Partial Charge Transfer State Research Articles

Distinctive Excited State Symmetry Breaking Dynamics in Typical Donor-Acceptor-Donor Fluorophore: Strong Photoluminescence and Ultrafast Charge Separation from a Partial Charge Transfer State.

Understanding the structure-property relationships in organic semiconductors is crucial for controlling their photophysical properties and developing new optoelectronic materials. Quadrupolar molecules, donor-acceptor-donor (DAD), have attracted extensive attention in various optoelectronic applications. However, the systematic studies on the differences on photophysical properties between DAD and simple donor-acceptor (DA) chromophores are rarely reported. Herein we present a comparative study on the excited state dynamics of DA and DAD fluorescence systems using theoretical calculation and transient absorption spectroscopy. Results show that DA and DAD molecules exhibit similar excited state dynamics, which are attributed to the distinctive excited-state symmetry breaking (ESSB) phenomenon observed in a DAD system. The strong photoluminescence (PL) and ultrafast charge separation (CS) from an ESSB-induced partial charge transfer (CT) state were clearly detected in different solvent environments. These results not only offer insight into the excited state dynamics of the DAD fluorescence system but also provide some basic guidelines for designing new optoelectronic materials.

Two-Step Charge Separation Passing Through the Partial Charge-Transfer State in a Molecular Dyad.

Charge separation (CS) in molecular systems usually takes place in weakly coupled donor-acceptor dyads where an electron charge moves from the donor to the acceptor in the local excited state of a chromophore. Herein, we present a two-step charge-separation process in a newly synthesized diketopyrrolopyrrole-pyrrolopyrrole (DPP-PP) dyad (AD), which starts from the initial photoexcited bright exciton and goes through a partial charge-transfer (CT) state before finally reaching the charge-separated (CS) state. The evolving CT character in the excited state is demonstrated through the complementary use of transient absorption, broad-band fluorescence upconversion, and transient impulsive stimulated Raman spectroscopy. The bright exciton state of the dyad relaxes to a partial CT state with 1 and 20 ps during solvent and structural fluctuations in toluene, respectively, and with 700 fs for the solvent fluctuations occurring in tetrahydrofuran. This is evident from the characteristic excited-state absorption spectra and the reduced fluorescence intensity observed on the adiabatic potential energy surface. AD in THF additionally evolves to the diabatic potential energy surface of the CS state, whose absorption spectrum converges to that of a DPP anion for which fluorescence is completely quenched. The trend of shifting for certain vibrational frequencies also supports the proposed CT dynamics and mechanism; furthermore, it gives quantitative insight into the CT characters of the bright state (0.1 e) and intermediate partial CT state (0.5 e), as determined by the linear relationship that exists between the vibrational frequency of the marker modes and the CT character. We have found that as the structure of the bridge between donor and acceptor enables an intermediate level of electronic communication, the charge-separation can rapidly occur through a distinct partial charge-transfer state. It seems that the exceptionally strong electronic communication at positions 2 and 5 of the pyrrolo[3,2-b]pyrrole core is a crucial element for this charge-transfer mechanism, which could be applied to organic photovoltaics or light-emitting diodes requiring efficient charge separation.

Improved Spectral Coverage and Fluorescence Quenching in Donor-acceptor Systems Involving Indolo[3-2-b]carbazole and Boron-dipyrromethene or Diketopyrrolopyrrole.

A novel π-conjugated triad and a polymer incorporating indolo[3,2-b]-carbazole (ICZ) and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) were synthesized via a Sonogashira coupling. Compared to the parent BODIPY the absorption and fluorescence spectrum were for both compounds broader and redshifted. The redshift of the fluorescence and the decrease of the fluorescence quantum yield and decay time upon increasing solvent polarity were attributed to the formation of a partial charge-transfer state. Upon excitation in the ICZ absorption band the ICZ fluorescence was quenched in both compounds mainly due to energy transfer to the BODIPY moiety. In a similar ICZ-π-DPP polymer (where DPP is diketopyrrolopyrrole), a smaller redshift of the absorption and fluorescence spectra compared to the parent DPP was observed. A less efficient quenching of the ICZ fluorescence in the ICZ-π-DPP polymer could be related to the unfavorable orientation of the transition dipoles of ICZ and DPP. The rate constant for energy transfer was for all compounds an order of magnitude smaller than predicted by Förster theory. While in a solid film of the triad a further redshift of the absorption maximum of nearly 100 nm was observed, no such shift was observed for the ICZ-π-BODIPY polymer.

Dynamics of the Formation of a Charge Transfer State in 1,2-Bis(9-anthryl)acetylene in Polar Solvents: Symmetry Reduction with the Participation of an Intramolecular Torsional Coordinate

We have studied 1,2-bis(9-anthryl)acetylene as a model compound for the characterization of the process of solvent-mediated symmetry reduction in an excited state. Thanks to the acetylenic bridge that joins the two anthracenic moieties, this system maintains minimal steric hindrance between the end chromophores in comparison with the classic 9,9'-bianthryl model compound. The acetylenic bridge also allows for significant electronic coupling across the molecule, which permits a redistribution of electron density after light absorption. Femtosecond resolved fluorescence measurements were used to determine the spectral evolution in acetonitrile and cyclohexane solutions. We observed that, for 1,2-bis(9-anthryl)acetylene, the formation of a charge transfer state occurs in a clear bimodal fashion with well separated time scales. Specifically, the evolution of the emission spectrum involves a first solvent-response mediated subpicosecond stage where the fluorescence changes from that typical of nonpolar solvents (locally excited) to an intermediate, partial charge transfer state. The second stage of the evolution into a full charge transfer state occurs with a much longer time constant of 37.3 ps. Since in this system the steric hindrance is minimized, this molecule can undergo much larger amplitude motions for the torsion between the two anthracenic moieties associated with the charge redistribution in comparison with the typical model compound 9,9'-bianthryl. Clearly, the larger range of motions of 1,2-bis(9-anthryl)acetylene gives the opportunity to study the electron transfer process with a good separation of the time scales for the formation of a partial charge transfer state, determined by the speed of solvent response, and the intramolecular changes associated with the formation of the fully equilibrated charge transfer state.

Tuning the Förster overlap integral: energy transfer over 20 Ångstroms from a pyrene-based donor to borondipyrromethene (Bodipy)

A linear molecular dyad was synthesised comprising a pyrene-thiophene energy donor linked via a triazole unit to a borondipyrromethene (Bodipy) energy acceptor. The donor to acceptor separation distance is around 20 Å. Emission from the donor originates from a mixed π-π* and partial charge-transfer state and overlaps favourably with the absorption profile for the acceptor. The level of spectral overlap is dependent on the solvent polarity. Rates for electronic energy transfer were measured by transient absorption spectroscopy in solvents of varying polarity and refractive index. The measured rates for Förster energy transfer (k(EET)) correlate fairly well with the calculated overlap integrals (J(F)). A sigmoidal relationship is observed between k(EET) and the solvent polarity function ΔF.

Two Different Charge Transfer States of Photoexcited 9,9′-Bianthryl in Polar and Nonpolar Solvents Characterized by Nanosecond Time-Resolved Near-IR Spectroscopy in the 4500−10 500 cm−1Region

Transient absorption spectra of 9,9'-bianthryl (BA) in heptane, in acetonitrile, and in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (bmimTf(2)N) are observed with a nanosecond time-resolved near-IR absorption spectrometer for the wavenumber range of 4500-10,500 cm(-1) (2200-950 nm). In nonpolar heptane solution, a broad absorption band is observed at 6700 cm(-1) (1500 nm), in addition to a strong absorption band of the locally excited (LE) state centered at 9800 cm(-1) (1020 nm). The broad band is assigned to a partial charge transfer (PCT) band. The decay time constants of the PCT band and the LE band are both (13 +/- 1) ns. The agreement of the two decay constants strongly suggests that the PCT state is in equilibrium with the LE state in heptane. In acetonitrile, an absorption band of the charge transfer (CT) state is observed at 8000 cm(-1) (1250 nm). This band decays in (41 +/- 2) ns. In bmimTf(2)N, the CT band appears at 8500 cm(-1) (1180 nm) and decays in (34 +/- 1) ns. The difference in peak position for the CT bands in acetonitrile and in bmimTf(2)N, and the PCT bands in heptane, is explained well by the model based on the charge resonance between the two equivalent electronic structures of the CT state.

Femtosecond time-resolved absorption anisotropy spectroscopy on 9,9′-bianthryl: Detection of partial intramolecular charge transfer in polar and nonpolar solvents

Femtosecond time-resolved absorption anisotropy spectroscopy by multichannel detection has been developed. The charge transfer (CT) character and dynamics of the UV-photoexcited 9,9(')-bianthryl (BA) in heptane, acetonitrile, and ethanol are revealed with this method. The transient absorption spectra are decomposed into two absorption components with different anisotropy values by the absorption anisotropy spectra. The decomposition results show two absorption bands having different anisotropy values or different directions of the transition dipole moment. One band that has the transition dipole perpendicular to the central C-C bond has almost an identical spectral shape with transient absorption of anthracene in the singlet excited state. This band is assigned to a transition in a locally excited anthracene ring. The other band is broad and structureless. This band is assigned to partial charge transfer (PCT) absorption because its transition dipole moment is parallel to the central C-C bond. Because the PCT band is observed in a nonpolar solvent heptane as well as in polar solvents, the PCT occurs in both nonpolar and polar solvents. The PCT band rises within the instrumental response, indicating that the PCT takes place immediately after the photoexcitation. In acetonitrile, the CT component shows a significant blueshift, indicating the formation of the stabilized CT state from the PCT state. In ethanol, the CT band does not show a spectral shift, suggesting that the stabilization is smaller than in acetonitrile. From these results, a new kinetic model on the intramolecular CT in BA is discussed.

Mixed valency in organic charge transfer complexes

Mixed-valence (partial charge transfer state) and segregated stacking are the key factors for constructing organic metals. Here, we discuss the ionicity phase diagrams for a variety of charge transfer systems to provide a strategy for the development of functional organic materials (Mott insulator, semiconductor, superconductor, metal, complex isomer, neutral-ionic system, etc.).

Transition State Spectroscopy of the Photoinduced Ca + CH3F Reaction. 2. Experimental and Ab Initio Studies of the Free Ca···FCH3 Complex

The Ca* + CH3F --> CaF + CH3 reaction was photoinduced in 1:1 Ca...CH3F complexes formed in a supersonic expansion. The transition state of the reaction was explored by monitoring the electronically excited product, CaF, while scanning the laser that turns on the reaction. Moreover, the electronic structure of the Ca...FCH3 system was studied using ab initio methods by associating a pseudopotential description of the [Ca2+] and [F7+] cores, a core polarization operator on calcium, an extensive Gaussian basis and a treatment of the electronic problem at the CCSD(T) (ground state) and RSPT2 (excited states) level. In this contribution we present experimental results for the free complex and a comparison with the results of a previous experiment where the Ca...CH3F complexes are deposited at the surface of large argon clusters. The ab initio calculations allowed an interpretation of the experimental data in terms of two reaction mechanisms, one involving a partial charge transfer state, the other involving the excitation of the C-F stretch in the CH3F moiety prior to charge transfer.

Host–Guest Interactions in the Supramolecular Incorporation of Fullerenes into Tailored Holes on Porphyrin‐Modified Gold Nanoparticles in Molecular Photovoltaics

Novel gold nanoparticles modified with a mixed self-assembled monolayer of porphyrin alkanethiol and short-chain alkanethiol were prepared (first step) to examine the size and shape effects of surface holes (host) on porphyrin-modified gold nanoparticles. The porphyrin-modified gold nanoparticles with a size of about 10 nm incorporated C60 molecules (guest) into the large, bucket-shaped holes, leading to the formation of a supramolecular complex of porphyrin-C60 composites (second step). Large composite clusters with a size of 200-400 nm were grown from the supramolecular complex of porphyrin-C60 composites in mixed solvents (third step) and deposited electrophoretically onto nanostructured SnO2 electrodes (fourth step). Differences in the porphyrin:C60 ratio were found to affect the structures and photoelectrochemical properties of the composite clusters in mixed solvents as well as on the SnO2 electrodes. The photoelectrochemical performance of a photoelectrochemical device consisting of SnO2 electrodes modified with the porphyrin-C60 composites was enhanced relative to a reference system with small, wedged-shaped surface holes on the gold nanoparticle. Time-resolved transient absorption spectroscopy with fluorescence lifetime measurements suggest the occurrence of ultrafast electron transfer from the porphyrin excited singlet states to C60 or the formation of a partial charge-transfer state in the composite clusters of supramolecular complexes formed between porphyrin and C60 leading to efficient photocurrent generation in the system. Elucidation of the relationship between host-guest interactions and photoelectrochemical function in the present system will provide valuable information on the design of molecular devices and machines including molecular photovoltaics.

Complex formation of ethylenedioxyethylenedithiotetrathiafulvalene (EDOEDT-TTF: EOET) and its self-assembling ability

36 kinds of donor(D)–acceptor(A) type charge transfer (CT) solids were prepared based on ethylenedioxyethylenedithiotetrathiafulvalene (EDOEDT-TTF or EOET), which is a hybrid molecule of bis(ethylenedioxy)-TTF (BEDO-TTF or BO) and bis(ethylenedithio)-TTF (BEDT-TTF or ET). A plot of the first CT absorption bands in solids against the difference in first redox potential between donor and acceptor molecules (ΔE) classified the complexes into five groups, A: highly conductive CT complexes with partial CT state and segregated stacks, B: partially ionic CT insulators with alternating stacks, C: essentially neutral clathrate complexes, D: neutral CT insulators with alternating stacks, and E: completely ionic insulators. Group A is spread out over an extensive ΔE range compared to one-dimensional organic metals such as the 1 ∶ 1 TTF–TCNQ system. This indicates that the EOET complexes among this group have a high electronic dimensionality arising from the self-aggregation of the EOET molecules in a fashion similar to the BO ones. However, the self-assembling ability of the EOET molecules is less pronounced than that of the BO ones, thereby giving a metal–insulator transition for many complexes among Group A. The reduced self-assembling ability also affords alternating stacks in (MeO)2TCNQ, BTDA-TCNQ (Group B) and p-iodanil (Group C) complexes. Two structurally distinct complexes were prepared with spherical C60 (Group D), one containing concave EOET molecules and one-dimensional C60 columns, and the other planar EOET and two-dimensional C60 layers. The F4TCNQ complex (Group E) exhibits an antiferromagnetic ordering below 7 K, which is intermediate between those in (BO)(F4TCNQ) (5.4 K) and (ET)(F4TCNQ) (14 K).

Preparation of metallic BEDT-TTF charge transfer complex of 3,3′,5,5′-tetranitro-4,4′-biphenyldiol dianion (TNBP2−) having flexible molecular shape

Two kinds of 3,3′,5,5′-tetranitro-4,4′-biphenyldiol dianion (TNBP2−) molecules having different dihedral angles (φ) between the two phenyl rings exist in the tetrabutylammonium salt of TNBP2− (φ = 0° and 35°), while only one kind of TNBP2− molecule exists in the tetraphenylphosphonium salt (φ = 0°). The 4∶1 cation radical salt of BEDT-TTF (ET) with TNBP2− asa counter anion is metallic down to 3 K. The ET molecules form a two-dimensional conducting layer which is sandwiched by the layers of flat TNBP2− molecules (φ = 0°). The crystal structure indicates the β″432-type stacking of tetramerized ET molecules which is caused by hydrogen bonds between the CH groups of ET and the oxygen atoms (nitro and phenol) of TNBP2−. The optical spectra in a KBr pellet are consistent with both the highly conductive and distorted structural nature. The magnetic susceptibility at room temperature is high (χ = 6.7 × 10−4 emu spin−1) compared with the conventional organic metal based on ET (χ = 3.4–5.5 × 10−4 emu spin−1) indicating strong electron correlation in the 4∶1 complex. The temperature dependence of the magnetic susceptibility suggests a semimetallic nature which is consistent with the calculated Fermi surface. The calculated band width is comparable to the estimated effective on-site Coulomb repulsive energy. A semiconductive behavior with high conductivity is observed in the nearly 2∶1 ET complex of TNBP2−. A partial charge transfer state of the ET molecules is supported by the optical spectra. The high conductivity and optical data suggest that the 2∶1 salt is not (ET1+)2(TNBP2−) but most plausibly (ET0.5+)2(HTNBP1−).

Structure and electronic properties of TTF bisannulated 24-crown-8 charge transfer complexes

Charge transfer (CT) complexes of tetrathiafulvalene bisannulated 24-crown-8 (1) with tetrafluoro (F 4)-, 2,5-dibromo (Br 2), 2,5-dichloro (Cl 2)-tetracyanoquinodimethane (TCNQ), and 2,3-dicyano-5,6-dichloro- p-benzoquinone (DDQ) were prepared. The electronic ground state of the acceptor varies from the completely ionic to partial charge transfer state. Within the CT complexes the donor 1 was completely oxidized to the divalent cationic state 1 2+. The molecular conformation of 1 2+ in Cl 2-TCNQ complex was an intra-dimer form folded over the flexible part of macrocyclic 24-crown-8.

Electrical and Nonlinear Optical Properties of Langmuir-Blodgett Films of Charge Transfer Complexes

A series of donors and acceptors are synthesized and, by combing these molecules, Langmuir-Blodgett (LB) films of the charge transfer (CT) complexes are prepared. Ionic CT complexes with partial CT state exhibit high conductivity in the form of LB films. The second nonlinear optical properties of the films are examined using electro-apsorption method.

Novel conductive charge-transfer complexes of 1,4,5,8-naphthalenetetrone (NTO) and 1,4,9,10-anthracenetetrone (ATO)

Abstract NTO, ATO, and each chlorinated derivative have been prepared. Some of the charge transfer complexes with TTF-type donors have been found to be electrically conductive. Most of the conductive solids have 1:1 composition, and the optical and structural studies support that a partial charge transfer state is achieved in the complexes.