The Hyperporphyrin Concept: A Contemporary Perspective.

The Gouterman four-orbital model conceptualizes porphyrin UV-visible spectra as dominated by four frontier molecular orbitals-two nearly degenerate HOMOs and two exactly degenerate LUMOS under D 4h symmetry. These are well separated from all the other molecular orbitals, and normal spectra involve transitions among these MOs. Unusual spectra occur when additional orbitals appear in this energy range, typically as a consequence of the central coordinated atom. For example, metals with empty d orbitals in a suitable energy range may lead to charge transfer from porphyrin (ligand) to metal, that is, so-called LMCT transitions. Metals with filled p or d orbitals may lead to charge transfer from metal to porphyrin, MLCT transitions. These cases lead to additional peaks and/or significant redshifts in the spectra and were classified as hyperporphyrins by Gouterman. Cases in which spectra are blueshifted were classified as hypsoporphyrins; they are common for relatively electronegative late transition metal porphyrins. Many of the same principles apply to porphyrin analogues, especially corroles. In this Perspective, we focus on two newer classes of hyperporphyrins: one reflecting substituent effects in protonated or deprotonated free-base tetraphenyporphyrins and the other reflecting "noninnocent" interactions between central metal ions and corroles. Hyperporphyrin effects on spectra can be dramatic, yet they can be generated by relatively simple changes and subtle structural variations, such as acid-base reactions or the selection of a central metal ion. These concepts suggest strategies for engineering porphyrin or porphyrinoid dyes for specific applications, especially those requiring far-red or near-infrared absorption or emission.

Understanding Hyperporphyrin Spectra: TDDFT Calculations on Diprotonated Tetrakis(p-aminophenyl)porphyrin.

A detailed TDDFT study (with all-electron STO-TZ2P basis sets and the COSMO solvation model) has been carried out on the effect of diprotonation on the UV-vis-NIR spectra of free-base tetraphenylporphyrin and tetrakis(p-aminophenyl)porphyrin. The diprotonated forms have been modeled as their bis-formate complexes, i.e., as so-called porphyrin diacids. The dramatic redshift of the Q-band of the TAPP diacid has been explained in terms of an elevated "a2u" HOMO and lowered LUMOs, both reflecting infusion of aminophenyl character into the otherwise classic Gouterman-type frontier MOs. The exercise has also yielded valuable information on the performance of different exchange-correlation functionals. Thus, the hybrid B3LYP functional was found to yield a substantially better description of key spectral features, especially the diprotonation-induced redshifts, than the pure OLYP functional. Use of the range-separated CAMY-B3LYP functional, on the other hand, did not result in improvements relative to B3LYP.

Gold Tris(carboxyphenyl)corroles as Multifunctional Materials: Room Temperature Near-IR Phosphorescence and Applications to Photodynamic Therapy and Dye-Sensitized Solar Cells.

Two amphiphilic corroles-5,10,15-tris(3-carboxyphenyl)corrole (H3[mTCPC]) and 5,10,15-tris(4-carboxyphenyl)corrole (H3[pTCPC])-and their gold complexes have been synthesized, and their photophysical properties and photovoltaic behavior have been investigated. Like other nonpolar gold corroles, Au[mTCPC] and Au[pTCPC] were both found to exhibit room temperature phosphorescence in deoxygenated solutions with quantum yields of ∼0.3% and triplet lifetimes of ∼75 µs. Both compounds exhibited significant activity as dyes in photodynamic therapy experiments and in dye-sensitized solar cells. Upon irradiation at 435 nm, both Au corroles exhibited significant phototoxicity against AY27 rat bladder cancer cells while the free-base corroles proved inactive. Dye-sensitized solar cells constructed using the free bases H3[mTCPC] and H3[pTCPC] exhibited low efficiencies (≪1%), well under that obtained with 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin, H2[pTCPP] (1.9%, cf. N719 9.5%). Likewise, Au[pTCPC] proved inefficient, with an efficiency of ∼0.2%. By contrast, Au[mTCPC] proved remarkably effective, exhibiting an open-circuit voltage (Voc) of 0.56 V, a short-circuit current of 8.7 mA cm(-2), a fill factor of 0.72, and an efficiency of 3.5%.

NMR Study of Hyperporphyrin Effects in the Protonations of Porphyrins with 4-Aminophenyl and 4-Pyridyl Meso Substituents.

Titrations for a series of porphyrins bearing either 4-aminophenyl or 4-pyridyl meso substituents were performed using methanesulfonic acid in DMSO and followed by proton NMR. Special emphasis was placed on identifying the intermediate protonation stages that are described as hyperporphyrins, that is, where there exist strong charge-transfer interactions between the peripheral aminophenyl groups and the protonated porphyrin ring. In particular, evidence was gathered to support the significance of a novel resonance form involving charge transfer between two peripheral substituents, an aminophenyl group and a protonated pyridinium group. (1)H NMR and NOESY spectra provide evidence for the importance of such resonance effects in the triply protonated triamino/monopyridyl hyperporphyrin (A3PyPH3(+3)).

Hyperporphyrin effects in the spectroscopy of protonated porphyrins with 4-aminophenyl and 4-pyridyl meso substituents.

Spectrophotometric titrations for a full series of 4-aminophenyl/4-pyridyl meso-substituted porphyrins were carried out using methanesulfonic acid in DMSO to study the hyperporphyrin effect across different substitution patterns. The series included zero, one, two (cis and trans), three, and four meso(4-aminophenyl) groups, with the remaining meso substituents being 4-pyridyl groups. The peripheral pyridyl groups consistently protonate before the interior porphyrin pyrrole nitrogens, which protonate before the aminophenyl groups. Aminophenyl substituents increase the basicity of the pyrrole nitrogens and lead to distinctive hyperporphyrin spectra with a broad Soret band and a strong red absorption. The structure proposed to give rise to these spectra is the previously proposed charge-transfer interaction between the aminophenyl and the protonated pyrrole. A novel hyperporphyrin structure involving charge-transfer interactions between two peripheral substituents is also proposed in one case, the triply protonated (+3) porphyrin with three aminophenyls and one pyridyl substituent; two of the aminophenyl groups delocalize the charges on the interior nitrogens, while the third aminophenyl group delocalizes with the protonated pyridyl.

Spectroscopy of protonated tetraphenylporphyrins with amino/carbomethoxy substituents: hyperporphyrin effects and evidence for a monoprotonated porphyrin.

Spectrophotometric titrations for a full series of para-amino/carbomethoxy-substituted tetraphenylporphyrins were carried out using methanesulfonic acid in DMSO to study the hyperporphyrin effect across different substitution patterns. The series included zero, one, two (cis and trans), three, and four amino groups, with the remaining para substituents carbomethoxy groups. With increasing numbers of aminophenyl groups, the relative basicity increased and the hyperporphyrin effect increased, marked by a strong red band and a split Soret band. The cis diamino derivative showed a stronger hyperporphyrin effect compared to the trans isomer, which can be explained based on simple resonance forms. For the monoamino derivative, an initial increase in the Soret band upon acid titration along with well-defined isosbestic points provided evidence for a monoprotonated porphyrin, distinct from the diprotonated and triprotonated states. The relative stability of this unusual intermediate is proposed to be due to charge delocalization of the first cation to the single amino group and destabilization of the second protonation by the electron-withdrawing carbomethoxy substituents.

Reaction of dichloromethane with pyridine derivatives under ambient conditions.

Pyridine derivatives and dichloromethane (DCM) are commonly used together in a variety of different applications. However, DCM slowly reacts with pyridine and a variety of other representative pyridine derivatives to form methylenebispyridinium dichloride compounds under ambient conditions. The proposed mechanism (two consecutive S(N)2 reactions) was studied by evaluating the kinetics of the reaction between 4-(dimethylamino)pyridine and DCM. The second-order rate constants for the first (k(1)) and second (k(2)) substitutions were found to be 2.56(+/-0.06) x 10(-8) and 4.29(+/-0.01) x 10(-4) M(-1) s(-1), respectively. Because the second substitution is so much faster than the first, the monosubstitution product could not be isolated or detected during the reaction; it was synthesized independently in order to observe its kinetics.

Photoelectropolymerization of aniline in a dye-sensitized solar cell.

A dye-sensitized solar cell was constructed using a porphyrin photosensitizer and, in place of the usual iodide redox system, a solution in aniline solvent containing lithium perchlorate electrolyte, camphorsulfonic acid, and poly(ethylene oxide) copolymer. Irradiation generated polyaniline within the cell, initially following a proposed photoelectropolymerization mechanism, and eventually operating as a solar cell with polyaniline as the hole transport medium. Overall energy conversion efficiency was 0.8% at moderate light intensities (14.6 mW cm(-2)) but lower at higher light intensities due to conductivity limitations.