The effect of two-exciton states on the linear absorption of the third molecular level in linear molecular aggregates

The engineering and manipulation of delocalized molecular exciton states is a key component for artificial biomimetic light harvesting complexes as well as alternative circuitry platforms based on exciton propagation. Here we examine the consequences of strong electronic coupling in cyanine homodimers on DNA duplex scaffolds. The most closely spaced dyes, attached to positions directly across the double-helix from one another, exhibit pronounced Davydov splitting due to strong electronic coupling. We demonstrate that the DNA scaffold is sufficiently robust to support observation of the transition from the lowest energy (J-like) one-exciton state to the nonlocal two-exciton state, where each cyanine dye is in the excited state. This transition proceeds via sequential photon absorption and persists for the lifetime of the exciton, establishing this as a controlled method for creating two-exciton states. Our observations suggest that DNA-organized dye networks have potential as platforms for molecular logic gates and entangled photon emission based on delocalized two-exciton states.

In nondegenerate FWM (NDFWM) experiments employing spectrally narrower pulses, we can limit the FWM processes to specific ones. Thus, NDFWM measurements will give new insights on the excitonic dynamics. In our previous study using spectrally-resolved DFWM techniques on a self-organized quantum-well material, (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/, it has been shown that two exciton states, including biexciton and weakly interacting two-exciton state, play an important role in FWM processes. However, the signals arising from various two-exciton states overlapped spectrally with each other in DFWM experiments. In this study, we have employed NDFWM techniques on (C/sub 6/H/sub 13/NH/sub 3/)/sub 2/PbI/sub 4/ to observe the contributions of two-exciton states separately.

We theoretically analyze the optical response from an ultrathin film built up of oriented molecular aggregates, the operating states of which are represented by Frenkel exciton states. A four-level model, involving transitions between the ground, one-exciton and two-exciton states, exciton-exciton annihilation from the two-exciton state as well as relaxation from the annihilation level back to the one-exciton and ground states, is used for describing the film optical response. It is proved that the exciton-exciton annihilation may act not as a destructive but, on the contrary, as a constructive factor tending towards the occurrence of bistability. In particular, the effect of inhomogeneous broadening of the exciton optical transition, preventing the bistable behavior, may be suppressed considerably due to a fast exciton-exciton annihilation.

A four-level model, involving the ground, one-exciton and two-exciton states as well as a high-lying molecular term through which the two-exciton state annihilates, is used for analyzing the bistable optical response of a thin film built up of oriented molecular aggregates. We focus on the effects of inhomogeneous broadening of the exciton optical transitions and exciton-exciton annihilation on bistability of the thin film response. It turns out that the inhomogeneous broadening, preventing generally the occurrence of bistability, may be suppressed considerably due to a fast exciton-exciton annihilation.

Molecular Materials with Short Radiative Lifetime for High-Speed Light-Emitting Devices

Organic semiconducting materials containing C60 molecules are efficient acceptors for planar perovskite solar cells. In this work, we theoretically investigate the optical and excitonic properties of C60 linear molecular aggregates (composed of 1 to 7 C60 molecules) via the real-time-propagation rt-TDDFT technique. In the case of a single C60 molecule, the photoabsorption peaks are dominated by localized molecular excitons. We furthermore demonstrate that, in the case of linear molecular aggregates, the photoabsorption peaks are contributed by localized molecular excitons, charge transfer excitons, and Wannier-like delocalized excitons. This result is different to the accepted theory that only localized molecular excitons or charge transfer excitons can be produced in organic semiconducting materials. This work provides additional insights into the exciton formation in C60 molecular aggregates and may help in the rational design of efficient solar cells.

Two-exciton states with lattice relaxation in one-dimensional semiconductors

The paper considers the differential pump–probe spectra due to excitons in linear molecular aggregates taking into account simultaneously effects of both exciton–exciton interaction and higher molecular levels. The theoretical analysis, carried out in terms of the Green function technique, provides analytical expressions for the line shape of the pump–probe spectrum valid for an arbitrary number N of molecules forming the aggregate. Furthermore, the theory can accommodate any number of molecular states with higher energies. This includes, inter alia, the most common situation in which the higher lying states form a dense set of sublevels of electronic, vibrational, etc. origin. It has been demonstrated that incorporation of such higher molecular levels introduces widths to biexciton peaks formed below the two-exciton continuum. In addition, the indirect interaction between the excitons via the higher molecular levels can facilitate formation of a biexciton at lower than usual values of the direct exciton–exciton coupling γ, in extreme cases even for negative γ values characterizing repulsion rather than attraction between the excitons. On the other hand, in the region around the exciton band-edge, the differential spectrum can be described reasonably well in terms of the model of noninteracting excitons for a wide range of parameters of the system, subject to the replacement of an actual number of molecules per aggregate N by the effective one Neff. The latter Neff is shown to be influenced both by the direct coupling between the excitons and also by the indirect coupling via the higher molecular levels.

The authors report on the photoluminescence spectroscopy of a single GaSb∕GaAs type II quantum dot (QD) at 8K. A sharp exciton emission with a linewidth of less than 250μeV was observed. Two-exciton emission at the higher energy side of the exciton emission indicates that the two excitons in a type II QD do not form a bound biexciton. The energies of the exciton and two-exciton states were calculated using an atomic pseudopotential model, which provides a quantitative description of the antibound nature of the two-exciton state in type II QDs.

There are two types of two-photon transitions in molecular aggregates: non-local excitations of two monomers and local double excitations to some higher excited intra-monomer electronic state. As a consequence of the inter-monomer Coulomb interaction, these different excitation states are coupled to each other. Higher excited intra-monomer states are rather short-lived owing to the efficient internal conversion of electronic into vibrational energy. Combining both the processes leads to annihilation of an electronic excitation state, which is a major loss channel for establishing high excitation densities in molecular aggregates. Applying theoretical pulse optimization techniques to a Frenkel exciton model, it is shown that the dynamics of two-exciton states in linear aggregates (dimer to tetramer) can be influenced by ultrafast-shaped laser pulses. In particular, we studied the extent to which the decay of the two-exciton population by inter-band transitions can be transiently suppressed. Intra-band dynamics is described by a dissipative hierarchy equation approach, which takes into account strong exciton–vibrational coupling in the non-Markovian regime.

We generalize our recent work on the optical bistability of thin films of molecular aggregates [J. A. Klugkist et al., J. Chem. Phys. 127, 164705 (2007)] by accounting for the optical transitions from the one-exciton manifold to the two-exciton manifold as well as the exciton-exciton annihilation of the two-exciton states via a high-lying molecular vibronic term. We also include the relaxation from the vibronic level back to both the one-exciton manifold and the ground state. By selecting the dominant optical transitions between the ground state, the one-exciton manifold, and the two-exciton manifold, we reduce the problem to four levels, enabling us to describe the nonlinear optical response of the film. The one- and two-exciton states are obtained by diagonalizing a Frenkel Hamiltonian with an uncorrelated on-site (diagonal) disorder. The optical dynamics is described by means of the density matrix equations coupled to the electromagnetic field in the film. We show that the one- to two-exciton transitions followed by a fast exciton-exciton annihilation promote the occurrence of bistability and reduce the switching intensity. We provide estimates of pertinent parameters for actual materials and conclude that the effect can be realized.

Exciton dephasing and thermal line broadening in molecular aggregates

Recent results of a study of the absorption lineshape, superradiant emission, and resonant light scattering measurements on aggregates of PIC are discussed in light of numerical simulations on linear molecular aggregates with diagonal and off-diagonal disorder.

Exciton–exciton annihilation in linear molecular aggregates at low temperature

Switching resonance character within merocyanine stacks and its impact on excited-state dynamics