| Cyclometalated platinum(II) complexes are fascinating phosphorescent luminophores due to the applications in electroluminescence and more recently in photocatalysis, luminescent molecular probes and triplet-triplet annihilation based upconversions. However, the conventional cyclometalated Pt(Ⅱ) complexes usually show weak absorption in the visible range and the lifetime of the T1excited state is short (usually less than5μs). With the development of the new application of the Pt(Ⅱ) complexes, it is highly desired that Pt(Ⅱ) complexes with strong absorption of visible light and long-lived triplet excited state are prepared. At present, the transition-metal complexes as triplet photosensitizers for TTA upconversion are limited to those show absorption in UV range and short-lived triplet excited state, with which the efficiency of triplet-triplet-energy-transfer (TTET) efficiency of the TTA upconversion is reduced. We propose that direct cyclometalation of an organic fluorophore showing intense visible light absorption is a powerful approach to prepare the transition-metal complexes that show strong absorption of visible light and long-lived triplet excited states. The photophysics of transition-metal complexes are closely related to the electronic structure of their excited states. Herein, we used DFT/TDDFT theoretical calculation to rationalize the photophysical properties of the novel Pt(Ⅱ) complexes. This complementary experimental and theoretical approach will be helpful in the rational design of new transition-metal complexes.Recently, we and others found that different connection profiles between the fluorophore and platinum coordination center exert significant effect on the emission wavelength of the complexes. Tuning the emission wavelength is attributed to either the elevated HOMO energy caused by the electron-donating diphenylamine (TPA) substituents at the ppy ligand (Pt-2, C-C single bond connection), or decreased LUMO energy caused by the electron trap effect of the electron withdrawing substitutions (Pt-3, a-diketo connection), both results in decreased HOMO-LUMO energy gap, and thus red-shifted emission. The complexes show extended luminescence lifetime (τ=3.0-5.5μs).Pyrene-containing cyclometalated Pt(Ⅱ) complexes, with pyrene moiety directly cyclometalated (Pt-4), or connected to the2-phenylpyridine (ppy) ligand via C-C single bond (Pt-5), or C=C triple bond (Pt-6) were prepared. We found that Pt-5, in which the pyrene moiety is not directly cyclometalated, shows intense pyrene-based phosphorescence, which is different from previous report that direct cyclometalation is necessary for observation of the phosphorescence of pyrene in cyclometalated complexes. The emission wavelengths of Pt-4, Pt-5and Pt-6have been extended to deep-red/near-IR (NIR) region. We propose that these emission are due to a triplet emissive state with a substantial3IL component, that is, the pyrene-localized excited state. And this assignment was supported by DFT/TDDFT calculations.We propose that the direct cyclometalation of an organic fluorophores is a powerful approach to access the transition-metal complexes, that show intense absorption in the visible range and at the same time, the long-lived triplet excited states. We would introduce direct metalation method according to different types of platinum complexes.Pt(N^C)(O^O) have been widely used in electroluminescence, etc. We studied the photophysical properties of three cyclometallated Pt(Ⅱ) complexes, in which the thiazo-coumarin ligands (Pt-9, Pt-10and Pt-11) were directly cycloplatinated. The emission wavelength is dependent on the size of the π-conjugation, not the intramolecular charge transfer (ICT) feature of the C^N ligands. DFT/TDDFT calculations and transient absorptions indicate coumarin-localized intraligand triplet excited state (3IL) for these complexes. Pt-9shows intense absorption in visible range (ε=47800M-1cm-1at496nm) and long-lived phosphorescence (τ=20.3μs), which are superior to other complexes. The complexes were used for luminescence O2sensing and TTA upconversion. The oxygen sensitivity of the emission of Pt-9is ca.89-fold of that of Pt-10. The TTA upconversion quantum yield with Pt-9as triplet photosensitizer is up to15.4%, and the TTET efficiency of Pt-9is34-fold of the model complex [Ru(dmb)3][PF6]2.Our results show that intense absorption in visible range and the long-lived emissive3IL excited states can be readily accessed by direct cyclometallation of organic fluorophores and this approach will be useful for rational design of new transition-metal complexes for the purpose of photocatalysis, phosphorescent molecular probes, etc.Based on our previous research, we improved the molecular structure of Pt (N^C)(O^O) with five-membered chelate ring. With4-pyrazolylnaphthalimide, six-membered (Pt-14) chelate ring was obtained. Compared to3-2, Pt-14with six-membered chelate ring show intense absorption of visible light (ε=21900M-1cm-1at443nm) and long-lived phosphorescence (τ=61.9μs), longer than the model complex ppyPt(acac)(τ=2.6μs). Time-resolved transient absorption spectra and spin density analysis indicated a NI-localized intraligand triplet excited state (3IL) for Pt-14. The complexes were used as triplet sensitizers for TTA upconversion, and the results show that Pt-14is an efficient sensitizer with an upconversion quantum yield of up to14.1%.For the bidentate (N^N) and tridentate cyclometalated platinum (Ⅱ) complexes, another two kinds of platinum complexes with functionalized acetylide ligands connected to their Pt centers were prepared. This is another direct metalation method, which improve the properties of (N^N) Pt≡L and (CAN*N)Pt=L. For example, long-lived phosphorescence of135.7μs was observed for Pt-20, respectively. Furthermore, the absorption of Pt-20was remarkably enhanced in the visible range.Our results are useful for design of visible light-harvesting transition-metal complexes that shows long-lived3IL triplet excited states and for their applications in TTA upconversion and photodynamic therapy, etc. Concerning these aspects, the direct metalation of organic fluorophores will be a useful method. |
PDF Full Text Request