Insights into the Mechanism of Enhanced Rhodamine 6G Dimer

Article pubs.acs.org/JPCC

Insights into the Mechanism of Enhanced Rhodamine 6G Dimer Fluorescence in Mesoscopic Pluronic-Silica Matrixes Aivaras Kazakevičius,† Domantas Peckus,†,‡ Oleksandr Boiko,† Leonas Valkunas,†,§ Evgen Leonenko,⊥ German Telbiz,⊥ and Vidmantas Gulbinas*,†,∥ †

Department of Molecular Compound Physics, Center for Physical Sciences and Technology, Savanoriu av. 231, 02300, Vilnius, Lithuania ‡ Institute of Materials Science, Kaunas University of Technology, Savanorių 271, 50131 Kaunas, Lithuania § Department of Theoretical Physics and ∥Department of General Physics and Spectroscopy, Vilnius University, Sauletekio al. 9, Build. 3, 10222 Vilnius, Lithuania ⊥ Department of Physico-Inorganic Chemistry, L. V. Pisarzhevsky Institute of Physical Chemistry of National Academy of Sciences of Ukraine, Prospect Nauki 31, Kiev 03128, Ukraine ABSTRACT: Comparative study of the spectroscopic properties of Rhodamine 6G dimers in ethanol solutions and in mesoscopic silica-Pluronic sol−gel films has been performed by means of steady state and transient absorption and fluorescence methods. The dimers act as fluorescence quenchers in solutions, while their fluorescence yield is about 36 times higher in the mesoscopic films where they dominate in the fluorescence spectra. The difference is caused by about 6 times slower nonradiative excited state decay and about 6 times stronger oscillator strength of the fluorescence transition of the dimers in the films in comparison with solutions. We demonstrate that the dimer fluorescence originates from the oblique sandwich-type dimers both in solutions and in mesoscopic films. We suggest that the higher fluorescence yield of the dimers in the mesoscopic films is mainly caused by the stronger deviation from the planar geometrical arrangement of the sandwich-type dimer, thus causing opening of the forbidden low energy excitonic transition. for mirrorless lasing.6,7 Excellent optical properties along with the ultrafast optical responses of the intercalated dye/acceptor complexes, can make these composite films suitable for use as photonic layers in all-optical ultrafast switching devices.8 Fixing the organic guests into the matrix in a way that would minimize the self-assembly aggregation process between individual dye molecules is the key problem in application of such hybrid films as photonic materials. Recent development in sol−gel procedure has opened new opportunities in use of mesostructured and mesoporous films as hosts for organic dyes.9,10 Rhodamine 6G (R6G) is the most widely studied and employed cationic xanthene dye. Due to strong absorption in the visible region, high fluorescence quantum yield reaching almost unity, as well as good optical and thermal stability, R6G has been extensively employed as lasing media in liquid and solid-state dye lasers. The majority of applications in which R6G is used exploit its strong absorption and fluorescence in the visible region; however, the self-assembly aggregation process between individual dye molecules often minimizes the performance efficiency. Dimerization of the R6G molecules in solutions leads to strong fluorescence quenching at high

1. INTRODUCTION Nanoscale thin films manifest a set of properties beneficial when used in dye lasers, amplifiers, switching devices, solar cells, and OLEDs. With increasing minuteness of these devices, highly accurate and homogeneous material structure is of primary importance. This can be fulfilled by using polymer materials such as polymethilmethacrylate1 or poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (usually called PEO20PPO70PEO20 or Pluronic P123)2 for the surfactant formation. Surfactants containing dye molecules, which demonstrate higher photostability than that in solutions, were effectively used for fabrication of solid state dye lasers.1,3 This allowed researchers to partially abandon solvent-based dye lasers, which required expensive maintenance and utilization of toxic wastes. Considering material-specific properties, the mesoscopic materials can be described as continuous and homogeneous substances on a lesser scale than an inherent nanostructure. The synthesis of surfactant-templated mesoporous silica stimulated an explosive technological development of mesoscopic materials: crystals, fibers, and films that have submicrometer thicknesses but macroscopic lengths and widths, and are composed of mesoscopic subunits with sizes of unit cells typical for micelles.4,5 Such films can also be used by constructing, for instance, sensors or mesoscopic waveguides © XXXX American Chemical Society

Received: May 11, 2015 Revised: July 27, 2015

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DOI: 10.1021/acs.jpcc.5b04514 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C concentrations. R6G dimers possessing very low fluorescence yield in solutions of the order of 10−3, act as fluorescence quenchers and their fluorescence is observable only at very high dye concentrations when the monomer fluorescence is substantially quenched.11−13 Because of the low fluorescence yield, such dimers were attributed to sandwich-type Haggregates. According to the exciton theory, the monomer units in the H-aggregate are stacked in a way that only the transition from the ground state to the upper exciton state is allowed, while deactivation occurs through a nonradiative internal conversion process. Sandwich-type dimers with ideally parallel molecular units are nonfluorescent and the weak fluorescence originates only due to deviation from the parallel arrangement. On the other hand, the R6G fluorescence yield in mesoscopic films drops down less dramatically with the dye concentration than in solutions. Moreover, the dimer fluorescence dominates at high concentrations. The enhanced efficiency of the dimer fluorescence has been attributed to the formation of another type, J-dimers as a result of adsorption of R36G molecules to the matrix surfaces.14−19 The fluorescent dimers may be either coplanar J-dimers with a strongly dominating red-shifted absorption band, or oblique oriented dimers.20 The oblique dimers have optically allowed transitions to both absorption bands including the J-band, and show fluorescence, which are sometimes also called J-dimers. Based on the exciton theory the angle between transition dipole orientations of monomeric units in oblique dimers of about 50−70° has been evaluated from the relative intensities of the H and J absorption bands. A quite similar R6G oblique dimer arrangement with an angle between monomeric units of about 50° has also been evaluated from the absorption spectrum of the high concentration R6G solution in ethylene glycol.11 However, the dimers in this case demonstrated a low fluorescence yield of about 0.42%. On the other hand, several publications suggested coexistence of H- and J-aggregates in mesoscopic films and discussed transformations from one type to the other.18,19,21 Thus, the difference between the R6G dimer fluorescence efficiencies in mesoscopic films and in solutions still has to be clarified. In general, photophysical properties of dye molecules within a framework of mesoscopic films depend on the host−guest interaction of dye molecules within the host material, on the self-organization, and on the dye−dye interaction that takes place in situ, responsible for the dye aggregation. Here we present study of Rhodamine 6G aggregates in solutions and in mesoscopic films performed by combining steady state and ultrafast time-resolved absorption and fluorescence techniques. We demonstrate that oblique sandwich-type dimers are responsible for the dimer fluorescence both in solutions and in solid films, and we attribute the stronger dimer fluorescence in films to stronger distortion of the planar dimer arrangement causing opening of the low energy excitonic transition.

ratios of TEOS, ethanol, distilled water, HCl, and Pluronic 123 in film-forming sol were 1:8:2:0.5:0.01, respectively. All reagents were of chemical pure grade and were employed without further purification. The R6G dye was added to the obtained solution with concentrations of 3.6, 10, and 73 g/L. This solution was mixed at ambient temperature for 20 min to form sol that was used for film formation. The colored films were prepared on glass substrates using a spin-coating technique with various rotation and withdrawal speeds (800− 2500 rev/min.). Standard procedure of substrate cleaning (in a hot chromic mixture, followed by rinsing with distilled water) was applied before coating. After preparation, the films were aged at ambient temperature for 48 h at atmospheric air conditions. The prepared films had average thickness of about 200 nm, which was measured by atomic force microscopy (AFM). As a result, four different sets of samples were produced using previously described techniques. Further on, we will use definitions CF3.6, CF10, and CF73 for the films prepared from SiO2 + P123 + R6G solutions with dye concentrations of 3.6, 10, and 73 g/L respectively. Optical absorption spectra were measured by Specord 210 spectrophotometer. Stationary fluorescence spectra and decay kinetics were measured with a time-correlated single photon counting (TCSPC) fluorescence spectrometer Edinburgh Instruments FL920. A 470 nm diode laser generating about 80 ps duration pulses at a 1 MHz repetition rate was used as an excitation source. A pair of short pass and long pass filters was additionally used to suppress the residual scattered laser radiation. The fluorescence decays were measured at few selected wavelength. Time-resolved fluorescence spectra with a time resolution of about 3 ps were measured using Hamamatsu streak camera with Princeton Instruments spectrograph (Acton SP2150i). The spectrograph is equipped with two gratings (50 and 150 g/mm), which provides the possibility to reach a proper wavelength resolution. The excitation source was a Yb:KGW PHAROS oscillator (OSC-11119) with a Kerr lens for mode locking, chirped mirrors, and a prism pair that was used for a group velocity and cavity length adjustment. Radiation at 1030 nm was emitted by the oscillator at 76 MHz of the repetition rate. A pulse picker with a Pockels cell was used to adjust the laser pulse repetition rate. Further the laser beam was guided into the light conversion harmonic generator HIRO (PH104L) which allows the fundamental laser radiation conversion into second, third and fourth harmonics. Therefore, radiation at 1030, 515, 343, and 258 nm could be used for time-resolved fluorescence spectra measurements. In this study, samples were excited at 515 nm, with 1 mW/cm2 of the average radiation power. No degradation of the samples was observed. The spectra were recorded in time ranges of 160 ps and 2.1 ns with average resolutions of 3.4 and 12 ps, respectively. The fluorescence quantum yields were evaluated by means of comparison of spectrally integrated fluorescence intensity with that of low concentration R6G solution used as etalon. Diode laser generating at 520 nm was used for the sample excitation, integrating sphere was used for the fluorescence collection, and fiber spectrometer Avantes Avaspec-HS-TEC was used for the fluorescence detection. Sample absorbance and spectrometer spectral sensitivity were accounted in evaluation of the fluorescence yield. Femtosecond transient absorption investigations were carried out by using the spectrometer based on the Ti:sapphire laser Quantronix Integra-C, generating pulses of 130 fs duration at

2. EXPERIMENTAL SECTION The mesoscopic hybrid SiO2 films were prepared by the one pot template sol−gel technique. Tetraethoxysilane (TEOS) with 208.33 g mol−1concentration, which served as a precursor of matrix was dissolved in ethanol using magnetic stirring for 120 min at 60 °C. Pluronic 123 (5800 Da × 1.660538921(73) × 10−27 kg.) served as template. These chemicals were dissolved in ethanol, distilled water and HCl solvent. Molar B


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The Journal of Physical Chemistry C

the H and J excitonic transitions getting value of about 1.2. This ratio is close to that obtained for R6G in the methanol solution.12,20 Figure 1a also shows the fluorescence spectra of the low and high concentration solutions. Fluorescence spectra of both solutions are typical for R6G monomers. However, the fluorescence yield strongly drops down with increase in the dye concentration; the fluorescence yield of 0.1 mol/L solution drops down to about 1.2%, that is, approximately 72 times in comparison with the 0.0073 mol/L solution. A similar trend was observed for the fluorescence decay rate (Figure 1b). The fluorescence lifetime for the 0.0073 mol/L solution was obtained as equal to 5.2 ns, which is slightly longer than typically reported values of 3.6−4 ns for R6G monomers. Similar prolongation observed in ref 22 was attributed to reabsorption. The fluorescence lifetime drops down to about 0.3 ns at 0.1 mol/L concentration, indicating the quenching of the monomer fluorescence by the dimers present at high concentration. It should be noted that the initial relaxation time measured with a streak camera was even shorter (not presented). Additionally the fluorescence decay has a weak slower decay component with a time constant of about 0.9 ns. Figure 1b also shows the fluorescence decay kinetics of the 0.1 mol/L concentration solution at 700 nm, where the dimer fluorescence dominates. In addition to the weak 0.3 ns decay component, attributed to monomers, the kinetics also shows relaxation with about 0.9 ns time constant, which should be attributed to dimers. It should be noted that more than 100 times shorter dimer relaxation time has been evaluated from the dimer absorption spectrum and its fluorescence yield.12 The fluorescence spectrum of the 0.1 mol/L solution has slightly stronger long wavelength tail, which should be attributed to dimers.11,21 We can evaluate the fluorescence spectrum of the dimer by normalizing the fluorescence spectra obtained at high and low concentrations and subtracting one from another. Figure 1 shows the resulting fluorescence spectrum, which is similar to those obtained in other solutions.11,12,20,21 Taking into account that the monomer fluorescence yield drops down about 72 times in high concentration solutions, we conclude that only about 1.4% of excited R6G monomers decay intrinsically, while remaining, approximately 98.6%, fraction of excitations is quenched by transferring the excitation energy to dimers. We can estimate the fluorescence yield of dimers by comparing fraction of excitations transferred to dimers and ratio of spectrally integrated dimer and monomer fluorescence intensities. Such estimation gives the dimer fluorescence yield of about 10−3. This value is in a good agreement with the fluorescence yield values of 6 × 10−4 and 8.5 × 10−4 in water and methanol, correspondingly,12,13,20 but it is about 4 times lower than that determined in ethylene glycol.11 This difference is not surprising if about 16 times larger viscosity of ethylene glycol, resulting in suppression of the nonradiative relaxation, is taken into account. Figure 2 shows the transient absorption spectrum of the 0.1 mol/L R6G solution. At short delay times after excitation, the transient absorption spectrum shows the negative band formed by the absorption bleaching and the stimulated emission of monomers and also bleaching of the dimer H-band. During several tens of picoseconds, the relative intensity of the monomer bleaching band drops down and the spectrum becomes similar to the negative dimer spectrum evaluated from the steady state absorption spectra, only the H-band intensity is

810 nm with a 1 kHz repetition rate. The laser output was converted into the 400 nm radiation used for the sample excitation via traveling wave parametric amplifier Topas-C. The sample probing was performed by continuum light generated in a sapphire plate. The spectra were registered using Avantes Avaspec-1650F spectrometer. All measurements were performed at room temperature in ambient atmospheric conditions.

3. EXPERIMENTAL RESULTS Absorption and Fluorescence Properties of R6G Solutions. At first, we will shortly discuss properties of R6G dimers in ethanol solutions. The absorption spectrum at low dye concentrations was typical for R6G monomers (Figure 1a).

Figure 1. (a) Absorption and fluorescence (470 nm excitation) spectra of R6G in ethanol at different concentrations normalized to the peak values. Magenta lines show calculated absorption and fluorescence spectra of dimers. (b) Fluorescence decay kinetics of the same solutions measured at the peak of the monomer band at 570 nm, and also at 700 nm for the high concentration solution.

Absorption changes caused by the dimer formation became evident by increasing the dye concentration in agreement with the previously reported results.11,12 We can estimate the absorption spectrum of the dimers (see Figure 1a) by appropriately normalizing the absorption spectra at different concentrations and subtracting one from another as was done for R6G solutions in other solvents.11,12 Such estimation is based on the assumption that the dimers do not absorb at the maximum of the monomer absorption band, which is probably not completely correct. From the obtained spectrum we are able to determine the ratio between the oscillator strengths of C


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Figure 2. Transient differential absorption spectra of 0.1 mol/L R6G solution measured at different delay times after excitation at 530 nm.

slightly lower. This situation is slightly unusual for the transient absorption. The absorption bleaching band usually moves to the longer wavelength side, revealing energy transfer to species with lower excited state energy. Here the energy transfer takes place from monomeric molecules to the low energy excitonic component of dimers, but it causes bleaching of both excitonic bands. At longer times, the transient absorption intensity decreases but the shape of the spectrum does not change further. This spectra dynamics is consistent with the fluorescence dynamics. We can unambiguously attribute the long-living spectrum to dimers. The relative intensities of the H- and J-band bleaching are slightly different than the evaluated steady state intensities; the H-band bleaching is about two times stronger. This disagreement should be attributed either to the inaccuracy of the estimation of the steady state spectrum of dimers, or to the excited state absorption, which may partly compensate the J-band bleaching. Thus, the transient absorption dynamics enables us to unambiguously conclude that both absorption bands of the dimers belong to the same species with partly allowed both low energy and high energy excitonic transitions. Moreover decay time of the H-band bleaching is very close to the evaluated 0.9 ns fluorescence decay time, which shows that the dimer fluorescence should be attributed to the same dimer type. Absorption and Fluorescence Properties of SiO2 + P123 + R6G Solutions. We investigated SiO2 + P123 + R6G solutions used for formation of the mesoscopic films in order to determine at which stage of the film preparation R6G molecules localize inside the pluronic micelles. The fluorescence spectra, their intensities, and their decay kinetics were identical to those of the R6G solutions at identical R6G concentrations. From this we conclude that the presence of P123 and SiO2 do not change the probability of the energy transfer from the dye monomers to the dimers. Consequently, we conclude that P123 and SiO2 do not affect aggregation of the dye molecules and deduce that R6G molecules in solutions do not localize inside the P123 micelles present in solutions. Absorption and Fluorescence Properties of Mesoscopic Films. Figure 3 shows absorption and fluorescence spectra of SiO2 + P123 + R6G films with different dye concentrations. Both absorption and fluorescence spectra clearly indicate the presence of dimers. As stated before, the 500 nm absorption band is attributed to the dimer H-band and to the vibronic band of the monomer absorption. With the increase of the dye concentration, the 500 nm absorption band

Figure 3. (a) Absorption and fluorescence spectra of films prepared from solutions with different R6G concentrations. Blue lines show evaluated absorption spectrum of dimers. (b) Normalized fluorescence spectra corresponding to the monomers, dimers, and crystallites. The fine line shows the fluorescence spectrum of the R6G powder. The fluorescence spectra were measured under excitation at 470 nm.

increases relative to the monomeric absorption band. This is the major proof of the dimer formation.12,20 The optical density of the CF73 film was too large to correctly measure its absorption spectrum. The long wavelength side of the major absorption band was also broadened extending to the 550−600 nm region. We used similar procedure as in the case of solution spectra by determining the absorption spectrum of dimers in the films. This spectrum is also presented in Figure 3. The dimer spectrum in the film is similar to the dimer spectrum in solutions, only the H-band is slightly stronger and broadened. Assuming that monomeric molecules in films have identical absorption spectra as in solutions and taking into account that the total spectrally integrated oscillator strength of the dimer should be twice as strong as that of the monomer, we estimate that about 11% and 25% of the dye molecules are dimerized in CF3.6 and CF10 films, respectively. These numbers are most probably slightly reduced, since, as was already mentioned, we assumed that dimers do not absorb at the maximum of the monomer absorption band. The fluorescence spectra and intensities of the films strongly depend on the dye concentration. Table 1 shows the fluorescence quantum yields of the films obtained by integration of the fluorescence over the entire spectrum. The fluorescence quantum yield drops down by about 20 times with increase in the R6G concentration in solutions used for the film preparation. Fluorescence spectra of the films are qualitatively D


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The Journal of Physical Chemistry C Table 1. Fluorescence Quantum Yields of the Films Prepared from SiO2 + P123 + R6G Solutions with Different Dye Concentrations film concn, g/L QY

CF3.6 3.6 4.0%

CF10 10 1.36%

CF73 73 1.17%

different from those of solutions. The spectra of the CF3.6 and CF10 films are dominated by the band peaking at 605 nm attributed to dimers, while the monomer band substantially contributes only to the spectrum of the lowest concentration CF3.6 film. An additional fluorescence band appears at about 680 nm and gains intensity with the increase in the dye concentration. Subtracting appropriately normalized fluorescence spectra of films with different dye concentrations one from another we decomposed the fluorescence spectra into three spectral components: the monomer band, the dimer band and the long wavelength band (see Figure 3b). This decomposition was made assuming that (a) only the longest wavelength component contributes to the fluorescence in the long wavelength region above 750 nm; (b) the shortest wavelength component is due to monomers, which have identical fluorescence to that in solutions, and (c) only the monomer fluorescence contributes to the short wavelength region below 560 nm. The obtained dimer band is also very similar to that determined in solutions. The long wavelength band, which appears at high dye concentrations, is very similar to the fluorescence spectrum of a R6G powder, which is also presented in Figure 3. Apparently, formation of larger aggregates, or, more probably, of small R6G dye crystallites, takes place in films during the solvent evaporation when the dye concentration increases. Thus, we have monomers, dimers, and crystallites in films with relative concentrations determined by the absolute dye concentration. We can estimate fluorescence yield of dimers in the films. According to the decomposition of the CF3.6 film fluorescence, the monomers contribute about 10% to the total fluorescence of this film with the lowest dye concentration. Thus, the average dimer and larger aggregate fluorescence yield is of about 3.6%. Significantly lower fluorescence yield of films with higher dye concentration containing more crystallites shows that the crystallites have lower fluorescence yield and act as quenchers of the dimer fluorescence. Unfortunately, we have no information about how strongly the dimer fluorescence is quenched; therefore, we can set only a low limit of the dimer fluorescence yield at about 3.6% assuming that crystallites do not quench the dimer fluorescence in the CF3.6 film. We conclude that the dimer fluorescence yield in the films is at least 36 times higher compared with dimer fluorescence yield in solutions. Time-Resolved Fluorescence of Mesoscopic Films. Figure 4 shows the time-resolved fluorescence spectra of the CF3.6 film measured by the streak camera under excitation at 530 nm. The monomer fluorescence band is dominating in the initial spectrum. It is gradually replaced by the dimer fluorescence. Quite similar fluorescence dynamics was observed in other samples (not presented), even in the film with the highest dye concentration. The crystallite fluorescence in the investigated 2 ns time domain was very weak even for the CF73 film. Figure 5 shows the fluorescence decay kinetics for films with different dye concentrations measured with the streak camera

Figure 4. Time resolved fluorescence spectra of the CF3.6 film measured with streak camera under excitation at 530 nm.

and the TCSPC spectrometer at different wavelengths corresponding to different species. Combination of the two measurement techniques enabled us to monitor the fluorescence decay in a wide time range. Kinetics at 555 nm is dominated by the monomers at 605 nm and at 655 nm by the dimers and crystallites. However, as a consequence of broadening and overlapping of the spectra, we cannot observe decay of only one particular molecular species. Fluorescence kinetics of all three species becomes faster with the increase of the dye concentration. This is apparently caused by the sequential energy transfer from monomers to dimers and from dimers to large aggregates that act as final excitation quenchers. We distinguish the monomer and dimer fluorescence with the streak camera and contributions of all species with the TCSPC spectrometer. The initial fluorescence measured with the streak camera was dominated by monomers in all samples and its intensity was very similar for all samples despite the strong difference in the integral fluorescence yield. In contrast to the fluorescence decay in solutions, the monomer fluorescence decay was strongly nonexponential. This is not surprising because the dye concentration in the solid films was much higher than in solutions, but the dye molecules were immobilized therefore fluorescence quenching was taking place by direct energy transfer between neighboring molecules. The monomer fluorescence predominantly decays during hundreds of picoseconds. However, the monomer contribution to the TCSPC decays was observed up to several nanoseconds. Thus, the fast initial monomer fluorescence quenching apparently corresponds to the monomer molecules, but their fluorescence was quenched by the nearby dimers or crystallites. The slow decay component similar as in the low concentration solutions corresponds to monomers with no fluorescence quenchers present in the vicinity. Both streak camera and TCSPC measurements clearly reveal the dimer fluorescence decay kinetics. Subtraction one from another appropriately normalized decay kinetics measured at different wavelengths enabled us to distinguish the dimer contributions to the kinetics and obtain the dimer relaxation times ranging between approximately 3 and 5 ns for the CF3.6 film, between 1.3 and 4 ns for the CF10 film, and between 0.7 and 2 ns for the CF73 film. This result correlates with our previous statement, that the dimer fluorescence is quenched by crystallites, the amount of which gradually increases in the films with higher dye concentration. Also, the relaxation time E


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Figure 5. Fluorescence decay kinetics for mesoscopic films with different dye concentrations at different wavelengths on short and long time scales measured with the streak camera (530 nm excitation) and TCSPC spectrometer (470 nm excitation) correspondingly.

distribution is relatively small in the lowest concentration film. This film shows a similar slow relaxation component as was observed for the CF10 film with about 3 times higher dye concentration. These observations allow us to conclude that the dimer fluorescence was nonquenched or only weakly quenched in the film of the lowest concentration. Thus, we estimate the intrinsic fluorescence relaxation time of the dimers in mesoscopic films of about 3−5 ns. In order to determine the crystallite fluorescence decay kinetics, we have additionally measured kinetics at 750 nm in the sample of the highest concentration where the crystallite fluorescence was well expressed. The fluorescence at this wavelength decayed exponentially with a 6.6 ns time constant; therefore, we attribute it to the intrinsic crystallite fluorescence decay. Figure 6a shows the transient absorption spectra of the CF3.6 film at different delay times, and Figure 6b shows the decay kinetics at the maxima of the monomer and dimer absorption bands. The transient absorption is initially dominated by the negative band at 550 nm corresponding to the bleaching of the monomer absorption band and related stimulated emission in agreement with the initial fluorescence spectra, and with the transient absorption dynamics in solution. At longer times, the intensity of the monomer band dropped down and the absorption bleaching band of the dimer at 500 nm gained intensity. Decay of the monomer absorption bleaching band and simultaneous growth of the dimer

absorption band at 500 nm reveals the energy transfer from the monomers to dimers, what is consistent with the monomer fluorescence decay dynamics. The J-band bleaching intensity is even weaker than in solutions, in agreement with the weaker evaluated J-band intensity in steady state absorption spectra. Consequently, we clearly observe energy transfer to dimers with strong H-band and can unambiguously relate it to the film fluorescence, but the transient absorption shows no any signs of the energy transfer to J-dimers, if they were present.

4. DISCUSSION The, R6G solutions and SiO2 + P123 + R6G hybrid films have significantly different fluorescence properties. Fluorescence of the films is dominated by the long wavelength bands, which should be attributed to aggregates, while the aggregate fluorescence in solutions is weak even at very high dye concentrations. Significantly stronger fluorescence of dimers in hybrid films than in solutions leads to a natural conclusion that the aggregates formed in solutions and in hybrid films are different, attributing higher fluorescence yield in films to additional formation of J-type aggregates.13,14,18,19,21,23 However, our fluorescence and particularly the transient absorption data clearly show that excitation energy in films gradually localizes in dimeric species with a strong H-band, and we do not see any evidence of the energy localization on the additionally formed J-dimers. Such localization would reveal bleaching of the J-dimer absorption band even if their F


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fluorescence is several times weaker than that of the monomer. Taking into account that the dimer relaxation time is about 6 times shorter than the monomer relaxation time, we estimate that the dimer fluorescence yield should be several tens of times lower than the monomer fluorescence yield. This value is more than by the order of magnitude higher than experimentally determined value of 10−3. Experimental data for the mesoscopic film also unambiguously show that sandwich-type dimers with a strong H-band rather than additionally formed J-dimers are responsible for the relatively strong dimer fluorescence. This also leads to the conclusion that the oblique sandwich-type dimers in mesoscopic films have much larger fluorescence yield than in solutions. According to the estimated dimer fluorescence yield of 3.6% and their relaxation time of 3−5 ns, we evaluate the oscillator strength of the low energy transition responsible for the fluorescence of the dimer in mesoscopic film as being equal to about 4−7% of the monomer oscillator strength. According to the eq 1, such oscillator strength value corresponds to about 16−21° of the angle between dipole moments of monomeric units. This oscillator strength value is also much lower than that determined from the absorption spectra providing estimation of the oscillator strength of the J-band equal to about 66% of the monomer oscillator strength. One of the possible explanations of the disagreement between experimentally determined and calculated fluorescence yields of the dimer could be that the dimer arrangement in the fluorescent state is different than that in the ground electronic state. The dimer configuration may change in the excited state, causing its planarization and corresponding weakening of the of the oscillator strength of the low energy excitonic transition. Our transient absorption data revealed some subpicosecond dynamics; however, we cannot unambiguously attribute it to the dimer planarization, because the energy transfer and solvation processes also take place simultaneously. On the other hand, we cannot also completely exclude that the experimentally determined J-band intensities are overestimated. This band appears at the slope of the monomer absorption band, and its broadening at high dye concentration may cause significant inaccuracy. The difference in the fluorescence yields in this case may be caused by the stronger distortion of the planar H-dimer configuration in mesoscopic films.

Figure 6. (a) Transient absorption spectra of the CF3.6 film at different delay times. (b) Transient absorption decay kinetics at different probe wavelengths.

concentration is low and they are not observed in conventional absorption spectra. Quantum chemistry calculations reported in ref 24 predicted formation of R6G dimers with two possible configurations, with almost parallel and almost perpendicular orientations of the monomer transition dipole moments attributed to H- and J-dimer types correspondingly. Our steady state absorption data suggest that the monomeric units in the dimers are arranged in a way that optical transitions to both excitonic states are allowed, however transition to the high energy excitonic state has higher intensity. According to the simple excitonic theory, oscillator strengths of the lower energy transition f+d and the higher energy transition f−d depend on the angle θ between the transition dipole moments of monomerric units as follows:25 f d+ = fm (1 ± cos θ )

5. CONCLUSIONS Comparative analysis of the steady state absorption, fluorescence and transient absorption properties of R6G molecules in solutions and in SiO2 + P123 + R6G films enabled us to evaluate the relative oscillator strengths of the J- and H-dimer absorption bands and radiative relaxation rates of the R6G dimers in solutions and in mesoscopic films. Transient absorption studies allow us to unambiguously attribute the dimer fluorescence to the sandwich-type dimers with partly allowed low energy excitonic transition both in solutions and in mesoscopic films. We attribute higher fluorescence yield of the dimers in mesoscopic films to the opening of the low energy excitonic transition caused by larger angle between transition dipole moments of monomeric units of the dimers, in the fluorescence state. We suggest that a very low transition dipole moment of the dimer fluorescent state in solutions may be caused by the planarization of the dimer in the excited state, while in mesoscopic films such planarization may be hampered. Alternatively, if the J-band oscillator strength is overestimated, the difference in the fluorescence yields is caused by the

(1)

where f m is the oscillator strength of the monomer. Taking into account the ratio of the H- and J-band intensities in solutions of 1.2 obtained from the absorption spectra, we estimate that θ ≈ 70°. According to the transient absorption data this angle should be several times smaller. Literature data for the oscillator strength of the J-band obtained from the absorption spectra reveal a strong distribution of this value. The oscillator strength for the R6G monomers and dimers in ethylene glycol were evaluated being equal to 0.74 and 0.16, while in aqueous solutions the corresponding values were obtained as equal to 0.605 and 0.027, respectively.11 Our data are closer to those obtained in ethylene glycol. Thus, these data suggest that the oscillator strength of the J-transition responsible for the dimer G


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(15) Vogel, R.; Meredith, P.; Harvey, M. D.; Rubinsztein-Dunlop, H. Absorption and Fluorescence Spectroscopy of Rhodamine 6g in Titanium Dioxide Nanocomposites. Spectrochim. Acta, Part A 2004, 60, 245−249. (16) Salleres, S.; Arbeloa, F. L.; Martinez, V.; Arbeloa, T.; Arbeloa, I. L. Adsorption of Fluorescent R6g Dye into Organophilic C12tma Laponite Films. J. Colloid Interface Sci. 2008, 321, 212−219. (17) Del Monte, F.; Mackenzie, J. D.; Levy, D. Rhodamine Fluorescent Dimers Adsorbed on the Porous Surface of Silica Gels. Langmuir 2000, 16, 7377−7382. (18) Malfatti, L.; Kidchob, T.; Aiello, D.; Aiello, R.; Testa, F.; Innocenzi, P. Aggregation States of Rhodamine 6G in Mesostructured Silica Films. J. Phys. Chem. C 2008, 112, 16225−16230. (19) Del Monte, F.; Levy, D. Formation of Fluorescent Rhodamine B J-Dimers in Sol-Gel Glasses Induced by the Adsorption Geometry on the Silica Surface. J. Phys. Chem. B 1998, 102, 8036−8041. (20) Anedda, A.; Carbonaro, C. M.; Corpino, R.; Ricci, P. C.; Grandi, S.; Mustarelli, P. C. Formation of Fluorescent Aggregates in Rhodamine 6g Doped Silica Glasses. J. Non-Cryst. Solids 2007, 353, 481−485. (21) Carbonaro, C. M.; Meinardi, F.; Ricci, P. C.; Salis, M.; Anedda, A. Light Assisted Dimer to Monomer Transformation in Heavily Doped Rhodamine 6G-Porous Silica Hybrids. J. Phys. Chem. B 2009, 113, 5111−5116. (22) Noginov, M. A.; Zhu, G.; Bahoura, M.; Small, C. E.; Davison, C.; Adegoke, J.; Drachev, V. P.; Nyga, P.; Shalaev, V. M. Enhancement of Spontaneous and Stimulated Emission of a Rhodamine 6g Dye by an Ag Aggregate. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 184203. (23) Bojarski, P. Concentration Quenching and Depolarization of Rhodamine 6g in the Presence of Fluorescent Dimers in Polyvinyl Alcohol Films. Chem. Phys. Lett. 1997, 278, 225−232. (24) Gavrilenko, V. I.; Noginov, M. A. Ab initio Study of Optical Properties of Rhodamine 6g Molecular Dimmers. J. Chem. Phys. 2006, 124, 044301. (25) Van Amerongen, H.; Valkunas, L.; van Grondelle, R. Photosynthetic Excitons; World Scientific Co.: Singapore, 2000.

stronger distortion of the planar H-dimer configuration in mesoscopic films. These results suggest the strategy to increase the dimer fluorescence in films further more by controlling intermolecular interactions which may cause stronger opening of the fluorescence transition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. P. A. Manorik for a useful discussion of data. O.B. acknowledges EU Structural Funds project ‘‘Postdoctoral Fellowship Implementation in Lithuania” L.V. acknowledges the support of the Research Council of Lithuania (Grant No. MIP-080/2015).

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REFERENCES

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Insights into the Mechanism of Enhanced Rhodamine 6G Dimer

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