Axial vs equatorial: Capturing the intramolecular charge transfer state geometry in conformational polymorphic crystals of a donor-bridge-acceptor dyad in nanosecond-time-scale

Two conformational polymorphs of a donor-bridge-acceptor (D-B-A) dyad, p-(CH3)2N-C6H4-(CH2)2-(1-pyrenyl)/PyCHDMA, were studied, where the electron donor (D) moiety p-(CH3)2N-C6H4/DMA is connected through a bridging group (B), -CH2-CH2-, to the electron acceptor (A) moiety pyrene. Though molecular dyads like PyCHDMA have the potential to change solar energy into electrical current through the process of photoinduced intramolecular charge transfer (ICT), the major challenge is the real-time investigation of the photoinduced ICT process in crystals, necessary to design solid-state optoelectronic materials. The time-correlated single photon counting (TCSPC) measurements with the single crystals showed that the ICT state lifetime of the thermodynamic form, PyCHDMA1 (pyrene and DMA: axial), is ∼3 ns, whereas, for the kinetic form, PyCHDMA20 (pyrene and DMA: equatorial), it is ∼7 ns, while photoexcited with 375 nm radiation. The polymorphic crystals were photo-excited and subsequently probed with a pink Laue x-ray beam in time-resolved x-ray diffraction (TRXRD) measurements. The TRXRD results suggest that in the ICT state, due to electron transfer from the tertiary N-atom in DMA moiety to the bridging group and pyrene moiety, a decreased repulsion between the lone-pair and the bond-pair at N-atom induces planarity in the C-N-(CH3)2 moiety, in both polymorphs. The Natural Bond Orbital calculations and partial atomic charge analysis by Hirshfeld partitioning also corroborated the same. Although the interfragment charge transfer (IFCT) analysis using the TDDFT results showed that for the charge transfer excitation in both conformers, the electrons were transferred from the DMA moiety to mostly the pyrene moiety, the bridging group has little role to play in that.

Thousand-Fold Enhancement of Charge-Separated State Lifetimes Caused by an Adamantane Bridge in Dimethylaniline-Anthracene and Dimethylaniline-Pyrene Dyads

A long-standing challenge in photoinduced electron transfer research is the design of compact donor-acceptor dyads that can generate long-lived charge-separated (CS) states for use as sensitizers in solar energy harvesting. Reports of dyads exhibiting CS state lifetimes in the microsecond time domain are very rare. Herein, we report two compact donor-bridge-acceptor dyads exhibiting lifetimes in the microsecond domain. We employed an adamantane moiety as a bridge, and the lifetimes obtained are nearly 1000-fold larger when compared to those of the same donor-acceptor dyads bridged through C₃-alkyl chains. In addition to long-lived CS state decays, slow formation of acceptor triplets was also observed via nanosecond flash photolysis. The long lifetime of the CS state is attributed to the extremely small value of the electronic coupling matrix element for the charge recombination compared to charge separation.

One-Pot Synthesis of (E)-2-(3-Oxoindolin-2-ylidene)-2-arylacetonitriles

A highly efficient and expeditious one-pot approach towards 2-(3-oxoindolin-2-yl)acetonitriles was designed, which involves a base-assisted aldol reaction of ortho-nitroacetophenones, followed by hydrocyanation, triggering an unusual reductive cyclization reaction.

Excited-state intermolecular proton transfer dependent on the substitution pattern of anthracene–diurea compounds involved in fluorescent ON1–OFF–ON2 response by the addition of acetate ions

Anthracene–diurea compounds exhibit different excited-state intermolecular proton transfer (ESIPT) reactions depending on the pattern of the substituents.

Precursor and interstitial Cajal cells in the human embryo liver

Interstitial Cajal Cells (ICCs) were only proven in human adult hepatic tissue. The immune phenotypes of various cell types in the human embryonic liver (HEL) are scarcely described. It was hypothesized that in HEL ICCs are present and distinctive to the precursor/progenitor cells populations. It was aimed and performed a qualitative study of HEL by use of antibodies against CD117/c-kit, CD31, CD34, CD90, CD105, DOG1, Ki67, and adiponectin. Five human embryos of 23-29 mm were used. Blasts and hematopoietic cells were comprising the two major cell populations in late stage embryos. The general population of blasts in the HEL was CD34-/CD105, although scarce CD117/c-kit+ and CD90+ such cells were found. Hematopoietic precursors were Ki67+. Adiponectin-positive plasmalemmas were found mostly in blasts. Endothelia were CD31+/CD34+. Interstitial cells with moniliform prolongations were found; such cells were scarcely CD117/c-kit+ but consistently DOG1+. They were diagnosed as ICCs but based on the morphology of their prolongations they can be equally viewed as being telocytes (TCs). Further studies should better correlate the precursor cell-types and immune phenotypes during human liver organogenesis. Liver ICCs and/or TCs should be also investigated in the human fetal liver.

Nanosecond to submillisecond dynamics in dye-labeled single-stranded DNA, as revealed by ensemble measurements and photon statistics at single-molecule level

Single-molecule and ensemble time-resolved fluorescence measurements were applied for the investigation of the conformational dynamics of single-stranded DNA, ssDNA, connected with a fluorescein dye by a C6 linker, where the motions both of DNA and the C6 linker affect the geometry of the system. From the ensemble measurement of the fluorescence quenching via photoinduced electron transfer with a guanine base in the DNA sequence, three main conformations were found in aqueous solution: a conformation unaffected by the guanine base in the excited state lifetime of fluorescein, a conformation in which the fluorescence is dynamically quenched in the excited-state lifetime, and a conformation leading to rapid quenching via nonfluorescent complex. The analysis by using the parameters acquired from the ensemble measurements for interphoton time distribution histograms and FCS autocorrelations by the single-molecule measurement revealed that interconversion in these three conformations took place with two characteristic time constants of several hundreds of nanoseconds and tens of microseconds. The advantage of the combination use of the ensemble measurements with the single-molecule detections for rather complex dynamic motions is discussed by integrating the experimental results with those obtained by molecular dynamics simulation.

Unique dual fluorescence of sterically congested hexaalkyl benzenehexacarboxylates: mechanism and application to viscosity probing

The static and dynamic fluorescence behavior of a series of hexaalkyl benzenehexacarboxylates (R(6)BHC; R = methyl (Me), tert-butyl (tBu), (-)-menthyl (Men), (-)-bornyl (Bor), (-)-1-methylheptyl (MHp), neopentyl (neoPn), and 2-adamantyl (Ad)) was studied by steady-state and time-resolved fluorescence spectroscopy. Dual fluorescence from both the partially relaxed metastable Franck-Condon-like (FC') and the fully relaxed (RX) state was observed for tBu(6)BHC, Men(6)BHC, Bor(6)BHC, MHp(6)BHC, neoPn(6)BHC, and Ad(6)BHC, whereas only single fluorescence from the RX state was observed for Me(6)BHC. Picosecond time-resolved fluorescence spectroscopic measurements clearly demonstrated that the initially formed Franck-Condon (FC) state sequentially converts to the FC' and then to RX state, with the relaxation hindered to such an extent that it shows variation with the steric bulk of the R groups. Thus, the fluorescence lifetimes (tau's) of FC' and RX are critically dependent on the bulkiness of the R groups, varying from 17 to 130 ps and from 0.6 to 1.1 ns, respectively. The relative intensity of FC' and RX fluorescence (I(RX)/I(FC)(')) was found to be dependent on the excitation wavelength, suggesting that the conformational relaxation from the FC' to RX state can compete with the vibrational relaxation of the FC' state. The temperature and pressure dependences were studied by steady-state fluorescence spectroscopy to give the activation energies of 1-3 kcal/mol for the FC'-to-RX relaxation of congested R(6)BHCs, as well as the activation volumes of 2.0, -0.62, and 7.4 mL/mol for tBu(6)BHC, Men(6)BHC, and Bor(6)BHC at room temperature. The fluorescence anisotropy (rho), as a measure of molecular motion, was also determined to be in the ranges of 0.03-0.3 for FC' and 0.003-0.01 for RX. The much larger rho's for the FC' fluorescence by a factor of 2-100 are attributed to the shorter tau's. The I(RX)/I(F' ratio was found to be insensitive to solvent polarity, but critically dependent on solvent viscosity, exhibiting an excellent linear relationship with the reciprocal viscosity. The potential use of these sterically congested R(6)BHCs as microenvironmental viscosity probes is proposed.