Dihedral-Angle-Controlled Crossover from Static Hole Delocalization to Dynamic Hopping in Biaryl Cation Radicals

In cases of coherent charge-transfer mechanism in biaryl compounds the rates follow a squared cosine trend with varying dihedral angle. Herein we demonstrate using a series of biaryl cation radicals with varying dihedral angles that the hole stabilization shows two different regimes where the mechanism of the hole stabilization switches over from (static) delocalization over both aryl rings to (dynamic) hopping. The experimental data and DFT calculations of biaryls with different dihedral angles unequivocally support that a crossover from delocalization to hopping occurs at a unique dihedral angle where the electronic coupling (Hab) is one half of reorganization (λ), that is, Hab=λ/2. The implication of this finding in non-coherent charge-transfer rates is being investigated

4-Hydr­oxy-1-oxo-1,2-dihydro­phthalazine-6,7-dicarboxylic acid dihydrate

In the crystal structure of the title compound, C10H6N2O6·2H2O, the OH and NH groups each serve as a hydrogen-bond donor to one acceptor site whereas the water mol­ecules each serve as a hydrogen-bond donor to two acceptor sites. The hydrogen-bonding scheme gives rise to a three-dimensional network

Ultrafast electronic energy transfer beyond the weak coupling limit in a proximal but orthogonal molecular dyad

Electronic energy transfer (EET) from a donor to an acceptor is an important mechanism that controls the light harvesting efficiency in a wide variety of systems, including artificial and natural photosynthesis and contemporary photovoltaic technologies. The detailed mechanism of BET at short distances or large angles between the donor and acceptor is poorly understood. Here the influence of the orientation between the donor and acceptor on EET is explored using a molecule with two nearly perpendicular chromophores. Very fast EET with a time constant of 120 fs is observed, which is at least 40 times faster than the time predicted by Coulombic coupling calculations. Depolarization of the emission signal indicates that the transition dipole rotates through ca. 64 degrees, indicating the near orthogonal nature of the EET event. The rate of EET is found to be similar to structural relaxation rates in the photoexcited oligothiophene donor alone, which suggests that this initial relaxation brings the dyad to a conical intersection where the excitation jumps to the acceptor.PostprintPeer reviewe

Functionalised azamacrocycles.

Potential uses of functionalised azamacrocycles have been investigated. Nine new azamacrocyclic ligands have been synthesised and characterised, some containing redox active centres, and these are identified on the following page. In Chapter 2 a series of redox active macrocycles L1 , L2 and L3, incorporating ferrocene as the redox active centre have been synthesised, based on the parent macrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam), and been investigated. Their potential as transition metal ion sensors has been studied. The ligand L1 shows irreversible electrochemistry in acetonitrile solution, with no reverse peak being observed in the cyclic voltammogram. However, on addition of Zn 2 + the system becomes reversible as shown by the reappearance of the reverse peak. The crystal structure of L2 reveals that the nitrogen atoms are not in suitable positions for chelation, in line with the observation that no metal complexes could be made of this macrocycle. The electrochemistry of L2 shows a single four electron cyclic voltammogram and there is no change on addition of transition metal ions. The ligand L3 does show a significant shift in the E½ value for the ferrocene/ferrocenium couple on addition of transition metal ions. In Chapter 3 studies of two macrocycles (L4 and L6) containing the redox active centre as part of the macrocyclic ring itself are described. In the case of L4 ferrocene was used, but owing to problems with the electrochemistry, it was not a good transition metal ion sensor. Metal complexes of this macrocycle were synthesised and are of the general formula [ML4](CH3 COO)2 .xH2O, {M = Zn2 + ,x = 2.5,Ni2 +,x = 4, Cu2 +,x = 3}. A more promising metal ion sensor was the cobalticenium macrocycle L6 and this shows very large shifts in the half wave potential, on the addition of transition metal ions like Zn2 + and Ni2 +. Problems with obtaining very pure samples at the moment hinder its useful application as a transition metal ion sensor. In Chapter 4, two pyrrolidinyl pendant arm triazamacrocycles are discussed L6 and L7, together with some of their transition metal complexes. Two crystal structures have been undertaken of the zinc(II) complexes of L6 and L7. A change in geometry from a trigonal bipyramid to a tetrahedron is brought about by an increase in length of the pendant arm in going from L6 to L7. In Chapter 5, a general route to N-alkylated macro cycles is described. Two new tetra-N-alkylated derivatives of cyclam (L8 and L9) have been synthesised, and their nickel(II) complexes studied. These metal complexes have been compared to the nickel(II) complexes of 1,4,8,11-tetramethyl, 1,4,8,11-tetraazacyclotetradecane (TMC). The complex [Ni(L8)]2 + was obtained as two isomers in nitromethane solution, these are tentatively assigned to the Trans-I and Trans-III conformations. The crystal structure of [Ni(L8)(NCS)2] is six co-ordinate and unusually the macrocycle adopts the Trans-I geometry. The nickel(II) complexes of L9 have the general formula [Ni(L9)(X)]2 + where X= DMSO or H2O. Only one isomer exists in solution as shown by 1 3 C n.m.r, and is assigned to the Trans-I conformation

Photocatalytic degradation of phenolic pollutants using N-methylquinolinium and 9-mesityl-10-methylacridinium salts

[EN] The photodegradation of a mixture of phenolic pollutants including: phenol (P), orto-phenylphenol (OPP), 2,4,6-trichlorophenol (TCP) and pentachlorophenol (PCP) was accomplished using two organic cationic photocatalysts, namely N-methylquinolinium (NMQ(+)) and 9-mesityl-10-methylacridinium (Mes-Acr-Me+) salts, due to their singular photophysical and redox properties. On one hand, NMQ+ exhibits more energetic excited states and accordingly more favorable redox potentials than Mes-Acr-Me+; on the other hand, NMQ(+) absorption reaches only up to 380 nm, while Mes-Acr-Me+ extends in the visible up to 480 nm. Evaluation of the efficiency of both photocatalysts, revealed that the highest level of photodegradation was achieved when they were employed at 20% mol. Specifically, with NMQ(+), removal of the pollutants was completed within 24 h of irradiation. Even more, irradiation time could be shortened from 24 to 8 h, since high levels of removal were already achieved (93%, 100%, 100% and 82% for P, OPP, TCP and PCP, respectively). Albeit, Mes-Acr-Me+ was not as effective, and best results were obtained using 20% mol upon 24 h of irradiation. Under these conditions, removal of PCP was 80%, while TCP was 40%, OPP 30% and P resulted in the most recalcitrant contaminant with only 10% of removal. Next, NMQ(+) and Mes-Acr-Me+ were separately supported onto Zeolite Y, an inert inorganic support (Y-NMQ(+) and Y-Mes-Acr-Me+), and elemental analyses revealed a loading of ca. 13% and 15% weight for NMQ(+) and Mes-Acr-Me+, respectively. Upon heterogenization, in the case of Y-NMQ(+), the extent of removal was lower than the one achieved in the homogeneous photodegradations. On the contrary, performance of Y-Mes-Acr-Me+ improved, because of its enhanced photostability; thus, upon 46 h irradiation, 98%, 80%, 40% and 26% for PCP, TCP, OPP and P, respectively, was achieved. Moreover, their efficiency was maintained upon second use. Steady-state and time-resolved fluorescence quenching revealed that every pollutant was able to quench the singlet excited state of both 1(NMQ(+))* and 1(Mes-Acr-Me+)*, with kinetic rate constants in the order of the diffusion limit. Thus, Type I photooxidation happening through the singlet excited state of either photocatalyst was the main operating process in the photodegradation of the studied pollutants.Financial support from Spanish Government (Grant SEV-20160683) is gratefully acknowledged. Financial support from VLC/Campus. R. Martinez-Haya thanks financial support for a predoctoral contract from Apadrina la Ciencia Association and Ford Espana/Ford Motor Company Fund.Martínez-Haya, R.; Luna, MM.; Hijarro, A.; Martinez-Valero, E.; Miranda Alonso, MÁ.; Marín García, ML. (2019). Photocatalytic degradation of phenolic pollutants using N-methylquinolinium and 9-mesityl-10-methylacridinium salts. Catalysis Today. 328:243-251. https://doi.org/10.1016/j.cattod.2019.01.045S24325132

Too much diversity—Multiple definitions of geodiversity hinder its potential in biodiversity research

Geodiversity—the diversity of abiotic features and pro-cesses of the Earth's surface and subsurface—is an increasingly used concept in ecological research. A growing body of scientific literature has provided evidence of positive links between geodiversity and biodiversity. These studies highlight the potential of geodiversity to improve our understanding of biodiversity patterns and to complement current biodiversity conservation practices and strategies. However, definitions of geodiversity in eco-logical research vary widely. This can hinder the progress of geodiversity–biodiversity research and make it difficult to synthesize findings across studies. We therefore call for greater awareness of how geodiversity is currently defined and for more consistent use of the term ‘geodi-versity’ in biodiversity research

Synthesis and photophysical studies of a pyrenylindole and a phenalenoindole obtained from dehydroamino acid derivatives : application as fluorescent probes for biological systems

Two pyrenyl-dehydroamino acid derivatives were cyclized by a metal-assisted C–N intramolecular cyclization developed in our research group, to give a pyrenylindole and a phenalenoindole. The pyrenylindole was inserted into a peptide by solid-phase coupling, with use of a 2-chlorotrityl chloride resin and a Fmoc strategy. The photophysical properties of the pyrenylindole and phenalenoindole in several solvents were studied and showed that these compounds can be used as fluorescence probes. The results obtained with the peptide labelled with the pyrenylindole moiety show potential for use of this compound as a fluorescence label avoiding the aggregation propensity of pyrene compounds. Photophysical studies of the pyrenylindole and of the phenalenoindole in lipid membranes were also carried out. Steady-state fluorescence anisotropy measurements revealed that both compounds adopt locations inside the lipid bilayers and are able to report the transition between the gel and liquid-crystalline phases. The results point to potential use of these compounds as fluorescent probes for biological systems.Fundação para a Ciência e a Tecnologia (FCT) - (SFRH/BPD/24548/2005), (SFRH/BD/38766/2007)Fundo Europeu de Desenvolvimento Regional (FEDER) - Project PTDC/QUI/81238/200