Phenylpyridyl‐Fused Boroles: A Unique Coordination Mode and Weak B−N Coordination‐Induced Dual Fluorescence

Abstract Using 4‐phenylpyridine or 2‐phenylpyridine in place of biphenyl, two electron‐poor phenylpyridyl‐fused boroles, [TipPBB1]4 and TipPBB2 were prepared. [TipPBB1]4 adopts a unique coordination mode and forms a tetramer with a cavity in both the solid state and solution. The boron center of TipPBB2 is 4‐coordinate in the solid state but the system dissociates in solution, leading to 3‐coordinate borole species. Compared to its borafluorene analogues, the electron‐accepting ability of TipPBB2 is largely enhanced by the pyridyl group. TipPBB2 exhibits dual fluorescence in solution due to an equilibrium between free TipPBB2 and a weak intermolecular coordination adduct with a second molecule. This equilibrium was further investigated by low‐temperature NMR spectroscopy and photophysical studies. Theoretical studies indicate that the highest occupied molecular orbital (HOMO) of TipPBB2 localizes at the Tip group, in contrast to its borafluorene derivatives, wherein the HOMOs are localized on the borafluorene cores.


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The concentration of [TipPBB1]4 was lower than 10 -5 M to minimize inner filter effects during fluorescence measurements. Due to the weak absorption of TipPBB2, the solutions for fluorescence measurements were measured at a concentration of ca. 2x10 -5 M.

Fluorescence quantum yield measurements
Fluorescence quantum yields were measured using a calibrated integrating sphere (150 mm inner diameter) from Edinburgh Instruments combined with the FLSP920 spectrometer described above. For solution-state measurements, the longest wavelength absorption maximum of the compound in the respective solvent was chosen for excitation.

Lifetime measurements
Fluorescence lifetimes were recorded via the time-correlated single photon counting (TCSPC) method using an Edinburgh Instruments FLS920 spectrometer equipped with a high-speed photomultiplier tube positioned after a single emission monochromator. Measurements were made in right-angle geometry mode, and the emission was collected through a polarizer set to the magic angle. Both compounds were excited with either pulsed diode lasers at wavelengths of 316 nm or 377 nm at repetition rates of 10 or 0.5 MHz, respectively. The full-width-at-half-maximum (FWHM) of the pulse from the diode laser was ca. 90 ps with an instrument response function (IRF) of ca. 1 ns FWHM, respectively. The IRFs were measured from the scatter from pure solvent. Decays were recorded to 10000 counts in the peak channel with a record length of at least 1000 channels. The band pass of the emission monochromator and a variable neutral density filter on the excitation side were adjusted to give a signal count rate of <60 kHz. Iterative reconvolution of the IRF with one decay function and non-linear least-squares analysis were used to analyze the data. The quality of decay fits was judged to be satisfactory based on the calculated values of the reduced χ 2 and Durbin-Watson parameters and visual inspection of the weighted residuals.

Electrochemical measurements
Cyclic voltammetry experiments were performed using a Gamry Instruments Reference 600 potentiostat. A standard three-electrode cell configuration was employed using a platinum disk working electrode, a platinum wire counter electrode, and a silver wire, separated by a Vycor tip, serving as the reference electrode. Formal redox potentials are referenced to the ferrocene/ferrocenium redox couple. Tetra-nbutylammonium hexafluorophosphate ([n-Bu4N][PF6]) was employed as the supporting electrolyte. Compensation for resistive losses (iR drop) was employed for all measurements.

Theoretical studies
All calculations (DFT and TD-DFT) were carried out with the Gaussian 09 (9.E.01) [7] program package and were performed on a parallel cluster system. GaussView (6.0.16), Avogadro (1.2.0) [8] and multiwfn [9] were used to visualize the results, to measure calculated structural parameters, and to plot orbital surfaces (isovalue: ± 0.030 [eÅ -3 ] 1/2 ). The ground-state geometries were optimized using the B3LYP functional [10] in combination with the 6-31G basis set. [11] The D3 dispersion correction of Grimme and co-workers was used. [12] The polarizable continuum model (PCM) was used to include solvent effects for the ground state structures. The ultrafine integration grid and symmetry constraints were used for all molecules. Frequency calculations were performed on the optimized structures to confirm them to be local minima showing no negative (imaginary) frequencies. Based on these optimized structures, the lowest-energy vertical transitions (gas-phase) were calculated (singlets, 25 states) by TD-DFT, using B3LYP in combination with the 6-31+G(d) basis set. [11] S5 Synthesis [9-(2,4,6-triisopropylphenyl)-9H-benzo [4,5]

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%LT is the relative value of the long lifetime component and %ST is the relative value of the short lifetime component.