Gating That Suppresses Charge Recombination-The Role of Mono-N-Arylated Diketopyrrolopyrrole.

Suppressing the charge recombination (CR) that follows an efficient charge separation (CS) is of key importance for energy, electronics, and photonics applications. We focus on the role of dynamic gating for impeding CR in a molecular rotor, comprising an electron donor and acceptor directly linked via a single bond. The media viscosity has an unusual dual effect on the dynamics of CS and CR in this dyad. For solvents with intermediate viscosity, CR is 1.5-3 times slower than CS. Lowering the viscosity below ∼0.6 mPa s or increasing it above ∼10 mPa s makes CR 10-30 times slower than CS. Ring rotation around the donor-acceptor bond can account only for the trends observed for nonviscous solvents. Media viscosity, however, affects not only torsional but also vibrational modes. Suppressing predominantly slow vibrational modes by viscous solvents can impact the rates of CS and CR to a different extent. That is, an increase in the viscosity can plausibly suppress modes that are involved in the transition from the charge-transfer (CT) to the ground state, i.e., CR, but at the same time are not important for the transition from the locally excited to the CT state, i.e., CS. These results provide a unique example of synergy between torsional and vibronic modes and their drastic effects on charge-transfer dynamics, thus setting paradigms for controlling CS and CR.

How To Reach Intense Luminescence for Compounds Capable of Excited-State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

What Makes Oxidized N-Acylanthranilamides Stable?

Oligoamides composed of anthranilic acid derivatives present a promising choice for mediating long-range charge transfer and controlling its directionality. Hole hopping, modulated by the anthranilamide (Aa) permanent dipoles, provides a plausible means for such rectified long-range charge transduction. All aliphatic and most aromatic amides, however, decompose upon oxidation, rendering them unacceptable for hole-hopping pathways. We, therefore, employ electrochemical and computational analysis to examine how to suppress oxidative degradation and stabilize the radical cations of N-acylated Aa derivatives. Our findings reveal two requirements for attaining long-lived radical cations of these aromatic amides: (1) keeping the reduction potentials for oxidizing the Aa residues under about 1.4 V vs SCE and (2) adding an electron-donating group para to the N-terminal amide of the aromatic ring, which prevents the electron spin density of the radical cation from extending over the C-terminal amide. These findings provide essential information for the design of hole-transfer amides.

Fluorescent hydrogels for embryoid body formation and osteogenic differentiation of embryonic stem cells.

Substrate mechanics (e.g., stiffness and topography of the microenvironment) are likely critical for driving normal morphogenesis and tissue development. As such, substrate mechanics imposed by hydrogels have been exploited to guide the lineage differentiation of stem cells and to drive stemness. In this work, we chemically modified gelatin hydrogels through glyceraldehyde cross-linking to render them suitable for cell culture. The modified hydrogels proved to be ideal for embryonic stem cell osteogenesis, initially providing a soft nonadhesive surface for the formation of embryoid bodies. They subsequently degraded in culture to afford a harder surface during osteoblast differentiation. The gels synthesized are highly fluorescent, relatively easy to prepare, and can potentially aid in overcoming the challenge of imaging changes to the microenvironments of cells during three-dimensional cell culture. Exploiting these materials could lead to the development of tissue-engineered products of increased complexity and rational treatment strategies.

Photoinduced dynamics of a cyanine dye: parallel pathways of non-radiative deactivation involving multiple excited-state twisted transients.

Cyanine dyes are broadly used for fluorescence imaging and other photonic applications. 3,3'-Diethylthiacyanine (THIA) is a cyanine dye composed of two identical aromatic heterocyclic moieties linked with a single methine, -CH[double bond, length as m-dash]. The torsional degrees of freedom around the methine bonds provide routes for non-radiative decay, responsible for the inherently low fluorescence quantum yields. Using transient absorption spectroscopy, we determined that upon photoexcitation, the excited state relaxes along two parallel pathways producing three excited-state transients that undergo internal conversion to the ground state. The media viscosity impedes the molecular modes of ring rotation and preferentially affects one of the pathways of non-radiative decay, exerting a dominant effect on the emission properties of THIA. Concurrently, the polarity affects the energy of the transients involved in the decay pathways and further modulates the kinetics of non-radiative deactivation.

Correction: Photoinduced dynamics of a cyanine dye: parallel pathways of non-radiative deactivation involving multiple excited-state twisted transients.

[This corrects the article DOI: 10.1039/C4SC02881C.].

Dipole-mediated rectification of intramolecular photoinduced charge separation and charge recombination.

Controlling charge transfer at a molecular scale is critical for efficient light harvesting, energy conversion, and nanoelectronics. Dipole-polarization electrets, the electrostatic analogue of magnets, provide a means for "steering" electron transduction via the local electric fields generated by their permanent electric dipoles. Here, we describe the first demonstration of the utility of anthranilamides, moieties with ordered dipoles, for controlling intramolecular charge transfer. Donor-acceptor dyads, each containing a single anthranilamide moiety, distinctly rectify both the forward photoinduced electron transfer and the subsequent charge recombination. Changes in the observed charge-transfer kinetics as a function of media polarity were consistent with the anticipated effects of the anthranilamide molecular dipoles on the rectification. The regioselectivity of electron transfer and the molecular dynamics of the dyads further modulated the observed kinetics, particularly for charge recombination. These findings reveal the underlying complexity of dipole-induced effects on electron transfer and demonstrate unexplored paradigms for molecular rectifiers.

Microfluidic space-domain time-resolved emission spectroscopy of terbium(III) and europium(III) chelates with pyridine-2,6-dicarboxylate.

This article describes the utilization of laminar microflows for time-resolved emission measurements with steady-state excitation and detection. Passing a laminar flow through a short illuminated section of a microchannel provided a means for pulsed-like photoexcitation of the moieties carried by the fluid. Imaging the microchannel flows carrying thus photoexcited chelates of lanthanide ions allowed us to extract their excited-state lifetimes from the spatial distribution of the changes in the emission intensity. The lifetime values obtained using this space-domain approach agreed well with the lifetimes from time-domain measurements. This validated space-domain microfluidic approach reveals a means for miniaturization of time-resolved emission spectroscopy.