Experimental and Theoretical Study on the Photophysical Properties of 90° and 60° Bimetallic Platinum Complexes

Abstract

The 90°and 60°bimetallic platinum complexes with special structures are widely used in coordination-driven selfassembled metallosupramolecular architectures, and these complexes are the key components of triangular, rectangular, and polygonal metallacycle and metallocage supramolecules. Therefore, spectroscopic techniques and quantum chemistry calculations were employed in this article to investigate the photophysical properties of these bimetallic platinum complexes. Compared with spectra for the ligands, the absorption spectra of these Pt complexes are redshifted, and the fluorescence spectra become wider and are also redshifted. Moreover, the reasons for the low fluorescence quantum yields and short fluorescence lifetimes of these compounds were investigated using quantum chemistry calculations. We demonstrate that the fluorescent states of the bimetallic platinum complexes can be considered as local excited states, and that they possess a ligand-centered π−π* transition feature. Meanwhile, the platinum metals act as perturbation for these transitions, whereas the nonfluorescent states are classified as intramolecular charge-transfer states. Furthermore, a new fluorescence modulation mechanism is developed to explain the different emission processes of these complexes with different ligands. ■ INTRODUCTIONCoordination-driven self-assembly employs directional metal− ligand interactions in the assembly process and provides a facile method of assembling a number of molecules into highly organized metallosupramolecular architectures. 1 These selfassembled macromolecules have been widely investigated in many fields, such as in synthetic chemistry, medicinal chemistry, molecular devices, chemical sensors, and so on. 1,2 Recently, Stang and co-workers have reported a series of supramolecules on the basis of the self-recognition processes. 3−5 Therrien et al. discovered that a trigonal prism metallacage host system can deliver drugs into cancer cells. 6−9 Chi and co-workers designed a number of two-dimensional polygons and three-dimensional cages that contain transition metals as fluorescent chemosensors for dicarboxylate anions and nitroaromatics, especially for picric acid. 2,10 Considering the many special electronic and optical properties of organometallic systems, increasing attentions are focused on the photophysical processes of organometallic materials. 11−16 Various experimental techniques, such as the two-photon absorption, steady-state fluorescence, and transient absorption, have been employed to study the photophysical and chargetransfer processes of self-assembled materials. 1 Yam, Wong, and others performed systematic investigations on the luminescence property of many platinum complexes. 17−27 Goodson and coworkers have employed femtosecond fluorescence upconversion and transient absorption to investigate different platinumcontaining metallacycles. 13 We have combined density functional theory (DFT)/time-dependent density functional theory (TDDFT) quantum chemistry calculations and spectros...

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