Synthesis, Characterization, and Computational Investigation of Bright Orange‐Emitting Benzothiadiazole [10]Cycloparaphenylene

Abstract

Conjugated aromatic macrocycles are attractive due to their unique photophysical and optoelectronic properties. In particular, the cyclic radially oriented π‐system of cycloparaphenylenes (CPPs) gives rise to photophysical properties unlike any other small molecule or carbon nanomaterial. CPPs have tunable emission, possess large extinction coefficients, wide effective Stokes shifts, and high quantum yields. However, accessing bright CPPs with emissions beyond 500 nm remains difficult. Herein, we present a novel and bright orange‐emitting CPP‐based fluorophore showing a dramatic 105 nm red‐shift in emission and striking 237 nm effective Stokes shift while retaining a large quantum yield of 0.59. We postulate, and experimentally and theoretically support, that the quantum yield remains large due to the lack of intramolecular charge transfer.

Effect of Curvature and Placement of Donor and Acceptor Units in Cycloparaphenylenes: A Computational Study

“…25,[28][29][30] Therefore, we chose [10]CPP as the main scaffold for this study as it has previously shown a high tolerance to strain based reactivity. 20,[23][24][25] The optimized ground state geometry, absorbance, optimized excited state geometry, and fluorescence of each nanohoop was calculated. The trends observed in the fluorescence were rationalized using supporting evidence from StrainViz and NICS calculations.…”

“…For example, [10]CPTcaq exhibits intramolecular charge transfer (ICT), which is responsible for decreasing the quantum yield. 25 [10]CPTcaq is a donor-acceptor molecule where the HOMO and LUMO of the S1 are separated. While the fluorescence is dramatically red-shifted, the quantum yield decreases by 92% to 0.05.…”

“…2,1,3-benzothiadiazole (BTD) is a classic benzazole acceptor used in conjugated polymers and is tolerated in CPP synthesis. 25 Therefore, we explored different BTD derivatives (S1-S3, 1-4). Most were found to show small changes in fluorescence, but addition of strongly withdrawing cyano groups, 1, causes a 70 nm (1775 cm -1 ) red-shift (bathochromic shift) to 664 nm.…”

“…However, from a previous study these highly strained nanohoops are predicted to be unstable. 25 Therefore, to elicit a further red-shift in fluorescence without dramatically increasing the strain, other promising electron acceptors were inserted into a m[10]CPP scaffold. shift, which is within error of 4.…”

“…Indeed, red-emitting CPPs have been obtained with this approach, however, only three acceptor units have been tested to date. [23][24][25] Herein, we computationally investigate the effect of donor and acceptor moieties on CPP photophysical properties using density functional theory (DFT, TD-DFT). This work illuminates viable synthetic targets for red-emitting CPPs and demonstrates strain effects on common conjugated polymer monomers.…”