Dual-mode photosensitizers: Iodinated thienyl-aza-BODIPYs with high singlet oxygen generation under one- and two-photon excitation

The synthesis, photophysical properties, two-photon absorption, and singlet oxygen ( 1 O 2 ) generation of three O-Hexyl thiophene substituted aza-BODIPYs are reported herein. The molecular scaffold was designed in an electronic configuration to promote optical activity in the red and near-infrared (NIR) region. These compounds ( AZB-4 , AZB-MY , and AZB-DY- thiophene ) exhibited maximum absorption in the range of 705–730 nm with a very narrow bandwidth (ω 1/2 –1100 cm −1 ) in various solvents, an emission centered around 750 nm; very high molar absorptivity with maximum value of 99 662 M −1 cm −1 was achieved for AZB-4 in toluene and somewhat reduced values for substituted derivatives. The 1 O 2 generation efficiency of these aza-BODIPYs was carefully evaluated by exploiting the heavy atom effect through iodine substitution at the core and peripheral thienyl rings. Single and double iodine substitution enhances both the intersystem crossing transitions S 1 → T 1 and the cross-section of two-photon absorption. A singlet oxygen quantum yield ( ϕ Δ ) as high as 61 % was achieved in the mono-iodinated AZB-MY derivative. As an example of the potential of these aza-BODIPYs as photosensitizers upon nonlinear absorption in the first near-infrared (NIR-I) window, we demonstrated efficient 1 O 2 generation under the action of two-photon excitation at 900 nm, resulting in up to 68 % degradation of the chemical traps within 240 s. Thus, 1 O 2 generation by these aza-BODIPYs can occur via two different photosensitive pathways: one-photon excitation using continuous-wave laser irradiation at 633 nm or two-photon excitation using a femtosecond pulsed laser at 900 nm. • Synthesized aza-BODIPYs with strong red/NIR absorption (705–730 nm) and high molar absorptivity. • Iodine incorporation enhances intersystem crossing and improves two-photon absorption. • Singlet oxygen generation under two-photon excitation at 900 nmhighlighting potential for dual-excitation photosensitizers.