Ground and Excited State Electronic Properties of Halogenated Tetraarylporphyrins: Tuning the Building Blocks for Porphyrin-based Photonic Devices

Abstract

<jats:p> The rational design of molecular photonic devices relies on the ability to select components with predictable electronic structure, excited state lifetimes and redox chemistry. Electronic communication in multiporphyrin arrays depends critically on the relative energies and electron density distributions of the frontier molecular orbitals, especially the energetically close highest occupied molecular orbitals (a<jats:sub>2u</jats:sub> and a<jats:sub>1u</jats:sub>). To explore how these ground and excited state properties can be modulated, we have synthesized and characterized 40 free base ( Fb ), magnesium and zinc tetraarylporphyrins. The porphyrins bear meso-substituents with the following substitution patterns: (1) four identical substituents (phenyl, o-chlorophenyl, p-chlorophenyl, o,o'-difluorophenyl, pentafluorophenyl, mesityl); (2) one, two, three or four o,o'-dichlorophenyl substituents; (3) one p-ethynylphenyl group and three mesityl or pentafluorophenyl groups; (4) one p-ethynyl-o,o″-dichlorophenyl or p-ethynyl-o,o″-dimethylphenyl and three phenyl groups. For each neutral complex the ground state electronic properties were investigated using electrochemical methods and optical absorption spectroscopy. Similarly the absorption, emission, and relaxation properties of the lowest singlet excited state were probed by time-resolved absorption and fluorescence methods. Each oxidized complex was investigated by static absorption and liquid and frozen solution EPR spectroscopy. The collective results of these investigations have provided insights into the direct (orbital overlap) and indirect (inductive/conjugative) mechanisms by which halogenated phenyl rings influence the static and dynamic electronic properties of neutral and oxidized porphyrinic chromophores. Three key findings are as follows. (1) The effective electron-withdrawing strength of halogenated phenyl rings required to reverse the ordering of the a<jats:sub>2u</jats:sub> and a<jats:sub>1u</jats:sub> HOMOs in Mg versus Zn tetraarylporphyrins has been elucidated. (2) Appropriate halogenation can significantly increase the excited state lifetime of a Zn porphyrin relative to the unsubstituted complex. (3) Halogenation can be used to modulate redox potentials in a manner that complements the enhancement of other electronic properties. The insights gained from study of this library of porphyrins provide a foundation for tuning the electronic properties of monomeric porphyrins as building blocks for multichromophoric assemblies in optoelectronics and other applications. </jats:p>

Synthesis, Spectra and Photophysics of some Free Base Tetrafluoroalkyl and Tetrafluoroaryl Porphyrins with Potential Applications in Imaging¶

“…Fluorescence lifetimes (τ) were determined for all four compounds and are also included in Table 2. The values for CF 3 P and C 3 F 7 P are in reasonable agreement with values previously reported for these compounds in dichloromethane solution (24), whereas our values for C 6 F 5 P and C 6 F 2 H 3 P are slightly lower than those given for these compounds in toluene solution (22). These slight differences in fluorescence yields and lifetimes of the fluoroaryl compounds may be because of solvent effects and do not significantly affect the discussion on the photophysics of these compounds.…”

“…Fluorescence quantum yields were also determined, and all fluorescence data is given in Table 2. The quantum yields for CF 3 P and C 3 F 7 P are similar to those previously reported for these compounds in dichloromethane (24), whereas the values of the two fluorinated arylporphyrins are comparable to, but slightly lower than, those previously reported for these compounds in toluene solution (22). Fluorescence lifetimes (τ) were determined for all four compounds and are also included in Table 2.…”

Engineering of relevant photodynamic processes through structural modifications of metallotetrapyrrolic photosensitizers

“…Halogen atoms are strongly σ-electron withdrawing and do not operate as π-electron donors. Fluorinating the phenyl groups of TPP in the meso positions also introduces electron withdrawing substituents that operate exclusively through the inductive effect, as shown after the phenyl group replacement by difluorophenyl rings (TF 2 PP), in free-base, Mg-and Zn-containing porphyrins[131]. Hence, halogen groups stabilize the reduced state of tetrapyrrolic macrocycles (especially in bacteriochlorins), both by electronic and steric effects.…”

Examination of Chlorin–Bacteriochlorin Energy‐transfer Dyads as Prototypes for Near‐infrared Molecular Imaging Probes†

“…Here the excited bacteriochlorin (produced essentially quantitatively from the excited chlorin) undergoes electron transfer to give the reduced chlorin and the oxidized bacteriochlorin. This process is expected to be more energetically favorable than the formation of C + – B − because bacteriochlorins are more easily oxidized and harder to reduce than chlorins for a given metalation state (60). Given that zinc chlorins are harder to reduce than free base chlorins (42,59), the product state C − – B + in will be higher in energy for ZnC‐FbB than in FbC‐FbB , which underlies the substantial diminution of the quenching process in the dyad containing the zinc chorin.…”