Cited by 711Photochemistry and Electron Transfer StudiesLuminescence and Fluorescent MaterialsSpectroscopy and Quantum Chemical Studies

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

Hestand, Nicholas J., Spano, Frank C.·2017·Accounts of Chemical Research

Abstract

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 Kasha's 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 Kasha's 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.

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Crossref · 10.1021/acs.accounts.6b00576

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Authors
Hestand, N. J., & Spano, F. C.
Affiliations
Temple University (US)
Corresponding author
Spano, Frank C.
Venue
Accounts of Chemical Research
Published
2017-02-01
Volume
50
Issue
2
Pages
341-350
ISSN
  • 0001-4842 (ISSN-L)
  • 1520-4898
OpenAlex
W2584403299
Cited by
711
References
51
Citations by year
2026: 94 · 2025: 87 · 2024: 89 · 2023: 69 · 2022: 78 · 2021: 71 · 2020: 82 · 2019: 60 · 2018: 55 · 2017: 26
FWCI
44.88
Type
journal article
Language
en
Open access
closed
Locations
  • Accounts of Chemical Research (publishedVersion)
  • PubMed (publishedVersion)
Funders
Division of Materials Research (DMR-1505437)
Topics
  • Physical Sciences › Chemistry › Physical and Theoretical Chemistry › Photochemistry and Electron Transfer Studies
  • Physical Sciences › Materials Science › Materials Chemistry › Luminescence and Fluorescent Materials
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Spectroscopy and Quantum Chemical Studies
Keywords
molecular aggregates, H-aggregates, J-aggregates, Coulomb coupling, perylene dyes, exciton transport, photophysical properties, molecular packing, organic solar cells

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