Vibronic exciton model for low bandgap donor-acceptor polymers.

A vibronic exciton model is introduced to describe the excited state band structure and associated absorption spectra of low bandgap donor-acceptor conjugated polymers. The Hamiltonian is represented in a diabatic basis consisting of Frenkel-like donor and acceptor fragment excitations as well as charge-transfer (CT) excitations between neighboring fragments. States are coupled to each other through electron and hole transfer as well as Coulombically, through interacting fragment transition dipole moments. Local vibronic coupling involving the prominent aromatic-quinoidal vibrational mode, which is responsible for pronounced vibronic progressions in most conjugated oligomers and polymers, is also included. The DAD repeat unit is shown to behave like a J-aggregate trimer, driven by both the sizable in-phase electron and hole transfer integrals between donor and acceptor fragments as well as negative Coulomb coupling between donor and acceptor fragment excitations. The J-aggregate behavior is enhanced in the polymer limit through inter-repeat unit coupling, with the 0-0 vibronic peak significantly enhanced in the lowest-energy near-IR band. In addition, the radiative rate is enhanced by the number of coherently connected repeat units. The near-IR band is shown to possess roughly equal admixtures of CT and Frenkel-like excitations. Applications are made to the polymer PffBT4T-2DT, with the simulated absorption spectrum quantitatively capturing the salient features of the measured spectrum.

Robust singlet fission in pentacene thin films with tuned charge transfer interactions.

Singlet fission, the spin-allowed photophysical process converting an excited singlet state into two triplet states, has attracted significant attention for device applications. Research so far has focused mainly on the understanding of singlet fission in pure materials, yet blends offer the promise of a controlled tuning of intermolecular interactions, impacting singlet fission efficiencies. Here we report a study of singlet fission in mixtures of pentacene with weakly interacting spacer molecules. Comparison of experimentally determined stationary optical properties and theoretical calculations indicates a reduction of charge-transfer interactions between pentacene molecules with increasing spacer molecule fraction. Theory predicts that the reduced interactions slow down singlet fission in these blends, but surprisingly we find that singlet fission occurs on a timescale comparable to that in pure crystalline pentacene. We explain the observed robustness of singlet fission in such mixed films by a mechanism of exciton diffusion to hot spots with closer intermolecular spacings.

Optical microcavities enhance the exciton coherence length and eliminate vibronic coupling in J-aggregates.

The properties of polaritons in J-aggregate microcavities are explored using a Hamiltonian which treats exciton-vibrational coupling and exciton-photon coupling on equal footing. When the cavity mode is resonant with the lowest-energy (0-0) transition in the J-aggregate, two polaritons are formed, the lowest-energy polariton (LP) and its higher-energy partner (P1), separated by the Rabi splitting. Strong coupling between the material and cavity modes leads to a decoupling of the exciton and vibrational degrees of freedom and an overall reduction of disorder within the LP. Such effects lead to an expanded material coherence length in the LP which leads to enhanced radiative decay rates. Additional spectral signatures include an amplification of the 0-0 peak coincident with a reduction in the 0-1 peak in the photoluminescence spectrum. It is also shown that the same cavity photon responsible for the LP/P1 splitting causes comparable splittings in the higher vibronic bands due to additional resonances between vibrationally excited states in the electronic ground state manifold and higher energy vibronic excitons.

Charge-transfer excitations steer the Davydov splitting and mediate singlet exciton fission in pentacene.

Quantum-chemical calculations are combined to a model Frenkel-Holstein Hamiltonian to assess the nature of the lowest electronic excitations in the pentacene crystal. We show that an admixture of charge-transfer excitations into the lowest singlet excited states form the origin of the Davydov splitting and mediate instantaneous singlet exciton fission by direct optical excitation of coherently coupled single and double exciton states, in agreement with recent experiments.

Interplay between intrachain and interchain interactions in semiconducting polymer assemblies: the HJ-aggregate model.

A new model for analyzing the photophysics of polymer aggregates is introduced taking into account exciton motion along a polymer chain and across polymer chains. Excitonic coupling and vibronic coupling are treated on equal footing using a Holstein-based Hamiltonian represented in a multi-particle basis set. In the HJ-aggregate model the competition between intrachain (through-bond) coupling leading to Wannier-Mott excitons, and interchain (through-space) coupling leading to Frenkel excitons, is studied in detail for two model dimers: one composed of red-phase polydiacetylene (PDA) chains and the other composed of regioregular P3HT chains. The resulting photophysical properties are shown to depend critically on the relative magnitudes of the intrachain and interchain exciton bandwidths. Dominant intraband (interband) coupling favors a photophysical response resembling J-aggregates (H-aggregates). In PDA dimers, where intrachain coupling prevails, the absorption spectrum is dominated by the 0-0 peak, as is characteristic of J-aggregates. The photoluminescence (PL) spectrum displays hybrid character: the ratio of the main (0-0) band to the first vibronic sideband intensities is initially zero at T = 0 K due to the forbidden nature of the 0-0 transition, but then increases with temperature in a manner characteristic of H-aggregates, peaking when kT ≈ ΔE, where ΔE is the interchain splitting. Further increases in temperature result in a decline of the PL ratio, as in a J-aggregate. This remarkable H to J transition is also predicted for the temperature dependence of the radiative decay rate, k(rad). The maximum (peak) rate scales as, k(rad) (max)∼(W(intra)/W(inter))(1/2), where W(intra) (W(inter)) is the intrachain (interchain) exciton bandwidth. Hence, when W(intra) is sufficiently larger than W(inter) the dimer displays thermally activated superradiance. In P3HT the intrachain coupling is far weaker than in PDA making the intrachain and interchain couplings comparable in the crystalline phase. Although the absorption spectral line shape is still well-accounted for by the conventional H-aggregate model, the photoluminescence is more sensitive, with H or J behavior tunable by changes in morphology. Long range intrachain order which coincides with weaker interchain interactions induces J-aggregate behavior, while short range intrachain order and the resulting stronger interchain coupling induces H-aggregate behavior. Our predictions neatly account for the H-like dominance exhibited by the PL from spin-cast films and the J-like dominance exhibited by the PL from highly ordered P3HT nanofibers self-assembled in toluene.

Absorption, circular dichroism, and photoluminescence in perylene diimide bichromophores: polarization-dependent H- and J-aggregate behavior.

Using a single-mode Holstein Hamiltonian with through-space excitonic couplings evaluated quantum mechanically, the absorption, circular dichroism, and photoluminescence spectral line shapes of a chiral perylene diimide dimer complex were accurately reproduced. In general, a dimer consisting of two chromophores related through a C(2) rotation is neither a J- nor an H-aggregate because oscillator strength is divided between the top and bottom of the exciton band. The division gives rise to the two Davydov components per vibronic band in the absorption spectrum. Nevertheless, it is shown that the vibronic structure of the absorption component polarized in the same direction as the lower (upper) Davydov component is identical to what one would obtain from an ideal J- (H-) aggregate. Emission generally contains both polarization components, but the component polarized in the same direction as the lower (upper) Davydov component behaves similarly to the emission from an ideal J- (H-) aggregate. The basic photophysical behavior also applies to molecular crystals containing two molecules per unit cell in which the interactions between inequivalent molecules dominate over interactions between equivalent molecules.

Vibronic coupling in quantum wires: applications to polydiacetylene.

A theory describing vibronic coupling in direct band gap, one-dimensional semiconductors is developed to account for the photophysical properties of isolated, defect-free conjugated polymers. A Holstein-like Hamiltonian represented in a multi-particle basis set is used to evaluate absorption and emission due to Wannier-Mott excitons. The photophysical properties of such quantum wires are shown to strongly resemble those of Frenkel exciton J-aggregates. The 1(1)B(u) exciton coherence length and effective mass are readily determined from the ratio of the 0-0 and 0-1 line strengths, I(0 - 0)/I(0 - 1), in the photoluminescence spectrum. I(0 - 0)/I(0 - 1) is shown to follow a T(-1/2) dependence, in an excellent agreement with experiments on the red-phase of polydiacteylene.

The nature of singlet excitons in oligoacene molecular crystals.

A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0-0) vibronic band of only -32 cm(-1), far smaller than the measured value of 631 cm(-1) and of the wrong sign-a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0-0 DS of 601 cm(-1) and a nearly quantitative reproduction of the relative spectral intensities of the 0-n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.

Multiple mode exciton-vibrational coupling in H-aggregates: synergistic enhancement of the quantum yield.

The impact of exciton-vibrational coupling involving fast and slow vibrational modes on absorption and emission in molecular H-aggregates is investigated using a multimode Holstein Hamiltonian. For H-aggregates composed of rigid molecules the radiative decay rate is rigorously zero due to the asymmetry of the lowest energy exciton. Increasing the Huang-Rhys factors of the coupled modes results in an increase of the radiative efficiency through increased sideband emission. Coupling to a spectrally unresolved slow mode leads to an apparent increase in the 0-0 intensity of the vibronic progression of the fast mode, thereby mimicking a distribution of molecular transition frequencies (diagonal disorder). In the intermediate coupling regime, the radiative efficiency of the aggregate exceeds the sum of the fast-mode-only and slow-mode-only radiative efficiencies. The mechanism underlying the synergistic enhancement of the radiative yield is similar to that which causes spectral splitting in the absorption spectrum of quaterthiophene crystals. [L. Silvestri et al., J. Chem. Phys. 130, 234701 (2009)]. The results here qualitatively account for the robust emission efficiency of hexaphenyl microcrystallites where the slow torsional motion involving the interphenyl dihedral angle in conjunction with the fast ring breathing mode conspire to enhance the quantum yield.

Reclassifying exciton-phonon coupling in molecular aggregates: evidence of strong nonadiabatic coupling in oligothiophene crystals.

Exciton-phonon (EP) coupling in molecular aggregates is reexamined in cases where extended intermolecular interactions result in low-energy excitons with high effective masses. The analysis is based on a single intramolecular vibrational mode with frequency omega0 and Huang-Rhys factor lambda2. When the curvature Jc at the exciton band bottom is much smaller than the free-exciton Davydov splitting W, the strength of the EP coupling is determined by comparing the nuclear relaxation energy lambda2omega0 with the curvature. In this way, weak (lambda2omega0<<4piJc), intermediate I (lambda2omega0 approximately 4piJc), and strong I (lambda2omega0>>4piJc) coupling regimes are introduced. The conventional intermediate (lambda2omega0 approximately W) and strong (lambda2omega0>>W) EP coupling regimes originally defined by Simpson and Peterson [J. Chem. Phys. 26, 588 (1957)] are based solely on the Davydov splitting and are referred to here as intermediate II and strong II regimes, respectively. Within the intermediate I and strong I regimes the near degeneracy of the low-energy excitons allows efficient nonadiabatic coupling, resulting in a spectral splitting between the b- and ac-polarized first replicas in the vibronic progression characterizing optical absorption. Such spectral signatures are clearly observed in OT4 thin films and crystals, where splittings for the lowest energy mode with omega0=161 cm(-1) are as large as 30 cm(-1) with a small variation due to sample disorder. Numerical calculations using a multiphonon BO basis set and a Hamiltonian including linear EP coupling yield excellent agreement with experiment.

Measurement of transition dipole moments in lithium dimers using electromagnetically induced transparency.

We have observed electromagnetically induced transparency in a Doppler broadened molecular cascade system using fluorescence detection. We demonstrate that the power-dependent splitting of lines in the upper-level fluorescence excitation spectrum can be used as a new spectroscopic tool for the measurement of molecular transition dipole moment functions.