Nicholas J. Hestand

Molecular Aggregate Photophysics beyond the Kasha Model: Novel Design Principles for Organic Materials

The transport and photophysical properties of organic molecular aggregates, films, and crystals continue to receive widespread attention, driven mainly by expanding commercial applications involving display and wearable technologies as well as the promise of efficient, large-area solar cells. The main blueprint for understanding how molecular packing impacts photophysical properties was drafted over five decades ago by Michael Kasha. Kasha showed that the Coulombic coupling between two molecules, as determined by the alignment of their transition dipoles, induces energetic shifts in the main absorption spectral peak and changes in the radiative decay rate when compared to uncoupled molecules. In H-aggregates, the transition dipole moments align side-by-side leading to a spectral blue-shift and suppressed radiative decay rate, while in J-aggregates, the transition dipole moments align head-to-tail leading to a spectral red-shift and an enhanced radiative decay rate. Although many examples of H- and J-aggregates have been discovered, there are also many unconventional aggregates, which are not understood within the confines of Kashas theory. Examples include nanopillars of 7,8,15,16-tetraazaterrylene, as well as several perylene-based dyes, which exhibit so-called H- to J-aggregate transformations. Such aggregates are typically characterized by significant wave function overlap between neighboring molecular orbitals as a result of small (∼4 Å) intermolecular distances, such as those found in rylene π-stacks and oligoacene herringbone lattices. Wave function overlap facilitates charge-transfer which creates an effective short-range exciton coupling that can also induce J- or H-aggregate behavior, depending on the sign. Unlike Coulomb coupling, short-range coupling is extremely sensitive to small (sub-Å) transverse displacements between neighboring chromophores. For perylene chromophores, the sign of the short-range coupling changes several times as two molecules are slipped from a side-by-side to head-to-tail configuration, in marked contrast to the sign of the Coulomb coupling, which changes only once. Such sensitivity allows J- to H-aggregate interconversions over distances several times smaller than those predicted by Kashas theory. Moreover, since the total coupling drives exciton transport and photophysical properties, interference between the short- and long-range (Coulomb) couplings, as manifest by their relative signs and magnitudes, gives rise to a host of new aggregate types, referred to as HH, HJ, JH, and JJ aggregates, with distinct photophysical properties. An extreme example is the null HJ-aggregate in which total destructive interference leads to absorption line shapes practically identical to uncoupled molecules. Moreover, the severely compromised exciton bandwidth effectively shuts down energy transport. Most importantly, the new aggregates types described herein can be exploited for electronic materials design. For example, the enhanced exciton bandwidth and weakly emissive properties of HH-aggregates make them ideal candidates for solar cell absorbers, while the enhanced charge mobility and strong emissive behavior of JJ-aggregates makes them excellent candidates for light-emitting diodes.

Two-dimensional spatial coherence of excitons in semicrystalline polymeric semiconductors: Effect of molecular weight

The electronic properties of macromolecular semiconductor thin films depend profoundly on their solid-state microstructure, which in turn is governed, among other things, by the processing conditions selected and the polymer chemical nature and molecular weight. Specifically, low-molecular-weight materials form crystalline domains of cofacially

The red-phase of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV): A disordered HJ-aggregate

pi

Interference between Coulombic and CT-mediated couplings in molecular aggregates: H- to J-aggregate transformation in perylene-based π-stacks

-stacked molecules, while the usually entangled nature of higher molecular-weight polymers leads to microstructures comprised of molecularly ordered crystallites interconnected by amorphous regions. Here, we examine the interplay between extended exciton states delocalized along the polymer backbones and across polymer chains within the

The Effect of Chain Bending on the Photophysical Properties of Conjugated Polymers

pi

Extended-Charge-Transfer Excitons in Crystalline Supramolecular Photocatalytic Scaffolds

-stack, depending on the structural development with molecular weight. We combine optical spectroscopies, thermal probes, and theoretical modeling, focusing on neat poly(3-hexylthiophene) (P3HT), one of the most extensively studied polymer semiconductors, of weight-average molecular weight of 3-450,kg/mol. The spatial coherence within the chain is significantly reduced (by nearly 30%). These observations give valuable structural information; they suggest that the macromolecules in aggregated regions of high-molecular-weight P3HT adopt a more planar conformation compared to low-molecular-weight materials. This results in the observed increase in intrachain exciton coherence. In contrast, shorter chains seem to lead to torsionally more disordered architectures. A rigorous, fundamental description of primary photoexcitations in

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

pi

Enhanced Davydov Splitting in Crystals of a Perylene Diimide Derivative

-conjugated polymers is hence developed: two-dimensional excitons are defined by the chain-length dependent molecular arrangement and interconnectivity of the conjugated macromolecules, leading to interplay between intramolecular and intermolecular spatial coherence.

The intermolecular charge transfer integral as an indicator of future success in organic photovoltaics(Conference Presentation)

The ratio of the 0-0 to 0-1 peak intensities in the photoluminescence (PL) spectrum of red-phase poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], better known as MEH-PPV, is significantly enhanced relative to the disordered blue-phase and is practically temperature independent in the range from T = 5 K to 180 K. The PL lifetime is similarly temperature independent. The measured trends are accounted for by modeling red-phase MEH-PPV as disordered π-stacks of elongated chains. Using the HJ-aggregate Hamiltonian expanded to include site disorder amongst electrons and holes, the absorption and PL spectra of cofacial MEH-PPV dimers are calculated. The PL 0-0/0-1 line strength ratio directly responds to the competition between intrachain interactions which promote J-aggregate-like behavior (enhanced PL ratio) and interchain interactions which promote H-aggregate-like behavior (attenuated PL ratio). In MEH-PPV aggregates, J-like behavior is favored by a relatively large intrachain exciton bandwidth--roughly an order of magnitude greater than the interchain bandwidth--and the presence of disorder. The latter is essential for allowing 0-0 emission at low temperatures, which is otherwise symmetry forbidden. For Gaussian disorder distributions consistent with the measured (inhomogeneous) line widths of the vibronic peaks in the absorption spectrum, calculations show that the 0-0 peak maintains its dominance over the 0-1 peak, with the PL ratio and radiative lifetime practically independent of temperature, in excellent agreement with experiment. Interestingly, interchain interactions lead only to about a 30% drop in the PL ratio, suggesting that the MEH-PPV π-stacks--and strongly disordered HJ-aggregates in general--can masquerade as single (elongated) chains. Our results may have important applications to other emissive conjugated polymers such as the β-phase of polyfluorenes.

Theory and assignment of intermolecular charge transfer states in squaraines and their impact on efficiency in bulk heterojunction solar cells (Presentation Recording)

The spectroscopic differences between J and H-aggregates are traditionally attributed to the spatial dependence of the Coulombic coupling, as originally proposed by Kasha. However, in tightly packed molecular aggregates wave functions on neighboring molecules overlap, leading to an additional charge transfer (CT) mediated exciton coupling with a vastly different spatial dependence. The latter is governed by the nodal patterns of the molecular LUMOs and HOMOs from which the electron (te) and hole (th) transfer integrals derive. The sign of the CT-mediated coupling depends on the sign of the product teth and is therefore highly sensitive to small (sub-Angstrom) transverse displacements or slips. Given that Coulombic and CT-mediated couplings exist simultaneously in tightly packed molecular systems, the interference between the two must be considered when defining J and H-aggregates. Generally, such π-stacked aggregates do not abide by the traditional classification scheme of Kasha: for example, even when the Coulomb coupling is strong the presence of a similarly strong but destructively interfering CT-mediated coupling results in null-aggregates which spectroscopically resemble uncoupled molecules. Based on a Frenkel/CT Holstein Hamiltonian that takes into account both sources of electronic coupling as well as intramolecular vibrations, vibronic spectral signatures are developed for integrated Frenkel/CT systems in both the perturbative and resonance regimes. In the perturbative regime, the sign of the lowest exciton band curvature, which rigorously defines J and H-aggregation, is directly tracked by the ratio of the first two vibronic peak intensities. Even in the resonance regime, the vibronic ratio remains a useful tool to evaluate the J or H nature of the system. The theory developed is applied to the reversible H to J-aggregate transformations recently observed in several perylene bisimide systems.