as electron transport layers, because their structures are easy to be synthesised and modified. In this paper, density functional theory and semi-classical Marcus electron transfer theory were used to explore the electron transport properties in three azaacene derivatives, including one experimentally reported molecule, 1,4,9,16-tetrakis((triisopropylsilyl)ethynyl)quinoxalino[2‴,3‴:4″,5″]cyclopenta[1″,2″,3″:5′,6′]acenaphtho[1′,2′:5,6]pyrazino[2,3-b]phenazine (1), and two theoretically designed molecules (2 and 3). Compound 2 is formed by substituting i-Pr groups in compound 1 with H atoms, which is designed to evaluate the effect of i-Pr groups on the electron transport properties. Compound 3 is designed by adding one more benzopyrazine group to the conjugation structure of compound 1. It shows that i-Pr group can increase HOMO and LUMO energy levels and improve solubility in organic solvent and hydrophobicity. Enlarging conjugation can not only decrease LUMO energy level and electron reorganisation energy, but also can increase solubility and electron mobility. So our designed compound 3 is expected to be a potential electron transport material in inverted PSCs. GRAPHICAL ABSTRACT

Theoretical studies on thermally activated delayed fluorescence mechanism of a series of organic light-emitting diodes emitters comprising 2,7-diphenylamino-9,9-dimethylacridine as electron donor: Theoretical Studies on Thermally Activated Delayed Fluorescence Mechanism of a Series of Organic Light-Emitting Diodes Emitters Comprising 2,7-Dipheny

According to one experimentally reported thermally activated delayed fluorescence (TADF) emitter (AcDPA‐2TP), two new molecules (AcDPA‐2PP and AcDPA‐TPP) have been designed theoretically to probe into the effect of different acceptor strengths on their TADF mechanisms. In this work, the rates of reverse intersystem crossing (kRISC) of the three targeted molecules were calculated by the semiclassical Marcus rate expression. The present results demonstrate that the kRISC rate of AcDPA‐2PP is estimated to be 5.56 × 106 s−1, about twice larger than that of AcDPA‐2TP (2.63 × 106 s−1), and especially AcDPA‐TPP is found to exhibit the largest kRISC value (6.97 × 106 s−1) among the three molecules. Considering that AcDPA‐2TP has been observed to be an efficient TADF emitter, our newly designed two molecules AcDPA‐2PP and AcDPA‐TPP are also expected to be potential TADF materials.