Ultrafast Exciton Self‐Trapping and Delocalization in Cycloparaphenylenes: The Role of Excited‐State Symmetry in Electron‐Vibrational Coupling

Abstract

Upon photon absorption, π‐conjugated organics are apt to undergo ultrafast structural reorganization via electron‐vibrational coupling during non‐adiabatic transitions. Ultrafast nuclear motions modulate local planarity and quinoid/benzenoid characters within conjugated backbones, which control primary events in the excited states, such as localization, energy transfer, and so on. Femtosecond broadband fluorescence upconversion measurements were conducted to investigate exciton self‐trapping and delocalization in cycloparaphenylenes as ultrafast structural reorganizations are achieved via excited‐state symmetry‐dependent electron‐vibrational coupling. By accessing two high‐lying excited states, one‐photon and two‐photon allowed states, a clear discrepancy in the initial time‐resolved fluorescence spectra and the temporal dynamics/spectral evolution of fluorescence spectra were monitored. Combined with quantum chemical calculations, a novel insight into the effect of the excited‐state symmetry on ultrafast structural reorganization and exciton self‐trapping in the emerging class of π‐conjugated materials is provided.

Triplet State Baird Aromaticity in Macrocycles: Scope, Limitations, and Complications

“…In contrast, with UM06-2X and UCAM-B3LYP these structures are 22–30 kcal/mol higher in energy. In the singlet excited states Kim, Nakano, Yamago, and co-workers recently revealed rapid interconversion between self-trapped exciton and delocalized states, 90 yet it has earlier been found through time-dependent CAM-B3LYP calculations that the S 1 state of [ n ]CPP ’s is more delocalized than the T 1 state. 25 Similar observations in the T 1 state of various [ n ]CPP ’s could give experimental support as to which functional gives the right description: UB3LYP or UCAM-B3LYP?…”

Photoexcited Energy Relaxation in a Zigzag Carbon Nanobelt

“…The main difference between carbon nanorings and nanobelts is the presence of single bonds in the former that connect benzene units allowing dihedral rotations between them . This peculiar structural aspect of nanorings introduces a competition between bending and torsional strains, disorder, and steric hindrances, − that affect the π-orbital overlaps between benzene units, thus limiting the conjugation length. In contrast, the fully fused edge-sharing arene rings of nanobelts provide strong structural rigidity, preventing significant deformations away from the original high-symmetry conformation .…”

Exciton-vibrational dynamics induces efficient self-trapping in a substituted nanoring

“…Since their initial synthesis, 1–3 they attracted the attention of the scientific community due to their unique structural properties that combine bending and torsional strains, disorder and steric hindrances. 4–6 These structural properties affect the π-orbital overlaps between phenylene units and conjugation lengths. After photoexcitation, [ n ]CPPs experience dynamical changes such as planarization of the chain, changes in their bond length alternation, exciton intramolecular spatial redistribution, and self trapping.…”

“…After photoexcitation, [ n ]CPPs experience dynamical changes such as planarization of the chain, changes in their bond length alternation, exciton intramolecular spatial redistribution, and self trapping. 5–7 These behaviours in turn impact their electronic and optical properties. 8–16 [ n ]CPPs are highly fluorescent compounds whose quantum yield significantly increases with n .…”