Electronic and steric control of meso-tetraaryl porphycene tautomerism via regioselective β-functionalization

Herein, we establish the first synthetic strategy for the direct β-C-H nitration of a free base meso-tetraaryl porphycene using HNO₃ in CHCl₃ in a selective and controlled manner. The reaction affords two regioisomers of the β-nitro porphycene (regioselectivity upto β : β' = 3 : 7) without the formation of any over-nitrated products or compromising the integrity of the macrocycle. Beyond synthetic access, β-functionalization enables precise electronic and steric modulation of the porphycene macrocycle, resulting in distinct optical properties, including enhanced Q-band absorption and red-shifted near-infrared (NIR) fluorescence extending up to 720 nm with good quantum yields. Moreover, comprehensive spectroscopy, X-ray crystallography, and computational studies revealed that the β-selective nitration shows a strong electronic bias on the intrinsically rapid N-H tautomerization, establishing a highly biased equilibrium that majorly favours a single tautomeric state at room temperature within the rectangular cavity of the porphycene. Furthermore, the reduction of the electron-deficient β-nitro porphycene to an electron-rich β-amino porphycene reverses the electronic perturbation and favours the opposite tautomeric form. This simple protocol provides electronically tuned β-functionalized porphycenes that not only serve as promising building blocks to design molecular systems for finely tuned NIR emissive optoelectronics, biological and material applications, but also act as model substrates to study the fundamentally important hydrogen transfer in porphycene macrocycles.