Observations of merocyanine J-aggregate domains in mixed molecular monolayers using SHG/fluorescence and atomic force microscopes

Optical second harmonic generation microscopy: application to the sensitive detection of cell membrane damage

Optical second harmonic generation (SHG) is a nonlinear optical process which is sensitive to the symmetry of media. SHG microscopy allows for selective probing of a non-centrosymmetric area of sample. This type of nonlinear optical microscope was first used to observe ferroelectric domains and has been applied to various specimens including the biological samples to date. Imaging of the endogenous SHG of biological tissue has been utilized for the selective observation of filament systems in tissues such as collagen, myosin, and microtubules, which exhibit a polar structure. The cellular membrane can be selectively observed by the SHG microscope through membrane staining with amphiphilic polar dye molecules. It has been reported that, by imaging exogenous SHG of the membrane, sensitive detection of membrane damage could be realized using the SHG microscope. Because the staining dye is fluorescent, both SHG and two-photon excited fluorescence (TPF) images can be obtained simultaneously. How the SHG intensity depends on the molecular alignment of the polar dye molecules that reflects the ordering of lipid molecules in the plasma membrane and the necessity of the normalization of the SHG intensity by the TPF intensity is discussed. Furthermore, the assessment of the membrane damage induced by exposing polycation to HeLa cells has been compared with the conventional cytotoxicity and cell viability tests to demonstrate the higher sensitivity of the present SHG-based assay.

Quantum Nonlinear Optics with Polar J-Aggregates in Microcavities

We predict that an ensemble of organic dye molecules with permanent electric dipole moments embedded in a microcavity can lead to strong optical nonlinearities at the single-photon level. The strong long-range electrostatic interaction between chromophores due to their permanent dipoles introduces the desired nonlinearity of the light-matter coupling in the microcavity. We develop a semiclassical model to obtain the absorption spectra of a weak probe field under the influence of strong exciton-photon coupling with the cavity field. Using realistic parameters, we demonstrate that a cavity field with an average photon number near unity can significantly modify the absorptive and dispersive response of the medium to a weak probe field at a different frequency. Finally, we show that the system is in the regime of cavity-induced transparency with a broad transparency window for dye dimers. We illustrate our findings using pseudoisocyanine chloride (PIC) J-aggregates in currently available optical microcavities.

Generation of disk-like domains with nanometer scale thickness in merocyanine dye LB film induced by hydrothermal treatment

We have characterized the binary LB films of merocyanine dye (MS) and arachidic acid (C20) before and after hydrothermal treatment (HTT), which is defined as a heat treatment under relative humidity of 100%, focusing on the morphology studied by bright field (BF) microscopy and fluorescence (FL) microscopy. BF microscopy observation has revealed that the as-deposited MS-C20 binary LB film is found to emit intense red fluorescence over the whole film area by 540-nm excitation. Since the surface image is almost featureless, it is considered that the crystallite sizes of J-aggregate are less than 10 μm. Interestingly, after HTT, round-shaped domains are observed in the LB systems, and the sizes are reaching 100 μm in diameter. Crystallites of J-aggregate, which are bluish in color and emit intense red fluorescence, tend to be in the round domains. We have observed two different types of domains, i.e., blue-rimmed domains and white-rimmed domains, which are postulated to be confined in the inner layers and located at the outermost layer, respectively. The thickness of the domains is equal to or less than that of the double layer of the MS-C20 mixed LB film, which is ca. 5.52 nm. The molecular order of MS in the J-aggregate is improved by the HTT process leading to the significant sharpening of the band shape together with the further red shift of the band (from 590 to 594 nm up to 597 to 599 nm). The reorganized J-band is considered to be 'apparently' isotropic owing to the random growth of the J-aggregate in the film plane. We consider that the lubrication effect by the presence of water molecules predominates in the HTT process.

Conversion of the aggregation state of merocyanine dye, modification of the subcell packing of arachidic acid, and removal of the majority of n-octadecane by hydrothermal treatment in the liquid phase in a mixed Langmuir-Blodgett film of the ternary system

We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir-Blodgett (LB) film of merocyanine dye with an octadecyl group (MS(18))-arachidic acid (C(20))-n-octadecane (AL(18)) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS(18) hydrocarbon chain, the slight orientation change in the C(20) hydrocarbon chain, and the complete evaporation of AL(18). The dissociation of MS(18) is probably ascribed to the complete evaporation of AL(18) from the mixed LB film and the increase in thermal mobility of the long axis of the MS(18) hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS(18) aggregation state from H- to J-aggregates, the modification of the C(20) subcell packing from hexagonal to orthorhombic, and the removal of most of the AL(18) molecules. The conversion of the MS(18) aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS(18) aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL(18) molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS(18) permanent dipole interaction. Moreover, the modification of the C(20) subcell packing is possibly due to the hydrophobic effect, where the C(20) hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTL is quite effective to reorganize the chromophore alignment of MS(18), to modify the subcell packing of C(20) and to erase the majority of AL(18) molecules in the mixed LB film of the MS(18)-C(20)-AL(18) ternary system in a short time.

Thermal behavior of J-aggregates in mixed Langmuir-Blodgett films composed of merocyanine dye and deuterated arachidic acid investigated by UV-visible and infrared absorption spectroscopy

We have investigated the thermal behavior of J-aggregates in the mixed Langmuir-Blodgett (LB) films composed of the merocyanine dye (MS18)-deuterated arachidic acid (C20-d) binary system in the temperature range from 25 to 250 degrees C by means of UV-visible and IR transmission absorption spectroscopy. The temperature-dependent variations in both UV-visible and IR absorption spectra indicate that the MS18 aggregation states are linked with the MS18 intramolecular charge transfer and the behavior of the packing, orientation, conformation, and thermal mobility of the MS18 hydrocarbon chain. The J-aggregate formed at 25 degrees C in the mixed LB films dissociates in the temperature range from 25 to 110 degrees C, which is mainly ascribed to the increase in the thermal mobility of MS18 hydrocarbon chain and the dissociation of the chelation by a cadmium ion to the MS18 keto group. A thermally induced blue-shifted band appears at around 515 nm from 110 to 160 degrees C. This band is attributed to oligomeric aggregation with side-by-side alignment of the MS18 transition dipole moments on the basis of the shift to a higher-energy side, broadening, and temporary increment of the MS18 intramolecular charge transfer of the band. Consequently, the appearance of the thermally induced blue-shifted band indicates the possibility that the MS18 aggregation states can be controlled from the red shift to the blue shift by the annealing method adopted in the present study.

Determination of a merocyanine J-aggregate structure and the significant contribution of the electric dipole interaction to the exciton band wavelength

The molecular alignment of a merocyanine (MC) J-aggregate monolayer at the air-water interface was determined by a grazing incidence x-ray diffraction method. The obtained molecular arrangement apparently shows that the conventional formula, which accounts only for the transition dipole interaction, is not sufficient to figure out the exciton band wavelength, suggesting the importance of the electric dipole (ED) interaction. We derived a simple formula for the ED interaction energy under an extended dipole approximation and clarified the ED contribution in the MC J aggregate.

Mesoscopic structures and dynamics of merocyanine J-aggregate studied by time-resolved fluorescence SNOM

Time-resolved fluorescence SNOM is used to probe the mesoscopic structure and dynamics of long-chain merocyanine (C18MC) J-aggregates on glass plates prepared by spin coating, casting, and casting of water-soluble polymer films. A globular structure with an average diameter of approximately 1 microm and a height of approximately 50 nm was attributed to the J-aggregate of C18MC in the spin-coating film. In polymer films, the bandwidth of the absorption of J-aggregate is much narrower in polyvinyl alcohol (PVA, approximately 20 nm) than that in polyvinyl sulphate (PVS, approximately 60 nm). We have demonstrated that the large bandwidth of the spectrum is due to the inhomogeneous distribution of the J-aggregate. The fluorescence image of the J-aggregate in PVA film was rather uniform, whereas non-uniform distribution of the fluorescence was observed in PVS film. The fluorescence of C18MC J-aggregate in a small domain of PVA film was a single exponential decay with a lifetime as short as 19 ps, which was shorter than that in PVS film with a two-exponential decay (average lifetime of approximately 25 ps). The fluorescence lifetime of the J-aggregate and its single exponential behaviour are considered to be indicators of the uniform distribution of the J-aggregate. The non-uniform distribution of the J-aggregate in PVS film was interpreted in terms of electrostatic interaction between PVS and merocyanine.