This review summarizes recent advances in the use of porphyrins, phthalocyanines, and related compounds as components of solar cells, including organic molecular solar cells, polymer cells, anddye-sensitized solar cells. The recent report of a porphyrin dye that achieves 11% power conversion efficiency in a dye-sensitized solar cell indicates that these classes of compounds can be as efficient as the more commonly used ruthenium bipyridyl derivatives.

Tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbs strongly onto nanoparticulate TiO2 and serves as an efficient photosensitizer for solar-energy conversion by TCPP-sensitized TiO2 electrodes. Nanoparticulate TiO2 electrodes were prepared from Degussa P25 TiO2 powder in the standard manner for a Grätzel cell. Adsorption studies of TCPP onto these sintered TiO2 electrodes gave a saturation surface coverage of 47 μmol/g. Adsorption studies of TCPP onto colloidal dispersions of Degussa P25 in ethanol gave a saturation surface coverage of 77 μmol/g. The difference between the saturation coverages is attributed to the reduction of the available surface area in the TiO2 films after sintering, from 55 m2/g as a free colloid to about 34 m2/g as a sintered electrode. The nature of the binding of TCPP onto the TiO2 electrodes was investigated using X-ray photoelectron spectroscopy (XPS) and Resonance Raman Spectroscopy (RRS). In the XPS spectra of TiO2 with adsorbed TCPP, the O (1s) and Ti (2p3/2) peaks of TiO2 were shifted to a higher binding energy value, by about 0.3 eV, and the O (1s) and N (1s) peaks of TCPP were shifted to a higher binding energy, by about 0.7 eV. Upon adsorption of TCPP, one of the Ti (2p3/2) peaks of TiO2 disappeared, suggesting complexation and removal of surface states. The RRS results indicated that for cases in which TCPP was adsorbed onto TiO2 films from ethanolic solutions of about 1 μM concentration, the porphyrin spectrum showed distinctive interactions with the surface, but for cases in which it was adsorbed from higher concentrations, the RRS spectra were similar to spectra of TCPP powder, indicating the dominance of porphyrin−porphyrin interactions. We conclude that lateral interactions between adsorbed TCPP are significant upon adsorption from all but the lowest (micromolar) initial concentrations. Photovoltaic cells with TCPP-sensitized TiO2 electrodes gave good solar-energy conversion efficiencies. At light simulating one sun (AM 1.5), a cell sensitized by TCPP gives a short-circuit photocurrent of about 6 mA/cm2 and an open-circuit photopotential of 485 mV. The incident photon-to-current conversion efficiency was 55% at the Soret peak and 25−45% at the Q-band peaks; the cells have a fill factor of 60−70% and an overall energy conversion efficiency of about 3%.

UV-visible spectra have been studied for a series of p-substituted tetraphenylporphyrins (TPPs) titrated with strong acid in various solvents. Substituent effects on the Soret and Q(I) absorption peaks and the fluorescence emission peaks have been treated by Hammett correlations. In general, there are only small effects with electron-withdrawing substituents, but electron-donating substituents lead to lower energy transitions, with especially strong effects observed in the case of the diprotonated porphyrins with good electron-donating substituents (hyperporphyrin effects). The hyperporphyrin effects are attributed to the crossing of a π molecular orbital on the substituted phenyl group above the usual porphyrin π highest occupied molecular orbital. For the neutral TPPs, this crossing is estimated to occur with a substituent as electron-donating as p-methoxy, and for the diprotonated TPPs, the crossing occurs at approximately unsubstituted TPP. Distinctive solvent effects on the spectra and the Hammett correlations are observed.

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.

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%.

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Porphyrins substituted at meso positions with aminophenyl groups undergo oxidative electropolymerization in a process analogous to the formation of polyaniline. Porphyrins that successfully generate polymer films on the electrode include those tetrasubstituted with four p-aminophenyl groups, trisubstituted, or transdisubstituted, but not cis-disubstituted or monosubstituted. The polymerization process is monitored independently by cyclic voltammetry, absorption spectroscopy, and an electrochemical quartz crystal microbalance. The mechanism is considered analogous to aniline polymerization, except that attack of electrophilic nitrogens must occur at ortho positions of another aminophenyl group. Reflectance FT-IR and resonance Raman spectroscopy detect the presence of diphenylamine, dihydrophenazine, and phenazine linkages in the porphyrin polymer film from tetra(4-aminophenyl)porphyrin (TAPP). TAPP polymerized in dichloromethane (DCM) with added pyridine gradually passivates; i.e., electronic conductivity diminishes and polymer growth levels off (the films are light yellow). Without added pyridine, electronic conductivity is sustained and film growth continues to a thick black film. Diphenylamine and dihydrophenazine linkages were common in films whose electroactivity remained constant throughout the film growth process while phenazine linkages were prevalent in films where passivation and loss of electrochemical activity had occurred. It is proposed that overoxidation to the phenazine structures leads to loss of electronic conductivity, analogous to formation of pernigraniline in polyaniline. The morphology of poly-TAPP is a highly interconnected nanofibrous network, with fiber diameters in the range 40-100 nm, with somewhat different structures depending on polymerization conditions.

This paper describes the results of instituting peer-led team learning workshop sessions as optional accompaniments for the first two years of chemistry (general chemistry and organic chemistry), courses taught in large-lecture format. About 30% of the students in the courses choose to add this one-credit, two-hour weekly course. Data have been collected for the organic chemistry course over the past five years to compare the outcomes for students who selected workshops with those for students who did not select the workshop option with respect to student success (achievement of a grade of C- or higher), student persistence (completing all three terms successfully in the same academic year), and student performance (percentage of total points accumulated). Students who elected to take the workshops had a somewhat higher overall grade point average (GPA), based on all courses taken (3.26 vs 3.14). However, the gains of the workshop students in each category—success rate (85% vs 69%), three-term persistence (57% vs 28%), and course performance (71% vs 65%, course GPA of 2.90 vs 2.51, ACS exam scores at the 77th vs 69th percentile)—are all significantly greater than can be attributed solely to differences in student GPA.