Role Of Charge Transfer States Research Articles

Unravelling the role of charge transfer state during ultrafast intersystem crossing in compact organic chromophores.

When organic electron donor (D) and acceptor (A) chromophores are linked together, an electron transfer (ET) state can take place. When a short bridge such as one Sigma bond is used to link the donor and the acceptor, complete charge separation is difficult to access and one usually observes an intramolecular charge transfer (CT) state instead. Due to the inevitable coupling between the donor and the acceptor in compact organic chromophores, the most common decay pathway for the CT state is charge recombination, which may lead to a distinct longer wavelength fluorescence emission or non-radiative dissipation of the excited state energy. However, recent studies have shown that unique excited state dynamics can be observed when the CT state is involved during both forward and backward intersystem crossing (ISC) from singlet excited states to triplet excited states in organic chromophores. Analysis of the mechanism for ISC involving the CT state has received much attention over the last decade. In this perspective, we present a collection of molecular design rationales, spectroscopy and theoretical investigations that provide insights into the mechanism of the ISC involving the CT state in compact organic chromophores. We hope that this perspective will prove beneficial for researchers to design novel compact organic chromophores with a predictable ISC property for future biochemical and optoelectronic applications.

A Quantum-Chemical Insight into the Role of Charge-Transfer States in Organic Emitters for Electroluminescence

The rapid progress of the design and development of efficient organic electroluminescent emitters without heavy transition metals has been achieved in recent experimental investigations. In this mi...

Comparison of ab initio HF and DFT calculations of the structure and spectroscopy of two dimeric systems of chloro Yb(III) mono-phthalocyanine in polymeric lattice

Abstract Lanthanide complexes with phthalocyanines (Pc) and porphyrins are of great interest because of their unique luminescence behavior and many applications. Our research is devoted to photophysical studies and theoretical calculations HF and DFT of the structure and spectroscopy of two dimeric systems, created by chloro Yb(III) monophthalocyanines and comparison of the calculated spectroscopic behaviour with the experimental one for monomeric chelate in polymeric lattice of PMMA. Ytterbium chelates were chosen because they have special properties appropriate for technology of different materials with unique spectroscopic behaviour in NIR region, which is not overlapped by very intensive emission of π-conjugated system of phthalocyanine or by upconversion transitions. The drastic variation in emission properties of materials depend on the energy of excitation, and only minor changes are observed in spectroscopic frequencies of dimeric systems compared to monomeric ones. Attention is paid to the radiative, non-radiative and non-linear processes, intramolecular energy transfer and the role of charge-transfer state in this process.

Singlet fission in tetracene: an excited state analysis

Singlet fission is a potential mechanism to enhance the performance of current solar cells. However, the actual mechanism is still a matter of debate, with charge transfer states believed to play an essential role. The probability of the overall process can be related to the electronic coupling between the electronic states. Here, we explore the excited states of three pairs of tetracene with different relative orientation in the crystal structure showing different electronic couplings and identify the role of charge transfer states. First, a suitable theoretical method for the study of the tetracene pairs is determined by comparing time-dependent density functional theory with wave function-based methods in terms of excitation energies, so-called exciton descriptors, and graphical tools such as electron-hole correlation plots and natural transition orbitals. The results show the presence of low-lying charge transfer states in those tetracene pairs with non-zero electronic coupling, suggesting a superexchange-mediated mechanism, and high-lying charge resonance states for the pair with zero electronic coupling. Finally, the lower electron-hole correlation coefficients for pairs with non-zero coupling speak in favour of the superexchange-mediated mechanism, as a weaker Coulombic attraction due to the mixing with charge transfer states further facilitates the formation of the state from the photoexcited molecule.

Role of charge transfer states into the formation of cyclobutane pyrimidine dimers in DNA

Photoinduced charge transfer between neighboring bases plays an important role in DNA. One of its important effects is shown in its ability to affect the photochemical yields of the formation of cyclobutane pyrimidine dimer (CPD) products between adjacent pyrimidine bases. In this work we examine how the energies of charge transfer states depend on the sequences of oligonucleotides using a hybrid quantum and molecular mechanics (QM/MM) methodology combined with the algebraic diagrammatic construction through a second order electronic structure method for excited states. Specifically, we examine 10 sequences with guanine being on the 5' or 3' position of two pyrimidine bases. The results show that the energies of charge transfer states are affected by the nature of the donor acceptor pair, by the distance between them, and by other electrostatic effects created by the surrounding environment.

Correction: Role of charge transfer states into the formation of cyclobutane pyrimidine dimers in DNA.

Correction: Role of charge transfer states into the formation of cyclobutane pyrimidine dimers in DNA.

The role of charge-transfer states in the spectral tuning of antenna complexes of purple bacteria.

The LH2 antenna complexes of purple bacteria occur, depending on light conditions, in various different spectroscopic forms, with a similar structure but different absorption spectra. The differences are related to point changes in the primary amino acid sequence, but the molecular-level relationship between these changes and the resulting spectrum is still not well understood. We undertook a systematic quantum chemical analysis of all the main factors that contribute to the exciton structure, looking at how the environment modulates site energies and couplings in the B800-850 and B800-820 spectroscopic forms of LH2. A multiscale approach combining quantum chemistry and an atomistic classical embedding has been used where mutual polarization effects between the two parts are taken into account. We find that the loss of hydrogen bonds following amino acid changes can only explain a part of the observed blue-shift in the B850 band. The coupling of excitonic states to charge-transfer states, which is different in the two forms, contributes with a similar amount to the overall blue-shift.

Femtosecond-Resolved Excited State Relaxation Dynamics of Copper (II) Tetraphenylporphyrin (CuTPP) After Soret Band Excitation

Excited state relaxation dynamics of Copper (II) tetraphenylporphyrin (CuTPP) after Soret band excitation have been investigated in various solvents by femtosecond broadband transient absorption spectroscopy. Significant role of charge transfer state has been confirmed from fast relaxation of triplet CuTPP in pyridine, giving τ ~ 26.5 ps. In piperidine, the transient measured at 480 nm shows biexponential behavior with distinct time constants of 300 fs and 27.4 ps. The fast component with τ ~ 300 fs is attributed to relaxation of the CuTPP-piperidine adduct populated in the ground state, giving the intrinsic relaxation rate of the CuTPP exciplex for the first time. For CuTPP in O-coordinating solvents of 1,4-dioxane and tetrahydrofuran (THF), a completely new relaxation channel via the 2[dz2, dx2−y2] state is opened. As the exciplex formation is diffusion controlled, triplet CuTPP lifetimes in pure solvents employed here are all measured to be more or less same to give ~30 ps, whereas the 2[dz2, dx2−y2] exciplex formed by the ligation with O-coordinating solvents is found to relax much slowly to the ground state, giving lifetimes of ~360 and ~270 ps in 1,4-dioxane and THF, respectively.

Absence of Intramolecular Singlet Fission in Pentacene-Perylenediimide Heterodimers: The Role of Charge Transfer State.

A new class of donor-acceptor heterodimers based on two singlet fission (SF)-active chromophores, i.e., pentacene (Pc) and perylenediimide (PDI), was developed to investigate the role of charge transfer (CT) state on the excitonic dynamics. The CT state is efficiently generated upon photoexcitation. However, the resulting CT state decays to different energy states depending on the energy levels of the CT state. It undergoes extremely rapid deactivation to the ground state in polar CH2Cl2, whereas it undergoes transformation to a Pc triplet in nonpolar toluene. The efficient triplet generation in toluene is not due to SF but CT-mediated intersystem crossing. In light of the energy landscape, it is suggested that the deep energy level of the CT state relative to that of the triplet pair state makes the CT state actually serve as a trap state that cannot undergoes an intramolecular singlet fission process. These results provide guidance for the design of SF materials and highlight the requisite for more widely applicable design principles.

Improved performance of inverted polymer solar cells using Cd2SSe/ZnS quantum dots

Quantum dots (QDs) materials have been widely used in polymer solar cells (PSCs) to improve device performance. In this paper, the enhanced efficiency of PSCs by incorporating Cd2SSe/ZnS (CSS-Z) QDs into active layer composed of Poly [N-9″-hepta-decanyl-2,7-carbazolealt- 5,5-(4′,7′-di-2-thienyl-2′,1′,3′-ben-zothiadiazole)] (PCDTBT): [6,6]-phenyl-C70-butyric acid methyl ester (PC71BM) were demonstrated, and the power conversion efficiency (PCE) of devices was increased from 5.72% to 6.75%, accounting for an 18% enhancement compared to the control device. The light trapping of optimized devices is higher than that of the control device, which is beneficial to boost the transport of photo-induced charges. Furthermore, the CSS-Z QDs play a role of charge transfer state, substantially improving exciton dissociation, charge transport, leading to an increased short-circuit current density (Jsc) and decreased series resistance (Rs).

Photophysical and theoretical studies of structure and spectroscopic behaviour of axially substituted Yb(III) mono-phthalocyanines in different media

Abstract Lanthanide complexes with phthalocyanines (Pc) and porphyrins belong to the group of most investigated compounds because of their unique luminescence properties and variety of applications. The special interest is directed to medical applications, mainly in photodynamic therapy. This paper is devoted to photophysical studies of acetato- and chloro-ytterbium monophthalocyanine complexes in the solid state, solutions, silica matrices and PMMA polymer which can design their applicability. The structures, IR, and Raman spectra were calculated applying Hartree–Fock and density functional theory methods and further correlated with experimental findings. The theory reproduces reasonably spectroscopic frequencies of YbPcOAc2DMSO chelate. Two molecules of DMSO with somewhat different spectroscopic behaviour were found to exist in this complex. The mechanism of the effect of conformation changes, steric obstacles, extra-coordination of solvent molecules and molecule immobilization in polymer and inorganic matrices on lanthanide and phthalocyanine emission spectra was discussed. Attention was paid to the radiative and non-radiative processes, intramolecular energy transfer and the role of charge-transfer state in this process, electron–phonon coupling, multiion cooperative interactions, non-linear proceses and dynamics in excited states. The role of solvent molecule exchange dynamic in possible interaction of Yb(III) complexes with biological systems was also analysed.

Role of Charge Transfer States in P3HT-Fullerene Solar Cells

By examining poly(3-hexylthiophene)-fullerene blend solar cells with controlled variations of the active layer using subgap external quantum efficiency combined with intensity dependent quantum efficiency at short circuit, a direct correlation between trap-assisted recombination parameters and the charge transfer state absorption strength is established. Most significantly, by comparing devices with different polymer molecular weight, regioregularity, C60 derivative chemistry, and film drying speed, we find that samples with higher trap-assisted recombination exhibit higher charge generation efficiency. Both findings are in support of the general view that the charge transfer states are involved in both processes. In addition, we find that the traps are full at about 100 mW/cm2, which suggests, together with the correlation with the charge transfer states, that the electrochemical potential difference in the bulk exceeds the open circuit voltage and that the loss of voltage is probably due to recombination associated with charge extraction at the electrodes.

The role of charge-transfer states in energy transfer and dissipation within natural and artificial bacteriochlorophyll proteins.

Understanding how specific protein environments affect the mechanisms of non-radiative energy dissipation within densely assembled chlorophylls in photosynthetic protein complexes is of great interest to the construction of bioinspired solar energy conversion devices. Mixing of charge-transfer and excitonic states in excitonically interacting chlorophylls was implicated in shortening excited states lifetimes but its relevance to active control of energy dissipation in natural systems is under considerable debate. Here we show that the degree of fluorescence quenching in two similar pairs of excitonically interacting bacteriochlorophyll derivatives is directly associated with increasing charge transfer character in the excited state, and that the protein environment may control non-radiative dissipation by affecting the mixing of charge transfer and excitonic states. The capability of local protein environments to determine the fate of excited states, and thereby to confer different functionalities to excitonically coupled dimers substantiates the dimer as the basic functional element of photosynthetic enzymes.

Theoretical Investigation of Singlet Fission in Molecular Dimers: The Role of Charge Transfer States and Quantum Interference

Singlet fission (SF) is a spin-allowed process by which a singlet excited state splits into a pair of triplet states. This process can potentially increase the efficiency of organic solar cells by a factor of 1.5. In this article, we study the dynamics of SF in different molecular aggregates of perylenediimide (PDI) derivatives, pentacene, and 1,3-diphenylisobenzofuran (DPB). To compute the SF rate, we have adopted a Markovian density matrix propagation approach to model SF in a molecular dimer. This approach allows accounting for both the coherent and incoherent processes that mediate the triplet formation. Our calculations show that SF can be much faster in PDI derivatives than in pentacene and DPB. Our analysis also indicates that SF is principally mediated by a superexchange mechanism that involves charge transfer states as virtual intermediates. In addition, because of the existence of different pathways for the formation of the triplet states, signatures of quantum interference are clearly observed.

Study on photophysical properties of Eu(III) complexes with aromatic β-diketones – Role of charge transfer states in the energy migration

We synthesized a set of aromatic β-diketones and measured the photophysical properties of their europium(III) complexes. According to these photophysical properties, the europium complexes can be divided into two groups: the complexes with or without the freely rotating amino-group (FRAG). On the basis of the experimental results, it can be concluded that in the FRAG complexes, the ligand-centered excitation energy is most probably transferred from a ligand to a coordinated europium via the intraligand charge transfer (ILCT) state. The temperature dependency of the lifetimes of the emissive 5D0 state revealed that in the FRAG complexes, the energy of the emissive 5D0 state is back-transferred to the ligand-to-metal charge transfer (LMCT) state and in the non-FRAG complexes, to the triplet state of the ligand. The most efficient complex synthesized was the europium complex of carbazole derivative L6 with the quantum yield of 47% and molar absorption coefficient of 70,400M−1cm−1.

Bichromophores bearing s-triazinyl ring spacer: A role of charge transfer states on fluorescence quenching of excitation energy acceptor moieties

Abstract Novel bichromophoric compounds bearing 2-aminoanthracene as the donor of excitation energy and 3-aminoperylene or 3-aminobenzanthrone as the acceptor of excitation energy and s-triazinyl ring as the spacer were synthesized and characterized. The UV/Vis absorption and fluorescence spectra as well as fluorescence quantum yields in different solvents were measured. On the basis of experimental results and semi-empirical quantum chemical calculations, the relationships between the chemical structure and photo-physical properties of prepared compounds were investigated.

Role of charge transfer state and host matrix in Eu3+-doped alkali and earth alkali fluoro-aluminoborate glasses

Judd–Ofelt parameters of Eu3+-doped alkali and earth alkali fluoro-aluminoborate glasses are determined from the fluorescence spectra. The dependence of these parameters on the compositional changes of the materials is discussed. The glass containing sulphate component shows the larger values of Ω compared with the fluoro-aluminoborate glasses because of higher ΔCT (Eu3+–F−) values. The difference of ionic radius and electronegativity between Na and Ca lead to higher Ω2 parameter of CaF2–Al2O3–B2O3:Eu3+ glass.Persistent spectral hole burning has been observed in Eu3+ ions doped alkali-and earth alkali fluoro-aluminoborate glasses with the compositions of 16NaF·73B2O3·8Al2O3·3Eu2O3 and 16CaF2·73B2O3·8Al2O3·3Eu2O3, which have been prepared using conventional melting technique.

The role of charge transfer states in deactivation of the electronic excitation energy of spirooxazines

338 Photochromic compounds, characterized by reversible photoinduced structural changes, are prom ising candidates for use in different areas of molecular electronics, photonics, and spintronics. Among the photochromic compounds, spirocyclic molecular sys tems, in particular, spirooxazines (SPOs), hold special position. SPOs have high photocoloration quantum yields and exhibit a wide variability of spectral and kinetic characteristics [1, 2].

Exciton/Charge-Transfer Electronic Couplings in Organic Semiconductors.

Charge transfer (CT) states and excitons are important in energy conversion processes that occur in organic light emitting devices (OLEDS) and organic solar cells. An ab initio density functional theory (DFT) method for obtaining CT-exciton electronic couplings between CT states and excitons is presented. This method is applied to two organic heterodimers to obtain their CT-exciton coupling and adiabatic energy surfaces near their CT-exciton diabatic surface crossings. The results show that the new method provides a new window into the role of CT states in exciton-exciton transitions within organic semiconductors.

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.