High quantum yield NIR fluorophores are rare. The energy gap law dictates that while radiative rate constants (k_R) 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 (PZn_n) 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 k_R, but that changes in solvent dielectric modulate the rate of intersystem crossing (k_ISC), providing a second handle to control the excited-state relaxation dynamics of NIR fluorophores. Not only is minimizing k_ISC important for augmenting fluorescence efficiency, but the ability to accelerate k_ISC would pave a way to the generation of high triplet exciton yields. Time-dependent density functional theory calculations show that modulation of k_ISC 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 k_ISC. Using a combination of ultrafast transient absorbance spectroscopy and computational analysis, I illustrate our ability to control k_ISC 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.

Dr. Erin J. Peterson

Postdoctoral Research Associate, Massachusetts Institute of Technology

Appearances