4′-(5′-R-pyrimidyl)-2,2′:6′,2-terpyridyl (R = H, OEt, Ph, Cl, CN) platinum(II) phenylacetylide complexes:: Synthesis and photophysics

A series of new square-planar 4'-(5'''-R-pyrimidyl)-2,2':6',2 ''-terpyridyl platinum(II) phenylacetylicle complexes (1a-5a) bearing different substituents (R = H, OB, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer ((MLCT)-M-1) transition, possibly mixed with some ligand-to-ligand charge transfer ((LLCT)-L-1) character, Photoluminescence arising from the (MLCT)-M-3 state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The (MLCT)-M-1/(LLCT)-L-1 absorption bands and the 3MLCT emission bands for 1a-5a red-shift in comparison to those of the corresponding 4'-toly-2,2':6',2 ''-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the (MLCT)-M-1/(LLCT)-L-1 absorption and the 3MLCT emission bands exhibit a linear correlation with the Hammett constant (sigma(p)) of the 5'-substituent on the pyrimidyl ring. The lifetime of the (MLCT)-M-3 emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a-5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH3CN than 2a, which has the -OEt substituent, does (lambda(max)(T1-Tn) = 720 nm, TT = 660 ns). The triplet excited-state absorption coefficients at the band maxima for la-5a vary from 36 600 L . 3 mol(-1) cm(-1) to 115 090 L . mol(-1) . cm(-1), and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH3CN solutions, with the singlet oxygen generation quantum yield (Phi(A)) varying from 0.24 to 0.46.