Blue-emitting acridine-tagged silver(i)-bis-N-heterocyclic carbene

Abstract

Herein, the photophysical properties of an acridine derivative of a bis-N-heterocyclic carbene silver complex were investigated. The HOMO and LUMO energy differences between 9-[(N-methyl imidazol-2-ylidene)]acridine and 4,5-bis[(N-methyl-imidazol-2-ylidene)methyl]acridine were theoretically compared. Based on the calculation, the 4,5-bis N-heterocyclic carbene-tethered acridine type of ligand was found to be a potential source for tuning the fluorescent nature of the resultant metal derivatives. Thus, a 4,5-bis N-heterocyclic carbene (NHC)-tethered acridine silver(i) salt was synthesized, and its photophysical properties were investigated. The 4,5-bis[(N-isopropylimidazol-2-ylidene)methyl]acridine silver(i) hexafluorophosphate complex was obtained from the reaction between [4,5-bis{(N-isopropylimidazolium)methyl}acridine] hexafluorophosphate and Ag₂O in very good yield; this molecule was characterized by elemental analysis and FTIR, multinuclear (¹H and ¹³C) NMR, UV-Vis, and fluorescence spectroscopic techniques. The molecular structure has been confirmed by single-crystal X-ray diffraction analysis, which has revealed that the complex is a homoleptic mononuclear silver(i) cationic solid. The charge of the Ag(i)-NHC cation is balanced by the hexafluorophosphate anion. The cationic moieties are closely packed in the chair and inverted chair forms where silver(i) possesses a quasi-linear geometry. Moreover, the silver complex provided blue emission from all the three excitations with good fluorescence quantum yield. The fluorescence lifetime of the silver(i) complex has been determined using the time-correlated single photon counting technique. Interestingly, the fluorescence decay pattern and the fluorescence lifetimes of the silver complex are largely different from those of the parent ligand acridine imidazolium salt. Moreover, the theoretical predictions have been found to be in good agreement with the experimental results.

Chart 1. The structurally characterized acridine-tagged NHC–silver derivatives.

Scheme 1. Synthesis of 2.

Fig. 1. (i) The solid-state structure of 2; (ii) orientation of the imidazolium moieties and acridine unit in 2. The hexafluorophosphate anion has been omitted for clarity. Selected bond lengths (Å): Ag(1)–C(1), 2.069(5); Ag(1)–C(4), 2.074(5); N(1)–C(1), 1.341(6); C(1)–N(2), 1.350(6), and selected bond angles (°): C(1)–Ag(1)–C(4), 177.80(19); N(2)–C(1)–N(1), 105.0(4); N(3)–C(4)–N(4), 105.8(5).

Fig. 2. UV-Vis absorption spectra of 1 (blue) and 2 (red) normalized at their longer wavelength absorption bands, inset: longer wavelength absorption spectra of 1 (blue) and 2 (red). Solvent: methanol.

Fig. 3. (i) The absorption spectra of 1 at various concentrations, (ii) the linear plot for the calculation of the molar extinction coefficient of 1, (iii) the absorption spectra of 2 at various concentrations, and (iv) the linear plot for the calculation of the molar extinction coefficient of 2. Solvent: methanol.

Fig. 4. Top: the photoluminescence spectra of 1 and 2 in methanol (13.7 μM); bottom: the excitation spectra of 1 and 2 in methanol (13.7 μM).

Fig. 5. Top: the 2D contour spectrum of 1 in methanol (13.7 μM), Middle: the 2D contour spectrum of 2 in methanol (13.7 μM); and bottom: the CIE chromaticity diagram of 1 (star) and 2 (triangle) in the solution state.