1-Nitro-7-Donor Functionalized Perylenediimides: Synthesis, Photophysical Properties, Redox Behavior, and Nanosecond...

A set of push-pull donor substituted nitro-perylenediimide (NO₂-PDI)-based derivatives NPDI 1-4 were designed and synthesized via nitration reaction of the mono-brominated-PDI (Br-PDI) followed by Pd(II)-catalyzed Sonogashira cross-coupling reaction in good yields. The nitro group was substituted at the bay position of the PDI-NO₂, and a variety of donors (phenothiazine (PTZ), carbazole(CBZ), and N,N-dimethylaniline(NND) were functionalized on the other bay side. The photophysical, electrochemical, spectroelectrochemical, computational calculations, and nanosecond transient absorption (ns-TA) studies were performed on the synthesized derivatives NPDI 1-4. The photophysical properties of NPDI 2-4 exhibit absorption in the NIR region due to strong push-pull interaction, resulting in stabilized LUMO energy levels to a greater extent. The electrochemical study reveals that the redox potentials of NPDI 1-4 derivatives shifted toward the anodic side due to the strong electron-withdrawing nature of the NO₂-PDI moiety. The spectroelectrochemical analysis revealed the absorption spectra shifted toward the bathochromic region during the reduction cycles, while a hypochromic shift occurred during the oxidation cycles. Nanosecond transient absorption spectral studies show that for the compounds containing strong electron donors (such as PTZ), either a short triplet state lifetime (τ_T = 0.47 µs for NPDI2) or no ISC was observed (NPDI4). For the compounds with moderate or no electron donor attached in the molecular structure, long triplet state lifetimes were observed (τ_T = 86.9 µs for NPDI1 and τ_T = 47.8 µs for NPDI3). For the compounds lacking ISC ability, an intermolecular triplet photosensitizing method, and long triplet state lifetimes were observed (τ_T = 46 µs for NPDI4).