氧族元素对D-A和D-π-A共聚物光吸收谱红移的第一性原理研究

D-A型共聚物作为有机聚合物太阳能电池的电子给体材料近年来引起广泛关注. 本文以苯并二噻吩(BDT)为电子给体单元, 苯并噻二唑(BT)为电子受体单元来模拟D-A共聚体; 并用噻吩环作为π桥, 构造出D-π-A(PBDT-DTBX, X = O, S, Se, Te)结构. 采用第一性原理的密度泛函理论, 系统地计算相应的电子结构和光吸收谱. 比较不同氧族元素和噻吩π-键桥对聚合物光吸收谱的影响. 研究结果表明: D-A共聚体中当X位元素以O, S, Se, T_e 替换时, 其体系的最高占有分子轨道(HOMO)能级变化不大, 最低未占有分子轨道(LUMO)能级逐渐靠近费米能级, 带隙逐渐减小. 在可见光区有两个较强的吸收峰, 随着X位元素原子序数增大, 位于4.0 eV左右的光吸收峰位基本不变, 另一光吸收峰强度明显增大并发生红移. 与D-A结构相比, D-π-A结构的带隙均有所减小, 其中X为Te时带隙最小; 光吸收峰强度随着氧族元素原子序数的增大也明显增大并发生红移. 通过比较光吸收系数和相应态密度, 结果表明, 4.0 eV 左右的光吸收峰主要是BDT单元的贡献, 氧族元素的改变主要影响519.4-703.9 nm范围的光吸收.     

Density functional theory design D-D-A type small molecule with 1.03 eV narrow band gap: effect of electron donor unit for organic photovoltaic solar cell

Six new low-band-gap copolymers of donor–donor–acceptor (D-D-A) architecture have been designed using density functional theory and time-dependent density functional theory methods in order to use them in organic photovoltaic cell (OPVC). Phenanthro[3,4-d:9,10-d′]bis([1,2,3]thiadiazole)-10,12-dicarbonitrile moiety has been used as an acceptor for all compounds. We insert benzo[1,2-b:4,5-b′]dithiophene and N,N-diphenylbenzo[1,2-b:4,5-b′]dithiophen-2-amine units as donor to complete designing of copolymers. In order to tuning the optical and electronic properties, we have modified the donor unit by substituted with amine, methoxyamine, N-methylenethiophen-2-amine, methoxy, alkoxy moieties. The band gap (E_g), HOMO and LUMO values and plots, open circuit voltage (V_OC) as well as optical properties have been analysed for designed copolymers. The optimised copolymers exhibit low-band-gap lying in the range of 1.03–2.24 eV. DPTD-6 copolymer presents the optimal properties to be used as an active layer due to its low E_g (1.03 eV) and a moderate V_OC (0.56 eV). Thus, OPVC based on this copolymer in bulk-heterojunction composites with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as an acceptor has been modelled. E_g and V_OC values of composite material DPTD-6:PCBM are found as 1.32 and 0.65 eV, respectively. A model band diagram has been established for OPVC, simulating the energy transfer between active layers.     

Photo-and thermally induced optical absorption and photoconductivity of sillenite crystals

A study is reported of photo-and thermally induced changes in the optical absorption and photo-conductivity spectra of Bi₁₂SiO₂₀ crystals in the 0.5–3.5 eV spectral interval, and of the temperature dependence of optical absorption for temperatures from 80 to 600 K. An analysis is made of the electron redistribution processes between the shallow and the deep donor and acceptor levels by invoking the configurational coordinate model.     

Intramolecular charge transfer in donor‐bridge‐acceptor compounds with paired linearly conjugated or cross‐conjugated pathways

An understanding of intramolecular charge transfer in 2‐D linearly conjugated and cross‐conjugated compounds is necessary for the rational design of molecular electronics, improved solar energy devices, semi‐conducting polymers, and materials with nonlinear optical properties. In this work, the femtosecond transient absorption spectra and kinetics of several donor‐bridge‐acceptor compounds containing cross‐conjugated or linearly conjugated bridging groups were obtained. The veratrole group was used as the donor, and the phthalimide group was used as an acceptor. 2‐D conjugation was achieved by involving two bridging groups arranged cyclically between the donor and acceptor. The donor and acceptor were bridged by m‐phenylene in the cross‐conjugated compounds or 2,5‐thiophene in the linearly conjugated compounds. We found slower charge separation times and slower charge recombination times in the compounds containing cyclic cross‐conjugated bridging groups than in those containing the cyclic linearly conjugated groups in polar solvent. Charge separation rates that were found to be dependent on solvent were observed in the donor‐bridge‐acceptor compounds. Copyright © 2014 John Wiley & Sons, Ltd.     

Nature of the Electron Excited States and Energy Transfer in Bichromophore Coumarin Molecules

Results of experimental and theoretical research for three bichromophore molecules, trans-stilbene-CH₂-coumarin 120 (I), 4-methylumbelliferone-CH₂-UC 17, and 4-(3-fluoro)-methylumbelliferone-CH₂-UC 17 (II, III), are presented. Schemes of photophysical processes in the bichromophore molecules based on quantum chemical calculations by the INDO method and theory of radiationless transitions in polyatomic organic molecules are suggested. After optical excitation to the strong donor absorption band, the fast internal conversion processes develop there. As a result, the molecule is found in the S ₁ ^* -state localized on the acceptor moiety. It is shown that a mechanism of intramolecular transfer energy in bichromophores different from that proposed by Förster may be realized. Excitation energy, initially located on D, will be transferred from the donor moiety to the acceptor chromophore in convenience of the internal conversion process. The intramolecular electronic energy transfer from energy donor to energy acceptor may be interpreted as the internal conversion process. The rate constants of internal conversion are calculated.     

Impurity absorption and luminescence of CuGaSe<Subscript>2</Subscript> crystals

A comparison of the experimental and calculated absorption spectra of CuGaSe₂ crystals revealed the existence of two acceptor levels with ionization energies of 66 and 167 meV in the samples under study. It was found that the luminescence spectra of CuGaSe₂ measured at 77 K exhibit four impurity-band transitions with impurity ionization energies of 66, 99, 136, and 190 meV. An analysis of the temperature dependence of the luminescence intensity in the temperature range 11–77 K revealed a band peaking at 1.671 eV due to the radiation of donor-acceptor pairs. The calculated sum of the ionization energies of the impurities responsible for the formation of donor-acceptor pairs and the temperature dependence of the relative intensities of impurity-band emission were used to determine the ionization energies of the corresponding donor and acceptor.     

Nonlinear optical properties of stilbene and azobenzene derivatives containing azaphosphane groups

We have studied the nonlinear optical (NLO) properties of some donor–acceptor molecules with stilbene and azobenzene molecules as backbone. We have used the nitro group as the acceptor and azaphosphane (R₃P=N–) as the donor group. To study the effect of variation of NLO properties, we have replaced the substituents (Rs) connected to the phosphorus atom by methyl, amine and phenyl groups. We find that both first-order polarizability and hyperpolarizabilities are larger for stilbene derivatives and is maximum for the phenyl substitution. Second-order polarizability is higher for methyl substitution. We have also obtained the two-photon absorption cross-section for these molecules. We find that both one-photon and two-photon absorption cross-sections are maximum for the same excited state (first excited state in the case of stilbene and second excited state in the case of azobenzene derivatives).     

Control of optical absorption edge of TiO2 through co-doped acceptors: The chemical trend

Tuning the optical adsorption edge of TiO₂ is attracting increasing attention as a potential solution to the worldwide energy shortage. A possible strategy to achieve high efficiency photocatalysis with TiO₂ is through dopants to modulate chemical composition. Based on first-principles calculations, we propose a hole-strain-mediated coupling mechanism between co-doped acceptors in anatase TiO₂. When the dopant complex on neighboring oxygen sites contains a large radius atom, and the doped system has at least one net hole, the dopants will strongly couple to form a pair through the local lattice strain induced by the large dopant. The coupling results in bandgap narrowing due to the appearance of the fully occupied mid-gap states, leading to a much more effective band gap reduction than that induced by mono-doping or conventional donor–acceptor codoping. The calculated absorption spectra show that acceptor–acceptor codopings could shift the absorption edge to the visible light region.     

The effect of twisted D–D–π–A configuration on electron transfer and photo-physics characteristics

Two D–D–π–A organic dyes (M45, M46) with dithieno[3,2-b:2′,3′-d]pyrrole (DTP) units as election donors in two perpendicular directions, were investigated using density functional theory (DFT) and time-dependent DFT. The ground-state geometries, the absorption, the electronic structures, the charge density difference and molecular electrostatic potential were obtained. To simulate a more realistic performance, all calculations were based on gas condition and dichloromethane solvent. Photoelectric parameters were evaluated by the following factors: the light harvesting efficiency, electron injection driving force, the excited lifetime and vertical dipole moment. Meanwhile, the polarisability and hyperpolarisability were investigated to further explain the relationship between non-linear optical properties and efficiency. The direction of the DTP obviously affects the twisted degree of molecule, forming a better coplanarity on the donor 2 of M45, which results in stronger charge transfer interactions. Furthermore, M45 possesses significant advantages in geometric structure, absorption band and intramolecular charge transfer mechanism. These critical parameters supported the higher performance of M45 in comparison with M46. Moreover, four dyes were designed by the substitution of donor 2, which further verify the influence of the twisted donor 2 on electron transfer and photoelectric properties of D–D–π–A configuration.     

Intermolecular energy transfer in mixed laser dyes: photophysical properties of triplet states

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N₂ laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼10³ dm³ mol⁻¹ s⁻¹ for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.     

Microsolvation of morpholine,a bidentate base – the importance of cooperativity

The structure, relative energies, and bonding in morpholine(water)_n (n = 1–4) clusters have been investigated at the Becke, three‐parameter, Lee–Yang–Parr/6–311 + G(3df,2p)//Becke, three‐parameter, Lee–Yang–Parr/6–311 + G(d,p) level of theory. Cooperative effects have been analyzed through the use of structural, energetic, and electron density indexes. Our analysis shows that these effects are crucial to trace the relative stability of the complexes formed. In all cases water molecules prefer to self‐associate forming chains in which each individual molecule behaves as a hydrogen bond (HB) donor and HB acceptor. The chain so formed behaves in turn as HB donor and HB acceptor with respect to morpholine, being the most stable arrangements those in which the NH group of morpholine behaves simultaneously as HB donor and HB acceptor. Higher in energy lie complexes in which the HB acceptor continues to be the NH group, but the HB donor is a CH group, or alternative structures in which the HB acceptor is the ether‐like oxygen of morpholine and the HB donor its NH group. Cooperativity increases with the number of solvent molecules, but there is a clear attenuation effect. Thus, whereas the additive interaction energy on going from dihydrated to trihydrated species increases by a factor of 3, this increase is about half on going from trihydrated to tetrahydrated complexes. Copyright © 2012 John Wiley & Sons, Ltd.     

Long-Range Energy Transfer Combined with Isotropic or Anisotropic Diffusion

Abstract

The effect of diffusion on long-range energy transfer by dipole-dipole interaction^1,2 from electronically excited donor molecules D? to (unexcited) acceptor molecules A may be important in solutions^3–7 and in molecular crystals^7–11. In solutions usually diffusion of matter occurs (the molecules perform a random walk) whereas in molecular crystals an excitation (exciton) migrates. Exciton migration can often be described in terms of an effective exciton diffusion tensor which may be highly anisotropic^12–17. Because of the restricted space only the time dependence of the donor fluorescence after δ-excitation will be given. The extension to general excitation forms and to the acceptor flouorescence is a straightforward matter^18,19.     

Impact of molecular tilt angle on the absorption spectra of pentacene:perfluoropentacene blends

We investigate the relation between the optical properties and the average molecular tilt angle for blends of pentacene and perfluoropentacene, which can be considered as a prototypical donor–acceptor complex. Combining near‐edge X‐ray absorption fine‐structure spectroscopy and optical spectroscopy we study thin films of these compounds prepared at three different substrate temperatures T_sub. For T_sub =180 K we observe a larger average tilt angle than for blends prepared at higher substrate temperatures. This orientational change has significant impact on the uniaxial anisotropic optical properties of the mixed films which we measure post growth as well as in real‐time during growth. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

PBE–DFT theoretical study of organic photovoltaic materials based on thiophene with 1D and 2D periodic boundary conditions

Conjugated organic systems such as thiophene are interesting topics in the field of organic solar cells. We theoretically investigate π-conjugated polymers constituted by n units (n = 1–11) based on the thiophene (Tn) molecule. The computations of the geometries and electronic structures of these compounds are performed using the density functional theory (DFT) at the 6–31 G(d, p) level of theory and the Perdew–Burke–Eenzerhof (PBE) formulation of the generalized gradient approximation with periodic boundary conditions (PBCs) in one (1D) and two (2D) dimensions. Moreover, the electronic properties (HOCO, LUCO, E _gap, V _oc, and V _bi) are determined from 1D and 2D PBC to understand the effect of the number of rings in polythiophene. The absorption properties—excitation energies (E _ex), the maximal absorption wavelength (λ_max), oscillator strengths, and light harvesting—efficiency are studied using the time-dependent DFT method. Our studies show that changing the number of thiophene units can effectively modulate the electronic and optical properties. On the other hand, our work demonstrates the efficiency of theoretical calculation in the PBCs.     

Weak interactions and cooperativity effects on disiloxane: a look at the building block of silicones

The behaviour of disiloxane 1 towards a set of Lewis acids (LA) and Lewis bases (LB) forming complexes through its oxygen and silicon atoms, respectively, was studied at the MP2/aug′-cc-pVTZ level of theory, exploring a wide variety of non-covalent interactions. Disiloxane is a moderate electron acceptor and a good electron donor, exhibiting in the latter case binding energies up to almost ?100 kJ/mol with BeCl₂. Cooperativity effects were also analysed by looking at ternary 1:LA:LB complexes. Shorter intermolecular distances than in the corresponding binary complexes and a negative contribution of the three-body term to the binding energy indicate that the non-covalent interactions allowed by disiloxane through its acid and basic centres cooperate between them to reinforce both donor–acceptor pairs. These effects are particularly strong in complexes involving beryllium and triel bonds, but are also relevant for complexes containing hydrogen bonds.     

Solvent effects on electronic absorption spectra of donor‐substituted 11,11,12,12‐tetracyano‐9, 10‐anthraquinodimethanes (TCAQs)

Solvent effects on the electronic absorption spectra of donor‐substituted 11,11,12,12‐tetracyano‐9, 10‐anthraquinodimethanes (TCAQs) 1 – 3 have been investigated in 32 well‐selected solvents. These compounds were chosen as model structures for charge‐transfer chromophores featuring second‐ and third‐order nonlinear optical properties. The resulting data were evaluated by means of theoretical models and (semi)empirical correlations determining the optical properties related to electron distribution and polarizability. We found that solvent effects on a polar D‐π‐A system do not depend on the donor/acceptor orientation (HOMO/LUMO localization) but especially on the length of the π‐system in between. The observed solvent effects are described with high accuracy by the applied theoretical models and linear combinations of physical quantities. Solvent polarization, permanent dipole moment, and molar volume substantially affect the longest‐wavelength absorption maxima. Solvent‐induced bathochromic shift resulting from the solvent polarity is described with high accuracy by the Born function. On the other hand, hypsochromic effects of the solvent permanent dipole moment are caused due to the slower reorganization of molecular dipoles compared with the rate of excitation. Solvent polarizability shifts the longest‐wavelength absorption maxima bathochromically with increasing length of the π‐conjugated system. Whereas this effect could be suitably described by the Onsager‐induced polarizability, the orientation polarizability was not found to be important. The solvent molar volume as a hypsochromic shift‐inducing factor is only relevant if the size of the solute and solvent molecules are comparable. If the size of the solute is considerably larger than that of the solvent molecules, the solvent behaves as a ‘shape continuum.’ Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis and photoinduced electron transfer characteristic of a bis (zinc porphyrin)‐perylene bisimide array

In this article, a donor–acceptor array consisting of two zinc porphyrin (ZnPOR) units attached to the 1,7‐positions of perylene‐3,4:9,10‐bis(dicarboximide) (PDI) was synthesized and characterized. This double‐wing molecule exhibits very broad absorption in the region from 300 to 900 nm. Especially, its lower energy absorption feature presumably arises from donor–acceptor interactions. The fluorescence emission spectra confirmed that photoinduced electron transfer occurred from POR to perylene bisimide moiety in this triad. In contrast to previously studied systems incorporating POR and PDI groups, this array shows the evidence of a relatively strong ground‐state electronic coupling between the donor and acceptor moieties. Additionally, highest occupied molecular orbital and lowest unoccupied molecular orbital values of the array were acquired by cyclic voltammetry (CV). The results showed that these energy values fulfill the energetic conditions required for the proposed electron transfer. More importantly, the photocurrent measurement showed that this molecule exhibits a high capacity to form a photoinduced charge‐separated state and to produce steady and prompt cathodic photocurrent responses. These results confirmed the role of this new array toward harvesting light energy and generating photocurrent during the operation of a photoelectrochemical cell. Copyright © 2011 John Wiley & Sons, Ltd.     

First-principles study on the effect of micro-modified D-A-π-A dyes with triphenylamine acting as a donor on its photoelectric properties

A triphenylamine-based dye sensitizer–the electron donor, added with the embellishment, was designed and analyzed, to serve as a backup molecule for the sensitized solar cells (DSSCs). For a more realistic performance, five sensitizer molecules (ND, NDSH-1～4) with triphenylamine (TPA) units as election donors (D), 4-benzo[c] [1, 2, 5] thiadiazol segment as acting as auxiliary acceptors (A), 4-ylethynyl as bridge (π) and benzoic acid as the electron acceptor (A) were explored in dichloromethane solvent by the methods of Density functional theory (DFT) and Time-dependent density functional theory (TD-DFT). Meanwhile, to understand the working process of DSSCs more systematically, these dyes adsorbed on (TiO₂)₁₆ were further explored. Finally, the effects of triphenylamine modification are summarized and disclosed from the phenomenon of simulation. The results show that the nitrogen-substituted hexyloxy dye exhibits a very narrow band gap and has a distinct red-shifted absorption band compared to the experimental dye ND.     

Third-Order Nonlinear Optical Properties of a Series of Polythiophenes

The third-order nonlinear optical properties of a series of polythiophenes are investigated with the Z-scan method under picosecond pulse laser irradiation at 532nm. The copolymers exhibit a good nonlinear response： large nonlinear refraction coefficients without nonlinear absorption. The signs of nonlinear refraction coefficients are positive, which are opposite to the negative signs of the polythiophene reported before. The mechanism accounting for the process of nonlinear refraction under pulse laser excitation is analyzed from the viewpoint of the electrondonor/acceptor units of polythiophenes. Moreover, changes of nonlinearity according to the lengths of main chains in polythiophene molecules are discussed.