Design rules for tuning charge-transfer emission in donor-acceptor nanohoops

Donor-acceptor (D-A) nanohoops provide a unique platform for tuning emission in curved π-systems through modulation of the frontier molecular orbitals as well as through charge-transfer (CT) interactions. Herein, we report a modular, building-block synthetic strategy enabling a systematic structure-property study of D-A nanohoops incorporating the electron acceptor benzothiadiazole (BT) and the electron donor thiophene (thio). Independent variations of ring size, D-A connectivity, and donor incorporation afford predictable tuning of the fluorescent emission across the visible spectrum, large effective Stokes shifts, and pronounced solvatofluorochromism. Combined experimental and computational analysis establishes design rules for fluorescent emission, demonstrating that increased acceptor strain, direct D-A connectivity, and increased content of electron-modulating units lead to bathochromic shifts in emission, enabling access to highly red-shifted emission as exemplified by compound 7. StrainViz calculations reveal that five-membered donor units redistribute strain across the nanohoop scaffold, influencing molecular geometry, while the observed photophysical trends are primarily governed by D-A charge-transfer interactions. Together, these findings position D-A nanohoops as a predictive platform for engineering curved π-systems with a range of different photophysical properties.