Novel 2,1,3-Benzothiadiazole-Based Red-Fluorescent Dyes with Enhanced Two-Photon Absorption Cross-Sections

This paper reports the two‐photon absorbing and orange‐red fluorescence emitting properties of a series of new 2,1,3‐benzothiadiazole (BTD)‐based D–π–A–π–D‐type and star‐burst‐type fluorescent dyes. In the D–π–A–π–D‐type dyes 1–6, a central BTD core was connected with two terminal N,N‐disubstituted amino groups via various π‐conjugated spacers. The star‐burst‐type dyes 8 and 10 have a three‐branched structure composed of a central core (benzene core in 8 and triphenylamine core in 10) and three triphenylamine‐containing BTD branches. All the BTD‐based dyes displayed intense orange‐red color fluorescence in a region of 550–689 nm, which was obtained by single‐photon excitation with good fluorescent quantum yield up to 0.98 as well as by two‐photon excitation. Large two‐photon absorption (TPA) cross‐sections (110–800 GM) of these BTD dyes were evaluated by open aperture Z‐scan technique with a femtosecond Ti/sapphire laser. The TPA cross‐sections of D–π–A–π–D‐type dyes 2–6 with a benzene, thiophene, ethene, ethyne, and styrene moiety, respectively, as an additional π‐conjugated spacer are about 1.5–2.5 times larger than that of 1 c with only a benzene spacer. The TPA cross‐sections significantly increased in three‐branched star‐burst‐type BTDs 8 (780 GM) with a benzene core and 10 (800 GM) with a triphenylamine core, which are about 3–5 times larger than those of the corresponding one‐dimensional sub‐units 9 (170 GM) and 11 (230 GM), respectively. The ratios of σ/eπ between three‐branched and one‐dimensional dyes were 6.5:3.8 (for 8 and 9) and 6.0:4.0 (for 10 and 11), which are larger than those predicted simply on the basis of the chromophore number density (1:1), according to a cooperative enhancement of the two‐photon absorbing nature in the three‐branched system. Attractive red fluorescence is observed by near‐IR irradiation of two‐photon absorbing materials, which are composed of an electron‐accepting benzothiadiazole ring and electron‐donating amino groups. The two‐photon‐induced red emission presented can be applied to biological fluorescence imaging with versatile advantages, increase of penetration depth in tissue, and less photodamage.