Theoretical Studies Of Electronic Absorption And Emission Spectra Of Pt(saloph) Compounds

The photophysical properties of metal Schiff base complexes have been extensively studied due to their important applications,especially some luminescent platinum(II) Schiff base complexes are supposed to be optical sensors or OLEDs (organic light-emitting diodes). Moreover, spectroscopic properties of platinum(II) Schiff base complexes have been the subject of many previous works, but the data on the photophysics of these complexes are very scattered and incomplete. We have synthesized the new luminescent Pt(II) Schiff base complex [Pt(saloph), (N,N'-bis(3-methoxy-salicylidene)-1,2-phenylenediamine platinum(II)] and obtained single crystals of Pt(saloph) whose crystal number is CCDC265459 in Cambridge Crystallographic Data Centre. The luminescence of the new Pt(II) Schiff base complex, which can be efficiently quenched by dissolved oxygen (DO), possesses a potential application such as novel dissolved oxygen sensitive materials. For explaining electronic absorption and emission spectra of the Pt(saloph), our present study is undertaken to make out the relation between molecule frame and spectra properties of the Pt(saloph) by comparing the experiment and the computation.All calculations in this work are carried out using the Gaussian 03 package. Vertical excitation energies at the optimized geometry of the ground state of the Pt(saloph) are computed by configuration interaction singles CIS and TD-B3LYP method, respectively. Calculations are carried out by the following basis sets (BS): LANL2DZ for Pt, 6-31G and 6-31G* for the other atoms, respectively. The optimization of S0 state geometry of the complex is performed by B3LYP. The LANL2DZ frozen-core basis set is chosen for Pt metal because this basis set is proved to give reliable geometries and ground state properties for molecular complex compounds. In the present study, excitation energies and transition intensities of the T1-Ti states are determined for the optimized geometry of the lowest triplet (T1) state. On the other hand, the fluorescence band is calculated from the optimized geometry in S1 excited state by CIS method.The result obtained from the predicted and the measured transitions indicates are very satisfactory. It should be observed that the energy transition of the corresponding UV Spectrum (251 nm, ca. 39800 cm-1) is only ofπ,π* characteristic approximately at 253.71 nm (ca. 39400 cm-1) with the 6-31G basis set and 251.34 nm (ca. 40000 cm-1) with the 6-31G* basis set by means of CIS method. TD-B3LYP method shows that the calculated transitions of the Pt(saloph), which are located at about 40400 cm-1(247 nm),26400 cm-1 (378 nm),25800 cm-1 (387 nm), are quite close to the available experimental values: 39800 cm-1(251 nm),27300 cm-1(366 nm),26000 cm-1(385 nm). The S0-Si and T1-Ti absorption spectra are calculated, and the transition between the ground S0 state and the excited S1 state involves the HOMO-2, HOMO-1, HOMO and LUMO. Moreover, calculations show that the emissive singlet is of mixed MLCT/LLCT characteristic. As a result, it is predicted that the absorption spectrum has a small red-shift atλmax = 200 ~ 300 nm (E = 33000 ~ 50000 cm-1). The absorption spectrum is expected to undergo a blue-shift atλmax = 300 ~ 400 nm (E = 25000 ~ 33000 cm-1) because these absorption bands have two charge transfers involving the phenoxide lone pair (L) and theπ* orbital of imine, i.e. [L(phenoxide O37, O38)→π*(imine)], mixed with [Pt(5d)→π*(saloph)] of MLCT characteristics. In the context, it should be kept in mind that the structure of the Pt complex in its MLCT/LLCT states is nonplanar, while the whole structure of the complex in ground state is planar. The structure of the complex is expected to embarrass the absorption since electronic transitions are hampered when the flexibility including excited-state distortions is restricted. We also notice that the rotations of the phenoxide grounds at the ligand take place in the ground state.