Selective Formation of a Novel Dequenched Aggregates of Rhodamines: Comparison with J- and H-Aggregates∗

Self-organized rhodamine aggregates prepared by wetting/dewetting process of an ethanol solution on a hydrophilic glass surface consistently exhibited fluorescence without quenching. Upon annealing, the aggregates exhibited an irreversible transition to quenched state, like H-aggregate, whereas the fluorescence spectrum was unchanged. The as grown fluorescent aggregate is a novel aggregate, neither a J-aggregate nor an H-aggregate, showing a blue-shifted excitation spectrum whereas the emission spectrum is similar to that of molecularly dispersed solution. This fluorescent aggregate was only realized in nonequilibrium by the rapid dewetting process. [DOI: 10.1380/ejssnt.2009.89]


I. INTRODUCTION
Much attention has been paid to organic materials in recent years for their potential applications in electronics and photonics [1].For photonic applications, πconjugated dyes are major concerns due to its superior optical responses: high oscillator strength of the π-π* transition of the conjugated π orbital and its wide tunability in wavelength via molecular design.Some dye species, e.g.those providing J-aggregates [2], of π-conjugated dyes show high quantum efficiency of photoluminescence, although others get completely quenched when placed in a solid state, or even in a concentrated solution [3].Among the latter quenched dyes, rhodamine 6G (Rh6G) is one of the most popular: both dilution and circulation of the solution are inevitable to avoid dye quenching and to induce a laser emission under photoexcitation.
There were only few reports on partial existence of fluorescent dimers of Rh6G [4] so far, where the mode of Rh6G aggregation was hypothetically discussed that brought about the dequenched state.In the present study, a reliable procedure is described to selectively yield a novel fluorescent Rh6G aggregate, demonstrating an in situ switching, for the first time, from this novel fluorescent state to a quenched state, during which every aggregate traced the identical change of fluorescence efficiency.The quasi-stable nature of the aggregate is discussed in line with the mode of dye ordering inside the aggregate, and is compared with the case of cyanine dye NK1420 J-aggregate [5,6].

II. MATERIALS AND METHODS
We adopted the π-conjugated organic dyes indicated in Fig. 1, Rh6G, rhodamine B (RhB), rhodamine 101 (Rh101) and NK1420 (5,6- 1-propenyl]-1,3-diethylbenzimidazolium iodide) for the present study because each dye has a static positive charge so that they are expected to interact strongly with a hydrophilic glass surface with an exposed O − group [7].The hydrophilic glass surface promoted a wetting / dewetting process [8] of polar organic solvent upon it, which was the primary motive force realizing the self-organizing formation of submicrometer-sized particle arrays of the dye [9].When a glass rod slid over a 100 µl ethanol solution of typically 30 µg/ml Rh6G (Fig. 2), a very thin layer of Rh6G solution was formed.Thickness of the solution layer became thinner while the solvent ethanol evaporated naturally from it.At certain thickness the solution layer became separated into droplets, and then dye particles were formed as precipitates after the solvent further evaporated.Since the particle size would be determined by the number of dye molecules which is proportional to the droplet size left on the surface, and therefore determined by the wetting / dewetting process of the organic solvent, the size was controlled by the degree of the surface hydrophilicity and by changing the retraction velocity of the solvent boundary via the glass rod movement [9].The same dewetting procedure was used to prepare other rhodamines and NK1420 dye particles, but NK1420 was dissolved in methanol for solubility.

A. Fluorescent self-organized particles of Rh6G and NK1420
Under an epifluorescence microscope, all the selforganized particles showed fluorescence as shown in Fig. 3(a), which is a unique characteristic of the present specimen: for example, Rh6G fluorescence is generally regarded to be quenched in a solid state.Dye ordering in the particle should be different from that in conventional quenched Rh6G precipitates.When a higher concentration solution was dewetted, number of particles with regular shape decreased and large irregular precipitates increased.We assumed that particles with circular shape and with diameter less than 2 µm, exhibiting an orientational order [10], have grown under self-organization process, whereas irregular shaped particles are random precipitates: area of the circular particles with diameter less than 2 µm, regarded to be proportional to the number of dye molecules inside, were measured, and the ratio was plotted in Fig. 3(b).Under high concentration (> 60 µg/ml), the ratio of self-organization shows a similar tendency between Rh6G and NK1420.However, below 60 µg/ml, Rh6G manifests perfect self-organization, whereas NK1420 indicates a saturation where the selforganization of the dewetting competes with the intrinsic capability to form J-aggregates [5,6].Figure 3(b) indicates that the self-organization process was favored by a low concentration of target dyes, which is reverse to the fact that conventional crystallization takes place only at the dye saturation.The dewetting of a volatile solution on a glass is a rapid and efficient process in accumulating dyes into a particle, which suggests that the aggregates have grown in nonequilibrium and that they are stable only under electrostatic interaction with the charged surface of the glass substrate.

B. Fluorescence change due to annealing
To evaluate the thermal stability, dye aggregates on a glass was sealed in an observation chamber filled with nitrogen and the fluorescence was monitored while the temperature was elevated from 23 • C to 200 • C, lowered down to 40 • C, and then repeatedly changed between 40 • C and 200 • C. Dye particles with a diameter less than 2 µm were traced because they were expected to show unidirectional dye orientation [10].In the Rh6G case, the fluorescence intensity decreased down to 5 % irreversibly at the first temperature raise, and thereafter showed reversible fluorescence between two states [Fig.4 The initial large and irreversible fluorescence decrease, therefore, suggests that the as grown dequenched (fluorescent) aggregate is in a quasi-stable state, which is consistent with the nonequilibrium nature of the rapid dewetting process.The evacuation of the specimen chamber alone decreased the fluorescence only few %, which may be due to a release of solvent ethanol incorporated in the aggregate.The majority of the initial large fluorescence decrease should be ascribed to an irreversible change of the aggregate electronic state, further suggesting an irreversible alteration of molecular ordering in the aggregate [Fig.6(a)].
Other rhodamines, RhB and Rh101, also showed an initial irreversible fluorescence decrease and subsequent reversible fluorescence change upon annealing [Figs.4(c)  and 4(d)].Every aggregate in a microscope field of view showed similar patterns of fluorescence change, indicating that the self-organized particles are composed of the identical dye aggregate with the same optical and thermodynamic property.Figure 4 indicates that the as grown aggregates are quasi-stable and consistently manifest a high fluorescence [Fig.6(a)].
The fluorescence of cyanine dye NK1420 increased to 400 % irreversibly at the initial temperature raise [Fig.5], which ruled out the possibility of thermal dye degradation.The time course of change was similar, although the initial fluorescence varied (the fluorescence at 40 • C after the first annealing was taken as 400 %, then the overall trace was most fitted between particles), suggesting that the degree of the initial dye ordering was imperfect, which is consistent with the broad J-aggregate peak, and also with the limited self-organization of NK1420 dye [Fig.3(b)].These observations indicate that the NK1420 aggregates were also in a quasi-stable state, showing a weaker fluorescence than a perfect stable J-aggregate that was attained after annealing [Fig.6(b)].

C. Fluorescence spectrum change due to annealing
The fluorescence excitation spectrum of the Rh6G aggregates was blue-shifted by 30 nm from the absorption peak of the molecularly dispersed specimen [Fig.7(a)], whereas the emission spectrum was similar [Fig.7(b)].Surprisingly, both the emission spectrum and the excitation spectrum were similar before and after the annealing.These features were consistent among other rhodamines as shown in Fig. 8. Blue-shift of the absorption peak was believed to be typical to the H-aggregate of conjugated dyes,where rod-like dye molecules are aligned vertically to the substrate, with the same electric charge at the rod  molecule end placed in proximity, elevating the energy of excitons (blue-shift).In this sense, the energy band of our aggregate may be similar to H-aggregates both before and after the annealing.However, contrary to the report [4] that hypothetical Rh6G H-aggregates manifested a strong quenching, our self-organized aggregates are in a fluorescent state.Drastic fluorescence decrease upon annealing may then be explained by two tentative models: (A) efficient relaxation processes in as grown aggregates may take the excited states from top of the band to the bottom where emission takes place, whereas the annealed aggregates lose these relaxation paths due to an emergence of dipole-inhibited triplet states.(B) The transition moment of the band top levels decreases upon annealing, reducing the absorbance of the aggregate.
Anyway the drastic fluorescence change with invariant emission spectrum may present a possibility of switching the fluorescence quantum efficiency of dye aggregates by a slight modification of molecular ordering.Artificial rearrangement of the substrate surface charge may trigger the fluorescence switching: this approach will open a new way to manipulate molecules to exhibit a macroscopic change of optical property.Our self-organized dye particles, manifesting both the quenched state and the dequenched state, will present a solid basis for this study.

IV. CONCLUSION
The dewetting process on a hydrophilic surface enabled a selective formation of dequenched rhodamine aggregates with identical dye ordering, having a blue-shifted exciton band structure.Upon annealing, the band structure was preserved, whereas the fluorescence decreased drastically, suggesting an irreversible transition to quenched state.The as grown rhodamine aggregate, exhibiting fluorescence, was quasi-stable, only realized by a rapid dewetting process.

FIG. 1 :
FIG. 1: The molecular structure of (a) Rh6G, (b) RhB, (c) Rh101, and (d) cyanine dye NK1420.Note that each molecule has a positive charge in either of the NH groups (or N atoms).

FIG. 2 :
FIG.2: Schematic diagram showing the wetting/dewetting procedure to prepare self-organized dye particles.To control the thickness of the ethanol solution of the rhodamine dye, a glass rod was pressed against a glass substrate surface and rolled over it.At a critical solution layer thickness during the evaporation of ethanol, the solution layer becomes separated into droplets and then dye particles are precipitated.The critical solution layer thickness was controlled by the hydrophilicity of the substrate surface, which is accomplished by a prior ozone processing.

FIG. 3 :FIG. 4 :
FIG. 3: (a) Conventional epifluorescence image of the selforganized Rh6G particles.Scale bar indicates 20 µm.(b) Molar ratios of self-organized particles with circular shape with the dependence on the original dye concentration.

FIG. 5 :
FIG. 5: (b) Time course of relative fluorescence intensities of four NK1420 particles [indicated in (a)] during repeated temperature increase and decrease.

FIG. 8 :
FIG.8: Comparison between the fluorescence emission spectra of RhB (a), and Rh101 (b), self-organized particles before (black solid curve) and after (gray and dotted curve) the annealing.