Oxidative C–H Homocoupling of Push–Pull Benzothiazoles: An Atom-Economical Route to Highly Emissive Quadrupolar Arylamine-Functionalized 2,2′-Bibenzothiazoles with Enhanced Two-Photon Absorption

Abstract

Copper(II)-catalyzed C-H/C-H coupling of dipolar 2-H-benzothiazoles end-capped with triphenylamine moieties affords highly fluorescent 2,2'-bibenzothiazoles with D-π-A-π-D architecture displaying large two-photon absorption (TPA) crosssections (5431252 GM) in the near-infrared region. The notably higher TPA performance as compared to quadrupolar π-systems with a widely-used 2,2'-bipyridine core, along with the ease of the synthesis and chelating N^N ability makes the title biheteroaryl platform an attractive building block for a large scope of functional dyes exploiting nonlinear optical phenomena.

Two‐Photon Excited Fluorescent 3,3’‐Diamino‐5,5’‐Diboryl‐2,2’‐Bithienyls Featuring a Quadrupolar Structure

“…Although the TPA property of BNMe 2 -BT corresponding to S 0 !S 3 excitation was not measured owing to the limitation of excitation laser wavelengths, it was expected the δ TPA of S 0 !S 3 transition would be much smaller than S 0 !S 2 transition since the involved molecular orbitals (HOMO-2 and LUMO) in S 0 !S 3 exhibit different symmetry. The TPA superiority of S 0 !S 2 over S 0 !S 3 is also supported by the estimation of δ TPA using the three-state model, [6] in which δ TPA of excitation from S 0 to the excited state S n , is proportional to j μ 01 j 2 j μ 1n j 2 /(E 1 À E n /2) 2 . The transition moment of S 1 !S 2 excitation (j μ 12 j = 6.54 D) is much larger than that of S 1 !S 3 (j μ 13 j = 1.08 D), the influence of which is Chemistry-A European Journal much more significant than the detuning energy (E 1 À E 2 /2 = 0.98 eV; E 1 À E 3 /2 = 0.88 eV).…”

“…Encouraged by the quadrupolar electronic structure of these two bithienyl derivatives, we were interested in their TPA properties since such chromophores are known to show large TPA cross sections. [6] The two-photon absorption spectra of BNMe 2 -BT and BNPh 2 -BT were measured via a two-photonexcited fluorescence method using coumarin 307 as the standard compound with femtosecond laser excitation at wavelengths of 730-860 nm (Figure 4b). It was found both of them exhibit high TPA cross sections.…”

“…In this context, a quest for the fluorophores exhibiting large δ TPA and high fluorescence quantum efficiency (Φ) is still an important topic. [4][5][6] In the continuous of our research on the triarylborane-based functional molecules, which have evoked remarkable research interest in material science and applications in the last few decades owing to their promising optoelectronic properties arising from the strong electron-accepting ability and great steric effect of boryl group, [7][8][9][10][11][12] we recently disclosed a new class of triarylborane, 2,2'-diamino-6,6'-diboryl-1,1'-binaphthyl, BNMe 2 -BNaph (Figure 1a). [13] Very different from the normal intramolecular charge-transfer (CT) emitting fluorophores with donor (D)-π-acceptor (A) structure, this compound actually consists of two D-π-A subunits, which are connected together through a single bond.…”

“…As a consequence, these two bithienyl derivatives behave more like a quadrupolar A-π-D-π-A system. Probably benefiting from this quadrupolar structure, which is beneficial to enhance TPA cross section, [6] they exhibit strong TPA with δ TPA up to 473 GM in THF, exceeding those of many one-photon fluorophores. [22] Although triarylboranes have well been studied for the TPEF, [23][24][25][26][27][28][29][30][31][32] these two bithienyl derivatives are the first examples that are characterized by amino-modified bithienyls as the electron-donor, which are expected to provide a new platform for TPEF fluorophores.…”

A new insight into the push‐pull effect of substituents via the stilbene‐like model compounds

“…Push‐pull effect of substituents is an important concept, which is often employed to improve the properties of organic compounds [1–24], such as UV absorption [4, 15], fluorescence emission [6, 16], intramolecular charge transfer [1, 3, 14], nonlinear optical properties [5, 13], photovoltaic performance [10], organic light‐emitting diode [19], and so on. If we input the word “push‐pull effect” into the Web of Science, we will obtain more than 100,000 items, which implies that the push‐pull effect of substituents is widely used in physics and chemistry.…”