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Block copolymers have the potential to self-assemble into thermodynamically stable nanostructures that are desirable for plastic electronic materials with prolonged lifetimes. Fulfillment of this potential requires the simultaneous optimisation of the spatial organisation and phase behaviour of heterogeneous thin films at the nanoscale. We demonstrate the controlled assembly of an all-conjugated diblock copolymer blended with fullerene. The crystallinity, nanophase separated morphology, and microscopic features are characterised for blends of poly(3-hexylthiophene-block-3-(2-ethylhexyl) thiophene) (P3HT-b-P3EHT) and phenyl-C61-butyric acid methyl ester (PCBM), with PCBM fractions varying from 0-65 wt%. We find that PCBM induces the P3HT block to crystallise, causing nanophase separation of the block copolymer. Resulting nanostructures range from ordered (lamellae) to disordered, depending on the amount of PCBM. We identify the key design parameters and propose a general mechanism for controlling thin film structure and crystallinity during the processing of semicrystalline block copolymers. [ABSTRACT FROM AUTHOR]

Internal conversion is the first step after photoexcitation to high lying electronic states, and plays a central role in many photoinduced processes. In this report, we demonstrate a truly ultrafast internal conversion (IC) in large molecules by time-resolved fluorescence (TF). Following photoexcitation to the S n ( n ≥ 2) state, TF of the S 1 state was recorded for two boron-dipyrromethene (BODIPY) derivatives in solution. IC to S 1 takes place nearly instantaneously within 20 fs for both molecules. Abundant nuclear wave packet motions in the S 1 state are manifest in the TF signals, which demonstrates that the IC in these BODIPY molecules is coherent with respect to most of the vibrational modes. Theoretical calculations assuming impulsive IC to S 1 account for the wave packet dynamics accurately.

Ultrafast intersystem crossing (ISC) in transition metal complexes leads to a long-lived active state with a high yield, which leads to efficient light energy conversion. The detailed mechanism of ISC may lead to a rational molecular design of superior transition metal complexes. Coherent nuclear wave packets observed in femtosecond time-resolved spectroscopies provide important information on the excited-state dynamics. In particular, analyzing the nuclear wave packets in both the reactant and the product may unveil the molecular dynamics of an ultrafast reaction. In this study, experimental evidence proving the reaction coordinates of the ultrafast ISC of ruthenium(ii) complexes is presented using coherent vibrational spectroscopy with a quantum chemical simulation of coherent vibrational motion. We observed vibrational modes strongly coupled to the ISC, whose vibrational coherences undergo remarkable attenuation after the ISC. The coupled modes contain metal-ligand stretching or symmetry breaking components, and the faster ISC rates of lower-symmetry ruthenium(ii) complexes support the significance of the latter.

Symmetry-breaking charge transfer (SBCT) is an important process at the early stages of the photoinduced processes in multichromophore systems such as the photosynthetic apparatus. We investigated the photoinduced SBCT dynamics of 9,9'-bianthracene (BA), a representative molecule showing SBCT, by time-resolved fluorescence (TF) with the highest time-resolution and excited-state quantum mechanics/effective fragment potential molecular dynamics (MD) simulation. TF experiments show that the SBCT kinetics matches quantitatively with the solvation function excluding the initial ultrafast component that is assigned to the inertial motion of the solvent. Therefore, it is established that the SBCT of BA is coupled solely with the rotational diffusion of solvent molecules excluding the inertial motion of solvents. MD simulations show that random rotational fluctuation of solvents mostly in the first solvation shell generates a transient electric field as high as 1.0 × 109 V m-1, which provides an asymmetric environment required for the generation of a CT state in this symmetric dimer. Once the CT state is formed, the dipole moment in the solute causes further rotation of solvent molecules leading to an augmented electric field, which in turn further stabilizes the CT state prohibiting the reverse reaction.

Detection of the biofilm of bacteria would be a counter strategy to detect hidden bacteria in their camouflage. Through unbiased screening of bacteria biofilm, we discovered a long wavelength probe CDr15 with extracellular DNA as the molecular target. CDr15 revealed a real-time geometric distribution of eDNA in a 3D bacterial colony.

Detailed molecular dynamics simulations of an acid-base reaction have been the subject of extensive investigations. Here we report the excited state proton transfer dynamics of pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) in acetate buffer by time-resolved fluorescence (TF) and quantum mechanical/effective fragment potential molecular dynamics (QM/EFP-MD) simulations. High time resolution in TF and TF spectra measurements allows the acquisition of accurate reaction kinetics. Upon the photoexcitation of HPTS, the proton (deuterium) is transferred coherently to acetate in 60 fs (80 fs) for a contact pair of HPTS (DPTS) and acetate by a hydrogen bond, which comprises approximately 28% of the population. ESPT proceeds slowly on a picosecond time scale for the remaining HPTS as reported previously. Coherent wave packet motions of the reactant (acid) and the product (conjugate base) enable the acquisition of the vibrational spectra of excited states via TF (VETF). A comparison of the VETFs of the reactant and the product and the calculation of the Huang-Rhys factors (vibrational reorganization energies) identify the vibrational modes that actively participate in the coherent proton transfer. In particular, the 246 cm -1 vibrational mode, which consists of in-plane skeletal stretching motion, promotes the ESPT by transferring the donor oxygen towards the acceptor oxygen in acetate. QM/EFP MD simulations corroborate the experiment and provide molecular details of the ESPT.

In this study, we found a (Py)A-modified adenine cluster (A-cluster), a minimum fluorescent unit for significant emission wavelength changes, and investigated its photophysical and structural properties. The basic A-cluster unit was an adenine-pentad duplex containing stacked (Py)A pairs in the center aligned in an antiparallel manner. Spectral analysis of the A-cluster revealed remarkable reddish fluorescence with a large Stokes shift (∼195 nm) and a long life-time constant (31 ns), originated from exciton states formed by (Py)A pairs and neighboring adenines. Structurally, the exciton state of the A-cluster exhibited unusually high stability, relative to that of other five-mismatched duplexes, as a result of stabilization through strong stacking interactions (zipper-like structure) of the mismatched A-A and (Py)A pairs, rather than through traditional Watson-Crick base pairing. These spectral and structural properties of the A-clusters were specific to the adenine bases and highly disturbed by introducing other bases (T, G, and especially C) or an abasic site into the A-cluster, whereas they were enhanced through synergistic effects in systems containing multiple A-clusters. As a minimum unit for these unique properties, finally, the A-cluster was exploited as a fluorescent probing system for specific nucleic acid sequences, such as miR-21, accompanying distinct fluorescence color changes from blue to red. These findings indicated the potential utility of the A-cluster as a part of fluorescent probes exhibiting clear signaling upon micro-environmental changes.

The donor-acceptor (D-A) type dipolar fluorophores, an important class of luminescent dyes with two-photon absorption behaviour, generally emit strongly in organic solvents but poorly in aqueous media. To understand and enhance the poor emission behaviour of dipolar dyes in aqueous media, we undertake a rational approach that includes a systematic structure variation of the donor, amino substituent of acedan, an important two-photon dye. We identify several factors that influence the emission behaviour of the dipolar dyes in aqueous media through computational and photophysical studies on new acedan derivatives. As a result, we can make acedan dyes emit bright fluorescence under one- and two-photon excitation in aqueous media by suppressing the liable factors for poor emission: 1,3-allylic strain, rotational freedom, and hydrogen bonding with water. We also validate that these findings can be generally extended to other dipolar fluorophores, as demonstrated for naphthalimide, coumarin and (4-nitro-2,1,3-benzoxadiazol-7-yl)amine (NBD) dyes. The new acedan and naphthalimide dyes thus allow us to obtain much brighter two-photon fluorescent images in cells and tissues than in their conventional forms. As an application of these findings, a thiol probe is synthesized based on a new naphthalimide dye, which shows greatly enhanced fluorescence from the widely used N , N -dimethyl analogue. The results disclosed here provide essential guidelines for the development of efficient dipolar dyes and fluorescence probes for studying biological systems, particularly by two-photon microscopy.

Time-resolved fluorescence (TRF) with a resolution higher than the periods of vibrations may provide the vibrational spectrum of an emitting species by directly recording the vibrational wave packet motions in time. We applied high-resolution TRF to investigate the excited-state dynamics of pigment yellow 101 (P.Y.101). The TRF spectra of P.Y.101 in dichloromethane showed that upon photoexcitation of the enol isomer, dynamics occur in the S1 state to form a product in two time constants at 30 and 140 fs. TRF signals were modulated due to the vibrational wave packet motions in the excited states, which provided the vibrational spectra of the emitting species. Depending on the emission wavelength, two different vibrational spectra were evident. With the help of theoretical calculations, the two spectra were assigned to the enol and keto isomers of P.Y.101 in the S1 state, leading to the conclusion that P.Y.101 undergoes ultrafast excited-state intramolecular proton transfer (ESIPT) with a quantum yield close to 1. Visible-pump infrared-probe transient absorption spectra were recorded to corroborate this conclusion.

We report photoluminescence (PL) modulation of quantum dots (QDs) by photoinduced electron transfers from acridine-1,8-dione derivative surface ligands. Reversible PL switching upon many repeated cycles was demonstrated, as alternating on and off of the UV excitation for the surface ligand has successfully resulted in the QD PL modulation.

Reaction-based fluorescent probes for monoamine oxidases A and B are developed based on a new two-photon absorbing compound and its precursor. The probes show turn-on fluorescence response to the enzymes owing to the two-photon absorbing compound produced by the enzymatic activity, as monitored by one- as well as two-photon microscopy for the first time.

The fluorescence changes of (Fl)U moieties located at the terminal and mid-loop positions of the human i-motif sequence allow its probing as well as the sensing of G-quadruplex sequences.

Blinking of zinc blende CdSe-based core/shell nanocrystals is studied as a function of shell materials and surface ligands. CdSe/ZnS, CdSe/ZnSe/ZnS and CdSe/CdS/ZnS core/shell nanocrystals are prepared by colloidal synthesis and six monolayers of larger bandgap shell materials are grown over the CdSe core. Organic-soluble nanocrystals covered with stearate are made water-soluble by ligand exchange with 3-mercaptopropionic acid. The light-emitting states of nanocrystals are characterized by absorption and emission spectroscopy as well as photoluminescence lifetime measurements in solution. The blinking time trace is recorded for single nanocrystals on a glass coverslip. Both on- and off-time distributions are fitted to the power law. The power-law exponents vary, depending on shell materials and surface ligands. The off-time exponents for organic and water-soluble nanocrystals are measured in the range of 1.36-1.55 and 1.25-1.37, respectively, while their on-time exponents are spread in the range of 1.53-1.86 and 1.85-2.17, respectively. Water-soluble surface passivation with thiolate prolongs the dark period regardless of shell materials and core/shell structures. Of the core/shell structures, CdSe/CdS/ZnS exhibits the longest bright state. The on/off-time exponents are inversely correlated, although the successive on/off events are not individually correlated. A two competing charge-tunneling model is presented to describe the variation of on- and off-time exponents with shell materials and surface ligands.

Homogeneous oligodeoxyguanylates incorporating small-molecule intercalator pyrene moieties in a 1,5 relationship form various structures of dimer, trimer, tetramer and internal hairpin form that are yellowish in color because of the intermolecular interactions between the pyrene units.

An ultra-fast chemical reaction can act as an impulsive excitation of the vibrations. Excited state intramolecular proton transfer in 10-hydroxybenzo[h]quinoline proceeds in 13 fs. As a result, product vibrational modes with their periods longer than (13 x 2) fs are coherently excited in the product potential surface, which can be observed most unambiguously by the oscillations in the time-resolved fluorescence signal of the product keto isomer.