High quantum yield NIR fluorophores are rare. The energy gap law dictates that while radiative rate constants (kR) increase proportional to the cube of the emission energy, nonradiative rate constants increase exponentially with decreasing S0-S1 energy gaps. Integrating benzothiadiazole (BTD) proquinoidal building blocks into the backbone of highly conjugated (porphinato)zinc(II) oligomers (PZnn) effectively minimizes the extent of excited-state structural relaxation relative to the ground-state conformation, defining neoteric classes of impressive fluorophores reaching 30% efficiency through the NIR. Herein I describe how varying the nature of proquinoidal linking motifs can be used not only to regulate kR, but that changes in solvent dielectric modulate the rate of intersystem crossing (kISC), providing a second handle to control the excited-state relaxation dynamics of NIR fluorophores. Not only is minimizing kISC important for augmenting fluorescence efficiency, but the ability to accelerate kISC would pave a way to the generation of high triplet exciton yields. Time-dependent density functional theory calculations show that modulation of kISC in these chromophores is driven by solvent-induced energetic modulation of the S1 state. We find that in BTD-conjugated PZn oligomers, the solvent-dependent electronic structural composition of higher lying triplet states correlates with changes in kISC. Using a combination of ultrafast transient absorbance spectroscopy and computational analysis, I illustrate our ability to control kISC in PZn-BTD systems that possess uniform S1-T1 energy gaps and spin-orbit coupling constants. This work highlights approaches to modulate the electronic structure of higher lying triplet states in highly conjugated oligomeric chromophores by specific integration of BTD units. We have shown that careful construction of proquinoidal-integrated supermolecular chromophores yields targeted, tunable control over excited-state relaxation processes and the magnitudes of their associated rate constants, thus laying the foundation for future development of exceptional NIR fluorophores which are not dependent on solvent polarity.

Unlocking structure-function relationships between proquinoidal integration motifs and excited-state relaxation dynamics in (porphinato)zinc(II) arrays

Fall 2022 abstracts, posters, and presentations will launch in mid-September.

ACS Fall 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.