Synthesis, Electrochemistry, and Photophysics of a Family of Phlorin Macrocycles That Display Cooperative Fluoride Binding

Abstract

A homologous set of 5,5-dimethylphlorin macrocycles in which the identity of one aryl ring is systematically varied has been prepared. These derivatives contain ancillary pentaflurophenyl (3H(PhlF)), mesityl (3H(PhlMes)), 2,6-bismethoxyphenyl (3H(PhlOMe)), 4-nitrophenyl (3H(PhlNO2)) or 4-tert-butylcarboxyphenyl (3H(PhlCO2tBu)) groups at the 15-meso-position. These porphyrinoids were prepared in good yields (35 – 50%) and display unusual multielectron redox and photochemical properties. Each phlorin can be oxidized up to three times at modest potentials and can be reduced twice. The electron-donating and releasing properties of the ancillary aryl substituent attenuate the potentials of these redox events; phlorins containing electron-donating aryl groups are easier to oxidize and harder to reduce, while the opposite trend is observed for phlorins containing electron-withdrawing functionalities. Phlorin substitution also has a pronounced effect on the observed photophysics, as introduction of electron-releasing aryl groups on the periphery of the macrocycle is manifest in larger emission quantum yields and longer fluorescence lifetimes. Each phlorin displays an intriguing supramolecular chemistry and can bind 2 equivalents of fluoride. This binding is allosteric in nature and the strength of halide binding correlates with the ability of the phlorin to stabilize the buildup of charge. Moreover, fluoride binding to generate complexes of the form 3H(PhlR)·2F− modulates the redox potentials of the parent phlorin. As such, titration of phlorin with a source of fluoride represents a facile method to tune the ability of this class of porphyrinoid to absorb light and engage in redox chemistry.

Reduction of CO2 using a rhenium bipyridine complex containing ancillary BODIPY moieties

“…The inability of these systems to harvest solar energy at wavelengths longer than ~450 nm is a critical impediment to the use of sunlight to drive CO 2 reduction, since the solar power spectrum is most intense from approximately 425 – 600 nm. Since BODIPY 47 – 51 and other polypyrrole derivatives 52–55 can absorb light throughout the visible region we rationalized that [Re(BB2)(CO) 3 Cl] might support an improved photocatalysis with CO 2 using sunlight, as compared to fac -Re I (CO) 3 frameworks lacking indacene units. 56 …”

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2

“…2 and agree with previously reported measurements on phlorins. 7,21 The phlorin, being a porphyrinoid, absorbs light with a Soret band at ~440 nm and in the Q band at ~650 nm. These absorption bands correspond to the S 0 → S 2 and S 0 → S 1 transitions, respectively.…”

“…Phlorin derivatives 3H(Phl F ) and 3H(Phl CO 2 t Bu ) were prepared using previously described methods. 7,18 Fig. 1 illustrates the phlorins employed in this study along with the solid state structure of 3H(Phl F ), which was previously obtained by X-ray diffraction.…”

Factors Controlling the Spectroscopic Properties and Supramolecular Chemistry of an Electron Deficient 5,5-Dimethylphlorin Architecture

“…These values are in line with those observed for other electron deficient phlorins. 36 Just as hydrogen-bonding to the N–H groups of 3H(Phl CF 3 ) perturbs the phorin spectral properties, the addition of either F – or OAc – to an MeCN solution of 3H(Phl CF 3 ) dramatically alters the phlorin redox properties. Formation of the F – and OAc – bound adducts ( 3H(Phl CF 3 )·2F – and 3H(Phl CF 3 )·OAc – , respectively) shifts all three phlorin oxidation waves to less positive potentials, which is expected upon going from the neutral phlorin to the negatively charged fluoride and acetate adducts.…”