Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(I)-tpy complexes.

A series of organometallic Re(I)(L)(CO)3Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO2 reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO2 electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)3Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75-0.95 V) with faradaic yields of 62-98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pKa of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.

Rewriting the Story of Excimer Formation in Liquid Benzene.

Formation of benzene excimer following UV excitation of the neat liquid is monitored with femtosecond spectroscopy. A prompt rise component in excimer transient absorption, which contradicts the classical scenario of gradual reorientation and pairing of the excited monomers, is observed. Three-pulse experiments in which the population of evolving excimers is depleted by a secondary dump pulse demonstrate that the excimer absorption band is polarized along the interfragment axis. The experiments furthermore prove that the subsequent 4-fold increase in excimer absorption over ∼50 ps is primarily due to an increase in the transition dipole of pairs which are formed early on, and not to excited monomers forming excimers in a delayed fashion due to unfavorable initial geometry. Results are analyzed in light of recent studies of local structure in the liquid benzene combined with advanced electronic structure calculations. The prompt absorption rise is ascribed to excited states delocalized over nearby benzene molecules, which are sufficiently close and nearly parallel in the pure liquid. Such low-symmetry structures, which differ considerably from the optimized structures of isolated benzene dimer and solid benzene, are sufficiently abundant in liquid benzene. Electronic structure calculations confirm the orientation of transition dipoles of the excimers along the interparticle axis and demonstrate how slow refinement of the intermolecular geometry leads to a significant increase in the excimer absorption strength.

Temperature Independence of Ultrafast Photoisomerization in Thermophilic Rhodopsin: Assessment versus Other Microbial Proton Pumps.

Primary photochemical events in the unusually thermostable proton pumping rhodopsin of Thermus thermophilus bacterium (TR) are reported for the first time. Internal conversion in this protein is shown to be significantly faster than in bacteriorhodopsin (BR), making it the most rapidly isomerizing microbial proton pump known. Internal conversion (IC) dynamics of TR and BR were recorded from room temperature to the verge of thermal denaturation at 70 °C and found to be totally independent of temperature in this range. This included the well documented multiexponential nature of IC in BR, suggesting that assignment of this to ground state structural inhomogeneity needs revision. TR photodynamics were also compared with that of the phylogenetically more similar proton pump Gloeobacter rhodopsin (GR). Despite this similarity GR has poor thermal stability, and the excited state decays significantly more slowly and exhibits very prominent stretched exponential behavior. Coherent torsional wave-packets induced by impulsive photoexcitation of TR and GR show marked resemblance to each other in frequency and amplitude and differ strikingly from similar signatures in pump-probe data of BR and other microbial retinal proteins. Possible correlations between IC rates and thermal stability and the promise of using torsional coherence signatures for understanding chromophore protein binding in microbial retinal proteins are discussed.

Efficient femtosecond energy transfer from carotenoid to retinal in gloeobacter rhodopsin-salinixanthin complex.

The retinal proton pump xanthorhodopsin (XR) was recently found to function with an attached carotenoid light harvesting antenna, salinixanthin (SX). It is intriguing to discover if this departure from single chromophore architecture is singular or if it has been adopted by other microbial rhodopsins. In search of other cases, retinal protein encoding genes in numerous bacteria have been identified containing sequences corresponding to carotenoid binding sites like that in XR. Gloeobacter rhodopsin (GR), exhibiting particularly close homology to XR, has been shown to attach SX, and fluorescence measurements suggest SX can function as a light harvesting (LH) antenna in GR as well. In this study, we test this suggestion in real time using ultrafast transient absorption. Results show that energy transfer indeed occurs from S2 of SX to retinal in the GR-SX composite with an efficiency of ∼40%, even higher than that in XR. This validates the earlier fluorescence study, and supports the notion that many microbial retinal proteins use carotenoid antennae to harvest light.

Microheterogeneity in native and cation-exchanged Nafion membranes.

2-(4'-Pyridyl)benzimidazole (4PBI) has been used to investigate the microheterogeneity of water nanochannels of Nafion membranes at two different hydration levels. Native as well as cation-exchanged Nafion membranes are found to protonate one of the two monoprotonated forms of 4PBI selectively. In native membranes and in those in which H(3)O(+) ions are replaced by (CH(3))(4)N(+) ions, the pyridyl nitrogen is protonated preferentially. In Na(+)-exchanged membranes, however, the benzimidazole nitrogen is protonated selectively. Unlike other fluorescent probes used in earlier studies, 4PBI can differentiate between the two different cation-exchanged membranes at lower as well as higher hydration levels.

(2,2'-bipyridyl)-3-3'-diol in nafion: stabilization of unusual ground and excited states.

(2,2'-Bipyridyl)-3-3'-diol (BP(OH)(2)) undergoes excited state intramolecular double proton transfer (ESIDPT). The photophysics of this molecule has been investigated, in the highly acidic water channels inside nafion membrane. The dianionic enolate form of the dye, which is formed only in alkaline conditions in neat aqueous solutions, is found to be present in its excited state in native nafion membrane at low water content. This surprising phenomenon has been explained in the light of loss of protons from the fluorophore to the medium, due to increased electrostatic interactions with the pendant sulfonate groups of nafion, at the lower water content. This observation fortifies the model of microscopic interactions that we have proposed in recent past. Further, the ground state of the diketo form, observed only in neat aqueous solutions so far, is found to be present in Na(+)-exchanged membranes at lower levels of hydration. The steady-state spectral features of the dye is different in (CH(3))(4)N(+)-exchanged membranes than that of Na(+)-exchanged membranes, unlike in our earlier studies with coumarin 102 and 2-(2'-pyridyl)benzimidazole, here we observed formation of an unusual ground state in Na(+)-exchanged membranes, while no such feature was observed in (CH(3))(4)N(+)-exchanged membranes.

2-(2'-Pyridyl)benzimidazole as a fluorescent probe of hydration of nafion membranes.

2,2'-(Pyridyl)benzimidazole is used as a probe to explore proton transfer through nafion membranes. The probe marks the availability of water in native as well as cation-exchanged membrane. Using steady state and time-resolved fluorescence studies, it has been shown that the rotation of the pyridyl and benzimidazole rings with respect to each other, which is ultrafast in higher water contents, is hindered as the water content in the membranes is decreased. In cation-exchanged membranes, it is observed that the formation of the ESPT (excited state proton transfer) state is reduced to a large extent. Thus, it may be inferred that the proton transport is observed to be hindered even in molecular dimensions of one water molecule thereby bolstering the contention that it may not be essential for water channels to break for proton conductivity to decrease.

Importance of electrostatic interactions in the mobility of cations in Nafion.

A molecular level understanding of the mobility of cations in the Nafion membrane has been attempted, using the excited state proton transfer (ESPT) process in the fluorescent probe Coumarin 102. ESPT is hindered significantly upon decreasing the water content. Using TRANES (time-resolved area normalized emission spectroscopy), the evolution of the ESPT state is clearly observed over hundreds of picoseconds in lower water content, implying that ESPT is hindered even in the nanovolume probed by the dye. Most remarkably, in the partially dried membrane, the predominant fluorescent species is the zwitterionic form, generated by excited state deprotonation of the cationic form. This implies that the molecule loses a proton from its nitrogen center in the excited state, as usual, but cannot recapture it readily at the oxygen center, at low water contents. This phenomenon is rationalized in light of an increased electrostatic attraction that is experienced by cations upon drying.

Ag(I) induced emission with azines having donor-acceptor-donor chromophore.

Two azine molecules, [3-{4-(dimethylamino)phenyl}prop-2-en-1-ylidene]hydrazone (HL) and [4-(dimethylamino)benzylidene]hydrazone (LL) synthesized via Schiff base condensation, show an absorption band due to intra-ligand charge transfer (ILCT). Both ligands show weak emission in the absence of a metal ion as input. When Ag(+) ions are added to either compound in THF, the metal ion gets bonded to the azine moiety resulting in a high intensity emission (approximately 400 fold). While Cu(+) shows a slight enhancement of emission, other first-row transition, alkali or alkaline-earth metal ions do not show any emission allowing Ag(+) ions to be detected in the presence of these metal ions. Both the Ag(+) complexes were characterised by X-ray crystallography and show solid-state spectra similar to their solution spectra. Time-resolved fluorescence measurements done on the complexes show two excited-state lifetimes.