Seung Joon Jeon

A two-photon fluorescent probe for lipid raft imaging : C-laurdan

The lipid‐rafts hypothesis proposes that naturally occurring lipid aggregates exist in the plane of membrane that are involved in signal transduction, protein sorting, and membrane transport. To understand their roles in cell biology, a direct visualization of such domains in living cells is essential. For this purpose, 6‐dodecanoyl‐2‐(dimethylamino)naphthalene (laurdan), a membrane probe that is sensitive to the polarity of the membrane, has often been used. We have synthesized and characterized 6‐dodecanoyl‐2‐[N‐methyl‐N‐(carboxymethyl)amino]naphthalene (C‐laurdan), which has the advantages of greater sensitivity to the membrane polarity, a brighter two‐photon fluorescence image, and reflecting the cell environment more accurately than laurdan. Lipid rafts can be visualized by two‐photon microscopy by using C‐laurdan as a probe. Our results show that the lipid rafts cover 38 % of the cell surface.

Femtosecond characterization of vibrational optical activity of chiral molecules

Optical activity is the result of chiral molecules interacting differently with left versus right circularly polarized light. Because of this intrinsic link to molecular structure, the determination of optical activity through circular dichroism (CD) spectroscopy has long served as a routine method for obtaining structural information about chemical and biological systems in condensed phases. A recent development is time-resolved CD spectroscopy, which can in principle map the structural changes associated with biomolecular function and thus lead to mechanistic insights into fundamental biological processes. But implementing time-resolved CD measurements is experimentally challenging because CD is a notoriously weak effect (a factor of 10-4–10-6 smaller than absorption). In fact, this problem has so far prevented time-resolved vibrational CD experiments. Here we show that vibrational CD spectroscopy with femtosecond time resolution can be realized when using heterodyned spectral interferometry to detect the phase and amplitude of the infrared optical activity free-induction-decay field in time (much like in a pulsed NMR experiment). We show that we can detect extremely weak signals in the presence of large achiral background contributions, by simultaneously measuring with a femtosecond laser pulse the vibrational CD and optical rotatory dispersion spectra of dissolved chiral limonene molecules. We have so far only targeted molecules in equilibrium, but it would be straightforward to extend the method for the observation of ultrafast structural changes such as those occurring during protein folding or asymmetric chemical reactions. That is, we should now be in a position to produce ‘molecular motion pictures’ of fundamental molecular processes from a chiral perspective.

Nonlinear optical and two-photon absorption properties of 1,3,5-tricyano-2,4,6-tris(styryl)benzene-containing octupolar oligomers

Octupolar oligomers containing 2-12 molecules of 1,3,5-tricyano-2,4,6-tris(styryl)benzene derivatives have been synthesized and their nonlinear optical and two-photon absorption (TPA) properties were determined. The beta(0) values are in the range of (85-1219) x 10(-30) esu and increase monotonically with the increasing number of the octupolar units within the molecule. The two-photon-induced fluorescence excitation spectra are quite similar to the single-photon absorption spectra except that the wavelength is doubled, indicating that the one- and two-photon allowed excited states are the same. The peak TPA cross-section values (delta(max)) measured with nanosecond pulses by the two-photon-induced fluorescence method are in the range (3010-62, 930) x 10(-50) cm(4)s photon(-1). The delta(max) increases as the number of the octupolar units in the molecules increases. A linear relationship is observed between delta(max) and beta, and this delta-beta relationship serves as a useful design strategy for the synthesis of novel octupolar oligomers and polymers with large TPA and beta.

Improved method for measuring the first-order hyperpolarizability of organic NLO materials in solution by using the hyper-Rayleigh scattering technique

Abstract We provide an improved method for measuring the first order hyperpolarizability of organic nonlinear optical materials in solution by the hyper-Rayleigh scattering technique. The method is free from the overestimation which is encountered in the conventional method due to the fluorescence induced by two-photon absorption.

UV-vis-NIR and Raman spectroelectrochemical studies on viologen cation radicals : evidence for the presence of various types of aggregate species

Abstract The spectral changes of 1-e − reduced species of unsymmetric viologens, 1-methyl-1′-alkyl-4,4′ -bipyridinium (C 1 C n V 2+ : n = 1, 8, 9, 12, 14, 16, 18) and 2-e − reduced species of bisviologens linked by polymethylene chains (C 1 V 2+ C n V 2+ C 1 : n = 3, 5, 8, 10) have been investigated spectroelectrochemically in the region from 350 to 1500 nm. Subtracting the contribution of the monomeric viologen cation radical from the spectra of reduced viologens, we obtained spectra of aggregates of viologen cation radicals. Three different spectral types were observed: Type I, the usual dimer spectrum of viologen cation radicals exhibiting structureless bands around 880 (band I), 530 (band II) and 360 nm (band III); Type II, red-shifts of the bands I and II of the Type I spectrum to as far as 1120 and 540 nm respectively; Type III, band positions similar to those of the Type I spectrum, but band II exhibiting a shoulder at around 510 nm and the bandwidth of band III much narrower than that of the corresponding band in the Type I spectrum. The Type I spectrum is shown with C 1 C n V .+ when n is less than 8. The Type II spectrum is observed with C 1 C n V .+ when n ≥ 9 and C 1 V .+ C n V .+ C 1 when n ≥ 8. Addition of cetyltrimethylammonium bromide (CTAB) to the solutions showing the Type II spectrum caused a spectral change towards Type I. The Type III spectrum is shown with C 4 C 4 V .+ in the presence of -γ-cyclodextrin and C 1 V .+ C 3 V .+ C 1 . The Raman spectra of viologen cation radicals showing Type I and II spectra exhibit doublet features at 1507/1527 and 1334/1351 cm −1 , whereas singlet bands are observed for solutions showing the Type III spectrum. We attributed the Type I spectrum to face-to-face viologen radical dimers, the Type II spectrum to higher aggregates of viologen radicals, and the Type III spectrum to a viologen radical dimer with oblique stacking of the viologen rings.

Vibrational contributions to the molecular first and second hyperpolarizabilities of a push-pull polyene

The vibrational contributions to the first and second hyperpolarizabilities of a push–pull polyene are theoretically studied by using the valence-bond charge-transfer model. The formal relationships between the vibrational and electronic contributions to the hyperpolarizabilities are derived. It is found that there exist strong correlations between the vibrational contributions and bond length alternation of a push–pull polyene. By calculating these contributions numerically, it is proven that the vibrational contributions, estimated from the IR, Raman, and hyper-Raman measurements, to the hyperpolarizabilities can also provide a measure of the nonlinear optical property.

2,6-Bis(styryl)anthracene derivatives with large two-photon cross-sectionsElectronic supplementary information (ESI) available: synthesis details; measurement of two-photon cross-sections. See http://www.rsc.org/suppdata/cc/b3/b309124d/

The first synthesis of 2,6-bis(styryl)anthrance derivatives with very large two-photon cross sections is reported.

Two-photon absorption properties of hexa-substituted benzene derivatives. Comparison between dipolar and octupolar molecules.

Hexa-substituted benzene derivatives show larger two-photon cross sections than the corresponding dipolar counterparts and the largest values of delta(max) = 2000 GM and delta(max)/MW = 2.25 GM g(-1) have been obtained with a compound having C[triple bond]C and C=C bonds in the conjugation bridge.

AN ELEMENTARY DESCRIPTION OF NONLINEAR OPTICAL PROPERTIES OF OCTUPOLAR MOLECULES : FOUR-STATE MODEL FOR GUANIDINIUM-TYPE MOLECULES

A theoretical description of the molecular polarizability (α) and first and second hyperpolarizabilities (β and γ) of a guanidinium-type octupolar molecule is presented. By using a valence bond and three charge-transfer states with nonzero transfer integrals, the electronic states are obtained as linear combinations of these basis states. It is found that a doubly degenerate excited state is only optically coupled to the ground state. Based on the analytic expressions for α, β, and γ obtained, it is shown that the resultant tensor elements of β satisfy the symmetry requirements of point group D3h. Unlike the linear push–pull polyenes, the magnitudes of α, β, and γ of the guanidinium-type octupolar molecules increase as the charge-transfer character of the electronic ground state increases. Also, their dependences on the distance (d) between the donor and acceptor are briefly discussed.

First order hyperpolarizabilities of 2-[2-(p-diethylaminophenyl)vinyl]-furan derivatives

Abstract The first evaluation of the β for 2-[2-(p-diethylaminophenyl)vinyl]furan derivatives is reported in comparison with the corresponding thiophene derivative.