Bhaskar Chilukuri

Comprehensive structure–function correlation of photoactive ionic π-conjugated supermolecular assemblies: an experimental and computational study

We provide a structure–function relationship study of an organic crystalline photoconductor composed of oppositely charged ionic porphyrins. Nano to millimeter size crystals with well-defined morphology composed of stoichiometric amounts of meso-tetra(N-methyl-4-pyridyl)porphyrin (TMPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) were grown in a controlled and reproducible manner. The rod shaped TMPyP:TSPP monoclinic P21/c crystals have a pseudo-hexagonal cross section and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions. Experimental characterization of the TMPyP:TSPP solid was performed using powder-XRD, AFM, SEM, DRS UV-visible, and photoconductivity measurements. For the first time the morphology of an ionic porphyrin solid is predicted. The TMPyP:TSPP crystals are non-conducting in the dark but become conductive with illumination. The n-type photoconductive response is significantly faster with excitation in the Q-band than with excitation in the Soret band. Quantum mechanical calculations were performed to determine the electronic band structure and density of states and to explain the photoconduction in TMPyP:TSPP. Based on these results we propose a model in which two types of photoconductivity occur: (1) band conduction which occurs at all excitation wavelengths and (2) hopping conductivity caused by metastable photoinduced defects that form primarily at higher energy excitations. This work combines the results from structural and theoretical studies and correlates them with electronic and optoelectronic properties thereby opening the road to the engineering of highly-organized functional materials from organic π-conjugated molecules.

Tuning the optoelectronic characteristics of ionic organic crystalline assemblies

The effect of selective metallation of free-base ionic porphyrin tectons on the structural, electronic, and optical properties of their crystalline self-assemblies is presented. Rod-like crystals were prepared under neutral pH conditions by combining meso-tetra(N-methyl-pyridyl)porphyrin, H2TMPyP, and meso-tetra(4-sulfonatophenyl)porphyrin, H2TSPP, with either a nickel or a copper ion contained in one of the synthons. These materials were characterized by optical microscopy, X-ray diffraction methods, thermogravimetric analysis, diffuse reflectance UV-visible and luminescence spectroscopies, and conductivity and photoconductivity measurements. All the porphyrin assemblies formed monoclinic P21/c crystals with pseudo-hexagonal cross-sections. Thermogravimetric experiments indicate that water molecules associated with crystals desorb at two different rates. In addition, temperature dependent XRD showed that the dehydration of the porphyrin solids causes modification in the crystals which is completely reversible up to 100 °C annealing (i.e., crystals return to their original structural geometry upon rehydration). All the metallated porphyrin crystals exhibit dark conductivity at moderately high temperatures and become more conductive upon photoexcitation. The photoresponse of the H2TMPyP:CuTSPP-substituted crystals is significantly higher than that of the CuTMPyP:H2TSPP and the Ni-substituted crystals. The Cu-substituted systems, like the parent free-base, exhibit persistent photoconductivity resulting from excitations into the Q and Soret bands. The primary charge carriers in these solids upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. Quantum mechanical calculations provide the electronic band structure and the density of states and explain the experimental prompt photoconductivity measurements of the porphyrin self-assemblies. This work provides evidence that optoelectronic properties of organic semiconductors can be effectively tuned by introducing transition metals into their crystal structures.