Stirring Effects on the Spontaneous Formation of Chirality in the Homoassociation of Diprotonated meso-Tetraphenylsulfonato Porphyrins

Abstract

The chirality of stepped sheetlike porphyrin aggregates, which are formed from achiral diprotonated sulfonato-substituted tetraphenylporphyrins, can be affected by the direction of the vortex during stirring or rotary evaporation (see scheme). This vortex effect can be attributed to enhancement of the chirality fluctuations that originate in the aggregation. Thus, in this sense, the phenomenon could be general for supramolecular systems that are obtained under kinetic control. Homoassociates of the achiral title porphyrins in acid solutions show spontaneous symmetry breaking, which can be detected by circular dichroism (CD). The CD spectra are due to differential scattering and differential absorption contributions, the relative significance of which is related to the shape and size of the homoassociate. When an earlier model, designed for the association of these diprotonated porphyrins (J aggregates with the geometry of stepped sheets of intramolecular-stabilised zwitterions), was applied to an exciton-coupling point-dipole approximation, the folding of the one-dimensional homoassociates explained the CD signals detected. An effect of the vortex direction, caused by stirring or rotary evaporation, upon the exciton chirality sign was detected. In the case of H2TPPS3, the number of experiments performed gave a statistical significance to this effect. This vortex effect can be attributed to enhancement of the chirality fluctuations that originate in the diffusion-limited aggregation to high-molecular-weight homoassociates. In this sense, the phenomenon could be general for supramolecular systems that are obtained under kinetic control, and its detection would be possible when inherent chiral chromophores were being generated in the association process. En solucions àcides, aquestes porfirines aquirals formen homoassociats que presenten trencament de simetria espontani, el qual hom detecta per DC. Els espectres de DC són deguts al conjunt de contribucions de dispersió diferencial i d'absorció diferencial, la importància relativa de les quals depèn de la mida i forma dels homoassociats. Segons un model publicat anteriorment per a l'associació d'aquestes porfirines diprotonades, llurs J agregats són zwitterions estabilitzats intermolecularment per formació de tires de porfirines escalonades. L'aplicació de l'aproximació punt-dipol del model d'acoblament excitònic a aquestes estructures explica el DC per la formació de colzes en els homoassociats monodimensionals lineals. Agitant o rotoevaporant hom detecta un efecte de la direcció del vòrtex en el signe de la quiralitat. En el cas de H2TPPS3 el nombre d'experiments efectuats dòna significat estadístic a l'efecte observat. Aquest efecte de la direcció del vortex podria esser atribuïble a una amplificació de les fluctuacions de quiralitat que es produeixen durant l'agregació limitada per difussió cap a homoassociats d'alt pes molecular. En aquest cas, el fenòmen podria ser general en la formació sota control cinètic d'homoassociats d'alt pes molecular, i detectar-lo fora possible quan, com en el cas que hom presenta, els cromòfors generats en el procés d'associació fossin inherenment quirals Supporting Information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2111/2001/f2124_s.pdf or from the author: Table with the first series of experiments (91) on H2TPPS4-f, H2TPPS4, H2TPPS3 H2TPPS2O and H2TPPS2A. Tables with the chirality signs at the BH, BJ and Q bands of all compounds and experiments. Example of the evolution of a metastable solution. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.