Palladium phthalocyanine (PdPc) covalently bonded to the internal framework of MCM-41 mesoporous structured silicates has been synthesized and studied by UV-vis steady-state absorption and fluorescence spectroscopy, and by pico-and femtosecond time-resolved emission spectroscopy in dichloromethane suspensions. We compare the results obtained for covalently bonded (MO-PdPc) and diffusion-formed (PdPc_MCM41) samples. A significant broadening of the diffuse transmittance spectra of both materials is observed. The effect is due to electrostatic and specific (through H-bonds) interactions of the guest with the host, but to some extent, formation of dimers also contributes to this broadening. However, only the MO-PdPc shows an additional absorption band around 708 nm. The static emission spectrum of MO-PdPc is different from that of PdPc, but it is similar to that of the metal-free phthalocyanine one. The results are explained in terms of confinement effect of the mesoporous material inducing deformation of the phthalocyanine molecule, and probably to the involvement of a photoinduced and reversible metal ejection process. The rise of new 1.4-ns component in the time-correlated single-photon counting emission decays of MO-PdPc reflects the nature of the confinement effect on the lifetimes of the guest. The ultrafast (fs regime) emission measurements show moderate dependence of the observed times on the type of inclusion (covalently bonded versus nonbonded). The origin of these times, ∼170 -500 fs and 1.5 -4.4 ps, is discussed according to the previous findings and nature of the nanosystem.

A new method for extending the utilizable range of Förster resonance energy transfer (FRET) is proposed and tested by the Monte Carlo technique. The obtained results indicate that the efficiency of FRET can be significantly enhanced at a given distance if the energy transfer takes place toward multiple acceptors that are closely located on a macromolecule instead of a single acceptor molecule as it is currently used in FRET analysis. On the other hand, reasonable FRET efficiency can be obtained at significantly longer distances than in the case of a single acceptor. Randomly distributed and parallel orientated acceptor transition moments with respect to the transition moment of the donor molecule have been analyzed as two extreme cases. As expected, a parallel orientation of donor and acceptor transition moments results in a more efficient excitation energy transfer. This finding could be used to directly reveal the assembly/deassembly of large protein complexes in a cell by fluorescence microscopy.

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The relaxation dynamics of 5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin (p-THPP) in tetrahydrofuran (THF) and encapsulated within the human serum albumin (HSA) protein in water solution was investigated. The protein environment affects the B→Q(y) and Q(x)→Q(y) transition dynamics (from 80 and 140-200 fs in THF to 50 and 100 fs in HSA, respectively) as well as the lifetime of the relaxed Q(x) state (9.1 vs 9.9 ns). The most prominent differences are observed in the relaxation dynamics in the hot Q(x) state in HSA, which includes the energy transfer to the protein in ∼1 ps and much slower solvent-assisted thermal equilibration component of about 20-30 ps.