Theory of optical transitions in π-conjugated macrocycles

Conformational Heterogeneity in Large Macrocyclic Thiophenes

For conjugated macrocycles, conformational disorder plays a key role in determining whether the unique form of excitons that are fully delocalized over the cyclic framework (cyclic excitons) is formed by photoexcitation. We have investigated the ring size dependence of conformations and photophysical properties of macrocyclic thiophenes of varying ring sizes (C-5NT_NV) by using single-molecule fluorescence spectroscopy. We measured modulation depth, M, values and fluorescence intensities. As the ring size increases, the correlation plots of the two parameters show bimodal distributions, revealing that larger macrocycles exhibit extremely congested linear structures. The size dependence of structural changes in macrocyclic thiophenes have been clearly confirmed by molecular dynamics simulation. The number of torsional defects from simulated structures, in conjunction with survival times from fluorescence intensity trajectories and photon coincidence measurements, demonstrated the existence of multiple acyclic chromophores in the larger macrocycles from the ground state due to complete deformation of circular structures.

Anomalous Linear Dichroism in Bent Chromophores of π-conjugated Polymers: Departure from the Franck-Condon Principle

We examine the influence of bending of π-conjugated chromophores on photoluminescence (PL) by spectrally resolving the depolarization of fluorescence on the single-molecule level. The effect of excited-state mixing mediated by molecular vibrations is manifested in the departure from the usual achromatic linear dichroism of fluorescence, with the polarization anisotropy decreasing in the vibronic progression. Bent chromophores reveal an overall increase in vibronic PL intensity with polarization orthogonal to the molecular long axis. This manifestation of the Renner-Herzberg-Teller (RHT) effect illustrates the breakdown of the Franck-Condon principle in macromolecules used in organic electronics, providing information on the orientation of transition-dipole moments and the origin of spectral broadening. While some of the spectral signatures of the RHT effect appear similar to those of H aggregation in molecular dimers, discrimination between the two phenomena is straightforward since H aggregation does not induce anomalous linear dichroism.

Expanded Theory of H- and J-Molecular Aggregates: The Effects of Vibronic Coupling and Intermolecular Charge Transfer

The electronic excited states of molecular aggregates and their photophysical signatures have long fascinated spectroscopists and theoreticians alike since the advent of Frenkel exciton theory almost 90 years ago. The influence of molecular packing on basic optical probes like absorption and photoluminescence was originally worked out by Kasha for aggregates dominated by Coulombic intermolecular interactions, eventually leading to the classification of J- and H-aggregates. This review outlines advances made in understanding the relationship between aggregate structure and photophysics when vibronic coupling and intermolecular charge transfer are incorporated. An assortment of packing geometries is considered from the humble molecular dimer to more exotic structures including linear and bent aggregates, two-dimensional herringbone and "HJ" aggregates, and chiral aggregates. The interplay between long-range Coulomb coupling and short-range charge-transfer-mediated coupling strongly depends on the aggregate architecture leading to a wide array of photophysical behaviors.

Excited-state structural relaxation and exciton delocalization dynamics in linear and cyclic π-conjugated oligothiophenes

π-Conjugated oligothiophene is considered a chain segment of its polymeric counterpart with simper excited-state dynamics and spectral signatures.

Interference of Interchromophoric Energy-Transfer Pathways in π-Conjugated Macrocycles

The interchromophoric energy-transfer pathways between weakly coupled units in a π-conjugated phenylene-ethynylene macrocycle and its half-ring analogue have been investigated using the nonadiabatic excited-state molecular dynamics approach. To track the flow of electronic transition density between macrocycle units, we formulate a transition density flux analysis adapted from the statistical minimum flow method previously developed to investigate vibrational energy flow. Following photoexcitation, transition density is primarily delocalized on two chromophore units and the system undergoes ultrafast energy transfer, creating a localized excited state on a single unit. In the macrocycle, distinct chromophore units donate transition density to a single acceptor unit but do not interchange transition density among each other. We find that energy transfer in the macrocycle is slower than in the corresponding half ring because of the presence of multiple interfering energy-transfer pathways. Simulation results are validated by modeling the fluorescence anisotropy decay.