Pt(II) Bipyridyl Complexes Bearing Substituted Fluorenyl Motif on the Bipyridyl and Acetylide Ligands: Synthesis, Photophysics, and Reverse Saturable Absorption

A series of Pt­(II) diimine complexes bearing benzothiazolylfluorenyl (BTZ-F₈), diphenylaminofluorenyl (NPh₂-F₈), or naphthalimidylfluorenyl (NI-F₈) motifs on the bipyridyl or acetylide ligands (<b>Pt-4</b>–<b>Pt-8</b>), (i.e., {4,4′-bis­[7-R₁-F₈-(≡)_<i>n</i>-]­bpy}­Pt­(7-R₂-F₈- ≡ -)₂, where F₈ = 9,9′-di­(2-ethylhexyl)­fluorene, bpy = 2,2′-bipyridine, <b>Pt-4</b>: R₁ = R₂ = BTZ, <i>n</i> = 0; <b>Pt-5</b>: R₁ = BTZ, R₂ = NI, <i>n</i> = 0; <b>Pt-6</b>: R₁ = R₂ = BTZ, <i>n</i> = 1; <b>Pt-7</b>: R₁ = BTZ, R₂ = NPh₂, <i>n</i> = 1; <b>Pt-8</b>: R₁ = NPh₂, R₂ = BTZ, <i>n</i> = 1) were synthesized. Their ground-state and excited-state properties and reverse saturable absorption performances were systematically investigated. The influence of these motifs on the photophysics of the complexes was investigated by spectroscopic methods and simulated by time-dependent density functional theory (TDDFT). The intense absorption bands below 410 nm for these complexes is assigned to predominantly ¹π,π* transitions localized on either the bipyridine or the acetylide ligands; while the broad low-energy absorption bands between 420 and 575 nm are attributed to essentially ¹MLCT (metal-to-ligand charge transfer)/¹LLCT (ligand-to-ligand charge transfer) transitions, likely mixed with some ¹ILCT (intraligand charge transfer) transition for <b>Pt-4</b>–<b>Pt-7</b>, and predominantly ¹ILCT transition admixing with minor ¹MLCT/¹LLCT characters for <b>Pt-8</b>. The different substituents on the acetylide and bipyridyl ligands, and the degrees of π-conjugation in the bipyridyl ligand influence both the ¹π,π* and charge transfer transitions pronouncedly. All complexes are emissive at room temperature. Upon excitation at their respective absorption band maxima, <b>Pt-4</b>, <b>Pt-6</b>, and <b>Pt-8</b> exhibit acetylide ligand localized ¹π,π* fluorescence and ³MLCT/³LLCT phosphorescence in CH₂Cl₂, while <b>Pt-5</b> manifests ¹ILCT fluorescence and ³ILCT phosphorescence. However, only ¹LLCT fluorescence was observed for <b>Pt-7</b> at room temperature. The nanosecond transient absorption study was carried out for <b>Pt-4</b>–<b>Pt-8</b> in CH₃CN. Except for <b>Pt-7</b> that contains NPh₂ at the acetylide ligands, <b>Pt-4</b>–<b>Pt-6</b> and <b>Pt-8</b> all exhibit weak to moderate excited-state absorption in the visible spectral region. Reverse saturable absorption (RSA) of these complexes was demonstrated at 532 nm using 4.1 ns laser pulses in a 2 mm cuvette. The strength of RSA follows this trend: <b>Pt-4</b> > <b>Pt-5</b> > <b>Pt-7</b> > <b>Pt-6</b> > <b>Pt-8</b>. Incorporation of electron-donating substituent NPh₂ on the bipyridyl ligand significantly decreases the RSA, while shorter π-conjugation in the bipyridyl ligand increases the RSA. Therefore, the substituent at either the acetylide ligands or the bipyridyl ligand could affect the singlet and triplet excited-state characteristics significantly, which strongly influences the RSA efficiency