Effect of Planarity of Aromatic Rings Appended to o-Carborane on Photophysical Properties: A Series of o-Carboranyl Compounds Based on 2-Phenylpyridine- and 2-(Benzo[b]thiophen-2-yl)pyridine

Abstract

Herein, we investigated the effect of ring planarity by fully characterizing four pyridine-based o-carboranyl compounds. o-Carborane was introduced to the C4 position of the pyridine rings of 2-phenylpyridine and 2-(benzo[b]thiophen-2-yl)pyridine (CB1 and CB2, respectively), and the compounds were subsequently borylated to obtain the corresponding CN-chelated compounds CB1B and CB2B. Single-crystal X-ray diffraction analysis of the molecular structures of CB2 and CB2B confirmed that o-carborane is appended to the aryl moiety. In photoluminescence experiments, CB2, but not CB1, showed an intense emission, assignable to intramolecular charge transfer (ICT) transition between the aryl and o-carborane moieties, in both solution and film states. On the other hand, in both solution and film states, CB1B and CB2B demonstrated a strong emission, originating from π-π * transition in the aryl groups, that tailed off to 650 nm owing to the ICT transition. All intramolecular electronic transitions in these o-carboranyl compounds were verified by theoretical calculations. These results distinctly suggest that the planarity of the aryl groups have a decisive effect on the efficiency of the radiative decay due to the ICT transition.

Keywords: intramolecular charge transfer; o-carborane; radiative decay; structural fluctuation.

Figure 1

Synthetic procedure for o-carboranyl compounds (CB1, CB2, CB1B, and CB2B). Reaction conditions: (i) 1-hexyne, CuI, PdCl₂(PPh₃)₂, NEt₃/toluene, r.t., 24 h. (ii) Pd(PPh₃)₄, Na₂CO₃, THF/H₂O (4:1, v/v), 80 °C, 24 h. (iii) B₁₀H₁₄, Et₂S, toluene, 110 °C, 72 h. (iv) BBr₃, (i-Pr)₂NEt, DCM, r.t., 48 h. (v) AlMe₃, toluene, r.t., 0.5 h.

Figure 2

X-ray crystal structures of CB2 (left) and CB2B (right) (50% thermal ellipsoids). H atoms are omitted for clarity.

Figure 3

(a) UV-vis absorption spectra in toluene (5.0 × 10⁻⁵ M). Photoluminescence (PL) spectra in toluene (5.0 × 10⁻⁵ M) at (b) 298 and (c) 77 K. (d) PL spectra in film state (poly(methylmethacrylate) (PMMA) film doped with 5 wt % o-carbonyl compound). Inset figures show the color of the emission of each state under UV irradiation (λ_ex = 354 nm).

Figure 4

Relative energies of frontier molecular orbitals of o-carboranyl compounds at the first excited singlet state (S₁) calculated by density functional theory (DFT) (isovalue = 0.04). The transition energy (in nm) was calculated at the TD-B3LYP/6-31G(d) level.