Electronic spectroscopy, photophysical properties, and emission quenching studies of an oxidatively robust perfluorinated platinum porphyrin

Abstract

The highly electron-deficient, β-octafluorinated meso-tetrakis(pentafluorophenyl)-porphyrin (H2F28TPP) was metalated with platinum to afford the oxidatively robust luminophore [PtF 28TPP], and its X-ray structure shows that the porphyrin core exists in a slightly saddle-shaped conformation. The absorption spectrum of [PtF 28TPP] in CH2Cl2 displays a near-UV Soret band (B) at 383 nm (ε = 2.85 × 108 dm3 mol -1 cm-1) and two visible Q(1,0) and Q(0,0) bands at 501 (ε = 1.45 × 104 dm3 mol-1 cm -1) and 533 (ε = 1.36 × 104 dm3 mol-1 cm-1) nm, respectively. These absorption bands of [PtF28TPP] are blue-shifted from those in [PtF20TPP] (390, 504, and 538 nm, respectively) and [PtTPP] (401, 509, and 539 nm, respectively). Excitation of [PtF28TPP] (complex concentration = 1.5 × 10-6 mol dm-3) in dichloromethane at the Soret or Q(1,0) or Q(0,0) band gave a phosphorescence with peak maximum at 650 nm (lifetime = 5.8 μs) and a weak shoulder at 712 nm. Both the emission lifetime and quantum yield vary with solvent polarity, and plots of τ versus EK and Φ versus EK (where EK is the empirical solvent polarity parameter based on the hypsochromic shift of the longest wavelength absorption of the [Mo(CO)4{(C5H 4N)HC=NCH2C6H5}] complex with increasing solvent polarity; see: Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog. Phys. Org. Chem. 1981, 13, pp 485-630) show linear correlation, indicating that the emission is sensitive to the local environment/ medium. Electrochemical studies on [PtF28TPP] by cyclic voltammetry showed no porphyrin-centered oxidation at potential ≤ 1.5 V versus Ag/AgNO 3, demonstrating that [PtF28TPP] is more resistant toward oxidation than PtF20- TPP] (E1/2 = 1.33 V) and [PtTPP] (E1/2 = 0.97 V). The porphyrin-centered reduction of [PtF 28TPP] occurs at -0.75 and -1.18 V, which is anodically shifted from those at -1.06 and -1.55 V in [PtF20TPP], and -1.51 V in [PtTPP], respectively. The excited-state reduction potential of [PtF28TPP] is estimated to be 1.49 V versus Ag/AgNO3. Over 97% of the emission intensity of [PtF28TPP] was retained after irradiation with a high power mercury arc lamp (500 W) for 14 h, compared to 90% and 12% for [PtF 20TPP] and [PtTPP], respectively; hence, [PtF28TPP] exhibits superior photostability. Quenching of the emission of [PtF 28TPP] by oxygen, alcohol, catechol, and butylamine reveals that [PtF28TPP] is an oxidatively robust material with medium-sensitive photoluminescence properties.