Platinum Chloride Complexes Containing 6‑[9,9-Di(2-ethylhexyl)-7-R‑9H‑fluoren-2-yl]-2,2′-bipyridine Ligand (R = NO₂, CHO, Benzothiazol-2-yl, n‑Bu, Carbazol-9-yl, NPh₂): Tunable Photophysics and Reverse Saturable Absorption

Six new platinum­(II) chloride complexes 1–6 containing a 6-[9,9-di­(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine (R = NO₂, CHO, benzothiazol-2-yl (BTZ), n-Bu, carbazol-9-yl (CBZ), NPh₂) ligand were synthesized and characterized. The influence of the electron-donating or electron-withdrawing substituent at the 7-position of the fluorenyl component on the photophysics of these complexes was systematically investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). Electron-withdrawing or -donating substituents exert distinct effects on the photophysics of the complexes. All complexes feature a low-energy, broad ¹MLCT (metal-to-ligand charge transfer)/¹ILCT (intraligand charge transfer)/¹π,π* absorption band (tail) above ca. 430 nm and a major absorption band(s) between 320 and 430 nm, which admix ¹MLCT, ¹π,π*, ¹ILCT, and/or ¹LLCT (ligand-to-ligand charge transfer) characters. The contributions of different configurations to the major absorption band(s) vary depending on the nature of the substituent. Strong electron-donating or -withdrawing substituents (NPh₂ and NO₂) and the aromatic substituent BTZ cause a pronounced red-shift of the absorption spectra of 1, 3, and 6. All complexes are emissive at room temperature and at 77 K. The emitting excited state is dominated by ³π,π* character in 1–3, with some contributions from ³MLCT in 1 and 2, while the emission is predominantly from the ³MLCT state for 4 and 5 but with some ³π,π* character. For 6, the emitting state is ³ILCT in nature. With the increased electron-donating ability of the substituent, the ³π,π* character diminishes while charge transfer character increases. All complexes exhibit broad and strong triplet excited-state absorption (TA) from the near-UV to the near-IR spectral region. The TA band maxima are red-shifted for complexes 1–3 (which possess the electron-withdrawing substituents) compared to those of 4–6 (which contain electron-donating substituents). All complexes manifest strong reverse saturable absorption (RSA) for a nanosecond laser pulse at 532 nm, which originates from the much stronger triplet excited-state absorption than the ground-state absorption of 1–6 in the visible spectral region. The strength of RSA follows this trend: 4 ≈ 5 < 1 ≈ 3 < 2 < 6, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground-state absorption (σ_ex/σ₀) at 532 nm. The σ_ex/σ₀ ratios (116–261) of 1–6 at 532 nm are much larger than those of most of the reverse saturable absorbers reported in the literature, with the ratio of 6 (σ_ex/σ₀ = 261) being among the largest values reported to date. This makes complexes 1–6, especially 6, very promising reverse saturable absorbers.