In order to better understand the nature of intramolecular charge and energy transfer in multibranched molecules, we have synthesized and studied the photophysical properties of a monomer quadrupolar chromophore with donor−acceptor−donor (D−A−D) electronic push−pull structure, together with its V-shaped dimer and star-shaped trimers. The comparison of steady-state absorption spectra and fluorescence excitation anisotropy spectra of these chromophores show evidence of weak interaction (such as charge and energy transfer) among the branches. Moreover, similar fluorescence and solvation behavior of monomer and branched chromophores (dimer and trimer) implies that the interaction among the branches is not strong enough to make a significant distinction between these molecules, due to the weak interaction and intrinsic structural disorder in branched molecules. Furthermore, the interaction between the branches can be enhanced by inserting π bridge spacers (−CC− or −CC−) between the core donor and the acceptor. This improvement leads to a remarkable enhancement of two-photon cross-sections, indicating that the interbranch interaction results in the amplification of transition dipole moments between ground states and excited states. The interpretations of the observed photophysical properties are further supported by theoretical investigation, which reveal that the changes of the transition dipole moments of the branched quadrupolar chromophores play a critical role in observed the two-photon absorption (2PA) cross-section for an intramolecular charge transfer (ICT) state interaction in the multibranched quadrupolar chromophores.

Recently, an organic synthetic strategy based on hybridized local and charge transfer (HLCT) character has been attracting much attention because of its potential for designing high-efficiency organic light-emitting diode materials. In this work, two novel molecules, N,N-diphenyl-4-phenol-(1-phenyl-1H-phenanthro­[9,10-d]­imidazol-2-yl)­biphenyl-4-amine (TPA-PPI-OH) and N,N-diphenyl-4′-(1-phenyl-1H-phenanthro­[9,10-d]­imidazol-2-yl)-[1,1′-biphenyl]-4-amine (TPA-PPI), were investigated by quantum chemical calculations, steady-state spectroscopy, and femtosecond transient absorption spectroscopy (fs-TA) to explore the nature of HLCT. Computational results and steady-state spectra suggest that the lowest excited state is dominated by local excitation (LE) character in low-polar toluene (TOL), whereas the charge transfer (CT) character plays the main role in high-polar acetonitrile (ACN) for both TPA-PPI-OH and TPA-PPI. Relative to TPA-PPI, TPA-PPI-OH shows less sensitivity to solvent polarity with higher quantum yields because of the more planar geometric structure, fabricated by inserting an additional intramolecular hydrogen bond (H-bond) to enhance the inflexibility of the molecule. Ultrafast fs-TA clearly shows the conversion of excited states from LE to CT with the increase of solvent polarity. The stimulated emission is mainly from the LE-dominated lowest excited state in low-polar TOL, whereas CT dominates the final relaxation process in high-polar ACN because of strong solvation. Furthermore, the excited states being dominated by LE and CT simultaneously in medium-polar tetrahydrofuran is observed, while the quick equilibrium LE ↔ CT is established just after a femtosecond pulse excitation, indicating the typical HLCT character. The excited state deactivation process of TPA-PPI-OH is faster than that of TPA-PPI, which is attributed to the higher proportion of the LE component and the additional vibrational decay paths induced by the H-bond in TPA-PPI-OH. The results herein offer a guidance to understand the solvent-modulated excited state deactivation mechanism of HLCT molecules.

We present direct observation of the velocity-selective optical pumping of the Rb ground state hyperfine levels induced by 5S(1/2) --> 5P(1/2) femtosecond pulse-train excitation. A modified direct frequency comb spectroscopy based on the fixed frequency comb and a weak cw scanning probe laser was developed. The femtosecond pulse-train excitation of a Doppler-broadened Rb four-level atomic vapor is investigated theoretically in the context of the density matrix formalism and the results are compared with the experiment.

We quantified the Manning free (uncondensed) counterions fraction θ for dilute aqueous solutions of rodlike polyions: 150 bp DNA fragments, in the presence of a very low concentration of monovalent salt c(salt)<0.05 mM. Conductivity measurements of these solutions for DNA base pair concentration range 0.015≤c≤8 mM were complemented by fluorescence correlation spectroscopy (FCS) measurements of the DNA polyion diffusion coefficient D(p)(c). We observed a crossover in the normalized conductivity σ(c)/c that nearly halved across the c=0.05-1 mM range, while D(p)(c) remained rather constant, as we established by FCS. Analyzing these data we extracted θ(c)=0.30-0.45, and taking the Manning asymmetry field effect on polyelectrolyte conductivity into account we got θ(c)=0.40-0.60. We relate the θ(c) variation to gradual DNA denaturation occurring, in the very low salt environment, with the decrease in DNA concentration itself. The extremes of the experimental θ(c) range occur toward the highest, above 1 mM, and the lowest, below 0.05 mM, DNA concentrations and correspond to the theoretical θ values for dsDNA and ssDNA, respectively. Therefore, we confirmed Manning condensation and conductivity models to be valuable in description of dilute solutions of rodlike polyions.

Single-molecule (SM) spectroscopy has been conducted to study exciton-like intramolecular charge transfer (ICT) coupling dynamics in two model dendritic systems containing branched ICT interactions. The strong coupling and stepwise photobleaching of the ICT branches in the dendrimers, which depend on the torsional disorder, have been demonstrated at the SM level. The fluorescence from the delocalized ICT excited state over two branches in push-pull molecules, which cannot be distinguished by means of conventional experiments probing the average behavior of large ensembles of molecules, has been observed at the SM level.

ABSTRACT:We correlate conformation and dynamics of the semirigid polyelectrolytes deoxyribonucleic acid (DNA) and hyaluronic acid (HA) in the semidilute regime, across a broad concentration range (10 −3 − 10 2 g/L). Salt-free polyelectrolytes are distinct from uncharged polymers as they presumably form a rather rigid, isotropic mesh. The polyelectrolyte characteristic mesh size is known as de Gennes correlation length ξ. We directly probed the mesh formed by DNA and HA, by employing fluorescence correlation spectroscopy (FCS) to measure the diffusion coefficient of fluorescently labeled DNA fragments added in trace amounts. For the salt-free solutions we found that the DNA or HA mesh size has to be 2−3 times larger than the fragments for them to start to diffuse freely (as if in the dilute solution). For a tighter mesh (concentrations 0.1−1 g/L), the fragment diffusion coefficient is only half the free diffusion value. Conversely, fragments show the free diffusion coefficientas if there is no meshin DNA or HA in 10 mM buffer. This complies well with the fact that the ξ fades for polyelectrolytes with added salt. The diffusion coefficient falls off further when the mesh size gets smaller than the fragment size (above 1 g/L) and a similar value is reached in buffer as well as in pure water, respectively, at the highest measured concentrations (10 g/L). We also performed small-angle X-ray scattering (SAXS) on HA and DNA (range 3−130 g/L) to complement our previous dielectric spectroscopy (DS) studies (range 0.01−5 g/L). Combined, these methods provide reference values of the de Gennes length ξ across the range studied by FCS.

No abstract

No abstract