Reductive Fluorescence Quenching of the Photoexcited Free Base meso-Tetrakis (Pentafluorophenyl) Porphyrin by Amines

Abstract

Steady state and time resolved fluorescence quenching behaviors of meso-Tetrakis (pentafluorophenyl) porphyrin (H(2)F(20)TPP) in presence of different aliphatic and aromatic amines have been executed in homogeneous dichloromethane (DCM) solution. At room temperature in DCM, free base (H(2)F(20)TPP) shows fluorescence with two distinct peaks at 640 and 711 nm and natural lifetime tauf=9.8 ns which are very similar to that of meso-tetraphenyl porphyrin (TPP). Unlike TPP, addition of both aliphatic and aromatic amines to a solution containing H(2)F(20)TPP results in an efficient decrease in fluorescence intensity without altering the shape and peak position of fluorescence emission. Upon addition of amines there was no change in optical absorption spectra of H(2)F(20)TPP. The fluorescence quenching rate constants ranged from 1 x 10(9) to 4 x 10(9) s(-1), which are one order below to the diffusion control limit, and temperature dependent quenching rate constants yield the activation energies which are found to be order of 0.1 eV. Femto second transient absorption studies reveal the existence of amine cation radical and porphyrin anion radicals with very short decay time (15 ps). The fluorescence quenching reaction follows Stern-Volmer kinetics. Steady state and time-resolved data are interpreted within general kinetic scheme of Marcus semi-classical model which attributes bimolecular electron transfer process between amines and the lowest excited singlet state of H(2)F(20)TPP. Calculated internal reorganization energies are found to be in between 0.04 and 0.22 ev. Variation of electron transfer rate as function of free energy change (DeltaG(0)) points the ET reactions in the present systems are in Marcus normal region. This is the first example of reductive fluorescence quenching of free base neutral porphyrins in homogeneous organic solvent ever known.

Diffusive and non-diffusive photo-induced proton coupled electron transfer from hydrogen bonded phenols to meso-tetrakis-5,10,15,20-pentafluorophenyl porphyrin

“…The steady state spectroscopic properties of H 2 F 20 TPP in DCM solution in terms of electronic absorption and fluorescence spectra have been described elsewhere. 19 In brief, it has a strong Soret band at 411 nm and two weak Q bands at 505 (Q y , 0-0) and 580 (Q x , 0-0) nm. Upon excitation either on a Soret band or on a Q band it shows fluorescence peaking at 640 nm and 707 nm with a fluorescence quantum yield (j f ) B 0.06 and natural singlet lifetime (t 0 ) B 10 ns obtained from TCSPC measurements.…”

“…Meso-tetrakis-pentafluorophenyl porphyrin (H 2 F 20 TPP), a free base porphyrin in which pentafluorophenyl groups are covalently linked to four meso-positions of the porphyrin macro-cycle acts as an excellent electron acceptor. Its reductive quenching behaviour with various aliphatic amines has been studied previously in our group 19 and we now intend to study the system for two different phenols, 4-methoxy phenol (4-MeOPhOH) and 2,6-dimethoxy phenol (2,, having nearly same oxidation potentials. In this article we report (Scheme 1) how the photo-induced electron transfer rate from phenols to H 2 F 20 TPP is enhanced by the addition of pyridine bases (Case-I) and how photo-induced electron transfer is ignited upon addition of bases (Case-II).…”

Corrole–ferrocene and corrole–anthraquinone dyads: synthesis, spectroscopy and photochemistry

“…However, the total reorganization energy (l) of tetrapyrrolic systems was found to be of the order of 1-1.5 eV. 60 So, for both the dyads ÀDG CR tends to be equal to l, and CR occurs in the Marcus optimal region, i.e. CR in both the dyads is essentially barrierless and the rate of CR is thus ultrafast in nature.…”

Kinetic analysis of nitroxide radical formation under oxygenated photolysis: toward quantitative singlet oxygen topology

“…48 Photo-oxidation of various tertiary amines or aniline by the electron deficient tetrakis( perfluorophenyl)porphyrin followed Marcus theory in dichloromethane, but no quenching was observed of porphyrins with more electron rich phenyl substituents. 49 Scheme 2-2b: energy transfer quenching to nitroxide Nitroxides like TEMPOne are efficient quenchers of excited triplet states, often with diffusion limited second-order rate constants (k NOq3 ∼ 10 10 s −1 ). 50,51 Build-up of nitroxide in the solution therefore provides an additional quenching pathway for the TPPS triplet.…”

“…deficient tetrakis(perfluorphenyl)porphyrin followed Marcus theory in dichloromethane, but no quenching was observed of porphyrins with more electron rich phenyl substituents 49 . 3a) 55.…”