Switching of a Single Boryl Center in π‐Conjugated Photochromic Polyboryl Compounds and Its Impact on Fluorescence Quenching

Abstract

Many materials consisting of a p-conjugated organic backbone attached to either a tetrahedral or trigonal-planar boron center display enhanced fluorescence and charge-transport properties. This phenomenon can be attributed to either chelation-enhanced p-conjugation of the organic backbone by a tetrahedral boron center [1][2] or to the strong electronaccepting ability of a trigonal-planar boron center.[3] Hence, boron-containing molecules have found applications in OLEDs, organic transistors, and nonlinear optical materials. [1][2][3] We recently reported an unusual photochromic switch involving tetrahedral N,C-chelating boron chromophores such as B(ppy)Mes 2 (ppy = 2-phenylpyridine, Mes = mesityl).[4] These compounds undergo a thermally reversible photoisomerization from a colorless or light colored state to a dark colored state upon irradiation with 350-450 nm light (Scheme 1). These compounds are highly fluorescent in the light-colored state with tunable emission colors, whereas the dark-colored isomers are nonemissive.We have recently shown that this photoisomerization can be suppressed when the system is conjugated to an olefinic bond, which can dissipate the excitation energy through trans-cis isomerization.[4b] While this conjugation has the benefit of stabilizing the system towards photodegradation and preserving its luminescence properties, it also renders the system photochromically inert. To elucidate the impact of incorporating multiple photochromic boron centers into these materials, we prepared a series of new p-conjugated molecules as shown in Scheme 2. Our study has shown that isomerization of one chromophore prevents isomerization of the others, leading to amplified and reversible fluorescence quenching of the entire molecule. The details of our study are presented herein.The synthesis of the monoboryl B1 was reported previously.[4b] Diboryl B2 and B2', triboryl B3, and hexaboryl B6 were synthesized from a common intermediate 1 (see the Supporting Information). B2' was prepared by Hay coupling, [5] and B2, B3, and B6 were prepared by Sonogashira coupling [6] with the appropriate aryl halides. The crystal structures of B2, B2', and B6 (Figure 1) [7] reveal several important features of these materials. B2' and B2 have good coplanarity between the two boron chromophores and dihedral angles of 08 between the two ppy rings for B2 ' and 23.98 between the ppy and the central phenyl ring for B2, which is consistent with strong p conjugation and electronic communication between the boron centers in these molecules. In contrast, the electronic communication between the boron centers in B6 appears to be much weaker as a result of the high degree of steric congestion, as evidenced by the large dihedral angles of 22.38, 59.58, and 82.88 between the ppy chelates and adjacent phenyl rings.These trends in electronic communication are manifested in the reduction potentials and absorption spectra of these materials. The polyboryl compounds except B3 all show two well-resolved reduction peaks, and the first redu...

Isomerization and rearrangement of boriranes: from chemical rarities to functional materials

“…The Φ PI vs. the number of the ppyBMes 2 moieties follows a reciprocal relationship, with decreasing chromophore content from P1 to P5 yielding higher isomerization efficiencies. This observation is explained according to a Förster resonance energy transfer (FRET) mechanism, where the excited states of the unreacted ppyBMes 2 units are quenched via inter-and intrachain energy transfer to the dark isomers within the polymer [55]. Due to the comparable color purity but higher Φ PI for polymers with less boron content, P5 was chosen for optical device applications.…”

Reversible Photothermal Isomerization of Carborane‐Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene‐Stabilized Carborane‐Fused Borirane

“…[3,4] They are usually Lewis-base-coordinated tetrahedral organoboranes. [3][4][5][6][7] A very recent example reported by the Wang group is illustrated in Scheme 1. [8] Such photo reactions serve as am ajor method for the preparation of Lewis-base-stabilized boriranes, [3][4][5][6][7][8][9] ac lass of three-membered rings much less studied than other heteroatom-containing analogues,s uch as epoxides and aziridines.…”

“…[3][4][5][6][7] A very recent example reported by the Wang group is illustrated in Scheme 1. [8] Such photo reactions serve as am ajor method for the preparation of Lewis-base-stabilized boriranes, [3][4][5][6][7][8][9] ac lass of three-membered rings much less studied than other heteroatom-containing analogues,s uch as epoxides and aziridines. [10] Other synthetic routes include the reductive deha-logenation or salt elimination of NHC-stabilized dihaloboranes (NHC = N-heterocyclic carbenes) reported by the Braunschweig group [11] and double hydroboration of alkynes with NHC-boranes demonstrated very recently by the Curran group.…”

Reversible Photothermal Isomerization of Carborane‐Fused Azaborole to Borirane: Synthesis and Reactivity of Carbene‐Stabilized Carborane‐Fused Borirane

“…Generally,t he three-coordinate trigonal-planar boron center has alow-lying empty po rbital that can either form p bonding interactions, which lead to conjugated systems,oraccept the electron pair of aLewis base to afford afour-coordinate tetrahedral boron. [3][4][5][6][7] A very recent example reported by the Wang group is illustrated in Scheme 1. [1,2] Recently,o rganoboranes have been reported to display very interesting photochromism, photochemical, and thermal structural transformations.…”

“…Such unique properties of organoboranes enable promising applications in optoelectronics and colorimetric chemosensors. [8] Such photo reactions serve as am ajor method for the preparation of Lewis-base-stabilized boriranes, [3][4][5][6][7][8][9] ac lass of three-membered rings much less studied than other heteroatom-containing analogues,s uch as epoxides and aziridines. [3,4] They are usually Lewis-base-coordinated tetrahedral organoboranes.…”