Intermolecular Charge-transfer States Research Articles

Solvation dynamics through Raman spectroscopy: Hydration of Br2 and Br3−, and solvation of Br2 in liquid bromine

Raman spectroscopy of bromine in the liquid phase and in water illustrates uncommon principles and yields insights regarding hydration. In liquid Br(2), resonant excitation over the B((3)Π(0u)(+)) ← X((1)Σ(g)(+)) valence transition at 532 nm produces a weak resonant Raman (RR) progression accompanied by a five-fold stronger non-resonant (NR) scattering. The latter is assigned to pre-resonance with the C-state, which in turn must be strongly mixed with inter-molecular charge transfer states. Despite the electronic resonance, RR of Br(2) in water is quenched. At 532 nm, the homogeneously broadened fundamental is observed, as in the NR case at 785 nm. The implications of the quenching of RR scattering are analyzed in a simple, semi-quantitative model, to conclude that the inertial evolution of the Raman packet in aqueous Br(2) occurs along multiple equivalent water-Br(2) coordinates. In distinct contrast with hydrophilic hydration in small clusters and hydrophobic hydration in clathrates, it is concluded that the hydration shell of bromine in water consists of dynamically equivalent fluxional water molecules. At 405 nm, the RR progression of Br(3)(-) is observed, accompanied by difference transitions between the breathing of the hydration shell and the symmetric stretch of the ion. The RR scattering process in this case can be regarded as the coherent photo-induced electron transfer to the solvent and its radiative back-transfer.

Modeling of photosynthetic light-harvesting: From structure to function

In order to bridge the gap between the crystal structure of photosynthetic pigment-protein complexes and the data gathered in optical experiments, two essential problems need to be solved. On one hand, theories of optical spectra and excitation energy transfer have to be developed that take into account the pigment-pigment (excitonic) and the pigment-protein (exciton-vibrational) coupling on an equal footing. On the other hand, the parameters entering these theories need to be calculated from the structural data. Good agreement between simulations and experimental data then allows to draw conclusions on structure-function relationships of these complexes and to make predictions. In the development of theory, a delicate question is how to describe the interplay between the quantum dynamics of excitons and the dephasing of coherences by the coupling of excitons to protein vibrations. Quantum mechanic coherences are utilized for efficient light harvesting. In the reaction centers of purple bacteria an energy sink is created by a coherent coupling of exciton states to intermolecular charge transfer states. The dephasing of coherences can be monitored, e.g., by the temperature dependent shift of optical lines. In the Fenna-Matthews-Olson protein, which acts as an excitation energy wire between the outer chlorosome antenna and the reaction center complex, an energy funnel for efficient light-harvesting is formed by the pigment-protein coupling. The protein shifts the local transition energies of the pigments, the so-called site energies in a specific way, such that pigments facing the reaction center are redshifted with respect to those on the chlorosome side. In the light-harvesting complex of higher plants an excitation energy funnel is created by the use of two different types of chlorophyll (Chl) pigments, Chla and Chlb and by the pigment-protein coupling that creates an energy sink at Chla 610 located in the stromal layer at the periphery of the complex. The close contact between Chla and Chlb gives rise to ultrafast subpicosecond exciton transfer, whereas dynamic localization effects are inferred to lead to long ps relaxation times between the majority of Chla pigments.

Molecular States of p-Dimethylaminobenzonitrile Coground with .BETA.-Cyclodextrin Investigated Using Solid-State Fluorescence Spectroscopy

Changes in molecular states of p-dimethylaminobenzonitrile (DMABN) coground with β-cyclodextrin (β-CD) were examined using solid-state fluorescence measurements. Formation of a DMABN/β-CD inclusion complex by coprecipitation was confirmed by powder X-ray diffraction measurement. The powder X-ray diffraction pattern of the ground mixture was a halo pattern and differed from the pattern of the mixture prepared by coprecipitation. Solid-state fluorescence measurements revealed emission by DMABN crystals in a twisted intermolecular charge-transfer state at 473 nm. DMABN in the DMABN/β-CD coprecipitate had a fluorescence emission peak at 393 nm due to its planar structure. In contrast, DMABN in a DMABN/β-CD ground mixture had an emission peak at 473 nm due to its twisted structure. Grinding time-dependent structural changes in DMABN were evaluated using fluorescence lifetime and relative quantum yield measurements. Structural changes in DMABN in the DMABN/β-CD coprecipitate from a planar to a twisted structure were observed with grinding. DMABN, dispersed in microcrystalline cellulose (CC) molecules in a DMABN/CC ground mixture, had a fluorescence emission peak at 473 nm. However, the excitation spectrum of a DMABN/β-CD ground mixture differed from that of DMABN in CC. These results indicated that the molecular state of DMABN accommodated in the β-CD cavity differs between the coprecipitate and the ground mixture.

Magneto-Optical Investigations on the Formation and Dissociation of Intermolecular Charge-Transfer Complexes at Donor−Acceptor Interfaces in Bulk-Heterojunction Organic Solar Cells

The electron-hole pairs can be formed in intermolecular charge-transfer (CT) states between two adjacent molecules due to Coulomb interaction in organic semiconducting materials. In general, the exciton dissociation can experience the intermediate states: intermolecular CT states at the donor-acceptor interfaces to generate a photocurrent in organic solar cells. This article reports the magneto-optical studies on intermolecular CT states in the generation of photocurrent by using magnetic field effects of photocurrent (MFE(PC)) and light-assisted dielectric response (LADR). The MFE(PC) and LADR studies reveal that internal electrical drifting and local Coulomb interaction can largely change the formation and dissociation of CT states by changing internal charge-transport channels and local Coulomb interaction through morphological development upon thermal annealing. Therefore, the MFE(PC) and LADR can be used as effective magneto-optical tools to investigate charge recombination, separation, and transport in organic solar cells.

Influence of an Intermolecular Charge-Transfer State on Excited-State Relaxation Dynamics: Solvent Effect on the Methylnaphthalene−Oxygen System and its Significance for Singlet Oxygen Production

The extent to which an intermolecular charge-transfer (CT) state can influence excited-state relaxation dynamics is examined for the system wherein 1-methylnaphthalene (MN) interacts with molecular oxygen. The MN-O2 system is ideally suited for such a study because excited states can be independently accessed by (i) irradiation into the discrete MN-O2 CT absorption band, (ii) direct irradiation of MN, and (iii) the photosensitized production of triplet state MN. Changing the solvent in which the MN-O2 system is dissolved influences the MN-dependent photoinduced production of singlet oxygen, O2(a1Delta(g)), which, in turn, yields information about fundamental concepts of state mixing. Results of experiments conducted in the polar solvent acetonitrile differ substantially from those obtained from the nonpolar solvent cyclohexane. The data reflect differences in the energy and behavior of the solvent-equilibrated MN-O2 CT state, CT(SE), and the extent to which this state couples to other states of the MN-O2 system. In particular, the data are consistent with a model where both the MN triplet state and the MN-O2 CT(SE) state are immediate precursors of O2(a1Delta(g)). Although the work reported herein is of direct and practical significance for the wide variety of systems in which O2(a1Delta(g)) can be produced upon irradiation, it also serves as an accessible model for a study of general issues pertinent to state mixing and the solvent-dependent dynamics of CT-mediated excited-state relaxation.

Deciphering the Influence of Short-Range Electronic Couplings on Optical Properties of Molecular Dimers: Application to “Special Pairs” in Photosynthesis

The excited states of chromophore dimers are, in general, delocalized, and the transition energies and transition dipoles are different from those of the monomers. The intermolecular interaction that is responsible for these effects has two contributions: Forster-type Coulomb coupling and a short-range coupling, which depends on the intermolecular overlap of electronic wave functions. The latter contains the Dexter-type exchange coupling and the coupling of excited states to intermolecular charge-transfer (CT) states. Recently, we developed a method (TrEsp) for an accurate and numerically efficient calculation of the Forster-type Coulomb part (Madjet et al. J. Phys. Chem. B 2006, 110, 17268). Here, we combine the latter with quantum chemical calculations to evaluate the short-range contribution, extending a method developed earlier by Scholes et al. (J. Phys. Chem. B 1999, 103, 2543). An effective two-state model is used, which relates the transition energies and transition dipole moments obtained by quantum chemical calculations of the monomers to those calculated for the dimer. From this relation, the short-range excitonic coupling and effective shifts of the local transition energies due to the coupling to intermolecular CT states can be inferred including a consistency check to evaluate quantum chemical methods that differ in the treatment of electron correlation. The method is applied to the special pairs of the reaction centers of purple bacteria (bRC) and photosystem I (PSI). We find that the short-range coupling represents the dominant contribution to the total excitonic coupling in both special pairs (80% in PSI and 70% in the bRC) and exhibits a monoexponential dependence on the distance between the pi-planes of the pigments with an attenuation factor of 2.8 A(-1). We obtain significant red-shifts of the local transition energies, which show a biexponential distance dependence with one attenuation factor being 2.8 A(-1) and another factor being in the range 0.3-0.7 A(-1) for PSI and 0.8-0.9 A(-1) for bRC. Both effects of the short-range coupling determine the excitation energy sink in the reaction centers at the special pairs.

Charge-Transfer Controlled Exchange Interaction in Radical-Triplet Encounter Pairs as Studied by FT-EPR Spectroscopy

The exchange interaction, J, producing quartet and doublet energy separation in radical-triplet excited molecule encounter pairs, was investigated in solution by measuring chemically induced dynamic electron polarization (CIDEP) created through the radical-triplet pair mechanism. A time-resolved FT-EPR method was utilized to measure CIDEP of galvinoxyl radical by recording FID signals and an absolute magnitude of CIDEP, P(n), was determined for each radical-triplet system by detailed analysis of the time evolution curves of CIDEP. A transient FT-EPR signal phase remarkably depends on the triplet molecule. The signal phase is related to the sign of J value, which is responsible for the radical-triplet pair interaction. Most of galvinoxyl-triplet systems showed normal negative sign. An unusual positive sign was found in some systems characterized by a small energy gap, DeltaG, between the radical-triplet pair and intermolecular charge transfer (CT) states. A theoretical calculation of J value for radical-triplet encounter pairs was carried out by considering exchange integral and intermolecular CT interaction. According to the calculated J value and the diffusion theory for CIDEP magnitude, experimental Pn values were theoretically reproduced as a function of DeltaG. The present results confirm our previously reported CT model explaining the complicated nature of the sign of J value in the galvinoxyl-triplet encounter pairs. According to the proposed model for CT effect on J value and CIDEP results, nature of J value in radical-triplet pairs is discussed.

Electronic structures of intermolecular charge-transfer states in fast electron transfers with tetrathiafulvalene donor. Thermal and photoactivation of [2 + 4] cycloaddition to o-chloranil acceptor

Tetrathiafulvalene (TTF) spontaneously forms a series of unusual charge-transfer complexes with various quinonoid acceptors such as o-chloranil (CA) that show pronounced near-IR absorption (lambda(CT) = 1100 nm). The successful isolation of the corresponding [1 : 1] donor-acceptor complex from solution and X-ray crystallographic analysis at low temperatures reveal the polarized charge-transfer state: [TTF(q+),CA(q-)] with high degree of charge-transfer (q = 0.6), which is spectrally and crystallographically distinguished from the separate redox (ion-pair) state: [TTF(+*) + CA(-*) ]. The unique interconversion of charge-transfer and electron-transfer states is theoretically well-accommodated by Mulliken theory using semi-empirical valence-bond and molecular-orbital methodologies. Mechanistic implications are discussed of both the thermally activated and the photochemically promoted processes via fast (intracomplex) electron transfer followed by collapse of the adiabatic and the non-adiabatic (vibrationally-excited) ion-pairs, respectively, to the [2 + 4] cycloadduct of tetrathiafulvalene and o-chloranil.

Control of Emission by Intermolecular Fluorescence Resonance Energy Transfer and Intermolecular Charge Transfer

Control of emission by intermolecular fluorescence resonant energy transfer (IFRET) and intermolecular charge transfer (ICT) is investigated with the quantum-chemistry method using two-dimensional (2D) and three-dimensional (3D) real space analysis methods. The work is based on the experiment of tunable emission from doped 1,3,5-triphenyl-2-pyrazoline (TPP) organic nanoparticles (Peng, A. D.; et al. Adv. Mater. 2005, 17, 2070). First, the excited-state properties of the molecules, which are studied (TPP and DCM) in that experiment, are investigated theoretically. The results of the 2D site representation reveal the electron-hole coherence and delocalization size on the excitation. The results of 3D cube representation analysis reveal the orientation and strength of the transition dipole moments and intramolecular or intermolecular charge transfer. Second, the photochemical quenching mechanism via IFRET is studied (here "resonance" means that the absorption spectrum of TPP overlaps with the fluorescence emission spectrum of DCM in the doping system) by comparing the orbital energies of the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of DCM and TPP in absorption and fluorescence. Third, for the DCM-TPP complex, the nonphotochemical quenching mechanism via ICT is investigated. The theoretical results show that the energetically lowest ICT state corresponds to a pure HOMO-LUMO transition, where the densities of the HOMO and LUMO are strictly located on the DCM and TPP moieties, respectively. Thus, the lowest ICT state corresponds to an excitation of an electron from the HOMO of DCM to the LUMO of TPP.

Enhanced intersystem crossings of S 1–T 1 and T 1–S 0 in coronene– and pyrene–galvinoxyl systems as studied by a pulsed ESR method

Radical enhanced intersystem crossing (EISC) of S 1–T 1 and T 1–S 0 in excited molecule–radical systems was investigated by observing chemically induced dynamic electron polarization of radicals with a time-resolved (TR) ESR method. Time profiles of CIDEP were observed in excited coronene–radical systems, where the radical was 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO), 1,1-diphenyl-2-picrylhydrazyl (DPPH), 1,3,5-triphenylverdazyl (TPV) or galvinoxyl (Galv). In two systems containing TEMPO and PTIO radicals, signals due to the quenching of coronene in both S 1 and T 1 states were observed, while in other three systems containing DPPH, TPV, and Galv radicals only the signals induced by triplet coronene–radical pair interaction could be observed. To make the quantitative analysis on CIDEP created in S 1- and T 1-quenching processes, a pulsed ESR method was applied for a coronene–Galv system chosen as a model system. The absolute magnitude of CIDEP of Galv radical created by S 1- and T 1-quenching processes ( | P n QP | and | P n DP | , respectively) were determined to be 7 P eq and 0.8 P eq, respectively, where P eq is the equilibrium Boltzmann polarization at room temperature. According to the CIDEP theory, the large | P n QP / P n DP | ratio of ca. 9 indicates that the S 1–T 1 EISC occurs through a long distance interaction which is mediated by intermolecular charge transfer state, while | P n QP | of 7 P eq indicates T 1–S 0 enhanced ISC is promoted by exchange interaction at the nearly contact coronene–Galv pair. Another system of pyrene–Galv pair was also discussed to confirm the proposed interaction model for the enhanced ISC.

Intermolecular charge and energy transfer in neurosporene and chlorophyll a derivative complex

Dye-sensitized solar cells using a chlorophyll a derivative (PPB a der.) as the sensitizer and carotenoids having different conjugation lengths as redox spacers was studied experimentally. In this Letter, the excited state properties of the neurosporene & PPB a der. complex are investigated theoretically with quantum chemistry method. The 2D real space analysis with the transition density matrix reveals that some excited states are locally excited states, and some is the intermolecular charge transfer (ICT) state. The 3D real space analysis reveals: (1) that the orientation and the strength of the dipole moment from transition density, and (2) the orientation and result of ICT in Neurosporene & PPB a der. complex from charge difference density.

Electroabsorption study of vacuum-evaporated films of Pt(II)octaethylporphyrin

Abstract Electric-field modulated absorption spectra in the range 270–580 nm have been studied in solid films of Pt(II)octaethylporphyrin (PtOEP) formed by vapour deposition onto room-temperature quartz substrates. The electroabsorption (EA) spectra were measured at first (1ω) and second (2ω) harmonic of fundamental frequency of modulating field (ω) applied to the sample in a sandwich cell arrangement with aluminum electrodes, Al/PtOEP/Al/quartz. The (2ω)EA spectra are interpreted in terms of the standard Stark spectroscopy analysis using a direct comparison of the (2ω)EA spectrum with first and second absorption derivatives. Both main absorption porphyrin bands (B and Q) give the first absorption derivative-like EA signals from molecular (Frenkel) excitons responding to an electric field with the similar values of an average polarizability change, Δ p ¯ ≅ 20 ± 5 A 3 / f 2 (f = 1.7 in the Lorentz field approximation). This result has been rationalized employing Gouterman’s “four orbital model”. A weakly resolved structure observed around 440 nm suggests a contribution of an intermolecular charge transfer state. The presence of an internal electric field, less than 105 V/cm, has been deduced from the relation between (1ω)EA and (2ω)EA spectra.

Intra‐ and intermolecular charge‐transfer phenomena in a triphenylamine‐containing anthrylenevinylene‐based copolymer and its model compound

AbstractOptical properties of a triphenylamine‐containing anthrylenevinylene‐based copolymer and its model compound were evaluated using absorption and steady‐state fluorescence spectroscopy. The polymer and model compound show a strong solvatochromic shift as a function of solvent polarity, indicating the emission of an additional charge‐transfer species. More direct and convincing evidences of the charge transfer, such as temperature and concentration‐dependent emission, are presented. Low temperature (77 K) fluorescence also confirmed the existence of charge transfer. Additionally, a twisted intramolecular charge transfer state was observed in the model compound when excited by higher energy, which could not be observed in the polymer whatever the excitation was. The results indicate that there are more charge transfer states including intramolecular, intermolecular and interchain charge transfer states in polymers than in the model compound. Copyright © 2003 John Wiley & Sons, Ltd.

Time resolved ESR study on energy difference of quartet and doublet states in radical-triplet encounter pairs

The energy difference J of quartet and doublet states in a radical-triplet pair (RTP) has been studied in pairs of triplet naphthalene (Np) derivative/galvinoxyl(Galv) and triplet biphenyl (Biph) derivative/Galv systems by time resolved ESR spectroscopy. Most of the systems studied show an unusual positive sign for J that corresponds to the relative location of the quartet state being below the doublet state. In order to explain this unusual sign of J in RTPs, the effects of intermolecular charge transfer (CT) states were examined. The observation was explained by introducing configuration interaction between zero-order RTP (RTP0) and CT (CT0) states in the encounter pair, where the RTP0 state is located above the CT0 state. Among the RTP systems examined, triplet 1-chloro-Np/Galv, triplet 1-bromo-Np/Galv and triplet 4-cyano-Biph/Galv systems show a normal negative value of J. On the basis of the estimated energy gap between the RTP0 and CT0 states, it was concluded that configuration interaction is not significant in these three RTP systems. The value of J in these RTP systems is dominated by exchange interaction, giving a negative sign. The solvent effect on the sign of J in a triplet Np/Galv system is also reported, as evidence that the sign of J is controlled by the CT0 state.

Electronic excitations and first hyperpolarizability of 2‐methyl‐4‐nitroaniline clusters

AbstractThe crystal packing effects upon the static and dynamic first hyperpolarizability of 2‐methyl‐4‐nitroaniline (MNA) clusters have been addressed at the TDHF/AM1 level. Due to electrostatic interactions, in 1‐D arrays of MNA unit cells extending along the a crystallographic axis, the effective hyperpolarizability strongly increases whereas stacking unit cells along the b or c axes leads to a reduction of these quantities. These effects are magnified in the case of frequency‐dependent quantities. The first hyperpolarizability variations can mostly be related to bathochromatic and hypsochromatic shifts as well as to changes in transition dipole moments of the low‐energy excited states that dominate the UV/visible absorption spectra. From calculations performed at the CIS/INDO/S level of approximation, these dominant excited states present an important intramolecular charge transfer character and bathochromatic (hypsochromatic) shifts are associated with increase (decrease) of the transition dipole moments. In addition, a large number of intermolecular charge‐transfer excited states have been detected. In the small MNA aggregates as well as in clusters extending only along the b or c crystallographic axes, these excited states possess small transition dipole moments. On the other hand, when stacking MNA unit cells along the a axis the electrostatic interactions induce a decrease of the excitation energies and an increase of the transition dipole moments of these intermolecular charge transfer states. As a consequence, they account for a nonnegligible contribution to the first hyperpolarizability. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Charge-transfer mediated photochemistry in alkene–O2 complexes

The photochemistry of a series of alkene–O2 complexes was studied in a supersonic expansion using a resonance enhanced multiphoton ionization probe of the O(3Pj) photoproduct at 226 nm. The relative yield of oxygen atoms from each complex was correlated to the ionization potential of the alkene species and indicates that initial excitation of an intermolecular charge-transfer state mediates the subsequent excited state chemistry. The behavior is similar to that observed previously for the C6H6–I2 system: a reverse electron-transfer step yields electronically excited O2 which subsequently dissociates. The kinetic energy release of the O(3Pj) fragment was also measured using a time-of-flight analysis and found to be small with an isotropic spatial distribution. No evidence for photo-oxidation of the alkenes was observed in the mass spectra. A comparison is made to the charge-transfer absorption spectra observed in cryogenic oxygen matrices of similar alkene complexes. Ab initio models were used to identify the stable ground state geometry of the C2H4–O2 complex and complete active space self-consistent-field calculations were performed to identify the energy of the charge-transfer state for several alkene–O2 complexes.

Excitation Dynamics in Solutions, Films and Crystals of Indandione-1,3 Pyridinium Betaine

The excitation decay kinetics of polar Indandione-1,3 Pyridinium Betaine (IPB) molecules in various environments: solvents, films and crystals, were studied using time-resolved luminescence and transient absorption measurements. A red-shift, depolarization and fast decay of the luminescence intensity were observed on a femtosecond time scale in dilute solutions and explained by excitation relaxation from the initially created intramolecular charge transfer state to the lower energy excited state of the indandione moiety. The first fast relaxation stage is followed by a slower one responsible for the relaxation to the ground state stage with the rate depending on the viscosity of the surrounding. By comparing the excitation properties of IPB in solution and those in the crystal form, the influence of the intermolecular interactions on the excited state dynamics is revealed. Excitation relaxation in the IPB deposited films of different thickness and in a single crystal is described as a competition between different relaxation paths of nonradiative recombination, these paths being the high lying Frenkel exciton states and the low lying intermolecular charge transfer states.

Photooxidation of Toluene and p-Xylene in Cation-Exchanged Zeolites X, Y, ZSM-5, and Beta: The Role of Zeolite Physicochemical Properties in Product Yield and Selectivity

The photooxidation of toluene and p-xylene with molecular oxygen and visible light has been investigated in several cation-exchanged zeolites. In general, the yield of the photooxidation products, for fixed irradiation time and intensity, was found to correlate with the electric field intensity at the cation sites within the zeolites. On the basis of measurements of CO vibrational frequencies, electric fields of approximately 3−7 V nm-1 are indicated for the cation-exchanged zeolites X, Y, ZSM-5, and Beta used in these studies. These large electric fields are thought to promote photooxidation by stabilizing an intermolecular charge transfer state (R+·O2-) that is formed upon excitation with visible light. The measured electric field was found to correlate with the product yield and was highest in divalent cation-exchanged zeolites with high Si/Al ratios, such as BaZSM-5 and BaBeta. For zeolites containing the same cation, the selectivity of toluene to form benzaldehyde and p-xylene to form p-tolualdehyde ...

Practical formulation of accurate many-body potentials through the perturbative extension of diatomics-in-ionic-systems: Applied to HF clusters

A perturbative extension of the diatomics-in-ionic-systems (DIIS) is formulated as a practical method for describing global many-body potential energy surfaces with accuracy and economy. The method is applied to HF clusters, generalizing the prior accurate DIIS treatment of the dimer [Grigorenko, Nemukhin, and Apkarian, J. Chem. Phys. 108, 4413 (1998)] to arbitrary numbers of HF molecules. The calculated geometries, energetics, and harmonic frequencies of (HF)n, n=2–6 clusters agree in detail with the available data on this well-studied system. The formulation is based on treating intermolecular interactions within perturbation theory. It is shown that second-order perturbation, which includes bimolecular excitations, is necessary and sufficient in describing the many-body potential energy surfaces with spectroscopic accuracy. The approach allows the analysis of H-bonding and its nonadditive induction and dispersion forces in terms of mixings and exchange between ground- and excited states of dimers including intra- and intermolecular charge-transfer states as well as molecular triplet states. The speed of evaluation of the potential scales is the cube of the number of molecules, providing a practical method for dynamical simulations of extended hydrogen-bonded networks.

Ground and excited state dipole moments of coumarin laser dyes: Investigation by electro-optical absorption and emission methods

Modified electro-optical absorption and emission methods were used to measure the dipole moments of four efficient coumarin laser dyes (CU1, CU4, CU120, CU334) in the equilibrated ground, excited Franck-Condon and equilibrated excited states. The measurements were performed in cyclohexane (CU1, CU120, CU334) and dioxane (all CUs) at room temperature. Our results show that the charge distribution in CU4 differs substantially from that in the other CUs. The equilibrated ground and excited state dipole moments measured by electro-optical methods are compared with those derived from other measurement techniques and from semiempirical calculations. The possible role of twisted intermolecular charge transfer (TICT) state formation is discussed.