A Switchable Near‐Infrared‐Absorbing Dye Based on Redox‐Bistable Benzitetraazaporphyrin

Abstract

Activatable near‐infrared (NIR) dyes responsive to external stimuli are used in medical and other applications. Here, we describe the design and synthesis of bench‐stable 18π‐ and 20π‐electron benzitetraazaporphyrins (BzTAPs) possessing redox‐switchable NIR properties. X‐Ray, NMR, and UV/Visible‐NIR analyses revealed that 20π‐electron BzTAP 1 exhibits NIR absorption and antiaromaticity with a paratropic ring‐current, while 18π‐electron BzTAP 2 shows weakly aromatic character with NIR inertness. Notably, the NIR‐silent BzTAP 2 was readily converted to the NIR‐active BzTAP 1 in the presence of mild reducing agents such as amine. The intense NIR absorption band of BzTAP 1 is in sharp contrast to the very weak absorption bands of previously reported antiaromatic porphyrinoids. Molecular orbital analysis revealed that symmetry‐lowering perturbation of the 20π‐electron porphyrinoid skeleton enables the HOMO–LUMO transition of 1 to be electric‐dipole‐allowed. BzTAPs are expected to be useful for constructing activatable NIR probes working in reductive environments.

20π‐Electron Antiaromatic Benziphthalocyanines with Absorption Reaching the Near‐Infrared‐II Region

“…The contour plots of NICS at 2.5 Å above the skeleton are shown in Figure 2b. [7,12] The values inside the BPc skeleton of 2' were positive, in contrast to the slightly negative values calculated for 1'. These computational results suggest a very weakly aromatic or nonaromatic character of 1 and a weakly antiaromatic character of 2, consistent with the NMR measurements.…”

“…This behavior is reminiscent of that reported for 18π-electron benzitetraazaporphyrin. [7] No detectable fluorescence was observed for 1. In contrast, 2 exhibited intense NIR absorption peaks at 861 and 782 nm, accompanied by a weak fluorescence peak at 890 nm (Figure S18).…”

“…The conversion of 1 to 2 using Hantzsch ester proceeded sluggishly with the formation of unidentified by-products, in contrast to the fast of the 18π-electron benzitetraazaporphyrin. [7] Instead, hydrogenation of 1 under a H 2 atmosphere with Pd/C afforded the desired reduced macrocycle 2 in 97 % yield. The expected molecular ion [M + H] + at m/z = 1534.9003 (calculated for C 102 H 116 N 7 O 6 m/z 1534.8982) was detected by ESI-TOF mass analysis.…”

“…This trend of aromaticity is similar to that reported for 18π-and 20π-electron benzitetraazaporphyrins. [7] Electronic absorption spectra of BPcs 1 and 2 were recorded in CH 2 Cl 2 (Figure 3a). 1 exhibited an absorption band at 356 nm and a broad band below 600 nm, likely attributable to its crossconjugated, 6π-aromatic/18π-nonaromatic properties.…”

“…[6g] Quite recently, a 20π-electron antiaromatic benzitetraazaporphyrin, a contracted analogue of BPc, was also synthesized. [7] This compound shows intense NIR absorption owing to the reduced structural symmetry, despite its 20π-antiaromaticity, and its absorption wavelength range is longer (650-800 nm) than that of regular 18π-electron tetraazaporphyrins (550-650 nm). [8] These previous studies led us to speculate that the absorption of 20π-electron antiaromatic BPc could reach the NIR-II region due to the expanded π-conjugation.…”