Luminescent dirhenium(I)-double-heterostranded helicate and mesocate.

The semirigid ligands 1,4-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)benzene (H2-pBC) and 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (H2-mBC), containing two hydroxyphenylbenzimidazolyl units as bis-chelating (or bis(bidentate)) N∩OH donor, were synthesized and were used to assemble neutral, luminescent heteroleptic, unsaturated double-hetero-stranded, rhenium(I)-based helicate (1) and mesocate (2) with the flexible bis(monodentate) nitrogen donor (1,4-bis(benzimidazol-1-ylmethyl)benzene/1,3-bis(benzimidazol-1-ylmethyl)benzene), and Re2(CO)10. The photophysical properties of the complexes were studied. Both complexes 1 and 2 exhibit dual emissions in both solution and solid state. In solution, these complexes show both fluorescence and phosphorescence. Complex 1 undergoes a predominantly ligand-centered oxidation, resulting in the generation of phenoxyl radicals.

Resolution of conformer-specific all-trans-1,6-diphenyl-1,3,5-hexatriene UV absorption spectra.

All-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) exists in solution as a mixture of s-trans,s-trans and s-cis,s-trans conformers. The latter is higher in energy, and its contribution increases with increasing temperature. ttt-DPH UV absorption spectra broaden with increasing temperature and undergo blue shifts with decreasing polarizability. We describe here the resolution of two spectrothermal matrices of ttt-DPH UV absorption spectra into two conformer-specific components. The first matrix consists of DPH spectra measured in n-dodecane in the 283 to 374 K T range and the second of ttt-DPH absorption spectra measured in the even numbered n-alkanes (n-C(8)-n-C(16)) at temperatures selected to achieve isopolarizability (284-372 K). Principal component analysis (PCA) treatments showed that reasonable two-component systems are attained by compensation for T-induced broadening and shifting in the pure conformer spectra. The self-modeling (SM) method used to resolve the n-C(12) matrix is successfully tested on a simulated matrix closely mimicking ttt-DPH experimental spectra in n-C(12). Compensation for nonlinear effects yields robust two-component matrices from the experimental spectra. Their resolution into pure component spectra is based on the application of the Lawton and Sylvestre (LS) non-negativity criterion at the spectral onset to define the spectrum of the low energy s-trans-conformer and the optimum linearity van't Hoff (vH) plot criterion to find the spectrum of the higher-energy s-cis-conformer. Resolved spectra are somewhat sensitive to the choice of the spectral region in which the LS criterion is applied. The surprising result is that both resolutions lead to the conclusion that the molar fraction of the s-cis-conformer equals, or even exceeds, the molar fraction of the s-trans-conformer as the highest T's employed in our study are approached.

Fluorescence spectroscopy of the retina for diagnosis of transmissible spongiform encephalopathies.

The feasibility of exploiting fluorescence spectra of the eye for diagnosis of transmissible spongiform encephalopathies (TSEs) was examined. Retinas from scrapie-positive sheep were compared with scrapie-negative sheep using fluorescence spectroscopy, and distinct differences in the fluorescence intensity and spectroscopic signatures were observed. The characteristic fluorescent signatures are thought to be the result of an accumulation of lipofuscin in the retina. It appears that the eye, in particular the retina, is a useful tissue for noninvasive examination of some neurological pathologies such as scrapie. The development of procedures based on examinations of the eye that permit the detection of neurological disorders in animals holds great promise.

Chlorin e6 decorated doxorubicin encapsulated chitosan nanoparticles for photo-controlled cancer drug delivery.

In the study we have reported the physico-chemical, photophysical and morphological properties of chlorin e6 (Ce6) decorated doxorubicin (DOX) encapsulated chitosan (CS)-tripolyphosphate (TPP) nanoparticles which prepared by ionotropic gelation method. The Ce6 physically loaded onto the nanoparticles by self-assembly of CS with TPP-DOX under aqueous conditions. The results from DLS studies highlights the prepared nanoparticles that possess the size in the range of 80-120 nm. with negatively charged of -6 mV. The SEM and AFM images showed 80-120 nm size while the average size of the Ce6 decorated nanoparticles was found to be around 100-130 nm. The absorption spectra of Ce6 decorated nanoparticles are similar when compared to free Ce6 which suggest there is no change in the Ce6 chromophore upon decoration. This nanoparticle showed high photostability and singlet oxygen generation (SOG). The Ce6 decorated and DOX encapsulated nanoparticles sizes and charges are in the range of 90-130 nm and -30 mV respectively. The nanoparticles showed high encapsulation efficiency towards DOX as well as pH controlled release. This has significant anti-proliferative activity against MCF-7 breast cancer cells after irradiation at near infra-red (NIR) ranges were evaluated. This could have potential applications in photo-controlled smart DOX delivery system for cancer treatment.

Comment on "Michael Addition Based Chemodosimeter for Serum Creatinine Detection Using ( E)-3-(Pyren-2-yl)-1-(3,4,5-trimethoxyphenyl)prop-2-en-1-one Chalcone".

Recently, a chalcone based fluorescence sensor has been developed for the detection of creatinine by Sundaram et al. ( ACS Sens. 2018, 3, 763-771). Though the efficiency of the sensor under clinical condition is appreciated, the proposed mechanism and the interpretations of the authors raise serious concerns. In the present work, DFT calculations were performed on the system. Based on the calculations and the spectral data reported earlier it is established that the Michael addition is not a photophsyical interaction as proposed by Sundaram et al.; instead, it is a ground state reaction. Several other misinterpretations of the authors including that in the ICT mechanism are properly inferred. Since the formula and the procedure followed for the calculation of the fluorescence quantum yield in ACS Sens. 2018, 3, 763-771 is not appropriate, the correct procedure is briefed with the proper formula.

Intramolecular proton transfer in 2-(2'-hydroxyphenyl)oxazolo[4,5-b]pyridine: evidence for tautomer in the ground state.

The intramolecular proton transfer in a newly synthesized molecule, 2-(2'-hydroxyphenyl)oxazolo[4,5-b]pyridine (HPOP) is studied using UV-visible absorption, fluorescence emission, fluorescence excitation and time-resolved fluorescence spectroscopy. In the ground state, the molecule exists as cis- and trans-enol in all the solvents. However, in dioxane, alcohols, acetonitrile, dimethylformamide and dimethylsulfoxide the keto tautomer is also observed in the ground state. Dual fluorescence is observed in HPOP where the large Stoke shifted emission is due to emission from the excited-state intramolecular proton transfer product, whereas the other emission is the normal emission from enol form. The fluorescence (both normal and tautomer emission) of HPOP is less than those of corresponding benzoxazole and imidazopyridine derivatives. This reveals that the nonradiative decay becomes more efficient upon substitution of electronegative atom on the charge acceptor group. The pH studies substantiate the conclusion that (unlike in its imidazole analog) the third ground state species is the keto tautomer and not the monoanion. The effect of temperature on cis-enol-trans-enol-keto equilibrium and the nonradiative deactivation from the excited state are also investigated.

Conformational isomerizations of symmetrically substituted styrenes. No-reaction photoreactions by Hula-twist.

Symmetrically substituted styrenes photochemically gave unstable conformers in low temperature organic glass in a manner consistent with the recently postulated Hula-twist mechanism of photoisomerization.

Temperature effect on dual fluorescence of 2-(2'-hydroxyphenyl)benzimidazole and its nitrogen substituted analogues.

The effects of temperature on the dual fluorescence of 2-(2'-hydroxyphenyl)benzimidazole (HPBI) and its nitrogen substituted analogues, viz., 2-(2'-hydroxyphenyl)-3H-imidazo[4,5-b]pyridine (HPIP-b) and 2-(2'-hydroxyphenyl)-1H-imidazo[4,5-c]pyridine (HPIP-c), were investigated in solvents of different polarity and hydrogen bonding capability. Absorption, steady-state, and time-resolved emission spectroscopic techniques were employed for the experimental study. Density functional theoretical calculations were performed to find the relative population of the conformers. The calculations predict that, with increase in temperature, the population of trans-enol increases, while that of cis-enol decreases. At all temperatures, the population ratio of cis-enol to trans-enol increases in the order HPIP-c < HPIP-b < HPBI. Except for HPBI in methanol and ethylene glycol, the fluorescence of both emissions decreases with an increase in temperature and is more pronounced in the tautomer band than in the normal band. The data are analyzed using the Arrhenius and van't Hoff equations. The change in the fluorescence with temperature is governed by (i) the change in the relative population of conformers and (ii) the increase in non-radiative decay from the excited states. The increase in non-radiative decay from the normal emission competes with the increase in the relative population of trans-enol with a rise in temperature.

Photoisomerization of trans-2-[4'-(dimethylamino)styryl]benzothiazole.

Photoreaction of trans-2-[4'-(dimethylamino)styryl]benzothiazole (t-DMASBT) under direct irradiation has been investigated in dioxane, chloroform, methanol and glycerol to understand the mechanism of photoisomerization. Contrary to an earlier report, isomerization takes place in all these solvents including glycerol. The results show that restriction on photoisomerization leads to the increase in fluorescence quantum yield in glycerol. The results are consistent with the theoretically simulated potential energy surface reported earlier using time-dependent density functional theory (TDDFT) calculations. DFT calculations on cis isomers under isolated condition have suggested that cis-B conformer is more stable than cis-A conformer due to hydrogen-bonding interaction. In the ground state, cis-DMASBT is predominantly present as cis-B. The fluorescence spectra of the irradiated t-DMASBT suggested that photoisomerization follows not the adiabatic path as proposed by Saha et al., but the nonadiabatic path.

Resolution of three fluorescence components in the spectra of all-trans-1,6-diphenyl-1,3,5-hexatriene under isopolarizability conditions.

all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (2(1)A(g)) and S2 (1(1)B(u)) states of the s-trans,s-trans conformer (s-t-DPH) and emission from the S1 state of the s-cis,s-trans conformer (s-c-DPH). The contribution of s-c-DPH fluorescence increases upon excitation at longer wavelengths, and both minor emissions, s-c-DPH and 1(1)B(u) s-t-DPH fluorescence, contribute more at higher temperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal component analysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individual components. We avoided differential polarizability-dependent spectral shifts by measuring the spectra in n-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, hence under isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolution of the spectral matrix into its three components. The optimum van't Hoff plot gives Delta H = 2.83 kcal/mol for s-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimum Boltzmann distribution law plot gives Delta E(ab) = 4.09 kcal/mol for 1(1)B(u)/2(1)A(g) equilibration. The 1(1)B(u) fluorescence spectrum bears mirror-image symmetry with the DPH absorption spectrum, and the energy gap, 1431 cm(-1), is consistent with the 1615 cm(-1) difference between the lowest energy bands in the 1(1)B(u) and 2(1)A(g) fluorescence spectra. The results give V(ab) = 198 +/- 12 cm(-1) for the vibronic matrix coupling element between the 2(1)A(g) and 1(1)B(u) states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increase in the effective radiative rate constant of s-t-DPH with increasing T.

The effect of solvent polarizability on the fluorescence of trans-1-(2-naphthyl)-2-phenylethene conformers. Conformer-specific fluorescence from the cis isomer.

A matrix of trans-1-(2-naphthyl)-1-phenylethene (t-NPE) fluorescence spectra obtained from benzene solutions at 20 degrees C by varying the excitation wavelength (lambdaexc) and the oxygen concentration is resolved into pure conformer fluorescence spectra by use of principal component analysis with self-modeling based on optimum global Stern Volmer constant criteria. The resulting fractional contributions of the two components to the fluorescence spectra are combined with observed fluorescence quantum yields as a function of lambdaexc to obtain the conformer-specific quantum yields. These quantum yields and fluorescence lifetimes are used to determine conformer-specific radiative and radiationless rate constants. Comparison with results from an analogous study in methylcyclohexane reveals pronounced, diferential enhancements of all these rate constants in benzene (Bz). Preliminary measurements of emission from a cis-1-(2-naphthyl)-1-phenylethene (c-NPE) solution in Bz at 20 degrees C, under static cell conditions, are dominated by sequential two-photon-induced t-NPE fluorescence due to excitation of t-NPE formed photochemically from the cis isomer. The spectra also reveal a weak structureless broad emission, which is assigned to 1c-NPE* by comparison with the published low temperature fluorescence spectrum of c-NPE in a hydrocarbon glass. The conformational origin of this emission is addressed.