Long memory effects in excitonic systems dynamics: Spectral relations and excitation transport.

The main quantity that controls excitation relaxation and transport in molecular systems is the environment-induced fluctuation correlation function. Commonly used models assume the exponentially decaying correlation function, characterized by a given characteristic time, which allows us to define the Markovian conditions and, hence, allows us to use rate equations for excitation dynamics. A long memory fractional correlation function is studied in this paper as an alternative model. Such a function has an infinite characteristic decay time, and thus, system decay to equilibrium becomes poorly defined. Consequently, it becomes impossible to define the Markovian regime. By assuming the weak system-bath coupling regime, we apply the non-Markovian equations of motion to describe the equilibration process in an excitonic molecular aggregate. The long memory model causes a weaker decay of coherent components in excitonic system relaxation dynamics. Nevertheless, the short time dynamics, which is important in optical spectroscopy, depends on the short time interval of the fluctuation correlation function. Excitation relaxation in this window appears to be well described by non-Markovian approaches.

Role of coherence and delocalization in photo-induced electron transfer at organic interfaces.

Photo-induced charge transfer at molecular heterojunctions has gained particular interest due to the development of organic solar cells (OSC) based on blends of electron donating and accepting materials. While charge transfer between donor and acceptor molecules can be described by Marcus theory, additional carrier delocalization and coherent propagation might play the dominant role. Here, we describe ultrafast charge separation at the interface of a conjugated polymer and an aggregate of the fullerene derivative PCBM using the stochastic Schrödinger equation (SSE) and reveal the complex time evolution of electron transfer, mediated by electronic coherence and delocalization. By fitting the model to ultrafast charge separation experiments, we estimate the extent of electron delocalization and establish the transition from coherent electron propagation to incoherent hopping. Our results indicate that even a relatively weak coupling between PCBM molecules is sufficient to facilitate electron delocalization and efficient charge separation at organic interfaces.

Modeling of ultrafast time-resolved fluorescence applied to a weakly coupled chromophore pair.

We present theory for calculating the third-order non-linear response function of a molecular aggregate in the weak inter-chromophore coupling regime. This approach is based on the perturbative expansion of the system evolution with respect to the resonance coupling, while the system-bath interaction is treated non-perturbatively by means of cumulant expansion. An explicit expression for the time-resolved fluorescence signal is then obtained. This allows us to investigate the ultrafast time-dependent Stokes shift, signatures of coherent dynamics, and the excitonic polaron formation in the excited state of the aggregate. Numerical simulations of the time-resolved fluorescence spectra of a pair of coupled molecules demonstrate these effects.

Visualizing charge separation in bulk heterojunction organic solar cells.

Solar cells based on conjugated polymer and fullerene blends have been developed as a low-cost alternative to silicon. For efficient solar cells, electron-hole pairs must separate into free mobile charges that can be extracted in high yield. We still lack good understanding of how, why and when carriers separate against the Coulomb attraction. Here we visualize the charge separation process in bulk heterojunction solar cells by directly measuring charge carrier drift in a polymer:fullerene blend with ultrafast time resolution. We show that initially only closely separated (<1 nm) charge pairs are created and they separate by several nanometres during the first several picoseconds. Charge pairs overcome Coulomb attraction and form free carriers on a subnanosecond time scale. Numerical simulations complementing the experimental data show that fast three-dimensional charge diffusion within an energetically disordered medium, increasing the entropy of the system, is sufficient to drive the charge separation process.

Excitation energy transfer and quenching in a heterodimer: applications to the carotenoid-phthalocyanine dyads.

The dynamics of a molecular heterodimer composed of a long-lived excitation donor and a short-lived acceptor (quencher) is examined. In order to consider various dynamical regimes without any restrictions on the system parameters, the energy transfer is modeled employing the hierarchical equations of motion, while the relaxation to the ground state is treated by assuming a phenomenological spontaneous nonradiative decay rate. Time scales of the resulting two-exponential evolution are investigated as functions of the energy gap and the resonance coupling between the monomeric constituents of the dimer. Relevance of the present analysis to the recent experimental findings on artificial carotenoid-phthalocyanine dyads is discussed. By examining the first two time scales of the reported time-resolved spectra, it is shown that upon the increase of carotenoid conjugation length its first excited state approaches the first excited state of phthalocyanine from above, thereby inducing a remarkable quenching. The proposed model also provides a unified treatment of quenching in the regimes previously distinguished as energy transfer and excitonic state formation.

Simulations of the two-dimensional electronic spectroscopy of the photosystem II reaction center.

We report simulations of the two-dimensional electronic spectroscopy of the Q(y) band of the D1-D2-Cyt b559 photosystem II reaction center at 77 K. We base the simulations on an existing Hamiltonian that was derived by simultaneous fitting to a wide range of linear spectroscopic measurements and described within modified Redfield theory. The model obtains reasonable agreement with most aspects of the two-dimensional spectra, including the overall peak shapes and excited state absorption features. It does not reproduce the rapid equilibration from high energy to low energy excitonic states evident by a strong cross-peak below the diagonal. We explore modifications to the model to incorporate new structural data and improve agreement with the two-dimensional spectra. We find that strengthening the system-bath coupling and lowering the degree of disorder significantly improves agreement with the cross-peak feature, while lessening agreement with the relative diagonal/antidiagonal width of the 2D spectra. We conclude that two-dimensional electronic spectroscopy provides a sensitive test of excitonic models of the photosystem II reaction center and discuss avenues for further refinement of such models.

Excitonic couplings and interband energy transfer in a double-wall molecular aggregate imaged by coherent two-dimensional electronic spectroscopy.

The early stage of molecular excitonics and its quantum-kinetic dynamics in the multiband, bitubular cyanine dye aggregate C(8)O(3) at room temperature are revealed by employing two-dimensional (2D) coherent electronic spectroscopy in the visible spectral region. The sub-20 fs measurements provide a direct look into the details of elementary electronic couplings by spreading spectroscopic transitions into two frequency axes. Correlation spectra of rephasing (k(I) = -k(1) + k(2) + k(3)) and nonrephasing (k(II) = +k(1) - k(2) + k(3)) data in emission (omega(3))-absorption (omega(1)) 2D-frequency space image interband excitons into cross-peak signals and unveil the quantum-dissipative regime of exciton relaxation. Spectral streaking of cross peaks directly reveals interband dephasing and exciton population relaxation on the road to tube-to-tube energy transfer without making recourse to an a priori model. Theory and simulations, based on an effective multilevel scheme and a quantum-dissipative model with experimental pulse envelopes, explain the origin of the cross peaks, reveal the underlying sequences of electronic transitions, recover the streaking patterns of relaxing cross peaks along omega(1), and reconstruct the space-energy pathways of electronic excitation flow.