Sharp J-band Research Articles

Integrating Highly Luminescent Lanthanides with Strongly Coupled Dye J-Aggregates on Nanotubes for Efficient Cascade Energy Transfer

Photoluminescent one-dimensional hybrid nanostructured materials having outstanding inorganic–organic advantages are gaining significant attention on account of their intriguing applications in nanoscale optoelectronic devices, (bio)sensors, and energy harvesting and conversion technologies. Here, we first report on the development of highly photoluminescent lanthanide organic hybrid nanotubular assemblies through in situ incorporation of a trivalent lanthanide ion, terbium (Tb3+), along with organic photosensitizers 2,3-dihydroxynaphthalene (DHN) or 1,10-phenanthroline (Phen) into the self-assembled nanotubes of sodium lithocholate (NaLC). Both the photosensitizers (DHN/Phen) are effective in sensitizing intense narrow emission peaks of Tb3+ on the nanotubes. Next, we utilize these luminescent lanthanides containing hybrid nanotubular assemblies as templates for spontaneous integration of strongly coupled pseudoisocyanine (PIC) dye J-aggregates with a sharp J-band absorption at 555 nm and strong fluorescence emission at 570 nm. The presence of the significant spectral overlap between the luminescence peak of Tb3+ at 545 nm and the J-aggregate absorption band results in efficient cascade energy transfer from photosensitizers to Tb3+ to the coherently coupled PIC dye J-aggregates. These NaLC nanotube-templated photosensitizer-Tb3+-J-aggregate hybrid systems have great potential for sensing and optoelectronic applications.

Photo-activated thin films of porphyrins for reactive oxygen species generation

The use of a single component porphyrin thin-film for photodynamic antimicrobial chemotherapy is unexplored. Herein, a porphyrin with good film formation properties is used to make thin layers, by physical vapor phase deposition, that are stable in aqueous media and can generate singlet oxygen upon 650 nm excitation. The synthesis and photophysical studies, in solution and as thin film, of free base 5,10,15,20-tetra-(4-hexyloxyphenyl)porphyrin are described. The porphyrin was synthesized via modified Adler-Longo conditions. Thin films of this porphyrin were made on quartz glass via spin coating and via vapour-phase deposition. The porphyrin was studied, in solution and as thin film (without a polymer component or matrix), with UV-Vis spectroscopy and with steady-state and time-resolved photoluminescence spectroscopy. The spin coated films display sharp J-band emission at 741 nm, indicating J-aggregate formation, but are not stable in aqueous media. The thin films obtained via vapour-phase deposition do not show the J-band emission, but are stable. The detection of singlet oxygen generated by the porphyrin thin film using a chemiluminescent water-soluble singlet oxygen probe is reported.

Study of lateral electron transfer in Langmuir–Blodgett films containing cyanine and viologen amphiphiles

The efficiency of the lateral photo-induced electron transfer between cyanine (electron donor) and viologen (electron acceptor) amphiphiles (Cy and V, respectively) was examined by observing the absorption and emission spectra of V doped Cy LB films. The sharp J-band observed in the absorption and fluorescence spectra indicated the efficient formation of J-aggregate of the cyanine chromophore even in the mixed LB monolayers with V. The fluorescence intensities of the J-band decreased monotonously with the increase of V concentration but the dependence of the electron transfer efficiency on the film composition was rather complicated. The efficiency of the lateral electron transfer estimated at the composition region of the lower V ratios, where V molecules are expected to disperse homogeneously into the Cy monolayer, was such that ca. 90 molecules of Cy would be quenched by one center molecule of V.

Spectroscopic properties of merocyanine mixed J-aggregate in wide temperature range

The monolayer assemblies incorporating merocyanine J-aggregates, MC(S), MC(Se), 6-Me-MC(S), and their combinations have been fabricated by the Langmuir–Blodgett (LB) technique. At room temperature, absorption and fluorescence spectra of the monolayer assemblies: MC(S), MC(Se), 6-Me-MC(S), are characterized by a sharp J-band and resonance fluorescence. The monolayer assemblies incorporating two dyes: MC(S)/MC(Se), 6-Me-MC(S)/MC(S), 6-Me-MC(S)/MC(Se), bear a single absorption band and accompanying fluorescence band, which are assigned to the homogeneous aggregate of amalgamation type (HA-aggregate). Detailed spectroscopic measurements: absorption spectra in wide temperature range and the fluorescence decay curves, of the MC(S)/MC(Se) assembly have been carried out as a prototype of the HA-aggregate. The continuous shift of the absorption and fluorescence peak positions with increasing x, from x = 0 (MC(S) assembly) to x = 1 (MC(Se) assembly), would indicate that the doping of MC(Se) molecules into the MC(S) J-aggregate does not induce serious structure change. An observation that the fluorescence lifetime of the MC(S)/MC(Se) assembly lies between those of the MC(S) assembly and the MC(Se) assembly implies that the inclusion of MC(Se) molecules does not result in the formation of defect states which may promote non-radiative decay of molecular excitons. The usefulness of the Urbach–Martienssen (UM) rule for the analysis of the low energy tail of absorption spectra of the MC(S)/MC(Se) assembly has been examined.

The role of cellulose acetate as a matrix for aggregation of pseudoisocyanine iodide: absorption and emission studies

Films of pseudoisocyanine iodide in a cellulose acetate matrix were prepared by spin coating and characterized by UV/Vis absorption and fluorescence spectroscopies. The comparison with self-supported films of the same dye enabled analysing the role of the matrix in the aggregation of pseudoisocyanine iodide ([PIC]I). It was proved that cellulose acetate is a suitable support for [PIC]I J-aggregates, which form during spinning, as shown by a very sharp J-band in the absorption spectra. This indicates a perfect coherence between stacked monomers in the supported J-aggregates. It was possible to individualize the emission spectrum of [PIC]I J-aggregates in cellulose acetate, by decomposition of the steady-state fluorescence spectra of the films. The dependence on the excitation wavelength of the relative emission intensities of monomers and J-aggregates, for λ em=587 nm, lead to confirm that the latter species have an absorption maximum at ∼500 nm in cellulose acetate. Finally, polarised absorption spectra of films obtained by the vertical spin coating technique showed that cellulose acetate allows a partial orientation of J-aggregates.

Controlled Layering of Two-Dimensional J-Aggregate of Anionic Cyanine Dye on Self-Assembled Cysteamine Monolayer on Au(111)

An anionic cyanine dye with disulfopropyl substituents allowed its two-dimensional (2D) J-aggregate of highest-ever quality to be successfully layered on top of the self-assembled monolayer (SAM) of cysteamine (CA) on atomically flat Au(111). The chemisorption of CA from millimolar ethanolic solutions was rapid, producing an estimated density of surface amino groups of ∼4 × 1014 cm-2 well within a minute. However, sufficient ordering of the CA SAM, which we found by scanning tunneling microscopy imaging to yield a characteristic 7 × √3 superlattice structure, took a considerably long extra immersion time of a few hours at room temperature. The highest-quality 2D J-aggregate with an extraordinary sharp J-band formed preferentially on densest but still not fully ordered CA SAMs, from typically 0.1 mM dye solutions in 1:1 mixed water and ethanol solvent at considerably high adsorption temperatures of 50−65 °C. A detailed analysis of the 2D J-aggregate and its molecular linkage with the CA SAM indicated that 1.3 × 1014 cm-2 dye molecules are individually anchored to the surface amino groups in the form of ammonium sulfonates. The resultant energy barrier for the 2D J-aggregation can still be overcome at the given high temperature of dye adsorption in the case of nonrigid (only partially ordered) CA SAMs because of the strong cohesive interactions between dye molecules possibly exceeding ∼50 kJ/mol. Other important questions such as the electric charge balance in the layered assembly and the overall chemistry of 2D J-aggregation are also addressed.

The formation of J-aggregates in mutual mixing merocyanine dyes LB films

The formation of J-aggregates in the mutual mixing LB films of [6Me-DS] 1− x [DO] x , [DS] 1− x [DO] x have been studied by optical absorbance, fluorescence and surface pressure-area isotherms. Here, 6Me-DS, DS and DO show three types of surface active merocyanine dyes. Both sharp J-band absorbance and fluorescence, being observed in the [6Me-DS] film, linearly shift to the longer wavelength for the replacement of [6Me-DS] by [DO]. On the other hand, in the [DS]-[DO] system, the shift of J-band peak is not monotonic and the peak intensity is once reduced by adding [DO], but enhanced at the 1:1 composition. At that composition a strong fluorescence is also detected, suggesting the well defined aggregates. In the pressure-area isotherms, characteristic reduction in the molecular occupying area is also observed in [DS] 0.5[DO] 0.5. The reduction may be ascribed to enhancement of the molecular ordering.

The Mutual Mixing Effect in the LB Films of Merocyanine Dyes

The mutual mixing effect in the LB films of four kinds of merocyanine dyes have been investigated by optical absorption and fluorescence measurements. In single component films, DSe, DS and 6-methyl DS (6Me-DS) can form J-like aggregates but not for DO. In [6Me-DS]-[DO] system, a sharp J-band peak linearly shifts to the longer wavelength for the replacement of 6Me-DS by DO. In [DS]-[DO] system, [DS]:[DO]=1:1 film shows a both sharp J-band absorption and fluorescence peak. The behavior are understood by the formation of the ‘solid solution’ for [6Me-DS]-[DO] and the ‘intermolecular compound’ for [DS]-[DO]. Furthermore, the spectrum of [DO]-[DSe] system shows the presence of the ‘phase separation’. These results would relate to an interesting problem, what we may call a ‘2-dimensional phase diagram’ in J-aggregates.

Optical absorption spectra of mixed merocyanine dye langmuir-blodgett films

Optical absorption and fluorescence, as well as electron spin resonance measurements were performed on Langmuir-Blodgett films of binary mixtures of three kinds of merocyanine dyes, i.e. DO, DS and DSe, where DS and DSe form J-like aggregates (but DO does not) in single-component films. The observed optical absorption spectra of mixed films were markedly dependent on the combination of dyes. [DS] 1− x [DSe] x LB films show a sharp red-shifted J-like band peak over the entire concentration range. In contrast, the spectrum of the [DSe] 1− x [DO] x system consists essentially of [DO]'s monomer peak and [DSe]'s J-like band peak, suggesting the presence of phase separation. In the [DS] 1− x [DO] x system, a J-like band peak is maintained up to a composition that contains 70% DO. In particular, [DS] 0.5[DO] 0.5 film shows a sharp J-like absorption and a sharp J-band fluorescence peak, and a small Stokes shift is observed. These results suggest that most of the DO molecules associate to form J-like aggregates in the [DS] 1− x [DO] x system.

Bandshapes of electronic absorption spectra in molecular aggregates

Qualitative analyses of bandshapes of molecular aggregates are made in terms of hypochromism (or hyperchromism) taking account of the TRK sum rule. The linear response theory for vibronic systems is applied to the analyses for the sharp J-band and the bands of the DNA-dye complexes as examples. It is numerically checked that the sum rule is satisfied within the absorption region under consideration. It is also emphasized that a particular bandshape in the region of interest is governed by not only its neighboring electronic levels but also by higher electronic levels outside the absorption region - this fact being implicated by the TRK sum rule. Finally, an analytical relationship for the bandwidths of polymer vibronic bands which are to be determined entirely by intermolecular interaction is derived and discussed in connection with the damping parameters.

EXTRINSIC COTTON EFFECTS ASSOCIATED WITH THE J- AND γ-BAND OF TRIPHENYLMETHANE DYES BOUND TO POLY-α,l-GLUTAMIC ACID

Abstract Spectroscopically “metachromatic” crystal violet shows a characteristic γ-band and, by comparison, “normal” malachite green gives rise to a sharp J-band, when each dye is bound to poly-α,l-glutamic acid in helical conformation. These absorption bands are associated with induced Cotton effects of very high magnitude hitherto not observed for acridine dyes.