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Space and Time Resolved Ultrafast Dynamics of Few Porphyrins Derivatives in Nanosystems

Final Report Summary - FENASY (Space and time resolved ultra-fast dynamics of few porphyrins derivatives in nanosystems)

The goals set forth in the original project (FENASY) were focused on obtaining comprehensive information and in-depth knowledge about the interaction of selected potential candidates for use in nanoscience, nanotechnology and nanomedicine with chemical and biological nanocavities, such as cyclodextrins, human serum albumin protein and mesoporous silicate nanomaterials. During the period of the contract we carried out extensive studies using picoseconds and femtosecond spectroscopic techniques to investigate the excited state behaviour of palladium pthalocynine interacting with MCM-41 mesoporous material and 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin (p-THPP) confined by the human serum albumin protein.

Following is an outline of the main results stemming from these studies.

1. Fast to ultrafast dynamics of palladium phthalocyanine covalently bonded to MCM-41 mesoporous material
In this work, we have shown that the steady-state absorption and emission spectra of a suspension (in dichloromethane) of palladium phthalocyanine (MO-PdPc) covalently bonded to the internal framework of MCM-41 are different from the nonbonded one prepared by physical sorption (PdPc_MCM41). The presence of a new absorption band in MO-PdPc at 708 nm suggests a large distortion of the PdPc molecular frame inside the nanochannels. Moreover, the emission decay also shows a new component with a lifetime of 1.4 ns.

The results of the femtosecond studies showed two times in the ultrafast dynamics. The shortest one changes from 170 to 500 fs, depending on the interrogated region and was assigned to intramolecular vibrational-energy redistribution, S2-S1 internal conversion and probably to a photoinduced Pd ion loosing. The other one has a time constant of 1.5 - 4.4 ps, which reflects vibrational relaxation/solvent cooling at the S1 manifold. At a longer time scale, the presence of Pd ion in the PdPc sample induces a 20 ps decay in addition to the nanosecond one (5.4 ns), observed in the free-base emission as a single exponential decay (6.1 ns). The results show that bonding PdPc to the framework of MCM-41 alters its chemical physics properties, spectroscopy, and longer-time relaxation.

The obtained results from this study suggest the possibility to tune and modify the excited state properties of palladium phthalocyanine, upon interaction with MCM-41 mesoporous material with potential applications in various important scientific areas, such as nanophotonics, photodynamic therapy and photovoltaic and solar cell design by encapsulation and by changing the mode of preparation of the selected complexes.

This work, of which Dr Anna Synak was the leading author, was published in the Journal of Physical Chemistry C, 2009, 113, 19199-19207 and was a direct result of the collaborative work within Prof. Douhal's group and the collaboration with Prof. Felix Sanchez from Instituto de Quimica Organica, CSIC in Madrid.

2. Femtosecond studies of a confined porphyrin derivative by human serum abumin potein
Through a combination of steady-state, and femtosecond and picosecond-resolved fluorescence spectroscopy we studied the excited state relaxation dynamics of p-THPP and p-THPP-human serum albumin protein complex. The dynamics of p-THPP in pure solvent can be described by four consecutive processes: a fast internal conversion from the B state to the Qy state taking place within 80 fs, internal conversion from the Qy to the Qx state in 140 - 200 fs (120 - 130 fs in MeOH), vibrational relaxation in the Qx state in 2 - 3.9 ps, and the intersystem crossing from Qx to the triplet state with a lifetime of 9.1 ns (8.3 ns).

When p-THPP was encapsulated within the HSA, the B to Qy and Qx to Qy internal conversion are slightly faster and occur within about 50 fs and 100 fs, respectively, while the lifetime of the relaxed Qx state is slightly longer, 9.9 ns. The most prominent changes are observed in the dynamics of the hot Qx state that are accompanied by energy transfer to the protein with the time constant of about 1 ps that decreases the final population of the relaxed Qx state when compared to the pure solvent. Further vibrational relaxation dynamics in the hot Qx state that takes place in even longer 17 - 32 ps time scale and its slowing down when compared to the pure solvent can be explained by the caging effect of the water molecules inside protein.

The results of this study have a wide impact since the metal phtalocyanines find broad applications in enzymatic catalysis, photonics and photodynamic therapy (PDT). For PDT p-THPP may be used as a potential drug for anticancer treatment. The porphyrins are usually introduced in the blood as relatively concentrated solutions, which may diminish its action or even cause adverse effects. Interactions with the human serum albumin protein may control their efficacy and biodistribution. Thus, our study of the interaction of those molecules with HSA might provide the basis for further investigations to help formulate safe drug and effective dosages. The manuscript corresponding to these results will be submitted soon for publication.

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