Bacteriochlorophyll in Electric Field

Regulation of organic solar cells performance through external electric field: From charge transfer mechanisms to photovoltaic properties

In organic solar cells (OSCs), comprehending the charge transfer mechanism at D/A interfaces is crucial for photoinduced charge generation and enhancing power conversion efficiency (PCE). The charge transfer mechanism and photovoltaic performance of the parallel stacking interface configuration of the PTQ10 polymer donor and T2EH non-fullerene acceptor (NFA) are systematically studied at the microscopic scale. The analysis of the electron-hole distribution of the PTQ10/T2EH excited states revealed the presence of multiple charge excitation modes and charge transfer pathways. Using Marcus theory, we examine the charge separation rate (KCS) of PTQ10/T2EH under external electric field (Fext) modulation, and it is clarified that reorganization energy (λ) is the main factor that affects the KCS. Our results show that Fext has a positive impact on the photovoltaic properties of PTQ10/T2EH thin films, as evidenced by the modulation of the open circuit voltage (VOC), voltage loss (VLOSS) and fill factor (FF). Overall, this study provides valuable theoretical insights for Fext to accelerate the charge separation process and enhance photovoltaic efficiency.

Slower Auger Recombination in 12-Faceted Dodecahedron CsPbBr3 Nanocrystals

Over the past two decades, intensive research efforts have been devoted to suppressions of Auger recombination in metal-chalcogenide and perovskite nanocrystals (PNCs) for the application of photovoltaics and light emitting devices (LEDs). Here, we have explored dodecahedron cesium lead bromide perovskite nanocrystals (DNCs), which show slower Auger recombination time compared to hexahedron nanocrystals (HNCs). We investigate many-body interactions that are manifested under high excitation flux density in both NCs using ultrafast spectroscopic pump-probe measurements. We demonstrate that the Auger recombination rate due to multiexciton recombinations are lower in DNCs than in HNCs. At low and intermediate excitation density, the majority of carriers recombine through biexcitonic recombination. However, at high excitation density (>10¹⁸ cm^-3) a higher number of many-body Auger process dominates over biexcitonic recombination. Compared to HNCs, high PLQY and slower Auger recombinations in DNCs are likely to be significant for the fabrication of highly efficient perovskite-based photonics and LEDs.

External electric field-dependent photoinduced charge transfer in non-fullerene organic solar cells

Based on Marcus theory, the photoinduced electron transfer properties of D-A type non-fullerene acceptor organic solar cells (OSCs) under the dependence of external electric field (F_ext) were investigated. The research results shown that the charge transfer mode under different F_ext intensities changes with certain regularity. Focusing on the important parameters (ΔG, λ, and V_DA) that affect the charge transfer rate, it was found that both charge separation (|ΔG_CS|>λ(1.3019 vs 0.8275 eV at F_ext = 0) and charge recombiation (|ΔG_CR|>λ, (1.9633 vs 0.8275 eV)) processes occur in the Marcus inverted region. The ΔG_CS is relatively sensitive to F_ext, and the calculated ΔG_CS at different F_ext intensities yields an increment of 0.0073 eV, which is also the main reason for the increase in the rate of charge separation. The ΔG_CR ranges between -1.9633 and -1.9637 eV, is insensitive to F_ext, and ΔG_CR is significantly smaller than ΔG_CS, which makes the charge recombination rate significantly smaller than the charge separation rate. For V_DA, it is found that V_DA will transition to a new level only when the F_ext intensity reaches a certain intensity, which also enables to obtain a faster charge separation rate. By studying the charge transfer parameters in different polar solvents, it is found that polar solvents can indeed increase the charge transfer rate. To a certain extent, our results also demonstrate that the addition of F_ext can further improve the performance of non-fullerene acceptor OSCs.

Theoretical exploration of diverse electron-deficient core and terminal groups in A–DA′D–A type non-fullerene acceptors for organic solar cells

The influences triggered by the structurally diverse electron-withdrawing terminal group and fuse-ring electron-deficient core on the performance of NFAs OSCs are comprehensively investigated by using DFT, TD-DFT and Marcus charge transfer theory.

Modification of NFA-Conjugated Bridges with Symmetric Structures for High-Efficiency Non-Fullerene PSCs

As electron acceptors, non-fullerene molecules can overcome the shortcomings of fullerenes and their derivatives (such as high cost, poor co-solubility, and weak light absorption). The photoelectric properties of two potential non-fullerene polymer solar cells (PSCs) PBDB-T:IF-TN (PB:IF) and PBDB-T:IDT-TN (PB:IDT) are studied by density functional theory (DFT) and time-dependent DFT (TD-DFT). Based on the optimized structure of the ground state, the effects of the electron donor (D) and electron acceptor (A) (D/A) interfaces PBDB-T/IF-TN (PB/IF) and PBDB-T/IDT-TN (PB/IDT) are studied by a quantum-chemical method (QM) and Marcus theory. Firstly, for two non-fullerene acceptors (NFAs) IF-TN and IDT-TN, the NFA IDT-TN has better optical absorption ability and better electron transport ability than IF-TN. Secondly, for the D/A interfaces PB/IF and PB/IDT, they both have high optical absorption and electron transfer abilities, and PB/IDT has better optical absorption and lower exciton binding energy. Finally, some important parameters (open-circuit voltage, voltage loss, fill factor, and power conversion efficiency) are calculated and simulated by establishing the theoretical model. From the above analysis, the results show that the non-fullerene PSC PB:IDT has better photoelectric characteristics than PB:IF.

Photoinduced electron transfer in conjugated molecules containing different donor in an external electric field

Photoinduced electron transfers in different conjugated donor polymer containing with PC₆₀BM as the acceptor (X3:PC₆₀BM and X6:PC₆₀BM) were theoretically investigated. A detailed theoretical analysis of Marcus electron transfer rate in an external electric field was also performed. By discussing the effect of electric field on ΔG, λ, V_DA and charge separation rate, we have come to some conclusions. The results reveal that the free energy change and electronic coupling matrix are significantly influence by the vector properties of external electric field. Thus, the electron transfer process appears critically influenced by external electric fields. Our results will provide meaningful information on enhancing the photoelectric conversion efficiencies of BHJ solar cells.

Photoinduced charge transfer by one and two-photon absorptions: physical mechanisms and applications

We review photoinduced charge transfer in organic solar cells without and with an external electric field and then we introduce the visualization methods of the transition density, charge difference density and transition density matrix for the analysis of the photoinduced charge transfer in a neutral system and a charged system excited by one-photon and two-photon absorption. This review will not only promote a deeper understanding of the available theories and methods of PICT, but also lead to further developments in this field.

Non-Fullerene Acceptor-Based Solar Cells: From Structural Design to Interface Charge Separation and Charge Transport

The development of non-fullerene small molecule as electron acceptors is critical for overcoming the shortcomings of fullerene and its derivatives (such as limited absorption of light, poor morphological stability and high cost). We investigated the electronic and optical properties of the two selected promising non-fullerene acceptors (NFAs), IDIC and IDTBR, and five conjugated donor polymers using quantum-chemical method (QM). Based on the optimized structures of the studied NFAs and the polymers, the ten donor/acceptor (D/A) interfaces were constructed and investigated using QM and Marcus semi-classical model. Firstly, for the two NFAs, IDTBR displays better electron transport capability, better optical absorption ability, and much greater electron mobility than IDIC. Secondly, the configurations of D/A yield the more bathochromic-shifted and broader sunlight absorption spectra than the single moiety. Surprisingly, although IDTBR has better optical properties than IDIC, the IDIC-based interfaces possess better electron injection abilities, optical absorption properties, smaller exciton binding energies and more effective electronic separation than the IDTBR-based interfaces. Finally, all the polymer/IDIC interfaces exhibit large charge separation rate (KCS) (up to 1012⁻1014 s-1) and low charge recombination rate (KCR) (<10⁶ s-1), which are more likely to result in high power conversion efficiencies (PCEs). From above analysis, it was found that the polymer/IDIC interfaces should display better performance in the utility of bulk-heterojunction solar cells (BHJ OSC) than polymer/IDTBR interfaces.

Photoinduced Electron Transfer in Organic Solar Cells

Electron transfer (ET) is the key process in light-driven charge separation reactions in organic solar cells. The current review summarizes the progress in theoretical modelling of ET in these materials. First we give an account of ET, with a description originating from Marcus theory. We systematically go through all the relevant parameters and show how they depend on different material properties, and discuss the consequences such dependencies have for the performance of the devices. Finally, we present a set of visualization methods which have proven to be very useful in analyzing the elementary processes in absorption and charge separation events. Such visualization tools help us to understand the properties of the photochemical and photobiological systems in solar cells.

Effects of different functional groups on the optical and charge transport properties of copolymers for polymer solar cells

Three novel copolymers were designed based on BSeTT, QTT, BDT-DTBTBPz and BDT-DTBTBQx. Their properties of ground state and excited state were calculated via DFT/TDDFT methods. The charge moves from polymers to PC₆₀BM.

Theoretical investigations on enhancing the performance of terminally diketopyrrolopyrrole-based small-molecular donors in organic solar cell applications

Diketopyrrolopyrrole (DPP)-based small molecules with acceptor-core-acceptor (A-core-A) type as donor materials have been used successfully in organic solar cells (OSC). In this work, based on the DPP-core-DPP type molecule SM1 consisting of a DPP unit as acceptor and benzene as the core, we replaced the benzene core with more electron-withdrawing groups in SM1 and further designed four new small-molecular donors (SM2-SM5) in order to improve the electrical properties, optical absorption and performance in OSC applications. The calculated results indicate that the designed small-molecular donors SM2-SM5 exhibit better performances in comparison with SM1, such as lower highest occupied molecular orbital (HOMO), narrower energy gap, larger absorption range, better electronic transfer between donor and acceptor and higher hole mobility. Moreover, the decreased HOMO levels and transition energy of small-molecular donors in OSC applications play an important role in the parameters of open-current voltage, fill factor and short-circuit current. Consequently, adjusting the electron-deficient ability of cores in DPP-core-DPP type small-molecular donors is an efficient approach that can be used to obtain high-efficiency DPP-based small-molecular donors for OSC applications. Graphical Abstract The designed small-molecules with good electronic and photophysical properties will act as a promising donor candidate for organic solar cell applications. Moreover, The decreased HOMO levels and transition energy of small-molecular donors in OSC applications play an important role in the parameters of open-current voltage, fill factor and short-circuit current.

Photoinduced Charge Transport in a BHJ Solar Cell Controlled by an External Electric Field

This study investigated theoretical photoinduced charge transport in a bulk heterojunction (BHJ) solar cell controlled by an external electric field. Our method for visualizing charge difference density identified the excited state properties of photoinduced charge transfer, and the charge transfer excited states were distinguished from local excited states during electronic transitions. Furthermore, the calculated rates for the charge transfer revealed that the charge transfer was strongly influenced by the external electric field. The external electric field accelerated the rate of charge transfer by up to one order when charge recombination was significantly restrained. Our research demonstrated that photoinduced charge transport controlled by an external electric field in a BHJ solar cell is efficient, and the exciton dissociation is not the limiting factor in organic solar cells.Our research should aid in the rational design of a novel conjugated system of organic solar cells.

Fullerene-Based Photoactive Layers for Heterojunction Solar Cells: Structure, Absorption Spectra and Charge Transfer Process

The electronic structure and optical absorption spectra of polymer APFO₃, [70]PCBM/APFO₃ and [60]PCBM/APFO₃, were studied with density functional theory (DFT), and the vertical excitation energies were calculated within the framework of the time-dependent DFT (TD-DFT). Visualized charge difference density analysis can be used to label the charge density redistribution for individual fullerene and fullerene/polymer complexes. The results of current work indicate that there is a difference between [60]PCBM and [70]PCBM, and a new charge transfer process is observed. Meanwhile, for the fullerene/polymer complex, all calculations of the twenty excited states were analyzed to reveal all possible charge transfer processes in depth. We also estimated the electronic coupling matrix, reorganization and Gibbs free energy to further calculate the rates of the charge transfer and the recombination. Our results give a clear picture of the structure, absorption spectra, charge transfer (CT) process and its influencing factors, and provide a theoretical guideline for designing further photoactive layers of solar cells.

Spectral shift mechanisms of chlorophylls in liquids and proteins

Origins of non-excitonic spectral shifts of chlorophylls that can reach -1,000 cm(-1) in pigment-protein complexes are actively debated in literature. We investigate possible shift mechanisms, basing on absorption and fluorescence measurements in large number of liquids. Transition wavelength in solvent-free state was estimated (±2 nm) for chlorophyll a (Chl a, 647 nm), Chl b (624 nm), bacteriochlorophyll a (BChl a, 752 nm), and pheophytines. The dispersive-repulsive shift is a predominating mechanism. It depends on polarizability difference between the ground and the excited state Δα and the Lorenz-Lorentz function of refractive index of solvent (n). The approximate (± 2Å(3)) increase of polarizability Δα is close to 15Å(3) for S(1) bands of Chl a, BChl a, and BPheo a, slightly larger for Chl b (18Å(3)), and less for Pheo a (11Å(3)). The effect of solvent polarity, expressed in terms of static dielectric permittivity (ε) is relatively minor, but characteristic for different pigments and transitions. Remarkably, maximum influence of ε on S(1) band of BChl a is less (-20 ± 10 cm(-1)) than that for Chl a (-50 ± 10 cm(-1)), and not correlated with dipole moment changes on excitation Δμ (∼2D and 0.1 ± 0.1D, respectively). Hydrogen bonding in protic solvents produces red shifts in Chl a (-60 cm(-1)) and BChl a (-100 cm(-1)), but not in Chl b. Second axial ligand of BChl a has no influence on the S(1) band, whereas the S(2) transition suffers a -400 to -600 cm(-1) down shift. Aromatic character of solvent is responsible for a ∼-100 cm(-1) red shift of both Q transitions in BChl a. The S(1) bands in chlorophylls are relatively insensitive with respect to dielectric properties and specific solvation. Therefore, nontrivial mechanisms, yielding large site-energy shifts are expected in photosynthetic chlorophyll-proteins.

Chiroptical nature of two-exciton states of light-harvesting complex: Doubly resonant three-wave-mixing spectroscopy

Photosynthetic light-harvesting complex is a coupled multichromophore system. Due to electronic couplings between neighboring chlorophylls in the complex, the one- and two-exciton states are delocalized and they can be written as linear combinations of singly and doubly excited configurations, respectively. Despite that the chiroptical properties of one-exciton states in such a multichromophore system have been investigated by using linear optical activity measurement techniques; those of two-exciton states have not been studied before due to a lack of appropriate measurement methods. Here, we present a theoretical description on chiroptical chi((2)) spectroscopy and show that it can be used to investigate such properties of a photosynthetic light-harvesting system, which is the Fenna-Matthews-Olson complex, consisting of seven bacteriochlorophylls in its protein subunit. To simulate the doubly resonant sum- and difference-frequency-generation spectra of the complex, one- and two-exciton transition dipoles were calculated. Carrying out quantum chemistry calculations of electronically excited states of a model bacteriochlorophyll system and taking into account the dipole-induced dipole electronic transition processes between the ground state and two-exciton states, we could calculate the two-dimensional sum-frequency-generation spectra revealing dominant second-order chiroptical transition pathways and involved one- and two-exciton states. It is believed that the present computational scheme and the theoretically proposed doubly resonant two-dimensional three-wave-mixing spectroscopy would be of use to shed light on the chiroptical natures of two-exciton states of arbitrary coupled multichromophore systems.

Probing the effect of the binding site on the electrostatic behavior of a series of carotenoids reconstituted into the light-harvesting 1 complex from purple photosynthetic bacterium Rhodospirillum rubrum detected by stark spectroscopy

Reconstitutions of the LH1 complexes from the purple photosynthetic bacterium Rhodospirillum rubrum S1 were performed with a range of carotenoid molecules having different numbers of C=C conjugated double bonds. Since, as we showed previously, some of the added carotenoids tended to aggregate and then to remain with the reconstituted LH1 complexes (Nakagawa, K.; Suzuki, S.; Fujii, R.; Gardiner, A.T.; Cogdell, R.J.; Nango, M.; Hashimoto, H. Photosynth. Res. 2008, 95, 339-344), a further purification step using a sucrose density gradient centrifugation was introduced to improve purity of the final reconstituted sample. The measured absorption, fluorescence-excitation, and Stark spectra of the LH1 complex reconstituted with spirilloxanthin were identical with those obtained with the native, spirilloxanthin-containing, LH1 complex of Rs. rubrum S1. This shows that the electrostatic environments surrounding the carotenoid and bacteriochlorophyll a (BChl a) molecules in both of these LH1 complexes were essentially the same. In the LH1 complexes reconstituted with either rhodopin or spheroidene, however, the wavelength maximum at the BChl a Qy absorption band was slightly different to that of the native LH1 complexes. These differences in the transition energy of the BChl a Qy absorption band can be explained using the values of the nonlinear optical parameters of this absorption band, i.e., the polarizability change Tr(Deltaalpha) and the static dipole-moment change |Deltamu| upon photoexcitation, as determined using Stark spectroscopy. The local electric field around the BChl a in the native LH1 complex (ES) was determined to be approximately 3.0x10(6) V/cm. Furthermore, on the basis of the values of the nonlinear optical parameters of the carotenoids in the reconstituted LH1 complexes, it is possible to suggest that the conformations of carotenoids, anhydrorhodovibrin and spheroidene, in the LH1 complex were similar to that of rhodopin glucoside in crystal structure of the LH2 complex from Rhodopseudomonas acidophila 10050.

Probing binding site of bacteriochlorophyll a and carotenoid in the reconstituted LH1 complex from Rhodospirillum rubrum S1 by Stark spectroscopy

Stark spectroscopy is a powerful technique to investigate the electrostatic interactions between pigments as well as between the pigments and the proteins in photosynthetic pigment-protein complexes. In this study, Stark spectroscopy has been used to determine two nonlinear optical parameters (polarizability change Tr(Deltaalpha) and static dipole-moment change |Deltamu| upon photoexcitation) of isolated and of reconstituted LH1 complexes from the purple photosynthetic bacterium, Rhodospirillum (Rs.) rubrum. The integral LH1 complex was prepared from Rs. rubrum S1, while the reconstituted complex was assembled by addition of purified carotenoid (all-trans-spirilloxanthin) to the monomeric subunit of LH1 from Rs. rubrum S1. The reconstituted LH1 complex has its Q(y) absorption maximum at 878 nm. This is shifted to the blue by 3 nm in comparison to the isolated LH1 complex. The energy transfer efficiency from carotenoid to bacteriochlorophyll a (BChl a), which was determined by fluorescence excitation spectroscopy of the reconstituted LH1 complex, is increased to 40%, while the efficiency in the isolated LH1 complex is only 28%. Based on the differences in the values of Tr(Deltaalpha) and |Deltamu|, between these two preparations, we can calculate the change in the electric field around the BChl a molecules in the two situations to be E (Delta) approximately 3.4 x 10(5) [V/cm]. This change can explain the 3 nm wavelength shift of the Q(y) absorption band in the reconstituted LH1 complex.

Density functional theory for charge transfer: the nature of the N-bands of porphyrins and chlorophylls revealed through CAM-B3LYP, CASPT2, and SAC-CI calculations

While density functional theory (DFT) has been proven to be extremely useful for the prediction of thermodynamic and spectroscopic properties of molecules, to date most functionals used in common implementations of DFT display a systematic failure to predict the properties of charge-transfer processes. While this is explicitly manifest in Rydberg transitions of atoms and molecules and in molecular charge-transfer spectroscopy, it also becomes critical for systems containing extended conjugation such as polyenes and other conducting polymers, porphyrins, chlorophylls, etc. A new density functional, a Coulomb-attenuated hybrid exchange-correlation functional (CAM-B3LYP), has recently been developed specifically to overcome these limitations, and it has been shown to properly predict molecular charge-transfer spectra. Here, we demonstrate that it predicts qualitatively reasonable spectra for porphyrin, some oligoporphyrins, and chlorophyll. However, alternate density functionals developed to overcome the same limitations such as current-density functional theory are shown, in their present implementation, to remain inadequate. The CAM-B3LYP results are shown to be in excellent agreement with complete-active-space plus second-order Møller-Plesset perturbation theory and symmetry-adapted cluster configuration interaction calculations: These depict the N and higher bands of porphyrins and chlorophylls as being charge-transfer bands associated with localization of molecular orbitals on individual pyrrole rings. The validity of the basic Gouterman model for the spectra of porphyrins and chlorophylls is confirmed, rejecting modern suggestions that non-Gouterman transitions lie close in energy to the Q-bands of chlorophylls. As porphyrins and chlorophylls provide useful paradigms for problems involving extended conjugation, the results obtained suggest that many significant areas of nanotechnology and biotechnology may now be realistically treated by cost-effective density-functional-based computational methods.

Photoabsorption of green and red fluorescent protein chromophore anions in vacuo

Photoabsorption properties of green and red fluorescent protein chromophore anions in vacuo were investigated theoretically, based on the experimental results in gas phase [Phys. Rev. Lett. 2001, 87, 228102; Phys. Rev. Lett. 2003, 90, 118103]. Their calculated transition energies in absorption with TD-DFT and ZINDO methods are directly compared to the experimental reports in gas phase, and the calculations with ZINDO method can correctly reproduce the absorption spectra. The orientation and strength of their transition dipole moments were revealed with transition density. We also showed the orientation and result of their intramolecular charge transfer with transition difference density. The calculated results show that with the increase of the extended conjugated system, the orientation of transition dipole moments and the orientation of charge transfer can be reversed. They are the linear responds with the external electric fields. These theoretical results reveal the insight understanding of the photoinduced dynamics of green and red fluorescent protein chromophore anions and cations in vacuo.

S1 and S2 Excited States of Gas-Phase Schiff-Base Retinal Chromophores: A Time-Dependent Density Functional Theoretical Investigation

In concert with the recent photoabsorption experiments of gas-phase Schiff-base retinal chromophores (Nielsen et al. Phys. Rev. Lett. 2006, 96, 018304), quantum chemical calculations using time-dependent density functional theory coupled with different functionals and under the Tamm-Dancoff approximation were made on the first two excited states (S1 and S2) of two retinal chromophores: 11-cis and all-trans protonated Schiff bases. The calculated vertical excitation energies (Tv) and oscillator strengths (f) are consistent with the experimental absorption bands. The experimentally observed phenomenon that the transition dipole moment (mu) of S2 is much smaller that of S1 was interpreted by 3D representation of transition densities. The different optical behaviors (linear and nonlinear optical responds) of the excited states were investigated by considering different strengths of external electric fields.

How proteins trigger excitation energy transfer in the FMO complex of green sulfur bacteria

A simple electrostatic method for the calculation of optical transition energies of pigments in protein environments is presented and applied to the Fenna-Matthews-Olson (FMO) complex of Prosthecochloris aestuarii and Chlorobium tepidum. The method, for the first time, allows us to reach agreement between experimental optical spectra and calculations based on transition energies of pigments that are calculated in large part independently, rather than fitted to the spectra. In this way it becomes possible to understand the molecular mechanism allowing the protein to trigger excitation energy transfer reactions. The relative shift in excitation energies of the seven bacteriochlorophyll-a pigments of the FMO complex of P. aestuarii and C. tepidum are obtained from calculations of electrochromic shifts due to charged amino acids, assuming a standard protonation pattern of the protein, and by taking into account the three different ligand types of the pigments. The calculations provide an explanation of some of the earlier results for the transition energies obtained from fits of optical spectra. In addition, those earlier fits are verified here by using a more advanced theory of optical spectra, a genetic algorithm, and excitonic couplings obtained from electrostatic calculations that take into account the influence of the dielectric protein environment. The two independent calculations of site energies strongly favor one of the two possible orientations of the FMO trimer relative to the photosynthetic membrane, which were identified by electron microscopic studies and linear dichroism experiments. Efficient transfer of excitation energy to the reaction center requires bacteriochlorophylls 3 and 4 to be the linker pigments. The temporal and spatial transfer of excitation energy through the FMO complex is calculated to proceed along two branches, with transfer times that differ by an order of magnitude.

Reorganization Asymmetry of Electron Transfer in Ferroelectric Media and Principles of Artificial Photosynthesis

This study considers electronic transitions within donor-acceptor complexes dissolved in media with macroscopic polarization. The change of the polarizability of the donor-acceptor complex in the course of electronic transition couples to the reaction field of the polar environment and the electric field created by the macroscopic polarization. An analytical theory developed to describe this situation predicts a significant asymmetry of the reorganization energy between charge separation and charge recombination transitions. This result is proved by Monte Carlo simulations of a model polarizable diatomic dissolved in a ferroelectric fluid of soft dipolar spheres. The ratio of the reorganization energies for the forward and backward reactions up to a factor of 25 is obtained in the simulations. This result, as well as the effect of the macroscopic electric field, is discussed in application to the design of efficient photosynthetic devices.

Time-dependent density functional theory investigation of electric field effects on absorption spectra of meso-meso-linked zinc porphyrin arrays: Role of charge-transfer States

By using time-dependent density functional theory, we calculated the transition energies of a zinc porphyrin monomer and its meso-meso-linked arrays. In line with the prediction of the molecular exciton model, the calculated splitting energy of the Soret band increased as the number of linked porphyrins increased. We then examined how the transition energies of the dimer array were shifted by an applied electric field. For reproduction of an electroabsorption spectrum (EA), i.e., the field-induced change in absorption intensity, a model Hamiltonian constructed from five states is proposed. It is concluded for the dimer that the field-induced coupling between the lower-energy Soret band Se and the lower-lying ionic character (charge-transfer) states is responsible for the experimentally observed blue shift of Se as well as the second-derivative profile in the EA spectrum.