The equilibria and conversions between three excited states: the LE state and two charge transfer states, in twisted pyrene-substituted tridurylboranesw

Triarylboron p-conjugated systems have received wide interest in the past decades due to many applications in materials chemistry as nonlinear optical and charge-transport materials, emitters in organic light-emitting devices (OLEDs), as well as selective and sensitive chemosensors for the detection of fluoride and cyanide ions. These intriguing electronic and photophysical properties of triarylboranes derive from the presence of the vacant p-orbital on the boron (p(B)) acting as an excellent electron acceptor in intramolecular charge transfer (ICT). Photoinduced ICT in a strong electron donor (D)– acceptor (A) substituted conjugated system normally displays a prominent solvatochromic effect. The representative example is N,N-dimethylaminobenzonitrile with a dual fluorescence emission, in which the long-band wavelengths strongly depend on solvent polarity. Since the first observation by Lippert et al., a large variety of mechanisms have been presented in the literature to explain this phenomenon. Among these, the TICT model has gained widespread acceptance and appears to be conclusion of the numerous debates over the past decades. However, there are still some points that arise leading to doubt or contrary interpretation. For example, both the twisted angle necessary and excited-state CT equilibria are not quite clear. In this work, we have observed three excited states from a series of synthesized pyrene-substituted tridurylboranes PB1–3 (Fig. 1, for synthesis details see ESIw), and gained some new insights into the ICT in a pretwisted D–A system. By comparing with free pyrene, the absorption spectra of PB1–3 show that the electron coupling between the pyrene moiety (D) and p(B) (A) unit is very weak. The absorption maxima of PB1–3 show only small red-shifts relative to free pyrene, and their absorption edges increase from PB1 to PB3 (Fig. 1). This is indicative of the conjugation becoming stronger from PB1 to PB3. DFT calculation results are similar to the crystal structure of a tridurylborane derivatives, that is, the central boron is completely trigonal planar, and three duryl groups are arranged in a propeller-like fashion. In addition, our calculations show the dihedral angles between the boron plane and the duryl planes decrease gradually from PB1 to PB3 from 55.9 to 50.11, which is in agreement with their absorption spectra, while the dihedral angles between the boron and pyrene planes increase from 31.2 to 45.31 (Table S2w). Hence, these pyrene-substituted tridurylboranes are pretwisted D–A molecules. The fluorescence spectra of PB1–3 in THF reveal a difference in the excited-state properties of these molecules, shown in Fig. 1. Similar to free pyrene, the structured spectrum with two peaks at 378 and 397 nm from PB1 should be assigned to the locally excited (LE) state, which is independent of solvent polarity (vide infra). Both PB2 and PB3 reveal a single-band emission, which gradually red-shift relative to PB1. This shows that excited-state energy levels of these compounds from PB1 to PB3 are reduced, in other words, there is a gradual increase in charge-separation degree or dipole moment. To obtain dipole moments of the excited states, fluorescence spectra of these compounds in various polarity solvents were recorded and are shown in Fig. 2. In most solvents, fluorescence spectra of PB1 exhibit the LE-state emission, and a new single long-band emission appears only in two strongly polar solvents, acetone and acetonitrile. This shows clearly the excited-state conversion of PB1 from the LE state to an ICT state. However, the fluorescence spectra of PB2 and PB3 exhibit single band Fig. 1 Left: Structures of PB1–3. Right: UV absorption (solid line) and fluorescence (dashed line) spectra of 10 5 M triarylboranes PB1–3 and pyrene as a comparison, in THF solutions.