Light absorption excited-state photophysics

A number of cyano-bridged complexes are included here even though they strictly do not fall in the general family-type defined for the section. The syntheses and photophysical properties of [(NC)(bpy)2Ru(/r-NC)Cr(CN)5] and [(NC)5Cr(Ai-CI Ru(bpy)2(M-NC)Cr(CN)5] have been described. Absorption of visible light by the Ru(bpy)2 unit results in phosphorescence from the Cr(CN)g luminophore, and the results evidence fast intramolecular exchange energy transfer from the MLCT state of the Ru(bpy)2 chromophore to the doublet state of the Cr -based unit. Time-resolved resonance Raman and transient UV-vis absorption spectroscopies have been employed to investigate the MLCT excited states of [(NC)(bpy)2Ru(//-CN)Ru (bpy)2(CN)], [(NC)(bpy)2Ru(//-CN)Ru(phen)2(CN)]+, [(NC)(phen)2Ru(//-CN)Ru (bpy)2(CN)]+, [(NC)(bpy)2... 

Once a molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes. Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or reacts with another molecule. Photophysical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state and nonradiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat. 

The Photoactive Yellow Protein (PYP) is the blue-light photoreceptor that presumably mediates negative phototaxis of the purple bacterium Halorhodospira halophila [1]. Its chromophore is the deprotonated trans-p-coumaric acid covalently linked, via a thioester bond, to the unique cystein residue of the protein. Like for rhodopsins, the trans to cis isomerization of the chromophore was shown to be the first overall step of the PYP photocycle, but the reaction path that leads to the formation of the cis isomer is not clear yet (for review see [2]). From time-resolved spectroscopy measurements on native PYP in solution, it came out that the excited-state deactivation involves a series of fast events on the subpicosecond and picosecond timescales correlated to the chromophore reconfiguration [3-7]. On the other hand, chromophore H-bonding to the nearest amino acids was shown to play a key role in the trans excited state decay kinetics [3,8]. In an attempt to evaluate further the role of the mesoscopic environment in the photophysics of PYP, we made a comparative study of the native and denatured PYP. The excited-state relaxation path and kinetics were monitored by subpicosecond time-resolved absorption and gain spectroscopy. 

Most photosensitizers, however, are reasonably photostable compounds, and their optical properties have been studied in depth. In particular, there has been much interest in ruthenium-based photosensitizers such as [Ru(bpy)3]2+ and [Ru(phen)3]2+, due to their stability and absorption of visible light. Detailed information on their optical properties, including ground and excited state information in relation to photosensitization, has been reviewed by Creutz et al. [16]. Similarly, the photochemistry and photophysics of rhenium complexes, as discussed here, have been reviewed in detail by Kirgan et al. [7]. 

The absorption of light by a substance causes the formation of excited-state molecules. This excitation is followed by various elementary transformations which eventually lead to the deactivation or to the disappearance of those excited molecules. The absorption of light as well as each one of the elementary transformations of the original molecule in an excited state is a primary step. Specifically, a primary step may be (a) a transformation of the excited molecule into a different chemical species, as in steps 24, 15, and 14 of Figure 1, or (b) a radiative or nonradiative transition between different energy levels of the molecule, e.g., steps 02, 21, 22, 23, 13, 11, and 16 of Figure 1. Those corresponding to (a) are photochemical primary steps, while those of (b) are photophysical primary steps. 

As previously mentioned, the interaction between protruding, doubly occupied dz orbitals critically affects the photophysical properties upon aggregate formation. Absorption and emission of light, excited-state lifetimes, and redox properties are dramatically affected. A particularly interesting feature is represented by the possibility of tuning the distance between the monomeric units and, consequently, the degree of electronic coupling... 

The presence of MEF, MEP and Metal-Enhanced superoxide anion radical generation in the same system seems surprising at first, as these processes are effectively competitive and ultimately provide a route for deactivation of electronic excited states. As recently shown by the authors, simultaneous photophysical mechanisms can be present within the same system when enhanced absorption effects of the fluorophore near to silver are present (i.e. an enhanced excitation rate). In this case, enhanced absorption of Acridine near-to the plasmon resonant particles facilitates MEF, MEP, ME Oa and also Metal-Enhanced superoxide generation simultaneously within the same system. Aaidine showed an enhanced absorption spectra near-to silver, similar to other probes reported by the authors, in essence acridine absorbs more light. ... 

---