Relaxed Charge Transfer State Research Articles

Femtosecond Fluorescence Upconversion Study of a Naphthalimide-Bithiophene-Triphenylamine Push-Pull Dye in Solution.

There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photoexcited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophene-naphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC) evolves toward a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher nonradiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in nonpolar solvents, hinting that the excited-state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.

Charge Generation via Relaxed Charge-Transfer States in Organic Photovoltaics by an Energy-Disorder-Driven Entropy Gain

In organic photovoltaics, efficient charge generation relies on our ability to convert excitons into free charges. Efficient charge separation from “energetic excitons” has been understood to be governed by delocalization effects promoted by molecular aggregation. A remaining puzzle is, however, the mechanism underlying charge generation via relaxed interfacial charge-transfer (CT) excitons that also exhibit an internal quantum efficiency close to unity. Here, we provide evidence for efficient charge generation via CT state absorption over a temperature range of 50–300 K, despite an intrinsically strong Coulomb binding energy of about 400 meV that cannot be modified by fullerene aggregation. We explain our observation by entropy-driven charge separation, with a key contribution from energy disorder. The energy disorder reduces the charge generation barrier by substantially gaining the entropy as electron–hole distance increases, resulting in efficient CT exciton dissociation. Our results underline an emer...

(Invited) Photoinduced Charge Transfer Reactions and Excited State Properties in Triphenylamine C60 Donor-Acceptor Conjugates

The molecular heterojunction triphenylamine/C60 is a potential material to construct high-efficient dye-sensitized solar cells. We have carried out a combined molecular dynamics and time-dependent density functional theory study to analyze charge separation and exciton delocalization in excited states of two constitutional isomers of this interface.[1] Our results reveal the presence of exciton states characterized by partial charge-transfer character (hybrid states); which are suggested to promote the direct charge separation process by excitation. These states may have a significant impact on the efficiency of the light-harvesting ensembles. That is to say, a greater amount of such hybrid states is found at short distances between the triphenylamine fragment and the C60 cage; as a result, charge transfer reactions can occur faster. This trend resembles the experimental evidence since rate constants for photoinduced charge transfer reactions kct are bigger for the constitutional isomer with the shortest distance between the triphenylamine fragment and the C60 cage.[2] We have also evaluated the performance of U-CAM-B3LYP/6-31G* and TD-CAM-B3LYP/6-31G* level of theories to compute the different parameters affecting kct ; that is the reorganization energy λ, the electronic coupling Vij , and free energies ΔG.[3] Our study is done under the theory of nonadiabatic electron transfer. By using the standard Marcus formulation, our results show that the outer-sphere contribution to λ and ΔG for the recombination reaction are the parameters that more uncertainty introduces to the estimation of kct . We implemented a low-cost methodology based on U-DFT for the evaluation of the outer-sphere part of λ and ΔG; wherein Franck-Condon and relaxed charge-transfer states are conveniently modeled, including properly the solvent reorganization energy, by U-CAM-B3LYP/6-31G* coupled to continuum solvation models. Finally, it is worth to mention that the replacement of the C60 cage by Ih -Sc3N@C80 generates longer-lived charge-transfer states; which is attributed to the lower ΔG of the charge recombination reaction.[2] Our group is currently analyzing this trend for the whole family of endohedral metallofullerenes; for which dynamics is expected to be in the normal region of the Marcus parabola. [1] J. P. Martínez, S. Osuna, M. Solà, A. Voityuk, Theor. Chem. Acc. 2015, 134, 12. [2] J. R. Pinzón, D. C. Gasca, S. G. Sankaranarayanan, G. Bottari, T. Torres, D. M. Guldi, L. Echegoyen, J. Am. Chem. Soc. 2009, 131, 7727–7734. [3] J. P. Martínez, M. Solà, A. Voityuk, 2015, in preparation. Figure 1

Efficient charge generation by relaxed charge-transfer states at organic interfaces

Interfaces between organic electron-donating (D) and electron-accepting (A) materials have the ability to generate charge carriers on illumination. Efficient organic solar cells require a high yield for this process, combined with a minimum of energy losses. Here, we investigate the role of the lowest energy emissive interfacial charge-transfer state (CT1) in the charge generation process. We measure the quantum yield and the electric field dependence of charge generation on excitation of the charge-transfer (CT) state manifold via weakly allowed, low-energy optical transitions. For a wide range of photovoltaic devices based on polymer:fullerene, small-molecule:C60 and polymer:polymer blends, our study reveals that the internal quantum efficiency (IQE) is essentially independent of whether or not D, A or CT states with an energy higher than that of CT1 are excited. The best materials systems show an IQE higher than 90% without the need for excess electronic or vibrational energy.

Femtosecond to millisecond studies of electron transfer processes in a donor–(π-spacer)–acceptor series of organic dyes for solar cells interacting with titania nanoparticles and ordered nanotube array films

Time-resolved emission and absorption spectroscopy are used to study the photoinduced dynamics of forward and back electron transfer processes taking place between a recently synthesized series of donor-(π-spacer)-acceptor organic dyes and semiconductor films. Results are obtained for vertically oriented titania nanotube arrays (inner diameters 36 nm and 70 nm), standard titania nanoparticles (25 nm diameter) and, as a reference, alumina nanoparticle (13 nm diameter) films. The studied dyes contain a triphenylamine group as an electron donor, cyanoacrylic acid part as an electron acceptor, and differ by the substituents in a spacer group that causes a shift of its absorption spectra. Despite a red-shift of the dye absorption band resulting in an improved response to the solar spectrum, smaller electron injection rates and smaller extinction coefficients result in reduced dye sensitized solar cell (DSSC) conversion efficiencies. For the most efficient dye, TPC1, electron injection from the hot locally excited state to titania on a time scale of about 100 fs is suggested, while from the relaxed charge transfer state it proceeds in a non-exponential way with time constants from 1 ps to 50 ps. Our results imply that the latter process involves the trap states below the conduction band edge (or the sub-bandgap tail of the acceptor states), localized close to the dye radical cation, and is accompanied by fast electron recombination to the parent dye's ground state. This process should limit the efficiency of DSSCs made using these types of organic dyes. The residual, slower recombination can be described by a stretched exponential decay with a characteristic time of 0.5 μs and a dispersion parameter of 0.33. Both the electron injection and back electron transfer dynamics are similar in titania nanoparticles and nanotubes. Variations between the two film types are only found in the time resolved emission transients, which are explained in terms of the difference in local electric fields affecting the position of the emission bands.

Analysis of heterogeneous fluorescence decays in proteins. Using fluorescence lifetime of 8-anilino-1-naphthalenesulfonate to probe apomyoglobin unfolding at equilibrium

The solvatochromic fluorescent dye 8-anilino-1-naphthalenesulfonate (ANS) is one of the popular probes of protein folding. Folding kinetics is tracked with ANS fluorescence intensity, usually interpreted as a reflection of protein structure—the hydrophobicity of the binding environments. Such simplistic view overlooks the complicated nature of ANS–protein complexes: the fluorescence characteristics are convoluted results of the ground state populational distribution of the probe–protein complex, the structural changes in the protein and the excited state photophysics of the probe. Understanding of the interplay of these aspects is crucial in accurate interpretation of the protein dynamics. In this work, the fluorescence decay of ANS complexed with apomyoglobin at different conformations denatured by pH is modeled. The fluorescence decay of the ANS–apomyoglobin complex contains information on not only apomyoglobin structure but also molecular populational distributions. The challenge in modeling fluorescence decay profiles originates from the convolution of heterogeneous binding and excited-state relaxation of the fluorescent probe. We analyzed frequency-domain fluorescence lifetime data of ANS-apomyoglobin with both maximum entropy methods (MEM) and nonlinear least squares methods (NLLS). MEM recovers a model of two expanding-and-merging lifetime distributions for ANS–apomyoglobin in the equilibrium transition from the native (N) through an intermediate (I-1) to the acid-unfolded state U A. At pH 6.5 and above, when apomyoglobin is mostly populated at the N-state, ANS–apomyoglobin emits a predominant long-lifetime fluorescence from a relaxed charge transfer state S 1,CT of ANS, and a short-lifetime fluorescence that is mainly from a nascent excited-state S 1,np of ANS stabilized by the strong ANS–apomyoglobin interaction. Lowering the pH diminishes the contribution from the S 1,np state. Meanwhile, more protein molecules become populated at the U A state, which exhibits a short lifetime that is not distinguishable from the S 1,np state. At pH 3.4, when the population of the U A becomes significant, the short-lifetime fluorescence comes predominantly from ANS binding to the U A. Further lowering the pH leads to more exposure of the bound ANS. The long lifetime shifts toward and finally merges with the short lifetime and becomes one broad distribution that stands for ANS binding to the U A below pH 2.4. The above expanding-and-merging model is consistent with F-statistic analysis of NLLS models. The consistency of this model with the knowledge from the literature, as well as the continuity of the decay parameters changing upon experimental conditions are also crucial in drawing the conclusions.

Mechanism of photoinduced intramolecular charge transfer in aminobenzethynes: an ab initio study

Ab initio electronic structure calculations were performed to characterize the charge transfer (CT) process in benzethyne (BE), 4-aminobenzethyne (ABE) and 4-dimethylamino-benzethyne (DMABE). The Hartree-Pock (HF), configuration interaction scheme (CIS ), complete active space self-consistent field (CASSCF) and CASPT2 (multiconfigurational second-order perturbation theory taking CASSCF function as a reference) methods were employed. Geometry optimization of the CT state at the CIS level predicts a planar configuration with a trans bent ethyne group to be the lowest excited singlet state in all systems. The significant stabilization of the CT state in the BE series can be understood as arising from the rehybridization of the carbon atoms of the ethyne group on transfer of an electron. The calculation of the relative energy of the locally excited state and the relaxed CT state using the CASPT2 method reveals that the intramolecular CT process is exothermic in ABEs under isolated molecule conditions, in contrast with the much studied aminohenzonitriles, where interaction with a polar solvent is required for sufficient stabilization of the CT state. Therefore BE and its amino derivatives potentially offer unique opportunities for the investigation of photoinduced intramolecular CT in isolated molecules.

On the ab-initio calculation ofd-d spectra in transition metal compounds: The importance of relaxed charge transfer states

By calculations on CuCl, CuBr, and NiO clusters it is shown that a first-order configuration interaction (CI) calculation significantly improves the d-d and charge transfer (CT) spectra of ionic transition metal compounds. The first-order CI introduces delocalization (covalency) effects in the dn states, thus increasing the effective ligand-field splitting which is always underestimated at the Hartree–Fock (HF) level. It is demonstrated that this HF + first-order CI treatment is strongly related to a valence bond model. In this model the delocalization is introduced by explicit interactions with relaxed CT states. After account has been taken of the physically very different atomic correlation effects, a very good agreement with experimental d-d spectra is obtained, using only a small cluster. The effect of first-order CI on CT states is to account for hole localization and polarization effects which lead to reductions in the CT excitation energies in the order of 2–3 eV.