Playing with light on molecular and hybrid nanomaterials

The aim of my thesis was the designing and the investigation of molecular antennas (dendrimer of nanometric size) and nanohybrid systems able to collect and use light. Most of the compounds studied are based on Ru(II)- or Os(II)-polypyridine subunits as chromophores and polyaromatic organic moieties as electron acceptor or energy donor in photoinduced processes. A new heptanuclear, mixed-metal luminescent compound has been assembled to act efficiently as light-harvesting device. In this nanosized molecular dendrimer all the light energy absorbed by the ancillary Ru(II) chromophores is efficiently funnelled to the central Os(II) one, which acts as energy trap. All the intrinsic photoinduced processes have been investigated in details by pump-probe transient absorption spectroscopy. A new class of light-harvesting antennae has been prepared by exploiting the template-assisted host-guest chemistry of mesoporous organosilica. A new blue light-absorbing species formed as a result of aggregation phenomena of BODIPY dye inside mesoporous silica. This species has been demonstrated to be able to transfer its excitation energy quantitatively to BODIPY monomers inside the nanopores of mesoporous silicas, in a time range between 20 and 80 ps depending on the relative localisation and distribution. Because of the dual properties of a piridinium species, able to act as luminophore and electron acceptor subunit, mixed hybrid nano-silica have been prepared loading both BODIPY dye and poly-aromatic pyridinium salt, into MCM-41. The excited-state equilibration that governs the photophysical behaviour of a pyrene-appended Ru(II) complex in solution has been demonstrated to be equally performing when the dyad is hosted in mesoporous silica nanoparticles. The excited state of this species, if protected from dynamic quenching when hosted by MCM-41 silicas, retains a quite long luminescence lifetime. The higher porosity of new silica nanospheres compared to the MCM-41 allows for higher permeability and so they have been used as support for the photochemical synthesis of IrOx nanoparticles as possible catalysts for photoinduced water oxidation. Moreover, it has been demonstrated that Ru-1P is able to cross the cellular membrane of erythrocytes and to be detected as luminophores in the cytosol at concentration higher than 1 mM, whereas if encapsulated into nano-silica the luminescence is detectable inside cells at concentration lower than 10 nM. The ability of permeating erythrocytes membrane has been revealed also for a Ru(II) dinuclear complex. This species, used as precursor in the synthesis of the heptanuclear Ru6Os antenna dendrimer, has been demonstrated to be able to intercalate into DNA and to play the role of DNA light-switch. All the obtained results on this topic indicate that the optical bandwidth and the energy collection in artificial antennae can be manipulated not only choosing different chomophores to link covalently but also by employing chromophores able to self-assemble into host-guest systems. Moreover, mesoporous silica materials prove to be an excellent playground for a fine tuning of the light-harvesting properties in artificial photo-systems, as well as vector for luminophores in bio-imaging applications. The obtained results could open the way for further investigations in order to fully unravel and control the mechanisms of formation of such systems and of energy migration within them.