Chromophores in Molecular Nanorings: When Is a Ring a Ring?

Abstract

The topology of a conjugated molecule plays a significant role in controlling both the electronic properties and the conformational manifold that the molecule may explore. Fully π-conjugated molecular nanorings are of particular interest, as their lowest electronic transition may be strongly suppressed as a result of symmetry constraints. In contrast, the simple Kasha model predicts an enhancement in the radiative rate for corresponding linear oligomers. Here we investigate such effects in linear and cyclic conjugated molecules containing between 6 and 42 butadiyne-linked porphyrin units (corresponding to 600 C-C bonds) as pure monodisperse oligomers. We demonstrate that as the diameter of the nanorings increases beyond ∼10 nm, its electronic properties tend toward those of a similarly sized linear molecule as a result of excitation localization on a subsegment of the ring. However, significant differences persist in the nature of the emitting dipole polarization even beyond this limit, arising from variations in molecular curvature and conformation.

A Molecular Nanotube with Three‐Dimensional π‐Conjugation

“…The time‐dependent fluorescence anisotropies (γ) at these two wavelengths, derived from the data in Figure 5 a, show no change over 0–15 ps after excitation (Figure 5 b). The average anisotropy on excitation at 830 nm is 0.07, which is similar to that recorded for c‐ P6⋅T6 12b, 25 and close to the theoretical value of 0.1 for an excited state delocalized over a two‐dimensional ring 26. This indicates that in t‐ P12⋅(T6) 2 , absorption at 830 nm is associated with transition dipoles in the plane of the six‐porphyrin rings ( xy plane, Figure 3), and that emission occurs from an excited state that is delocalized over this same xy plane.…”

Excited‐State Dynamic Planarization of Cyclic Oligothiophenes in the Vicinity of a Ring‐to‐Linear Excitonic Behavioral Turning Point

“…[23,27,29,32] However, continued studies have collectively demonstrated that the common characteristics of ac yclic system can be lost depending on the size of the system. [35] In line with previous studies,o ur group recently published ac omparative study on the exciton delocalization process of linear and cyclic oligothiophenes. [33] In CPPs,A damska et al have shown that excitons localized to form distorted structures in large CPPs,w hereas uniform cyclic structures are maintained in small CPPs as ar esult of complete delocalization of excitons.…”

Excited‐State Dynamic Planarization of Cyclic Oligothiophenes in the Vicinity of a Ring‐to‐Linear Excitonic Behavioral Turning Point

“…[34] Parkinson et al studied the difference in the size of the absorbing and emitting states of porphyrin nanorings and demonstrated that exciton localization in large porphyrin nanorings results in aloss of ring-like behaviors. [35] In line with previous studies,o ur group recently published ac omparative study on the exciton delocalization process of linear and cyclic oligothiophenes. [36] By analyzing the evolution of transient fluorescence spectra, we suggested that the conformational disorder is significantly decreased by forming acyclic geometry.Inacyclic oligothiophene composed of ten thiophene units (C-10 T 2V ), ad ynamic planarization process allows complete delocalization of the exciton over the cyclic framework (herein termed formation of ac yclic exciton), whereas in larger rings (from C-15 T 3V to C-30 T 6V ), the same process fails to produce circular excitons.Aconsecutive study on C-10 T 2V by single-molecule spectroscopy has differentiated the spectral features originating from torsional defects and small torsional disorder,which led to the conclusion that the ensemble of C-10 T 2V contains an appreciable portion of excitons which cannot completely delocalize over the cyclic framework (herein termed acyclice xcitons).…”