Modification of [5]Helicene with Dimesitylboryl: One Way To Enhance the Fluorescence Efficiency

Abstract

The efficient synthetic route was disclosed to prepare structurally asymmetric [5]helicenes, which are substituted with either BMes 2 (7B-HC) or both BMes 2 and NMe 2 (8B5NMe 2 -HC, 7B5NMe 2 -HC). Compared with the parent [5]helicene, these compounds show greatly enhanced fluorescence. In addition, they still retain fairly strong fluorescence in the solid state. Moreover, the complexation of 8B5NMe 2 -HC and 7B5NMe 2 -HC with fluoride can induce significant blue shift in fluorescence and the formed complexes are also highly fluorescent.

Triarylborane‐Based Helical Donor–Acceptor Compounds: Synthesis, Photophysical, and Electronic Properties

“…Compounds 1 and 2 exhibit high quantum yields of up to unity in the solid state which are much highert han those of the recently reported compounds 8B5NMe 2 -HC and 7B5NMe 2 -HC. [22] This is presumably due to CÀH···C interactions between the helical backbone, the mesityl rings,a nd the tolyl rings that restrict molecular motion. Upon excitation with UV light, the interplanar angle of the [5]helixc ore of 2 is contracted by approximately 78 (S 1 state comparedw ithS 0 state).…”

“…[25] The absorption spectrum of 2 has two bands in the range of 400-450 nm in hexane and THF (410, 435 nm;4 10, 433 nm, respectively), but the second band (432 nm) was nearly invisible in acetonitrile. The absorption maximao f8B5NMe 2 -HC and 7B5NMe 2 -HC reported by Zhao and co-authors [22] are redshifted compared with 2,a nd corresponding the extinction coefficients (2800-2300 m À1 cm À1 ) are only aq uarter that of 2.T he l abs of 1 and 2 are hardly affected by the solventp olarities.F rom hexane to acetonitrile, a blue shift of only about 300 cm À1 was observed, indicatingt he absence of al arge dipole momentint he ground state. In contrast, the emission maximum strongly depends on the solventp olarity.…”

“…While our manuscript was in preparation, Zhao and co-workers reported the [5]helicene-based D-p-A triarylborane compounds 8B5NMe 2 -HC and 7B5NMe 2 -HC ( Figure 1). [22] In Zhao's work, aM e 2 Ng roup was introduced at the 5-or 10-positions as electron donor,a nd a( Mes) 2 Bg roup was attached at the 7position( Figure 2). For our [5]helicene-based compound 2, (Mes) 2 Ba nd (p-Tol) 2 Ng roups reside at the 3a nd 12 positions.…”

“…Herein, nonplanar aromatic rigid helicenes as π‐bridges of D –π–A triarylborane compounds are disclosed. While our manuscript was in preparation, Zhao and co‐workers reported the [5]helicene‐based D –π–A triarylborane compounds 8B5NMe 2 ‐HC and 7B5NMe 2 ‐HC (Figure ) . In Zhao's work, a Me 2 N group was introduced at the 5‐ or 10‐positions as electron donor, and a (Mes) 2 B group was attached at the 7‐position (Figure ).…”

Photochemical Functionalization of Helicenes

“…[14][15][16] Regardless of the widespread applications, helicenes are still synthesized following conventional methods using pre-functionalized starting materials, [11,[17][18][19] which makes the further derivatization to finetune the properties of the target compounds at a later stage difficult. Therefore, the late-stage introduction of functional groups onto the helicene scaffold is highly desirable, which, however, remains challenging owing to the lack of regioselectivity, [20][21][22] and only a limited number of methods for the synthesis of functionalized helicenes on a larger scale.…”