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

Six new platinum­(II) chloride complexes <b>1</b>–<b>6</b> containing a 6-[9,9-di­(2-ethylhexyl)-7-R-9<i>H</i>-fluoren-2-yl]-2,2′-bipyridine (R = NO₂, CHO, benzothiazol-2-yl (BTZ), <i>n</i>-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)/¹<i>π</i>,<i>π</i>* absorption band (tail) above ca. 430 nm and a major absorption band(s) between 320 and 430 nm, which admix ¹MLCT, ¹<i>π</i>,<i>π</i>*, 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 <b>1</b>, <b>3</b>, and <b>6</b>. All complexes are emissive at room temperature and at 77 K. The emitting excited state is dominated by ³<i>π</i>,<i>π</i>* character in <b>1</b>–<b>3</b>, with some contributions from ³MLCT in <b>1</b> and <b>2</b>, while the emission is predominantly from the ³MLCT state for <b>4</b> and <b>5</b> but with some ³<i>π</i>,<i>π</i>* character. For <b>6</b>, the emitting state is ³ILCT in nature. With the increased electron-donating ability of the substituent, the ³<i>π</i>,<i>π</i>* 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 <b>1</b>–<b>3</b> (which possess the electron-withdrawing substituents) compared to those of <b>4</b>–<b>6</b> (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 <b>1</b>–<b>6</b> in the visible spectral region. The strength of RSA follows this trend: <b>4</b> ≈ <b>5</b> < <b>1</b> ≈ <b>3</b> < <b>2</b> < <b>6</b>, 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 <b>1</b>–<b>6</b> at 532 nm are much larger than those of most of the reverse saturable absorbers reported in the literature, with the ratio of <b>6</b> (σ_ex/σ₀ = 261) being among the largest values reported to date. This makes complexes <b>1</b>–<b>6</b>, especially <b>6</b>, very promising reverse saturable absorbers.