Self-Assembly of Linear Porphyrin Oligomers into Well-Defined Aggregates

Abstract

Conjugated zinc porphyrin oligomers of various lengths are shown to form well-defined planar aggregates at low temperatures. The aggregation occurs over a narrow temperature interval (170–150 K) and is accompanied by dramatic changes in the electronic absorption and emission spectra. Similar changes are found in J-aggregates in which the transition dipole moments of aggregated chromophores couple to form a new and intense transition in the absorption spectrum, red shifted from the monomeric chromophore band. For the present porphyrin oligomers, the dramatic absorption changes are not associated with the formation of large aggregates, but rather with the dimerization accompanied by planarization of the oligomers. Free oligomers have a broad distribution of porphyrin–porphyrin dihedral angles and show a broad and unstructured absorption spectrum. As the oligomers stack to form aggregates, they planarize and the width of the conformational distribution is reduced to include virtually only the planar conformers, resulting in the observed change of the absorption spectrum. No experimental evidence for the formation of large aggregates was found, while a small aggregate, probably only dimer, is supported by the minor changes of the fluorescence rate constant upon aggregation and the fact that pyridine has no significant effect on the formation of this aggregate, which otherwise is very effective for inhibiting aggregation of zinc porphyrin oligomers. Compared to most porphyrin aggregates, which show broad absorption spectra and quenched fluorescence, these aggregates give sharp absorption and emission spectra with little change in the fluorescence quantum yield. Similar aggregates were also observed for oligomers substituted with both a fullerene electron acceptor and a ferrocene donor. The results presented here will be potentially useful as tools to understand how electron transfer and delocalization processes are influenced by molecular order/disorder transitions.

Photogenerated triplet states in supramolecular porphyrin ladder assemblies: an EPR study

“…In linear porphyrin oligomers with 3,5-bis(octyloxy)phenyl side groups aggregation was observed in non-coordinating solvents. 31,32 The relatively low concentration employed in this EPR study and the similarity of the obtained triplet state parameters to those obtained for porphyrins with THS side groups in a MTHF:py solvent mixture, 26 suggests, however, that aggregation is not a confounding variable in the present case. To further support this statement, a spectrum of P3 with the bigger and bulkier THS side groups, which are less prone to aggregation, was also recorded in toluene.…”

Towards efficient near-infrared fluorescent organic light-emitting diodes

“…Additionally, the rigidity of the oligomers in the solid-state matrix might also be responsible for the minimum Stokes’ shift (<10 nm) on all samples, analogous to what was observed in porphyrin oligomer complexes 39 and ladder-like conjugated polymers such as poly( para -phenylene) 51 . However, this minimal Stokes’ shift, together with the red-shift in the absorption (and PL) spectrum, may also be related to either intraoligomer, oligomer–oligomer, or polymer–oligomer dipole coupling in a J-aggregated manner 52 .…”

Semiconducting π-extended porphyrin dimer and its characteristics in OFET and OPVC

“…The field-effect mobilities of 1.06 Â 10 À4 and 7.65 Â 10 À5 cm 2 V À1 s À1 for the solutions of PD-1 in toluene and THF, respectively, were observed when each device was annealed at 120 C for 20 min, together with high on/off current ratios (I on /I off = 1.0 Â 10 4 and 1.0 Â 10 3 , respectively) and threshold voltages (V th % -7.54 and -3.32 V, respectively). For PD-1, the molecular planarity and extended p-conjugation of porphyrin framework generate strong p-p interactions with adjacent molecules in the solid state to afford self-assembled J-aggregated porphyrin arrays [20]. However, considerably high crystallinity is found in the case of device made of the mixed solvents of toluene and THF.…”