Search

Exciton-phonon (EP) coupling in molecular crystals is investigated in the case where two intramolecular vibrational modes are involved and a theoretical model is presented which applies when one of the modes is strongly coupled to crystal excitons. The model is used to simulate the low energy portion of the absorption spectra of quaterthiophene (4T) single crystals, for which we find it appropriate to consider a low energy vibrational mode at 161 cm-1 and an effective strongly coupled high energy mode at 1470 cm-1. Our numerical results demonstrate that the high energy mode renormalizes the excitonic band, thereby strongly affecting the environment seen by the low energy mode and the overall EP coupling regime. Numerical simulations also confirm the existence of the new coupling regimes “intermediate-I” and “strong-I” already introduced for oligothiophene aggregates [Spano et al., J. Chem. Phys. 127, 184703 (2007)], which arise as a consequence of the large effective mass of low energy excitons in 4T crystals. Comparison with experimental high resolution absorption spectra is also reported and shown to support the model predictions. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) spectral line shape of regioregular poly(3-hexylthiophene) thin films is analyzed using a model which treats the polymer π-stacks as H-aggregates with exciton-vibrational coupling and spatially correlated site disorder. The Stokes shift, linewidth, and relative vibronic peak intensities in the low-temperature PL spectrum (T=10 K) are accurately reproduced, allowing the coherence function corresponding to the lowest energy (emitting) exciton to be determined from the ratio of the 0-0 to 0-1 peak intensities. The exciton migration length is determined from the N-dependent Stokes shift, where N is the number of segments comprising the stack. Based on the temperature dependence of the PL spectrum it is concluded that emission arises from a low concentration of aggregates which are more disordered than the dominant species responsible for absorption. The emissive aggregates are characterized by shorter average conjugation lengths and hence greater exciton bandwidths. The coherence length of the emitting exciton is estimated to be only three lattice spacings (≈1.1 nm) along the π-stacking direction. By contrast, the exciton migration length for incoherent hopping between coherent domains is estimated to be approximately 15 nm. [ABSTRACT FROM AUTHOR]

Circularly polarized emission from helical MOPV4 aggregates is studied theoretically based on a Hamiltonian including excitonic coupling, exciton phonon coupling, and site disorder. The latter is modeled via a Gaussian distribution of site energies. The frequency dependence of the circularly polarized luminescence dissymmetry glum(ω) contains structural information about the low-energy-neutral (excitonic) polaron from which emission originates. Near the 0-0 emission frequency, glum(ω) provides a measure of the exciton coherence length, while at lower energies, in the vicinity of the sideband frequencies, glum(ω) probes the polaron radius. The present work focuses on how the 0-0 dissymmetry, glum0-0, relates to the emitting exciton’s coherence function, from which the coherence length is deduced. In the strong disorder limit where the exciton is localized on a single chromophore, glum0-0 is zero. As disorder is reduced and the coherence function expands, |glum0-0| increases more rapidly than the sideband dissymmetries, resulting in a pronounced surge in glum(ω) near the 0-0 transition frequency. The resulting spectral shape of glum(ω) is in excellent agreement with recent experiments on MOPV4 aggregates. In the limit of very weak disorder, corresponding to the motional narrowing regime, the coherence function extends over the entire helix. In this region, glum0-0 undergoes a surprising sign reversal but only for helices which are between n+12 and n+1 complete turns (n=0,1,...). This unusual sign change is due to the dependence of the rotational line strength on long-range exciton coherences which are also responsible for a heightened sensitivity of glum(ω) to long-range excitonic coupling. [ABSTRACT FROM AUTHOR]

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton’s coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes. [ABSTRACT FROM AUTHOR]

Absorption and emission in polymer aggregates is studied theoretically, taking into account excitonic (intermolecular) coupling, exciton-phonon (EP) coupling, and disorder, all treated on equal footing within a generalized Holstein Hamiltonian with numerically generated eigenmodes and energies. The disorder is modeled as a Gaussian distribution of molecular transition frequency offsets of width σ and spatial correlation length l0. Both herringbone (HB) and lamellar aggregate morphologies are considered. The emission spectral line shape is shown to undergo marked changes in response to increasing disorder, with the intensity of the ac-polarized 0-0 emission peak generally increasing relative to the replica intensities (0-1,0-2,...) as σ increases and/or as l0 decreases. This is contrary to the behavior of the b-polarized component of the 0-0 intensity, which, in HB aggregates, decreases with increasing disorder. Comparisons are made to analogous trends in oligomer aggregates. Analytical results are obtained in the strong EP coupling regime appropriate for conjugated polymers while treating the disorder perturbatively. A method for uniquely determining σ and l0 from the emission and absorption spectra is presented. Applications are made to absorption and low-temperature emission in thin films of regioregular poly(3-hexylthiophene), with excellent agreement between theory and experiment obtained for a spatial correlation length of only 3–4 molecules. [ABSTRACT FROM AUTHOR]

A multimode Holstein Hamiltonian is used to describe optical excitations in quaterthiophene pinwheel aggregates. The Hamiltonian includes the coupling of excitons originating from the 1Ag→1Bu electronic transition to phonons originating from the five intramolecular vibrational modes known from oligothiophene solution absorption/emission spectroscopy. The resulting eigenstates with lowest energy are best described as hybrid polaron phonons. The polarons are formed by coupling excitons with the higher frequency (688, 1235, and 1551 cm-1) vibrational modes, while the (optical) phonons arise from the lower frequency (161 and 333 cm-1) modes. The polaron phonons are responsible for the fine structure defining the A1 band in the low-energy region of the absorption spectrum, ranging from the band origin to ≈1500 cm-1 beyond. The calculated A1 band of quaterthiophene aggregates agrees favorably with that observed from thin films. [ABSTRACT FROM AUTHOR]

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (glum) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and glum are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. glum for the 0–0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions |glum| decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, |glum| is almost constant throughout the series of vibronic transitions but for weak coupling |glum| becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates. [ABSTRACT FROM AUTHOR]

In this work, the effect of temperature, exciton bandwidth, and size on the photoluminescence spectra of defect-free two-dimensional herringbone aggregates of π-conjugated oligomers such as oligophenylene vinylene and oligothiophene is investigated theoretically. The model is based on exciton-phonon coupling in two-dimensional herringbone lattices with the exciton deriving from the lowest optical (1Ag→1Bu) transition and the phonon from the most strongly coupled intramolecular vibrational mode with frequency ω0. Simple analytical expressions are obtained for the line strengths of the emission origin (0-0) and first replica (0-1) as a function of the number of molecules comprising the aggregate, N, the free exciton bandwidth, WD, and the temperature, T. At a given temperature, the 0-0 emission intensity initially scales as N/Nth, where Nth is the superradiant threshold number, but eventually converges to NT/Nth, where NT is the size independent thermal coherence number. NT is inversely proportional to temperature and proportional to the exciton band curvature (ωc) near the band bottom; NT=1+4πωc/kbT. In striking contrast, the 0-1 line strength is relatively insensitive to temperature and size, but scales as the inverse square of WD+ω0. The insensitivity of the first replica to the exciton coherence number makes the ratio of the 0-0 to 0-1 line strengths a measure of the exciton coherence number. The ratio can be used to test for crystal purity. Comparison to experiments on thin films of quaterthiophene shows that the thermal coherence size is given by NT≈1+450/T (K) and that superradiance, which requires NT>Nth, can only be observed at temperatures less than 1 K. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photophysical properties of defect-free herringbone aggregates of π-conjugated oligomers are investigated theoretically using a two-particle basis set consisting of vibronic excitons and coupled vibronic-vibrational excitons. Incorporation of periodic boundary conditions allows the treatment of aggregates containing up to 1000 molecules. The vibrational distortion fields for the optically allowed excitons, including those responsible for the upper and lower Davydov components, are evaluated. The herringbone lattice supports both vibrationally dressed, heavy excitons as well as nearly free, light excitons. The former are responsible for the b-polarized absorption origin as well as two ac-polarized peaks slightly higher in energy. The strongly blueshifted main absorption peak is due to an exciton which travels with almost no nuclear distortion. The main absorption features are studied as a function of aggregate size and exciton bandwidth. The vibronic replicas in the aggregate emission spectrum are found to be strongly dependent on a destructive interference between one and two particle emissions. The primarily ac polarized replica intensities initially decrease with the number of molecules comprising the aggregate, N, converging to a nonzero value in the large N limit. By contrast, the b-polarized 0-0 line intensity increases linearly with N, eventually dominating the rest of the vibronic progression when N surpasses approximately 10. Beyond this size the aggregate radiative decay rate, γ[sub agg], scales linearly with N, eventually driving the quantum yield to unity when γ[sub agg] surpasses the nonradiative decay rate. The relative magnitude of the 0-0 emission line versus the rest of the progression generally increases with increasing excitonic interactions. The sum of the (dimensionless) replica intensities diminishes from 1- exp(-λ²) in the weak excitonic coupling regime to approximately zero in the strong coupling regime. By... [ABSTRACT FROM AUTHOR]

The impact of exciton-phonon coupling and defect states on the photophysical properties of p-distyrylbenzene nanoaggregates is studied numerically. Molecular packing within aggregates is based on the known crystal structures of poly-p-phenylene vinylene (Type I) and the five phenyl group oligomer (Type II). Calculations of absorption and emission are conducted using a reduced basis set consisting of all one- and two-particle vibronic states. The calculated spectra are very similar for both aggregate types, the only substantial difference being the polarization directions for the J-band and 0-0 emission line. Under the noninteracting domains approximation the calculated nanoaggregate absorption spectrum is in excellent agreement with experiment, assuming an exciton coherence length of approximately 20 Å. In the calculated emission spectrum the 0-0 emission is uniquely polarized compared with the rest of the vibronic progression, also in agreement with experiment. The 0-0 emission intensity in defect-free Type I and II aggregates is linearly proportional to the total number of molecules, becoming superradiant beyond a certain size threshold. The 0-0 emission is highly sensitive to stacking faults and dislocations. These defects account for the measured Stokes shift, but quench the 0-0 emission (and superradiance) while only slightly affecting the rest of the vibronic progression. Adding orientational point defects to an aggregate with stacking faults and/or dislocations enhances the 0-0 oscillator strength, bringing the 0-0 emission intensity into good agreement with experiment. [ABSTRACT FROM AUTHOR]

The effects of exciton-vibrational coupling and point defects on the absorption and emission of distyrylbenzene nanoaggregates are treated theoretically. Two aggregate types based on a two-dimensional array of cyclic tetramers (pinwheels) are considered: type A aggregates, composed of chiral pinwheels, and type B aggregates, composed of achiral pinwheels. The low-energy vibronic features in the experimental excitation spectrum arise from vibrationally dressed K=(0,0) excitons, while the more intense blue shifted H-band is due to nearly free K=(0,0) excitons. The K=(0,0) features are polarized primarily along the herringbone plane normal. The lowest Davydov component is polarized in the herringbone plane and is due to the lowest energy K=(π,π) exciton. This state is also responsible for the aggregate emission. The 0-v peaks for v>0 are mainly due to indirect transitions to the ground electronic state containing v phonons, with wave vector sum equal to (π,π). These peaks are largely independent of defect fraction and are polarized primarily along the herringbone plane normal. In stark contrast, the 0-0 emission critically depends on the concentration of point defects and is polarized entirely in the herringbone plane. This wavelength dependent emission polarization is in full agreement with experimental observations. Type A aggregates are weakly emissive, with the 0-0 emission peak vanishing in defect-free aggregates and increasing with defect concentration. The reverse holds for type B aggregates: the 0-0 intensity scales with the number of molecules in the aggregate and decreases with defect concentration. Sufficiently large type B aggregates are superradiant, and may be used to enhance the quantum yield in optical devices such as light-emitting diodes. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Vibronic excitation theory is used for evaluating the absorption and CD spectra of lutein and lutein diacetate aggregates, which are described as card packed H-aggregates and head-to-tail J-aggregates. The analytical expressions for the adsorption and CD spectra have highlighted the spectral signatures of weakly coupled H (and J)-aggregation arising from distortions in the single-molecule Franck-Condon progression.

The photoexcitations in helical aggregates of a functionalized, chiral oligophenylenevinylene (MOPV) are described, which have allowed for accurate modeling of the wavelength dependence of the circular polarization in fluorescence. It is shown that disorder and electron-vibration (EV) coupling have acted synergistically in localizing the emitting exciton to a single MOPV in the aggregate with 95% probability.

Examines the absorption and spontaneous emission in aggregates of conjugated polymers. Relationship between the radiative decay rate of exciton and the number of polymers; Effects of intermolecular interactions on exciton sizes and energies; Implication of J-aggregate formation for the absorption and emission of polymer aggregates.

Using a density matrix formalism we derive a general expression for the resonant response of an ensemble of molecular aggregates to two optical pulses. The aggregates, which can be of arbitrary geometry, are described by a Frenkel exciton Hamiltonian including exciton–exciton interactions. A hierarchy of resonant coherent transient signals is generated having wave vectors sk1+(1-s)k2, where ki is the wave vector of the ith pulse and s is an integer. General expressions for the differential pump–probe absorption spectrum D(ω;tD) (s=0) and the photon echo Iecho(tD) (s=-1) are presented for arbitrary pulse intensities. Applications are made to a linear chain of interacting Frenkel excitons with nearest neighbor exciton–exciton interaction A and exciton transfer J. Analytical expressions for D(ω;tD) and Iecho(tD) are obtained which are valid to third order in the aggregate-field interaction. The influence of the optically active red (blue) biexciton which detaches from the bottom (top) of the two-exciton band when A<-2|J| (A>=2|J|) is investigated. Biexcitons appear as extra peaks in D(ω;tD) and as oscillations in Iecho(tD). Finite size effects are responsible for quantum beat oscillations in D(ω;tD) and long time oscillations in the echo decay. Quantum beats arise predominantly from the difference between the lowest one-exciton frequencies. Our theory successfully describes the salient spectral features in the pump–probe spectra of J-aggregates. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We analyze the size and alternation dependence of the second static hyperpolarizability γ(0;δ,N) of N-site Hückel rings with transfer integrals t(1±δ). Regular (δ=0) rings lead to a simple γ(0;0,N) N5 dependence, while alternating (δ>=0) chains show a δ-6 divergence in the band limit. Analytical results for noninteracting electrons clarify the evolution from molecules to polymers, show strong enhancement of γ(0;δ,N) in the region Nδ≊1, and connect previous oligomer and polymer calculations. [ABSTRACT FROM AUTHOR]

An analytical expression for the third order hyperpolarizability of an infinite one-dimensional molecular aggregate is derived, and compared with the same quantity calculated under the local field approximation. It is found that the peak nonlinear absorption is greatly overestimated in this approximation by a factor of (|J|/Γ)1/2, where J is the near neighbor dipole–dipole coupling and Γ is the molecular excited state lifetime. [ABSTRACT FROM AUTHOR]

Using a reduced equation of motion for the density matrix which accounts for spontaneous emission and superradiance, we analyze the fluorescence and transient grating (TG) decays from a dilute, optically thin distribution of molecular aggregates. We find that the fluorescence is a limited form of superradiance, where cooperativity is restricted to the number N of two level systems which make up the aggregate. The dependence of the linear aggregate decay rate on the amount of inhomogeneous broadening, which is randomly distributed according to Gaussian line shape of width (1/e)2σ, is calculated. It is shown that σ must be comparable in magnitude to the nearest neighbor dipole–dipole coupling V and not the superradiant decay rate, in order to quench the superradiance. [ABSTRACT FROM AUTHOR]

We numerically investigate optical dephasing from as many as 500 interacting two-level absorbers by making the semiclassical approximation on the radiation–matter interaction which includes retarded dipole–dipole coupling and superradiant coupling. We calculate the superradiant decay rate following single pulse optical excitation, the photon echo decay and the fluorescence. By carrying out an expansion to the second order in time we demonstrate a fundamental difference between the homogeneous free-induction decay and photon echo decay which is due to radiation damping. We show that an echo sequence is capable of reversing superradiant damping in some cases. Specific computations for pentacene doped mixed molecular crystals, where ‘‘domains’’ of resonant impurities are likely to exist, are presented and compared with experimental results. [ABSTRACT FROM AUTHOR]

We show that a fundamental assumption used in analyzing photon echo decays from interacting molecules—the assumption that the ‘‘pure’’ dephasing rate can be obtained by subtracting one half of the fluorescence decay rate (2T1)-1 from the photon echo decay rate—is generally inconsistent with a fully quantum mechanical treatment. The density matrix evolution for a system of noninteracting dimers is calculated using the retarded electromagnetic coupling between two oscillating dipoles. We are able to properly include the effects of spontaneous emission and Dicke superradiance on the echo decay. [ABSTRACT FROM AUTHOR]

Studies the effects of intermacrocycle interactions on the second hyperpolaribilities of cofacial phthalocyanine dimers and trimers. Presentation of theoretical analysis based on the Frenkel exciton model for a chain of three level molecules; Identification of two mechanisms which lead to enhancements in the dimer and trimer value; Description of the first mechanism.

Studies spontaneous emission and absorption in an aggregate of pi-conjugated oligomers. Aggregate Hamiltonian; Aggregate-field interaction Hamiltonian; Evaluation of absorption spectra for aggregates; Radiative emission; Radiative decay in an isolated oligomer; Radiative decay in aggregates.

The properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into poly(3-hexylthiophene-2,5-diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X-ray data suggest that F4TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π-stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6-8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm2 V−1 s−1 in the most crystalline P3HT samples. Temperature-dependent conductivity measurements show that polaron mobility in SqP-doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature. [ABSTRACT FROM AUTHOR]