Click Reaction Synthesis and Photophysical Studies of Dendritic Metalloporphyrins

Several dendritic zinc(II)-porphyrins bearing carbazole units at the terminals have been prepared through click reaction of azide-substituted Zn-porphyrin precursors and carbazole-based alkynes under [Cu(NCCH 3)4][PF6] catalysis. This family of new dendritic metalloporphyrins shows dual luminescence from both the upper S 2 and the lowest S1 singlet states. The observed trends in the spectroscopic data and the photophysical properties of the dendrimers have been rationalized in terms of the type of meso-spacer between the macrocycle and dendritic shell. The key feature of the meso-spacer is proposed to be the degree of hindrance towards aryl ring rotation relative to the mean porphyrin plane. Based on the observed differences in the S1→S0 fluorescence quantum yield on going from four- to five-coordinate dendrimers, it was shown that dendrimer architectures with all four meso-aryl spacers sterically hindered, are most appropriate to monitor processes related to axial ligation of the dendrimer core, since they provide larger luminescence response differences between the four-coordinate and five-coordinate forms. The blue S2→S0 fluorescence quantum yield has been measured and the observed trend has been rationalized in terms of the S 2→S1 energy gap law. No significant differences were observed between the compounds either with different rotational degree of freedom of the meso-spacers or with different ligation states of the dendrimer core. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.status: publishe

Femtosecond resonance enhanced CARS for background-free detection of organic molecules

We study the coherent excitation profile (CEP) of resonance enhanced femtosecond CARS in a model system zinc phthalocyanine in a polymer film host as a prospective technique for detection and identification of molecular species in ambient environments. A new method of suppressing the non-resonant FWM background is demonstrated. Transform theory is applied to calculate CEP based on the absorption spectrum, and good agreement between theory and experiment is obtained

Porphyrin Core Dendrimers with Ether-Linked Carbazole Dendrons: Dual Luminescence of Core and Conformational Flexibility of Dendritic Shell

The ground state absorption spectra, spectra and lifetimes of the luminescence originating from the lowest S-1 (Q(0,0)) and higher S-2 (B) excited states of porphyrin core in the dendrimers with ether-linked carbazole dendrons have been studied in detail. The relationship between the molecular design of the dendritic shell and the optical features of the dendrimer core in solution is presented. The influence of the axial ligation of the porphyrin core and the conformational flexibility of the dendritic shell on the luminescent properties of the studied dendrimers is revealed

Quantum chemical insights into the dependence of porphyrin basicity on the meso-aryl substituents: thermodynamics, buckling, reaction sites and molecular flexibility

The chemical and sensing properties of porphyrins are frequently tuned via the introduction of peripheral substituents. In the context of the exceptionally fast second protonation step in the case of 5,10,15,20- tetraphenylporphyrin (TPP), as compared to porphin and 5,10,15,20-tetramesitylporphyrin (TMesP), we investigated the macrocycle-substituent interactions of these three porphyrin derivatives in detail. Using quantum chemical thermodynamics calculations, the analysis of geometric structures, torsional profiles, electrostatic potential distributions, and particularly the analysis of molecular flexibilities via ab initio molecular dynamics simulations, we obtained a comprehensive picture of the reactivities of the studied porphyrins and how these are influenced by the meso-substituents. As compared to porphin and TMesP the second protonation of TPP is energetically more favorable and is particularly energetically comparable to its first protonation, instead of being significantly less favorable like in the case of porphyrin and TMesP. Additionally, the second TPP protonation is facilitated by an interplay between out-of-plane (oop) distortion of the protonation site and a pronounced electrostatic binding spot at the protonation site. Furthermore, the second protonation is particularly facilitated in the case of TPP by the large oop-flexibility of the diprotonated species as unraveled by ab initio molecular dynamics simulations