Solvent dielectric delimited nitro-nitrito photorearrangement in a perylenediimide derivative

Abstract

The discovery of vibrant excited-state dynamics and distinctive photochemistry has established nitrated polycyclic aromatic hydrocarbons as an exhilarating class of organic compounds. Herein, we report the atypical photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI), triggered by visible-light excitation and giving rise to linkage isomers in the polar aprotic solvent acetonitrile. ONO-PDI has been isolated and unambiguously characterized using standard spectroscopic, spectrometric, and elemental composition techniques. Although nitritoaromatic compounds are conventionally considered to be crucial intermediates in the photodissociation of nitroaromatics, experimental evidence for this has not been observed heretofore. Ultrafast transient absorption spectroscopy combined with computational investigations revealed the prominence of a conformationally relaxed singlet excited-state (S^CR ₁) of NO₂-PDI in the photoisomerization pathway. Theoretical transition state (TS) analysis indicated the presence of a six-membered cyclic TS, which is pivotal in connecting the S^CR ₁ state to the photoproduct state. This article addresses prevailing knowledge gaps in the field of organic linkage isomers and provides a comprehensive understanding of the unprecedented photoisomerization mechanism operating in the case of NO₂-PDI.

Fig. 1. Schematic representations of inorganic (left) and organic (right) linkage isomers.

Fig. 2. Core-level N (1s) XPS spectra for (a) NO₂-PDI and (b) ONO-PDI in the powder state.

Fig. 3. FTIR spectra of NO₂-PDI (top) and ONO-PDI (bottom) in KBr disks.

Fig. 4. Time-dependent laser-irradiated UV-vis absorption spectra of NO₂-PDI in (a) acetonitrile and (c) toluene. Time-dependent laser irradiated emission spectra (exc. at 532 nm) of NO₂-PDI in (b) acetonitrile and (d) toluene.

Fig. 5. (top) fsTA spectra (λ_ex = 532 nm) of NO₂-PDI in (a) TOL and (b) ACN showing the excited-state dynamics after photoexcitation at 532 nm. (middle) Evolution associated spectra reconstructed from global analysis of the time vs. wavelength based three-dimensional fsTA spectrograph. (bottom) Relative population profiles of the excited states fitted using kinetic models. (c) (top) nsTA spectra (λ_ex = 532 nm) of NO₂-PDI in TOL. (middle) Evolution associated spectra reconstructed from global analysis of the time vs. wavelength based three-dimensional nsTA spectrograph. (bottom) Relative population profile of the excited-state (B) fitted using kinetic models.

Fig. 6. (a) A pictorial illustration presenting a plausible kinetic mechanism explaining the excited-state dynamics and associated photochemistry of NO₂-PDI in the polar aprotic solvent acetonitrile and non-polar solvent toluene. Here, the S₀ state represents the ground state of NO₂-PDI and ONO-PDI, the S₁^FC state represents the Franck–Condon state of NO₂-PDI, and the S₁^CR(CT) state is a long-lived transient species observed in nsTA measurements, representing the conformationally relaxed singlet excited-state of NO₂-PDI having charge-transfer character. (b) Optimized geometries showing the transition from the S₁^FC state first to the S₁^CR(CT) state and finally to the six-membered TS through the nitro-aromatic torsion relaxation pathway computed at the CAM-B3LYP/6-311++G(d,p) level (the –R group was replaced by a –H atom to reduce the computational cost and increase clarity).