Rongxing He

Franck-Condon simulation of vibrationally resolved optical spectra for zinc complexes of phthalocyanine and tetrabenzoporphyrin including the Duschinsky and Herzberg-Teller effects

High resolved absorption and fluorescence spectra of zinc complexes of phthalocyanine (ZnPc) and tetrabenzoporphyrin (ZnTBP) in the region of Q states were reported. Few theoretical investigations were performed to simulate the well-resolved spectra and assigned the vibrational bands of the large molecules, especially for high symmetrical characteristic molecules, on account of the difficulties to optimize the excited states and analyze a large number of final vibrational-normal modes. In the present work, the S(0) ↔ S(1) absorption and fluorescence spectra (that is, the Q band) of ZnPc and ZnTBP were simulated using time-dependent density functional theory with the inclusions of Duschinsky and Herzberg-Teller contributions to the electronic transition dipole moments. The theoretical results provide a good description of the optical spectra and are proved to be in excellent agreement with experimental spectra in inert-gas matrices or in supersonic expansion. This study focused attentions on the optical spectral similarities and contrasts between ZnPc and ZnTBP, in particular the noticeable Duschinsky and Herzberg-Teller effects on the high-resolved absorption and fluorescence spectra were considered. Substitution of meso-tetraaza on the porphyrin macrocycle framework could affect the ground state geometry and alter the electron density distributions, the orbital energies that accessible in the Q band region of the spectrum. The results were used to help interpret both the nature of the electronic transitions in Q band region, and the spectral discrepancies between phthalocyanine and porphyrin systems.

Exploring photophysical properties of metal-free coumarin sensitizers: an efficient strategy to improve the performance of dye-sensitized solar cells

An efficient strategy was provided by adopting different numbers of electron-deficient units (pyrimidyl and quinolyl) into parent coumarin sensitizers to obtain excellent absorption in the short-wavelength region (B2 band), which eventually improves the performance of DSSCs. Density functional theory calculations were performed on both free dyes and dye–TiO2 complexes. As expected, introducing a single electron-deficient unit results in a positive influence on the power conversion efficiency (η) of DSSCs because of the larger short-circuit current density (Jsc is proportional to optical absorption (φLHE), charge separation, dye regeneration (φreg) and electron injection (φinject)) and the higher open circuit voltage (Voc). The introduction of more pyrimidine facilitates charge separation and favors effective electron injection, whereas the second quinoline displays the opposite effect. The results give guidance to design promising candidates for future DSSCs applications.

Exploring the Photodeactivation Pathways of Pt[O^N^C^N] Complexes: A Theoretical Perspective

In this article, the influence of the tert-butyl unit on the photodeactivation pathways of Pt[O^N^C^N] (O^N^C^N=2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-l)phenyl) pyridin-2-yl)phenolate) is investigated by DFT/TDDFT calculations. To further explore the factors that determine the radiative processes, the transition dipole moments of the singlet excited states, spin-orbit coupling (SOC) matrix elements, and energy gaps between the lowest triplet excited states and singlet excited states are calculated. As demonstrated by the results, compared with Pt-3, Pt-1 and Pt-2 have larger SOC matrix elements between the lowest triplet excited states and singlet excited states, an indicator that they have faster radiative decay processes. In addition, the SOC matrix elements between the lowest triplet excited states and ground states are also computed to elucidate the temperature-independent non-radiative decay processes. Moreover, the temperature-dependent non-radiative decay mechanisms are also explored via the potential energy profiles.

Exploring the role of varied-length spacers in charge transfer: a theoretical investigation on pyrimidine-bridged porphyrin dyes

A series of dyes based on a porphyrin donor and a cyanoacrylic acid anchor/acceptor group for solar cell application are investigated with regards to varied-length π-spacers affecting the photo-to-electric conversion efficiency (PCE). Investigations are firstly performed on three porphyrin sensitizers with 1–3 conjugated phenylethynyl (PE) units, which have experimentally proved that the efficiency of power conversion decreases systematically with increasing spacer length. The distances and amounts of charge transfer after photoexcitation are calculated. In the PE bridged porphyrin dyes, the calculated electron injection driving forces and the regeneration driving forces gradually decrease as the distance of the π-spacer increases. Our theoretical calculations can reproduce well the experimental conclusion, showing that the photo-to-electric efficiency has a strong distance dependence for the electron-rich phenyl spacer. Then we replace the phenyl group with a pyrimidyl (PM) group to uncover how the characteristics of the π-spacer affect the performance of optical absorption, charge separation, and the regeneration process, to further improve the power conversion. We find that the adoption of electron-deficient pyrimidyl can break and even remove the distance dependence of the π-spacer. Some integral factors affecting the dye performance, such as short-circuit photocurrent, open-circuit voltage and charge collection efficiency are analyzed. It would help to interpret what role the electron deficient π-spacers with varied lengths will play and how they are expected to behave in the performance of sensitizers. In this regard, this study presents us with a promising way to design novel functional dyes and to utilize the potential advantages of the lengthy spacer dyes.

Theoretical investigation of dihydroacridine and diphenylsulphone derivatives as thermally activated delayed fluorescence emitters for organic light-emitting diodes

A series of donor–acceptor–donor type compounds containing 9,9-dimethyl-9,10-dihydroacridine and diphenylsulphone as thermally activated delayed fluorescence emitters are designed and investigated, and their broad application prospects in organic light emitting diodes are predicted by density functional theory (DFT). The results show that the orbital interaction of the atom between the acceptor and the donor is an important factor to influence the singlet–triplet energy difference. Effective intermolecular singlet–singlet, singlet–triplet and triplet–triplet energy transfers from hosts to emitters, in which donors and acceptors are linked by C–N bonds, would occur. The para- and meta-linked compounds exhibit blue emission and the ortho-linked compounds show green emission in these emitters.

Mechanistic Insights into the Cu(I)- and Cu(II)-Catalyzed Cyclization of o-Alkynylbenzaldehydes: The Solvent DMF and Oxidation State of Copper Affect the Reaction Mechanism

A computational study with the BhandHLYP density functional is conducted to elucidate the mechanisms of Cu(I)- and Cu(II)-catalyzed reactions of o-alkynylbenzaldehydes with a nucleophile (MeOH). Our calculations suggest the following. (a) The use of CuCl as a catalyst deceases significantly the energy barrier and promotes intramolecular cyclization. (b) Solvent DMF is critical in the stepwise hydrogen-transport process involved in an intermolecular nucleophilic addition because it can greatly reduce the free energy barrier of the hydrogen-transfer process as a proton shuttle. In addition, we find that substrate MeOH also plays a role similar to that of DMF in the hydrogen-transport reaction. (c) The 6-endo product P1 is formed exclusively using a catalytic system consisting of CuCl and DMF, whereas a mixture of 6-endo product P1 and 5-exo product P2 in a ratio of ∼1:1 is produced using CuCl2 and DMF as a catalytic system. Our theoretical calculations reproduce the experimental results very well. This study is expected to improve our understanding of Cu(I)- and Cu(II)-catalyzed reactions involving Lewis base solvents and to provide guidance for the future design of new catalysts and new reactions.

Influence of position of auxiliary acceptor in D–A–π–A photosensitizes on photovoltaic performances of dye-sensitized solar cells

Several novel indoline dyes configured with donor–acceptor–bridge–acceptor (D–A–π–A) structures were designed and applied to organic dye-sensitized solar cells. These D–A–π–A dye molecules are composed of indoline (electron donating group), benzothiadiazole (BDT) (auxiliary acceptor), two furan rings (π-conjugated group), and 2-cyanoacrylic acid (electron accepting group). The influence of position of auxiliary acceptor in D–A–π–A organic sensitizer on the performance of photosensitize is investigated in detail. Calculated results show that the sensitizer could achieve a red-shifted absorption in long-wavelength region and a stronger absorption in short-wavelength region when the position of auxiliary acceptor changes from the donor to the acceptor. Moreover, among these dyes, WS-12, whose auxiliary acceptor nearing the 2-cyanoacrylic acid, possesses the better performance in terms of the charge transfer characteristics, lifetime of excited state as well as the vertical dipole moment when compared with WS-1 and WS-11. We hope that the present results could provide theoretical guidance for designing photosensitizes with higher efficiencies.

Influence of Duschinsky and Herzberg-Teller effects on S0 → S1 vibrationally resolved absorption spectra of several porphyrin-like compounds

The S0 → S1 (Q band) high-resolved absorption spectra of three porphyrin-like compounds, porphycene, magnesium porphyrin, and zinc tetraazaporphyrin, were simulated in the framework of the Franck-Condon approximation including the Duschinsky and Herzberg-Teller (HT) contributions. Substitution of meso-aza on porphyrin macrocycle framework could change severely the absorption energy, vibrational intensity, and spectral profile of Q band. Therefore, we focused attention on the spectral similarities and contrasts among the three compounds based on the density functional theory and its time-dependent extension calculations. The simulated spectra agreed well with the experimental ones and further confirmed that the HT and Duschinsky effects have significant influence on the weakly allowed or forbidden transition of sizable organic molecules. The pure HT and Duschinsky effects were explored separately to clarify their contributions on changing vibrational intensities of different modes. Moreover, we tentatively assigned most of the vibrational modes which appeared in the experimental spectra but corresponding assignments were not given. The present work provided a useful method to simulate and interpret the absorption spectra of porphyrin-like compounds.

Effect of “push–pull” sensitizers with modified conjugation bridges on the performance of p-type dye-sensitized solar cells

A series of “push–pull” sensitizers with modified conjugation bridges are designed and investigated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT), with the purpose of revealing the effect of different linker moieties on the performance of p-type dye-sensitized solar cells (DSSCs). Creatively, the electron-rich unit (thiophene) and the electron-deficient unit (pyrimidine) are studied as the linking groups in p-type sensitizers from a comparative perspective, two special bridge-sites and the lengths of conjugation bridges are also taken into account. Calculations of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicate that there is efficient hole injection and dye regeneration for all the sensitizers. Importantly, the influence of the number of thiophene and pyrimidine moieties is seen mainly on the long-wavelength region and the short-wavelength region, respectively. According to the charge transfer properties and the driving forces of hole injection, dye regeneration and charge recombination (ΔGinj, ΔGreg and ΔGCR, respectively), the increased length of the thiophene-based bridge close to the carboxyl group has a positive impact on the device performance. Likewise, for the pyrimidine-based bridges, it is probably the increased conjugation length between the donor and acceptor that significantly improves the device efficiency. Our intensive analysis on the π-bridges provides assistance for designing more efficient p-type photosensitizers, which contributes to the rational design for tandem DSSCs.

Theoretical Strategy To Design Novel n-Type Copolymers Based on Anthracene Diimide and Pyrido[2,3-g]quinoline Diimide for Organic Solar Cells

The design and synthesis of efficient electron-transporting materials have been an active area of research in the area of organic solar cells (OSCs), organic field-effect transistors (OFETs), and organic light-emitting diodes (OLEDs). This paper is focused on designing novel n-type donor-acceptor (D-A) copolymers as electron-transporting materials for replacing the widely used fullerene acceptor materials in OSC applications. We first present a strategy which can remarkably improve the photovoltaic performances of D-A copolymer acceptors by means of adjusting the molecular planarity and intensifying the electron-withdrawing ability of electron-deficient units. Then we further analyze the role played by the D-A copolymer acceptor in the light-absorbing performance of the active layer. On the basis of two reported two D-A copolymer acceptors (PNDIT and P(NDI2OD-T2)) which are composed of an electron-deficient naphthalene diimide (NDI) unit and different electron-rich units of thiophene or bithiophene, replacement of the NDI unit with an anthracene diimide (ADI) unit and a pyrido[2,3-g]quinoline diimide (PQD) unit can produce two types of copolymer acceptors (P2, P3 and P2a, P3a). From the calculated results, the introduction of ADI and PQD units to replace the NDI unit can significantly improve the optoelectronic properties, light-absorbing efficiencies, and intermolecular electron transport abilities of the copolymers as well as exciton separation efficiencies at donor/acceptor interface. Finally, this study would give us a theoretical guidance to design efficient D-A copolymer acceptors for replacing fullerene acceptors in organic solar cells.