Photochemical Transformations Involving Organoboron

Abstract

Boron gets its name from the term borax, the mineral from which it was originally isolated, by analogy with carbon due to their chemical similarities. Unlike its neighbor on the periodic table, however, boron only possesses enough valence electrons to form three covalent bonds. This means that organoboron compounds are inherently electron deficient and typically adopt a trigonal‐planar (sp 2 ‐hybridized) geometry with the empty p z orbital of boron lying orthogonal to the bonding plane. In this form, boron is isoelectronic and isostructural with a positively charged carbocation, which gives rise to many of the applications often associated with trivalent organoboranes (e.g., organic synthons and electron‐transport materials). Coordinatively saturating these systems with anionic or neutral Lewis bases yields tetrahedral, four‐coordinated boron compounds with a formal negative charge. This new bonding arrangement significantly alters the orbital composition of the resulting molecules, imparting new features such as intense charge transfer (CT) luminescence and, in some cases, unique photochemical reactivities such as photochromism or photoelimination. This chapter will document recent progress in photochemical transformation based on organoboron compounds. A brief overview of concepts related to photophysical and photochemical processes will be presented, followed by an introduction to photochromic systems and a comprehensive review of organoboron photochemistry. Several recently discovered classes of photoresponsive organoboron molecules and their potential applications will be presented.

Highly Emissive 9‐Borafluorene Derivatives: Synthesis, Photophysical Properties and Device Fabrication

“…Three-coordinate boron-containing materials have attracted considerable attention over the last several decades due to the conjugation between the vacant po rbital on boron and the p electrons of conjugated systems.T his conjugationl eads to desirable optical and electronic properties that, in turn, enable applications in optoelectronics and sensing materials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] For example, severalg roupsh ave shown that three-coordinate boron,a sa ne lectron-acceptor,c ombined with electron-donor groups, giving D-A systems, can exhibit thermally activated delayed fluorescence (TADF), which is favourable for organic light-emitting diode (OLED) devices. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] Amongt hree-coordinate boranes,b orolesa re unique because of their 4p-electron five-membered-ring structure, which presents antiaromatic character,according to Hückel's rule.…”

Millisecond Time-scale Photoluminescence of B–N-doped Tetrathienonaphthalene with Borane/Amine Substituents

“…Three-coordinated boron-containing materials have attracted considerable attention over the last several decades due to the conjugation between the vacant p orbital on boron and π electrons of conjugated systems. This conjugation leads to desirable optical and electronic properties that, in turn, enable applications in optoelectronics and sensing materials. − An emerging strategy of incorporation of boron in organic structures is the substitution of a CC bond with an isoelectronic and isosteric B–N unit (BN/CC isosterism). The isoelectronic nature between the BN and CC bonding arises from the fact that boron has three valence electrons and nitrogen has five valence electrons, and consequently, a B–N unit has the same valence electron count (that is, eight valence electrons) as a corresponding CC unit in which each carbon contributes four valence electrons; however, the strong dipole moment of the B–N unit makes it exhibit quite different properties. − Since pioneering work on singly BN-substituted aromatic compounds by Dewar and co-workers in 1958, a large number of BN-doped PAHs have been developed which have exhibited potentials in applications of sensory materials, light-emitting materials, and organic electronics. − Among them, in 2011, Nakamura and co-workers prepared a BN-doped PAH and studied the intrinsic hole mobility via time-resolved microwave conductivity measurements, which was higher than that of a carbon analog, meaning that BN-doped PAHs are potential candidates for organic electronic materials .…”

Dipole Effect of BN-Doped Tetrathienonaphthalene on Photo-Physical Properties and Lewis Acidity of the D-π-A Derivatives

“…Boron is adjacent to carbon in the periodic table, which usually utilizes sp 2 hybridization to form neutral tricoordinated boron compounds, thus retaining an empty p orbital, showing obvious electron deficiency. The empty p orbital on boron is prone to form p−π* conjugation with lone-pair electrons or π-electrons of adjacent conjugated systems, which gives it unique optical and electronic properties. − Many methods of incorporating boron into conjugated systems have been reported. Among them, using the isoelectric property between the B–N bond and the CC bond is an effective method to prepare boron-containing conjugated systems.…”