Both bands were found to include asymmetric stretching vibrations

Both bands were found to include asymmetric stretching vibrations of the methine bridges [17]. A number of fine structure bands, including combination of two vibrational quanta, were obtained and compared with available site-selected spectra from Shpolskii and noble-gas matrices. Both absorption and fluorescence spectra could be interpreted on the basis of the linear coupling model and a good applicability of the mirror-symmetry rule was established [17].Dendritic encapsulated metalloporphyrins mimic efficiently a number of functions expressed in biological systems. These are hemoglobin- and myoglobin-like gas-binding ability, heme mono-oxygenase activity, electron-acceptor capacity in light-harvesting antenna systems, and shell-modulated redox potentials as found in cytochromes [5].

One very interesting property of the dendritic molecules is their ability to create a microenvironment inside. Such dendron coating can protect porphyrins from the surrounding environment [21�C25]. The site isolation can be used for protecting an active pigment photo-center from de-excitation by oxygen [23] or potentially even change monomolecular photophysical parameters, hence to some extent controlling the lifetimes of the excited states. Such controlled molecular photosystems could be of use for applications like optical power limiting devices [26,27] or in sensing applications [28]. For such and related purposes, porphyrins decorated with bis-MPA dendrons were prepared [29]. Specifically, Bis-MPA (2,2-bis(methyolol)propionic acid) repeating units were used as building block in the synthesis of dendron-coated meso-tetraphenyl porphyrins (TPP).

They were further functionalized both as free-base porphyrin (TPPH2) and with a central zinc ion (TPPZn). Different sizes of molecules in terms of a systematic variation of the size of the dendrimer substituent were prepared, and their basic properties investigated [29]. For example, the hydrodynamic volume of the dendrimers could be Anacetrapib determined from polarization anisotropy decay data, and it was established that the bis-MPA dendrimers are significantly smaller than the same generation Fr��chet-type [30] benzyl ether TPP dendrimer. The larger dendrimer substituents of the zinc ion case gave rise to entirely new features in the absorption and fluorescence spectra [29]: A broad shoulder at longer wavelengths was more prominent in the emission spectra of the larger dendrimers however, only in the case with the zinc ion in the center. The proto-porphyrin analogue did not show this size-effect. Since a large substituent could impose a larger ��stress�� to the molecule than a small one, we anticipate that this could also affect the porphyrin ring configuration and its associated vibronic structure to different extent.

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