In the Ni-substituted chlorophylls, an ultrafast (<60 fs) deactivation channel is created, which is not present in Ni-porphyrins. This observation prompted us to investigate in detail the mechanism of excitation-to-heat conversion in Ni-substituted chlorophylls, experimentally, using time-resolved laser-induced optoacoustic spectroscopy, and theoretically, using group theory approach. The Ni-substituted chlorophylls show exceptional photostability and the optoacoustic measurements confirm the prompt and very efficient (100%) excitation-into-heat conversion in these complexes. Considering their excellent spectral properties and the loss-free excitation-into-heat conversion they are likely to become a new class of versatile photocalorimetric references. The curious features of the Ni-substituted chlorophylls originate from the symmetry of a ligand field created in the central cavity. The central N-Ni(2+) bonds, formed via the donation of two electrons from each of the sp(2) orbitals of two central nitrogens to an empty [Formula: see text] hybrid centered on Ni(2+), have a considerable covalent character. The extreme rate of excited state relaxation is then not due to a ladder of the metal centered d-states, often invoked in metalloporphyrins, but seems to result from a peculiar topology of the potential energy surface (a saddle-shaped crossing) due to the covalent character of the N-Ni(2+) bonds. This is confirmed by a strong 0→0 character of electronic transitions in these complexes indicating a similarity of their equilibrium geometries in the ground (S(0)) and the excited states (both Q(X) and Q(Y)). The excitation energy is very efficiently converted into molecular vibrations and dissipated as heat, involving the central Ni(2+). These Ni-substituted pigments pose a fine exemplification of symmetry control over properties of excited states of transition metal complexes.

Gold nanorods deserve special attention as they exhibit tunable longitudinal localized surface plasmon resonances (LSPRs). In our study, gold nanorods of the aspect ratio of 2.25 (maximum of LSPR band at 660 nm) and of controllable SiO 2 thickness in the range of 6−14 nm were mixed with pheophorbide (chlorophyll derivative) in order to create a hybrid system. Energy transfer and singlet oxygen generation were studied for different SiO 2 thicknesses of the nanorod shell. The spectral properties of the hybrid mixture were characterized, and the overlapping of the pheophorbide fluorescence and the longitudinal LSPR band of nanorods on the fluorescence emission, energy transfer, and generation of singlet oxygen were studied. Two independent approaches were used to determine the quantum yield and enhanced factor of singlet oxygen generation. For a certain thickness of the SiO 2 shell and for certain concentrations of gold nanorods, the effect of the plasmon-enhanced singlet oxygen production was observed. Moreover, the enhanced of singlet oxygen yield enhancement was correlated with the far-field optical properties of the gold nanorods. The results obtained indicate the significance of further studies of dye-photosensitizers in hybrid mixtures, taking into account the spectral overlap between dye emission and longitudinal LSPR bands as well as the character of coatings (type and thickness) and scattering yields of gold nanorods.

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Photosynthetic energy conversion competes with the formation of chlorophyll triplet states and the generation of reactive oxygen species. These may, especially under high light stress, damage the photosynthetic apparatus. Many sophisticated photoprotective mechanisms have evolved to secure a harmless flow of excitation energy through the photosynthetic complexes. Time-resolved laser-induced optoacoustic spectroscopy was used to compare the properties of the T states of pheophytin a and its metallocomplexes. The lowest quantum yield of the T state is always observed in the Mg complex, which also shows the least efficient energy transfer to O . Axial coordination to the central Mg further lowers the yield of both T and singlet oxygen. These results reveal the existence of intrinsic photoprotective mechanisms in chlorophylls, embedded in their molecular design, which substantially suppress the formation of triplet states and the efficiency of energy transfer to O , each by 20-25 %. Such intrinsic photoprotective effects must have created a large evolutionary advantage for the Mg complexes during their evolution as the principal photoactive cofactors of photosynthetic proteins.

recommended by the European Agency for Safety and Health at Work (EU-OSHA) is 5 ppmv [2]. The permissible exposure limit value for H 2 S is 10 ppmv, the Immediately Dangerous to Life and Health (IDLH) level is 300 ppmv and lethal concentrations are in the range of 2000 ppmv [2]. In practice, concentrations ranging from sub-ppmv levels at low pressures to several per cents at atmospheric conditions need to be monitored. Despite a variety of online monitoring options for gaseous H 2 S, its reliable quantitative and selective determination still remains challenging in the field of chemical sensors [3][4][5].In the field of laser spectroscopy, the constant improvement of quantum cascade lasers (QCLs) has led to their application as reliable sources of coherent light ranging from the mid-infrared (MIR) to the terahertz spectral region for sensitive detection of molecular species on their fundamental vibrational, respectively, rotational bands [6][7][8][9]. Due to their tailorable emission wavelength, high output power, compactness, narrow spectral linewidth, and wavelength tuneability, QCLs are optimal choices for spectroscopic applications. In addition, optical resonator designs are constantly improved over the years with the distributed feedback (DFB) [10] and the external cavity (EC) [11] approach being the most prominent ones. A general aim with respect to the ongoing development of QCLs for sensing applications is to reduce the line width of the emitted radiation to a minimum while achieving a spectral coverage as large as possible. So far, EC-QCLs offer the largest tuning range which, depending on the employed gain medium, may cover up to several hundreds of wavenumbers. The external cavity design facilitates broadband spectral tuning by an external diffraction grating, while the selection of the emission wavelength takes place by changing the grating angle relative to the QCL chip.Abstract Hydrogen sulfide (H 2 S) trace gas detection based on off-beam quartz-enhanced photoacoustic spectroscopy using a continuous wave (CW), mode-hop-free external cavity (EC) quantum cascade laser tunable from 1310 to 1210 cm −1 was performed. A 1σ minimum detection limit of 492 parts per billion by volume (ppbv) using a 1 s lock-in time constant was obtained by targeting the line centered at 1234.58 cm . This value corresponds to a normalized noise equivalent absorption coefficient for H 2 S of 3.05 × 10 −9 W cm −1 Hz −1/2 .

In a hybrid mixture of organic (dye) and inorganic (metallic nanoparticles) components, the optical properties of a dye can be easily controlled by tailoring the shape or the concentration of the noble metal nanoparticles (NPs). The influences of multiexcitation (multiwavelength excitation) of photosensitizers (pheophorbide a and hematoporphyrin) on the interactions with pegylated Au-NPs and on the photophysical parameters of the dyes are studied. Detailed, systematic fluorescence quenching studies were performed in the mixtures of different contents of Au-NPs, and interpreted together with the results of quantum singlet oxygen yield examinations. According to the results, the fluorescence of the two dyes studied was effectively quenched in the presence of Au-NPs, mainly because of the resonance energy transfer between the donor (dye) and the acceptor (Au-NPs). Stern-Volmer quenching constants were determined by a few orders of magnitude higher than those describing the photochemical quenching process. In hybrid mixtures analyzed, the mechanism of energy transfer between the donor and the acceptor was nanometal surface energy transfer. Furthermore, different behavior of the mixtures on excitation with the wavelengths from the Soret and Q bands of the dyes and with those corresponding to the surface plasmon resonance band of Au-NPs was analyzed. Moreover, for certain concentrations of Au-NPs and for certain excitation wavelengths, an increase in singlet oxygen generation was observed. The results obtained indicate the significance of further studies of photosensitizers in hybrid mixtures with NPs.

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