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.
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