- Email: [email protected]
Configuration of organic dye excimers in nanoporous SiO2 matrices
Journal of Luminescence 179 (2016) 171–177
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Configuration of organic dye excimers in nanoporous SiO2 matrices A.V. Sorokin n, B.A. Gnap, I.I. Bespalova, S.L. Yefimova, Yu.V. Malyukin Institute for Scintillation Materials, STC “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin ave., 61001 Kharkov, Ukraine
art ic l e i nf o
a b s t r a c t
Article history: Received 15 December 2015 Received in revised form 13 May 2016 Accepted 12 June 2016 Available online 18 June 2016
The effect of cyanine dye 3,30 -dioctadecyloxacarbocyanine perchlorate (DiO) and benzimidazole dye 4dimethylamino-1,8-naphthoylene-10 ,20 -benzimidazole (DNBI) accumulation in nanoporous silica matrices on the dyes luminescence properties has been studied. For both dyes, ground state dimer formation with perpendicular transition dipoles at high dye concentrations has been considered as a result of restricted geometry of the nanoscale pores. The dimer excitation leads to excimer formation revealing by appearance of new long-wavelength luminescence band and shortening the dye luminescence lifetime. In the excimer luminescence excitation spectra two additional bands have been observed, one of which is bathochromically shifted relatively to the absorption band and another one is hypsocromically shifted. Using the Kasha exciton model it was shown that the excimers possess oblique transition dipoles configuration. & 2016 Elsevier B.V. All rights reserved.
Keywords: Excimer Silica matrix Luminescence Oblique dimer Cyanine Benzimidazole
1. Introduction Low cost, relative simplicity and low temperatures of the sol– gel synthesis make it a very popular method for the production of wide range of materials including glass highly-porous films or bulk matrices which could be a host for organic and inorganic molecules [1,2]. Composite materials based on sol–gel matrices with incorporated organic luminophores were successfully applied as solar concentrators, tunable lasers, active waveguides, dyesensitized solar cells, etc. [1–8]. The main advantage of such composite materials is enhanced photostability of the incorporated dye molecules [1–5,9,10]. There are two ways to embed dye molecules into the sol–gel matrices: (i) addition the dye at a stage of the matrix synthesis and (ii) adsorption of the dye molecules by the synthesized matrix dipped into the dye solution [1,2]. The first approach allows to obtain the matrices with high dye loading capacity and high degree of homogeneity [1,2,11]. However, as the dye-loaded matrices prepared by this way are not annealed at high temperatures to prevent dye destruction, they are very fragile and have uncontrolled porosity. The second approach allows imparting reasonable mechanical properties and controlled porosity to the synthesized sol–gel matrices by high temperature treatment prior the dye loading [3–8]. But in this case, a heterogeneous dye n
Corresponding author. E-mail address: [email protected] (A.V. Sorokin).
http://dx.doi.org/10.1016/j.jlumin.2016.06.025 0022-2313/& 2016 Elsevier B.V. All rights reserved.
distribution in the matrix and the dye intermolecular interaction should be taken into account [6,12,13]. Indeed, the apparent fractal distribution of dye molecules in a SiO2 sol–gel matrix with dimensional parameter do 3 was shown using non-radiative energy transfer [6,13]. Furthermore, spatial restriction and non-uniform distribution of dye molecules within pores of the matrix can cause the dye aggregation [11–17] with a preferential formation of dimers [14,15] and excited dimers, i.e. excimers [16,17]. The dye exсimerization in sol–gel matrices were reported for some polycyclic aromatic molecules, such as pyrene, naphthalene, fluorene, acenaphthene, which tend to form excimers in solutions as well [17–25]. Recently, we have report the cyanine dyes excimer formation in porous SiO2 matrices [26,27]. Although the dimer and aggregate formation is well-known phenomenon for cyanine dyes [14,28] and were also observed in sol–gel matrices [11,29,30], the cyanine dyes excimerization has not be reported up to now. Moreover, cyanine dyes DiI and DiD used in our experiments are well-known fluorescent probes for biological applications [31], for which aggregation is hindered due to their structure. So, the main reason for these dyes excimerization was supposed a spatial restriction by nanovolume of silica pores [26,27]. However the excimer configuration wasn't established. In the present paper we report the formation of excimers and studying their configurations for one more representative of cyanine dye family, 3,30 -dioctadecyloxacarbocyanine perchlorate (DiO), and for the dye belonging to the benzimidazole compounds, 4-dimethylamino-1,8-naphthoylene-10 ,20 -benzimidazole (DNBI), which are embedded by adsorption into nanoporous SiO2 matrices (Fig. 1).
172
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
(IRF) full width at half maximum (ΔFWHM) for the whole setup was about 100 ps. Decay time analysis has been performed using FluoFit software (PicoQuant, Germany). To study the matrix porosity, Physisorption Analyzer ASAP 2000 (Micromeritics, USA) with nitrogen as an analysis gas (the BET-BJH methods) was used. Transmission electron microscopy (TEM) images were obtained using a PEM-125 (Selmi, Ukraine) microscope at 100 V accelerating voltage.
3. Results and discussion
Fig. 1. Structural formulas of the dyes: a) DiO, b) DNBI and an image of SiO2 matrix (c).
2. Experimental Nanoporous SiO2 matrices were prepared by sol–gel hydrolysis and condensation of TMOS (Si(OCH3)4) in a water–methanol solution [32,33]. Briefly, 3.15 mL of methanol was mixed with 3.75 mL of TMOS for 5 min. Then 4.5 mL of distilled water and 0.525 mL of hydrochloric acid were added to the mixture, which was agitated for the next 5 min. The sol was poured in 35 10 mm plastic Petri dishes and held for 24 h at room temperature until a gel was formed, which was then dried for 120 h at temperature of 45 °C. Subsequently, the dried samples were annealed at 750 °C for 3 h. The resulted samples were transparent colorless disks with 20 mm diameter and 1–1.5 mm high (Fig. 1c). DiO dye was obtained from Sigma-Aldrich (USA) and used as received. DNBI dye was a kind gift of Dr. I.A. Borovoy (Institute for Scintillation Materials, NAS of Ukraine). To embed the dyes, the matrices were dipped into the dye chloroform solution for at least one hour. After that samples were dried at 60 °C for at least three hours. Luminescence and luminescence excitation spectra were recorded using fluorescence spectrometer Lumina (Thermo Scientific, USA) equipped with a solid sample holder. Luminescence was registered in front excitation geometry to avoid reabsorption effects. Absorption spectra was registered using a microspectrometer USB4000 (Ocean Optics, USA) supplied with an incandescent and deuterium lamps. Luminescence decay spectra were registered using FluoTime 200 fluorescence lifetime spectrometer (PicoQuant, Germany) equipped with a 439 nm picosecond pulsed laser diode head. The instrument response function
The synthesis method used for SiO2 matrix formation in a combination with following high temperature treatment at 750 °C allows to obtain the matrices with high mechanical properties. The average microhardness of the matrices is 170 kg/mm2, which is practically twice as high as the values reported by the other authors [34]. The density and porosity of the matrices determined using the hydrostatic weighing method are 1.10 g/cm3 and 54 vol%, respectively [33]. According to X-ray phase analysis and atomicforce microscopy data, the matrices have an amorphous structure and consist of close-packed spherical particles with an average size of 35 nm [32]. These findings are supported by TEM images (See Supplementary Material, Fig. S1). Also some quite large pores with diameter of 20–30 nm could be found on the image. From other hand it is known that the higher temperature of the postsynthesis treatment the smaller pores have to be [1,2,4]. To clear the pore details of the SiO2 matrix a gas adsorption method has been used [35]. It reveals the average pore diameter of 2.1 nm with average micropore volume of 0.2 cm3/g and average pore area of 390 m2/g. It should be noted that a physisorption isotherm (See Supplementary Material, Fig. S2) reveals a tiny adsorption– desorption hysteresis and belongs to Type I according to standard classification [35]. The latter indicates that the very fine pores ( 2 nm) cause the material porosity [35]. As the pores are open and the matrix porosity is 54 vol% [32,33] it could be concluded that the matrix pore system is bicontinuous and consisted of randomly arranged spherical interconnected fine pores with a very small contribution of meso- and macropores with diameter 2– 50 nm and larger than 50 nm, respectively. To embed the studied dyes into the SiO2 matrix the dye adsorption from a chloroform solution up to the full matrix saturation was used as previously [12,26,27]. As the used dyes have a better chemical affinity to the matrix comparing with the chloroform, the adsorption process appeared to be more preferred than a back process of the dye dissolution in the chloroform. As a result the matrix became strongly colored and the solvent is practically colorless with the residual dye amount (See Supplementary Material, Fig. S3). Therefore, for the sample preparation a volume of the dye solution was calculated in such a way that the dye with the largest concentration (typically 10–4 M) was almost fully adsorbed by the matrix. Then, the same volume was used for preparation of samples with other dye concentrations. As the local dye concentration in the SiO2 matrices was much larger as compared to the solution and taking to account the apparent fractal distribution of dye molecules in SiO2 matrices [13], to specify the samples with different dye load we will use the concentration of dyes in a stock chloroform solution. DiO is amphiphilic dye with two long hydrophobic tails C18H37 (Fig. 1a), similarly to DiI and DiD dyes used in our previous study [26,27]. Due to trimethine chain and oxygen heteroatoms, its absorption and luminescence spectra (Fig. 2) are shifted toward shorter wavelengths (λabs ¼ 488 nm, λlum ¼505 nm, in DMF) as compared to DiI and DiD [31,36]. Similar to other cyanine dyes with two long hydrophobic tails DiO hardly aggregates in solutions
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
173
Fig. 2. Absorption (1) and luminescence (2, λexc ¼ 470 nm) spectra of the DiO dye in DMF.
but it could aggregate under some restriction conditions like formation in LB films [37]. A set of the samples with the different dye loads was prepared with the stock dye concentrations ranged from 10–6 M to 10–4 M (Fig. 3). The matrix loaded with the smallest dye amount revealed typical (e.g., see Fig. 2) monomer luminescence and absorption spectra with the maxima position determined by the “solvent” polarity (so called solvatochromic effect) [38]: λabs ¼ 483.5 nm and λlum ¼498 nm (Fig. 3, curves 1). The increase in the dye load causes a remarkable transformation of the DiO luminescence spectrum: the appearance of new broad long-wavelength band (λlum ¼ 532 nm) and redistribution of monomer and new band intensities depending on the dye concentration has been observed (Fig. 3a). At the stock dye concentration of 10–4 M, the new band becomes dominant, whereas the monomer one manifests itself only as a shoulder (Fig. 3a, curve 5). Further increase in the amount of loaded dye did not provoke any changes in the luminescence spectrum. At the same time, we did not reveal any changes in the DiO absorption spectrum within the whole dye concentration range (Fig. 3b). Notice that in a case of a large amount of the dye loaded in the matrix, we observed reabsorption effect and did not present these spectra. Similarly to the previous results [26,27], the new longwavelength luminescence band is supposed to belong DiO excimers formed in nanopores of the SiO2 matrix at high dye concentrations. Indeed, the strong concentration dependence of novel luminescence band indicates on some aggregation processes rather than other effects like strong deviation from the mirror symmetry between absorption and luminescence spectra due to conformational transitions of the molecules [39]. To prove the dye excimer formation, luminescence decays were registered at the luminescence band maxima for different dye concentrations (Fig. 4). The DiO luminescence decay measured in the matrix with the lowest dye concentration (monomeric luminescence) follows a non-exponential law (Fig. 4, curve 1) with the best fitting with three exponential curves and decay parameters (amplitude weighted): τ1 0.39 ns (45.4%), τ2 1.62 ns (51.4%), τ3 4.65 ns (3.2%) and τAV 1.16 ns. The multi-exponential pattern of the monomer luminescence decay could be explained by strong heterogeneity of the dye molecules distribution in the nanoporous matrix [11,13]. However, the attempts to fit the decay curve using Gaussian or Lorentzian distributions of decay times which are
Fig. 3. Luminescence (a, λexc ¼ 470 nm) and absorption (b) spectra of the DiO dye in a SiO2 matrix at different dye concentrations in a stock chloroform solution: 1 – 10 6 M, 2 – 5 10 6 M, 3 – 10 5 M, 4 – 5 10 5 M, 5 – 10 4 M.
widely applied for heterogeneous media [4] were unsuccessful indicating complicated mechanisms of excitation relaxation of DiO molecules adsorbed by the matrix. It is well-known that cyanine dye luminescence decay time is strongly depended on the solvent polarity [11,36,38] and the less polar solvent results in larger decay time (See Supplementary Material, Fig. S4). The sol–gel matrix annealed at temperatures higher than 600 °C usually considered as nonpolar due to polar silanol groups on its pore surface are transformed to nonpolar siloxane bridges [4] and the adsorbed dye decay time should be long. From other hand, the matrix contact with the environment hasn't been prevented and an adsorbed water vapor could change the pore interior polarity leading to the decay time shortening. Also, for the dye molecules restricted in the small pores cis-trans isomerization is hindered causing the decay time increasing due to radiationless deactivation prevented [11]. Yet another possibility for the presence of molecule fraction with the decreased decay time is an excitation energy transfer (EET) between adjacent molecules which are situated close enough socalled homo-EET [40,41]. Indeed, due to restricted geometry of the matrix porous system the EET is a very efficient between the adjacent molecules [13]. As the energy donor and the energy
174
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
Fig. 4. Luminescence decay curves (λexc ¼439 nm) of the DiO dye in SiO2 matrix at different dye concentrations in a stock chloroform solution and different registration wavelengths: 1 – 10 6 M, λreg ¼ 495 nm; 2 – 10 5 M, λreg ¼ 505 nm; 3 – 10 4 M, λreg ¼ 530 nm; 4 – IRF.
Fig. 5. Absorption (dashed curves) and luminescence (solid curves, λexc ¼ 415 nm) spectra of the DNBI dye in different solvents: 1 – toluene, 2 – DMF.
acceptor are the similar molecules the homo-EET couldn't manifest itself in stationary spectra but causes the luminescence quenching and decay time shortening due to more efficient radiativeless relaxation [40,41]. Except the adsorbed dye excitation dynamics the silica matrix photophysical properties should be taken into account [42]. Indeed, there is an excitation energy transfer could occur in the case of dye interaction with porous silica [42]. Due to amorphous structure our SiO2 matrices reveal absorption (λmax ¼307 nm) and luminescence (λmax ¼361 nm) bands only in UV range (see Supplementary Material, Fig. S5a). However, due to many energy levels of silica some luminescence also excited at λexc ¼440 nm (See Supplementary Material, Fig. S5b) resulting in nonexponential decay with τAV 2.8 ns (See Supplementary Material, Fig. S5c). Due to off-band excitation the SiO2 luminescence contribution to the composite spectrum is very small so any energy transfer effects related to the SiO2 intrinsic luminescence [42] could be neglected. Nevertheless, we could attribute the longest time component (τ3) with the smallest fractional amplitude of DiO monomer decay (Fig. 4, curve 1) to the SiO2 luminescence contribution. So, we could resume that in the DiO/SiO2 composite luminescence dynamics at the low dye concentration contribute three kinds of species: one of them is the SiO2 matrix intrinsic luminescence and two others are related with DiO molecules affected by unclear factors. One fraction of the dye molecules reveals short decay time and the second one reveals long decay time with the about the same fraction contributions. There is no physical sense to distinguish other dye fractions and fit the decay curve by more than 3 exponents [41]. The appearance of a new band in the DiO luminescence spectrum is attended by gradual shortening of the luminescence decay time (Fig. 4). The luminescence decay curve measured at the longwavelength band maxima (the largest dye concentration) reveals a stronger deviation from the monoexponential decay law (Fig. 4, curve 3) as compared to that for monomer band (Fig. 4, curve 1) and also was fitted by three exponents with the decay parameters (amplitude weighted): τ1red 0.35 ns (56.3%), τ2red 1.2 ns (42.1%), τ3red 6.2 ns (1.6%) and τAV red 0.8 ns. As previously, the smallest amplitude component τ3 with the largest decay time we ascribe to the SiO2 intrinsic luminescence. The analysis of fitting parameters shows that the decay time components τ1 and τ2 for the long-
wavelength band are a bit shorter, while the fractional amplitude of τ1 component is slightly increased. As a result, the average lifetime for the long-wavelength band is shorter than that for the monomer one. Since the luminescence lifetime shortening is a feature of a dye aggregation process [16,28], we can ascribe the long-wavelength band to the dye excimers. For the cyanine dye aggregates it is well-known that decreasing the luminescence decay time comparing with the monomer one is proportional to the aggregation degree [28,43]. So, comparing the monomer and excimer average decay time we could assume that only smallest aggregates, i.e. dimers, formed at the highest dye concentration. It should be noted, that taking into account shortening τ1 component of the DiO decay time at the high dye concentration (Fig. 4, curve 3) simultaneously with its fractional amplitude growth indicates that the cis-trans isomerization couldn't be considered as an origin of this DiO molecules fracture. Indeed, with the dye concentration growth one could expect strong decreasing of the dye molecule fraction for which rotation around the polymethine bridge is allowable especially in the case of the long hydrophobic tails of DiO molecule (Fig. 1a). For further clearing the origins of two DiO fractions revealed by the luminescence decay analysis we examine the polarity of the SiO2 matrix pores using another organic dye DNBI (Fig. 1b), which is known to be a polarity-sensitive molecule [44]. In polar solvents the luminescence spectrum of DNBI reveals a second emission peak with λmax ¼596 nm (Fig. 5, curve 2 solid) which is red-shifted comparing with the luminescence band with λmax ¼483.5 nm characteristic for the nonpolar solutions case (Fig. 5, curve 1 solid). The appearance of this peak does not depend of the dye concentration and was associated with a formation of so-called TICT (twisted intramolecular charge transfer) state in the excited state of the molecule [44]. SiO2 matrix loaded with the DNBI dye molecules exhibits the same features as it was described above for DiO dye (Fig. 6). At low dye concentrations, both absorption (not shown) and luminescence (λmax ¼505 nm, Fig. 6a, curve 1) spectra could be assigned to the monomer dye molecules. The TICT state emission band (Fig. 5, curve 2 solid) is absent in the luminescence spectrum of the DNBI adsorbed by the matrix (Fig. 6a). So, the water vapor from the environment is not affected on the dye spectral properties. The luminescence decay curve for the monomeric DNBI luminescence in the matrix was also nonexponential (Fig. 6b, curve 1)
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
Fig. 6. a) Luminescence spectra (λexc ¼ 440 nm) of the DNBI dye in a SiO2 matrix at different dye concentrations in a stock chloroform solution: 1 – 10 6 M, 2 – 5 10 6 M, 3 – 10 5 M, 4 – 5 10 5 M, 5 – 10 4 M; b) luminescence decay curves (λexc ¼439 nm) at different dye concentrations and different registration wavelengths: 1 – 10 6 M, λreg ¼ 515 nm; 2 – 10 4 M, λreg ¼550 nm.
similar to the DiO monomers with the best fitting by three exponential curves and following decay parameters (amplitude weighted): τ1 0.25 ns (46.6%), τ2 2.15 ns (34.0%), τ3 6.3 ns (19.4%) and τAV 2.0 ns. As previously the third decay component with the largest time we attribute with the SiO2 intrinsic luminescence. It much larger contribution could be assigned with much less extinction coefficient and luminescence quantum yield of DNBI comparing with DiO and, hence, less intense DNBI luminescence. The cis-trans isomerization for DNBI molecules is not possible due to its structure (Fig. 1b). So, a presence of two luminescent fractions with short and long decay times similarly to the DiO case proves our assumption concerning the luminescence quenching for part of DNBI molecules due to homo-EET between adjacent molecules. Increasing the DNBI concentration causes the luminescence maximum gradual shift toward longer wavelengths and the appearance of new luminescence band (λmax ¼ 534 nm, Fig. 6a). The luminescence decay registered at the long wavelength band was also shorter (Fig. 6b, curve 2) with fitting parameters (amplitude weighted): τ1red 0.24 ns (60.2%), τ2red 1.68 ns (31.15%), τ3red 8.6 ns (8.65%) and τAV red 1.15 ns. Similarly to DiO
175
Fig. 7. Luminescence excitation spectra of the DiO (a) and the DNBI (b) dyes in the SiO2 matrix at different dye concentrations in the stock chloroform solution and different registration wavelengths: 1 – 10 6 M (λregDiO ¼515 nm and λregDNBI ¼ 530 nm); 2 – 10 4 M (λregDiO ¼ 560 nm and λregDNBI ¼575 nm); 3 – subtracted spectrum of the oblique dimer.
the DNBI decay times are shortened and the excimerization within the pores of SiO2 matrix was considered. Note, that SiO2 intrinsic luminescence contribution becomes smaller because of the DNBI luminescence intensity increasing for the larger dye concentration. To determine the DiO and DNBI excimer configuration the luminescence excitation spectra collected at the monomer and excimer bands (samples with low and high dye concentrations, respectively) were analyzed (Fig. 7). The monomer luminescence excitation spectra for both dyes were similar to the correspondent absorption spectra (Fig. 7, curve 1). Contrarily, the excimer luminescence excitation spectra differ significantly from the absorption spectra (Fig. 7, curve 2). Similarly to our previous observation for DiI excimers [27], in the DiO and DNBI excimer luminescence excitation spectra two bands can be distinguished. One band is bathochromically shifted with respect to the monomer excitation band (we will call it J-band similarly to J-aggregates [28]) with λJDiO ¼511 nm and λJDNBI ¼512 nm (Fig. 7, curve 2). Another band is hypsochromically shifted with respect to the monomer excitation band (we will call it H-band) with λHDiO ¼423.5 nm and
176
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
Fig. 8. Classical Kasha scheme of the oblique dimer configuration.
Table 1 Excitonic parameters of the dys excimers in nanoporous SiO2 matrices.
DiO DNBI
|M|2, D2
ΔE, cm 1
α, °
θ, °
R, Å
20.4 18.8
4060 6385
55 77
62.5 51.5
3.9 3.5
λHDNBI ¼377 nm (Fig. 5a, curve 2). Despite the similar excimer excitation spectral features for both dyes studied much larger contribution of the excimers comparing with that for the monomers is observed for DiO dye with respect to DNBI (Fig. 7) supposing more favorable excimerization process for the former dye. Subtracting the monomer band contributions from the excimer excitation spectra we obtain well-separated H- and J-bands (Fig. 7, curve 3). According to the Kasha exciton band splitting model [45], the observed two bands correspond to an oblique dimer configuration (Fig. 8). For the oblique dimers, the Kasha exciton model could be successfully applied to estimate the dimer geometrical parameters [46,47]. So, here we applied it for the excimer configuration evaluation also. According to [45,46] the angles α and θ (Fig. 8) can be calculated as follows: α f 180 3 α J ¼ ; θ¼ ð1Þ tg 2 2 fH 2 where fH and fJ are the oscillation strengths of the corresponding electronic transitions. In our case, the ratio fJ/fH is proportional to the ratio of areas under the correspondent bands in the excitation spectrum (Fig. 7). The center-to-center distance R (Fig. 8) can be found as [45,46]:
ΔE ¼
2jM j2 R3
ð cos α þ3 cos 2 θÞ;
ð2Þ
where ΔE is the exciton splitting energy and M is the transition moment for the singlet–singlet transition in the monomer. The latter can be calculated as [40]: Z 9n εðνÞ jM j2 ¼ 9:186 10 3 dν; ð3Þ ν ðn2 þ 2Þ2 R where ε(ν) is the molar extinction coefficient (with εðνÞdν is the area of the monomer absorption band) and n is the refractive index of the medium. So, using the H- and J-bands in the DiO and DNBI excitation spectra (Fig. 7, curve 3) and the monomer bands in the absorption spectra (Figs. 3b and 5) we estimated the excimer configuration for both dyes in the nanoporous SiO2 matrices (Table 1). As it could be seen from Table 1, molecules forming the excimers are located very close to each other at the distances for DiO 3.9 Å and DNBI 3.5 Å. Taking into account a quite large value of the dyes
Fig. 9. Modified scheme of the oblique dimer configuration proposed in [47]: a) side view, b) top view. Reprinted with permission from [47]. Copyright © 2006, John Wiley and Sons.
transition moments ( 4.5 D) and lengths of the chromophores (410 Å) we could suggested a twisted face-to-face arrangement of the molecules forming excimers (Fig. 9). Since we did not observe any changes in the absorption spectrum as a result of dye excimerization (Fig. 3b) and the same growing-in profiles of the time-resolved luminescence for the excimer and monomers bands (Figs. 4 and 6b) we assume a ground state dimer formation with a perpendicular dipole moment arrangement for both dyes [18,19]. Under excitation of one molecule of the ground state dimer, its dipole moment is reoriented forming angle α with the second molecule dipole moment (Fig. 9) and the excimer forms. Such excimer configuration is unique and was not found in a literature for typical excimer forming dyes. As a mean diameter of the pores in SiO2 matrices is only 2 nm while the studied dyes are quite large we consider that the spatial restriction in the pores is the main factor that provokes the dyes excimer formation with the oblique configuration.
4. Conclusions Using two different organic dye molecules belonging to cyanine and benzimidazole families, we have studied the features of their intermolecular interactions in nanoporous silica matrices. In both cases, increasing the dye amount loaded to the matrix causes the second red-shifted emission peak appearance and shortening luminescence lifetime, whereas the dyes absorption spectra did not change. Basing on these facts, dyes excimer formation was considered as the origin of the additional luminescence band. The excimer configuration for the both dyes was evaluated using the Kasha of exciton splitting model for oblique dimers. The short distance and large angles between molecules forming the excimers allow us to suggested ground dimers formation with twisted face-to-face configuration. It was considered that the excimer
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
formation is caused by spatial confinement in the nanoscale silica pores.
Acknowledgments We thank Dr. I.A. Borovoy (Institute for Scintillation Materials of NAS of Ukraine, Kharkiv) for providing DNBI used in this study. Authors are grateful to Prof. A.V. Ragulya (V.Ye. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine, Kyiv) for the help with porosity measurements and Dr. O.M. Vovk (Institute for Single Crystals of NAS of Ukraine, Kharkiv) for the help with TEM imaging.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.06. 025.
References [1] M.A. Aegerter, M. Mennig (Eds.), Sol-Gel Technologies for Glass Producers and Users, Springer Science þBusiness Media, New York, 2004. [2] G.Q. Lu, X.S. Zhao (Eds.), Nanoporous Materials: Science and Engineering, Imperial College Press, London, 2004. [3] R. Reisfeld, E. Yariv, H. Minti, Opt. Materials 8 (1997) 31. [4] J.K. Thomas, E.H. Ellison, Adv. Coll. Interf. Science 89-90 (2001) 195. [5] R. Reisfeld, J. Fluoresc. 12 (2002) 317. [6] T.V. Antropova, Yu.A. Gordeeva, B.D. Ryzhikov, A.M. Saletskii, J. Appl. Spectrosc. 72 (2005) 478. [7] J.A. Anta, E. Guillen, R. Tena-Zaera, J. Phys. Chem. C 116 (2012) 11413. [8] O.N. Bezkrovnaya, I.M. Pritula, A.G. Plaksyi, V.F. Tkachenko, ОМ Vovk, Yu. L. Slominskii, A.D. Kachkovskiy, Yu.A. Gurkalenko, S.N. Krivonogov, A.V. Lopin, J. Non-Cryst. Sol. 389 (2014) 11. [9] A. Dubois, M. Canva, A. Brun, F. Chaput, J.-P. Boilot, Appl. Optics 35 (1996) 3193. [10] E. Yariv, S. Schultheiss, Ts Saraidarov, R. Reisfeld, Opt. Mat 16 (2001) 29. [11] U. De Rossi, S. Daehne, R. Reisfeld, Chem. Phys. Lett. 251 (1996) 259. [12] B.A. Gnap, I.I. Bespalova, S.L. Yefimova, A.V. Sorokin, Funct. Mat. 18 (2011) 487. [13] S.L. Yefimova, A.M. Rekalo, B.A. Gnap, A.V. Sorokin, O.G. Viagin, Yu.V. Malyukin, Appl. Phys. A 116 (2014) 2131. [14] W. West, S. Pearce, J. Phys. Chem. 69 (1965) 1894. [15] Yu. A. Gordeeva, A.M. Saletskii, J. Appl. Spectrosc. 73 (2006) 924. [16] J.B. Birks, Rep. Prog. Phys. 38 (1975) 903. [17] K.A. Zachariasse, Excimer formation with pyrenes on silica surfaces, in: M. Anpo, T. Matsuura (Eds.), Photochemistry on Solid Surfaces, Elsevier Science Publishers B.V., Amsterdam, 1989, pp. 48–78.
177
[18] E. Wellner, M. Ottolenghi, D. Avnir, D. Huppert, Langmuir 2 (1986) 616. [19] T. Fujii, E. Shimizu, Chem. Phys. Lett. 137 (1987) 448. [20] T. Fujii, A. Ishii, N. Takusagawa, H. Yamashita, M. Anpo, J. Photochem. Photobiol. A 86 (1995) 219. [21] J.T. Barbas, M.E. Sigman, R. Arce, R. Dabestani, J. Photochem. Photobiol. A 109 (1997) 229. [22] C. Reyes, M.E. Sigman, R. Arce, J.T. Barbas, R. Dabestani, J. Photochem. Photobiol. 112 (1998) 277. [23] Yu Tozuka, E. Tashiro, E. Yonemochi, T. Oguchi, K. Yamamoto, J. Coll. Interf. Sci. 248 (2002) 239. [24] A. Thomas, S. Polarz, M. Antonietti, J. Phys. Chem. B 107 (2003) 5081. [25] R. Dabestani, M. Kidder, A.C. Buchanan III, J. Phys. Chem. C 112 (2008) 11468. [26] B.A. Gnap, I.I. Bespalova, S.L. Yefimova, A.V. Sorokin, Funct. Mat. 20 (2013) 407. [27] A.V. Sorokin, B.A. Gnap, I.I. Bespalova, S.L. Yefimova, Funct. Mat. 22 (2015) 207. [28] F. Wurthner, T.E. Kaiser, Ch.R. Saha-Muller, Angew. Chem. Int. Ed. 50 (2011) 3376. [29] F. Nüesch, J.E. Moser, V. Shklover, M. Grätzel, J. Am. Chem. Soc. 118 (1996) 5420. [30] N. Kato, K. Yamamoto, Y. Uesu, Jpn. J. Appl. Phys. 46 (2007) 5318. [31] I. Johnson, M.T.Z. Spence (Eds.), The Molecular Probes Handbook, A Guide to Fluorescent Probes and Labeling Technologies, 11th ed., Life Technologies Corporation, New York, 2010. [32] T.A. Blank, I.I. Ganina, Yu.V. Malyukin, L.P. Eksperiandova, Funct. Materials 16 (2009) 517. [33] V.V. Seminko, A.A. Masalov, O.G. Viagin, I.I. Bespalova, L.I. Voloshina, O. V. Zelenskaya, V.A. Tarasov, Yu. V. Malyukin, Development of nanocomposite alpha-detectors based on silica matrices and organic scintillators, in: P. Petkov, D. Tsiulyanu, W. Kulisch, C. Popov (Eds.), Nanoscience Advances in CBRN Agents Detection, Information and Energy Security, Springer Science þBusiness Media, Dordrecht, 2015, pp. 415–419. [34] K.S. Lam, X.-L. Zhu, D. Lo, Appl. Phys. B 68 (1999) 1151. [35] J.B. Condon, Surface Area and Porosity Determinations by Physisorption. Measurements and Theory, Elsevier, Amsterdam, 2006. [36] A.A. Ishchenko, Russ. Chem. Bull. 43 (1994) 1161. [37] S. Chakraborty, P. Debnath, D. Dey, D. Bhattacharjee, S.A. Hussain, J. Photochem. Photobiol. A 293 (2014) 57. [38] C. Reichardt, T. Welton, Solvents and Solvent Effects in Organic Chemistry, Fourth ed., Wiley-VCH Verlag, Weinheim, 2011. [39] A.V. Naumov, N.L. Naumova, I.S. Osad'ko, Opt. Spectrosc. 91 (2001) 704. [40] W.W. Parson, Modern Optical Spectroscopy, Springer-Verlag, Berlin, 2007. [41] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer Science þBusiness Media, Singapore, 2006. [42] V. Pranculis, I. Šimkiene, M. Treideris, V. Gulbinas, Phys. Status Solidi A 210 (2013) 2617. [43] A. Mishra, R.K. Behera, P.K. Behera, B.K. Mishra, G.B. Behera, Chem. Rev. 100 (2000) 1973. [44] S.L. Efimova, Yu.V. Malyukin, Funct. Mat. 5 (1998) 349. [45] M. Kasha, H.R. Rawls, M. Ashraf El-Bayoumi, Pure Appl. Chem. 11 (1965) 371. [46] G. Grygorova, V. Klochkov, S. Yefimova, Yu Malyukin, Chem. Phys. Lett. 621 (2015) 46. [47] U. Rösch, Sh Yao, R. Wortmann, F. Würthner, Angew. Chem. Int. Ed. 45 (2006) 7026.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Configuration of organic dye excimers in nanoporous SiO2 matrices A.V. Sorokin n, B.A. Gnap, I.I. Bespalova, S.L. Yefimova, Yu.V. Malyukin Institute for Scintillation Materials, STC “Institute for Single Crystals”, NAS of Ukraine, 60 Lenin ave., 61001 Kharkov, Ukraine
art ic l e i nf o
a b s t r a c t
Article history: Received 15 December 2015 Received in revised form 13 May 2016 Accepted 12 June 2016 Available online 18 June 2016
The effect of cyanine dye 3,30 -dioctadecyloxacarbocyanine perchlorate (DiO) and benzimidazole dye 4dimethylamino-1,8-naphthoylene-10 ,20 -benzimidazole (DNBI) accumulation in nanoporous silica matrices on the dyes luminescence properties has been studied. For both dyes, ground state dimer formation with perpendicular transition dipoles at high dye concentrations has been considered as a result of restricted geometry of the nanoscale pores. The dimer excitation leads to excimer formation revealing by appearance of new long-wavelength luminescence band and shortening the dye luminescence lifetime. In the excimer luminescence excitation spectra two additional bands have been observed, one of which is bathochromically shifted relatively to the absorption band and another one is hypsocromically shifted. Using the Kasha exciton model it was shown that the excimers possess oblique transition dipoles configuration. & 2016 Elsevier B.V. All rights reserved.
Keywords: Excimer Silica matrix Luminescence Oblique dimer Cyanine Benzimidazole
1. Introduction Low cost, relative simplicity and low temperatures of the sol– gel synthesis make it a very popular method for the production of wide range of materials including glass highly-porous films or bulk matrices which could be a host for organic and inorganic molecules [1,2]. Composite materials based on sol–gel matrices with incorporated organic luminophores were successfully applied as solar concentrators, tunable lasers, active waveguides, dyesensitized solar cells, etc. [1–8]. The main advantage of such composite materials is enhanced photostability of the incorporated dye molecules [1–5,9,10]. There are two ways to embed dye molecules into the sol–gel matrices: (i) addition the dye at a stage of the matrix synthesis and (ii) adsorption of the dye molecules by the synthesized matrix dipped into the dye solution [1,2]. The first approach allows to obtain the matrices with high dye loading capacity and high degree of homogeneity [1,2,11]. However, as the dye-loaded matrices prepared by this way are not annealed at high temperatures to prevent dye destruction, they are very fragile and have uncontrolled porosity. The second approach allows imparting reasonable mechanical properties and controlled porosity to the synthesized sol–gel matrices by high temperature treatment prior the dye loading [3–8]. But in this case, a heterogeneous dye n
Corresponding author. E-mail address: [email protected] (A.V. Sorokin).
http://dx.doi.org/10.1016/j.jlumin.2016.06.025 0022-2313/& 2016 Elsevier B.V. All rights reserved.
distribution in the matrix and the dye intermolecular interaction should be taken into account [6,12,13]. Indeed, the apparent fractal distribution of dye molecules in a SiO2 sol–gel matrix with dimensional parameter do 3 was shown using non-radiative energy transfer [6,13]. Furthermore, spatial restriction and non-uniform distribution of dye molecules within pores of the matrix can cause the dye aggregation [11–17] with a preferential formation of dimers [14,15] and excited dimers, i.e. excimers [16,17]. The dye exсimerization in sol–gel matrices were reported for some polycyclic aromatic molecules, such as pyrene, naphthalene, fluorene, acenaphthene, which tend to form excimers in solutions as well [17–25]. Recently, we have report the cyanine dyes excimer formation in porous SiO2 matrices [26,27]. Although the dimer and aggregate formation is well-known phenomenon for cyanine dyes [14,28] and were also observed in sol–gel matrices [11,29,30], the cyanine dyes excimerization has not be reported up to now. Moreover, cyanine dyes DiI and DiD used in our experiments are well-known fluorescent probes for biological applications [31], for which aggregation is hindered due to their structure. So, the main reason for these dyes excimerization was supposed a spatial restriction by nanovolume of silica pores [26,27]. However the excimer configuration wasn't established. In the present paper we report the formation of excimers and studying their configurations for one more representative of cyanine dye family, 3,30 -dioctadecyloxacarbocyanine perchlorate (DiO), and for the dye belonging to the benzimidazole compounds, 4-dimethylamino-1,8-naphthoylene-10 ,20 -benzimidazole (DNBI), which are embedded by adsorption into nanoporous SiO2 matrices (Fig. 1).
172
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
(IRF) full width at half maximum (ΔFWHM) for the whole setup was about 100 ps. Decay time analysis has been performed using FluoFit software (PicoQuant, Germany). To study the matrix porosity, Physisorption Analyzer ASAP 2000 (Micromeritics, USA) with nitrogen as an analysis gas (the BET-BJH methods) was used. Transmission electron microscopy (TEM) images were obtained using a PEM-125 (Selmi, Ukraine) microscope at 100 V accelerating voltage.
3. Results and discussion
Fig. 1. Structural formulas of the dyes: a) DiO, b) DNBI and an image of SiO2 matrix (c).
2. Experimental Nanoporous SiO2 matrices were prepared by sol–gel hydrolysis and condensation of TMOS (Si(OCH3)4) in a water–methanol solution [32,33]. Briefly, 3.15 mL of methanol was mixed with 3.75 mL of TMOS for 5 min. Then 4.5 mL of distilled water and 0.525 mL of hydrochloric acid were added to the mixture, which was agitated for the next 5 min. The sol was poured in 35 10 mm plastic Petri dishes and held for 24 h at room temperature until a gel was formed, which was then dried for 120 h at temperature of 45 °C. Subsequently, the dried samples were annealed at 750 °C for 3 h. The resulted samples were transparent colorless disks with 20 mm diameter and 1–1.5 mm high (Fig. 1c). DiO dye was obtained from Sigma-Aldrich (USA) and used as received. DNBI dye was a kind gift of Dr. I.A. Borovoy (Institute for Scintillation Materials, NAS of Ukraine). To embed the dyes, the matrices were dipped into the dye chloroform solution for at least one hour. After that samples were dried at 60 °C for at least three hours. Luminescence and luminescence excitation spectra were recorded using fluorescence spectrometer Lumina (Thermo Scientific, USA) equipped with a solid sample holder. Luminescence was registered in front excitation geometry to avoid reabsorption effects. Absorption spectra was registered using a microspectrometer USB4000 (Ocean Optics, USA) supplied with an incandescent and deuterium lamps. Luminescence decay spectra were registered using FluoTime 200 fluorescence lifetime spectrometer (PicoQuant, Germany) equipped with a 439 nm picosecond pulsed laser diode head. The instrument response function
The synthesis method used for SiO2 matrix formation in a combination with following high temperature treatment at 750 °C allows to obtain the matrices with high mechanical properties. The average microhardness of the matrices is 170 kg/mm2, which is practically twice as high as the values reported by the other authors [34]. The density and porosity of the matrices determined using the hydrostatic weighing method are 1.10 g/cm3 and 54 vol%, respectively [33]. According to X-ray phase analysis and atomicforce microscopy data, the matrices have an amorphous structure and consist of close-packed spherical particles with an average size of 35 nm [32]. These findings are supported by TEM images (See Supplementary Material, Fig. S1). Also some quite large pores with diameter of 20–30 nm could be found on the image. From other hand it is known that the higher temperature of the postsynthesis treatment the smaller pores have to be [1,2,4]. To clear the pore details of the SiO2 matrix a gas adsorption method has been used [35]. It reveals the average pore diameter of 2.1 nm with average micropore volume of 0.2 cm3/g and average pore area of 390 m2/g. It should be noted that a physisorption isotherm (See Supplementary Material, Fig. S2) reveals a tiny adsorption– desorption hysteresis and belongs to Type I according to standard classification [35]. The latter indicates that the very fine pores ( 2 nm) cause the material porosity [35]. As the pores are open and the matrix porosity is 54 vol% [32,33] it could be concluded that the matrix pore system is bicontinuous and consisted of randomly arranged spherical interconnected fine pores with a very small contribution of meso- and macropores with diameter 2– 50 nm and larger than 50 nm, respectively. To embed the studied dyes into the SiO2 matrix the dye adsorption from a chloroform solution up to the full matrix saturation was used as previously [12,26,27]. As the used dyes have a better chemical affinity to the matrix comparing with the chloroform, the adsorption process appeared to be more preferred than a back process of the dye dissolution in the chloroform. As a result the matrix became strongly colored and the solvent is practically colorless with the residual dye amount (See Supplementary Material, Fig. S3). Therefore, for the sample preparation a volume of the dye solution was calculated in such a way that the dye with the largest concentration (typically 10–4 M) was almost fully adsorbed by the matrix. Then, the same volume was used for preparation of samples with other dye concentrations. As the local dye concentration in the SiO2 matrices was much larger as compared to the solution and taking to account the apparent fractal distribution of dye molecules in SiO2 matrices [13], to specify the samples with different dye load we will use the concentration of dyes in a stock chloroform solution. DiO is amphiphilic dye with two long hydrophobic tails C18H37 (Fig. 1a), similarly to DiI and DiD dyes used in our previous study [26,27]. Due to trimethine chain and oxygen heteroatoms, its absorption and luminescence spectra (Fig. 2) are shifted toward shorter wavelengths (λabs ¼ 488 nm, λlum ¼505 nm, in DMF) as compared to DiI and DiD [31,36]. Similar to other cyanine dyes with two long hydrophobic tails DiO hardly aggregates in solutions
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
173
Fig. 2. Absorption (1) and luminescence (2, λexc ¼ 470 nm) spectra of the DiO dye in DMF.
but it could aggregate under some restriction conditions like formation in LB films [37]. A set of the samples with the different dye loads was prepared with the stock dye concentrations ranged from 10–6 M to 10–4 M (Fig. 3). The matrix loaded with the smallest dye amount revealed typical (e.g., see Fig. 2) monomer luminescence and absorption spectra with the maxima position determined by the “solvent” polarity (so called solvatochromic effect) [38]: λabs ¼ 483.5 nm and λlum ¼498 nm (Fig. 3, curves 1). The increase in the dye load causes a remarkable transformation of the DiO luminescence spectrum: the appearance of new broad long-wavelength band (λlum ¼ 532 nm) and redistribution of monomer and new band intensities depending on the dye concentration has been observed (Fig. 3a). At the stock dye concentration of 10–4 M, the new band becomes dominant, whereas the monomer one manifests itself only as a shoulder (Fig. 3a, curve 5). Further increase in the amount of loaded dye did not provoke any changes in the luminescence spectrum. At the same time, we did not reveal any changes in the DiO absorption spectrum within the whole dye concentration range (Fig. 3b). Notice that in a case of a large amount of the dye loaded in the matrix, we observed reabsorption effect and did not present these spectra. Similarly to the previous results [26,27], the new longwavelength luminescence band is supposed to belong DiO excimers formed in nanopores of the SiO2 matrix at high dye concentrations. Indeed, the strong concentration dependence of novel luminescence band indicates on some aggregation processes rather than other effects like strong deviation from the mirror symmetry between absorption and luminescence spectra due to conformational transitions of the molecules [39]. To prove the dye excimer formation, luminescence decays were registered at the luminescence band maxima for different dye concentrations (Fig. 4). The DiO luminescence decay measured in the matrix with the lowest dye concentration (monomeric luminescence) follows a non-exponential law (Fig. 4, curve 1) with the best fitting with three exponential curves and decay parameters (amplitude weighted): τ1 0.39 ns (45.4%), τ2 1.62 ns (51.4%), τ3 4.65 ns (3.2%) and τAV 1.16 ns. The multi-exponential pattern of the monomer luminescence decay could be explained by strong heterogeneity of the dye molecules distribution in the nanoporous matrix [11,13]. However, the attempts to fit the decay curve using Gaussian or Lorentzian distributions of decay times which are
Fig. 3. Luminescence (a, λexc ¼ 470 nm) and absorption (b) spectra of the DiO dye in a SiO2 matrix at different dye concentrations in a stock chloroform solution: 1 – 10 6 M, 2 – 5 10 6 M, 3 – 10 5 M, 4 – 5 10 5 M, 5 – 10 4 M.
widely applied for heterogeneous media [4] were unsuccessful indicating complicated mechanisms of excitation relaxation of DiO molecules adsorbed by the matrix. It is well-known that cyanine dye luminescence decay time is strongly depended on the solvent polarity [11,36,38] and the less polar solvent results in larger decay time (See Supplementary Material, Fig. S4). The sol–gel matrix annealed at temperatures higher than 600 °C usually considered as nonpolar due to polar silanol groups on its pore surface are transformed to nonpolar siloxane bridges [4] and the adsorbed dye decay time should be long. From other hand, the matrix contact with the environment hasn't been prevented and an adsorbed water vapor could change the pore interior polarity leading to the decay time shortening. Also, for the dye molecules restricted in the small pores cis-trans isomerization is hindered causing the decay time increasing due to radiationless deactivation prevented [11]. Yet another possibility for the presence of molecule fraction with the decreased decay time is an excitation energy transfer (EET) between adjacent molecules which are situated close enough socalled homo-EET [40,41]. Indeed, due to restricted geometry of the matrix porous system the EET is a very efficient between the adjacent molecules [13]. As the energy donor and the energy
174
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
Fig. 4. Luminescence decay curves (λexc ¼439 nm) of the DiO dye in SiO2 matrix at different dye concentrations in a stock chloroform solution and different registration wavelengths: 1 – 10 6 M, λreg ¼ 495 nm; 2 – 10 5 M, λreg ¼ 505 nm; 3 – 10 4 M, λreg ¼ 530 nm; 4 – IRF.
Fig. 5. Absorption (dashed curves) and luminescence (solid curves, λexc ¼ 415 nm) spectra of the DNBI dye in different solvents: 1 – toluene, 2 – DMF.
acceptor are the similar molecules the homo-EET couldn't manifest itself in stationary spectra but causes the luminescence quenching and decay time shortening due to more efficient radiativeless relaxation [40,41]. Except the adsorbed dye excitation dynamics the silica matrix photophysical properties should be taken into account [42]. Indeed, there is an excitation energy transfer could occur in the case of dye interaction with porous silica [42]. Due to amorphous structure our SiO2 matrices reveal absorption (λmax ¼307 nm) and luminescence (λmax ¼361 nm) bands only in UV range (see Supplementary Material, Fig. S5a). However, due to many energy levels of silica some luminescence also excited at λexc ¼440 nm (See Supplementary Material, Fig. S5b) resulting in nonexponential decay with τAV 2.8 ns (See Supplementary Material, Fig. S5c). Due to off-band excitation the SiO2 luminescence contribution to the composite spectrum is very small so any energy transfer effects related to the SiO2 intrinsic luminescence [42] could be neglected. Nevertheless, we could attribute the longest time component (τ3) with the smallest fractional amplitude of DiO monomer decay (Fig. 4, curve 1) to the SiO2 luminescence contribution. So, we could resume that in the DiO/SiO2 composite luminescence dynamics at the low dye concentration contribute three kinds of species: one of them is the SiO2 matrix intrinsic luminescence and two others are related with DiO molecules affected by unclear factors. One fraction of the dye molecules reveals short decay time and the second one reveals long decay time with the about the same fraction contributions. There is no physical sense to distinguish other dye fractions and fit the decay curve by more than 3 exponents [41]. The appearance of a new band in the DiO luminescence spectrum is attended by gradual shortening of the luminescence decay time (Fig. 4). The luminescence decay curve measured at the longwavelength band maxima (the largest dye concentration) reveals a stronger deviation from the monoexponential decay law (Fig. 4, curve 3) as compared to that for monomer band (Fig. 4, curve 1) and also was fitted by three exponents with the decay parameters (amplitude weighted): τ1red 0.35 ns (56.3%), τ2red 1.2 ns (42.1%), τ3red 6.2 ns (1.6%) and τAV red 0.8 ns. As previously, the smallest amplitude component τ3 with the largest decay time we ascribe to the SiO2 intrinsic luminescence. The analysis of fitting parameters shows that the decay time components τ1 and τ2 for the long-
wavelength band are a bit shorter, while the fractional amplitude of τ1 component is slightly increased. As a result, the average lifetime for the long-wavelength band is shorter than that for the monomer one. Since the luminescence lifetime shortening is a feature of a dye aggregation process [16,28], we can ascribe the long-wavelength band to the dye excimers. For the cyanine dye aggregates it is well-known that decreasing the luminescence decay time comparing with the monomer one is proportional to the aggregation degree [28,43]. So, comparing the monomer and excimer average decay time we could assume that only smallest aggregates, i.e. dimers, formed at the highest dye concentration. It should be noted, that taking into account shortening τ1 component of the DiO decay time at the high dye concentration (Fig. 4, curve 3) simultaneously with its fractional amplitude growth indicates that the cis-trans isomerization couldn't be considered as an origin of this DiO molecules fracture. Indeed, with the dye concentration growth one could expect strong decreasing of the dye molecule fraction for which rotation around the polymethine bridge is allowable especially in the case of the long hydrophobic tails of DiO molecule (Fig. 1a). For further clearing the origins of two DiO fractions revealed by the luminescence decay analysis we examine the polarity of the SiO2 matrix pores using another organic dye DNBI (Fig. 1b), which is known to be a polarity-sensitive molecule [44]. In polar solvents the luminescence spectrum of DNBI reveals a second emission peak with λmax ¼596 nm (Fig. 5, curve 2 solid) which is red-shifted comparing with the luminescence band with λmax ¼483.5 nm characteristic for the nonpolar solutions case (Fig. 5, curve 1 solid). The appearance of this peak does not depend of the dye concentration and was associated with a formation of so-called TICT (twisted intramolecular charge transfer) state in the excited state of the molecule [44]. SiO2 matrix loaded with the DNBI dye molecules exhibits the same features as it was described above for DiO dye (Fig. 6). At low dye concentrations, both absorption (not shown) and luminescence (λmax ¼505 nm, Fig. 6a, curve 1) spectra could be assigned to the monomer dye molecules. The TICT state emission band (Fig. 5, curve 2 solid) is absent in the luminescence spectrum of the DNBI adsorbed by the matrix (Fig. 6a). So, the water vapor from the environment is not affected on the dye spectral properties. The luminescence decay curve for the monomeric DNBI luminescence in the matrix was also nonexponential (Fig. 6b, curve 1)
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
Fig. 6. a) Luminescence spectra (λexc ¼ 440 nm) of the DNBI dye in a SiO2 matrix at different dye concentrations in a stock chloroform solution: 1 – 10 6 M, 2 – 5 10 6 M, 3 – 10 5 M, 4 – 5 10 5 M, 5 – 10 4 M; b) luminescence decay curves (λexc ¼439 nm) at different dye concentrations and different registration wavelengths: 1 – 10 6 M, λreg ¼ 515 nm; 2 – 10 4 M, λreg ¼550 nm.
similar to the DiO monomers with the best fitting by three exponential curves and following decay parameters (amplitude weighted): τ1 0.25 ns (46.6%), τ2 2.15 ns (34.0%), τ3 6.3 ns (19.4%) and τAV 2.0 ns. As previously the third decay component with the largest time we attribute with the SiO2 intrinsic luminescence. It much larger contribution could be assigned with much less extinction coefficient and luminescence quantum yield of DNBI comparing with DiO and, hence, less intense DNBI luminescence. The cis-trans isomerization for DNBI molecules is not possible due to its structure (Fig. 1b). So, a presence of two luminescent fractions with short and long decay times similarly to the DiO case proves our assumption concerning the luminescence quenching for part of DNBI molecules due to homo-EET between adjacent molecules. Increasing the DNBI concentration causes the luminescence maximum gradual shift toward longer wavelengths and the appearance of new luminescence band (λmax ¼ 534 nm, Fig. 6a). The luminescence decay registered at the long wavelength band was also shorter (Fig. 6b, curve 2) with fitting parameters (amplitude weighted): τ1red 0.24 ns (60.2%), τ2red 1.68 ns (31.15%), τ3red 8.6 ns (8.65%) and τAV red 1.15 ns. Similarly to DiO
175
Fig. 7. Luminescence excitation spectra of the DiO (a) and the DNBI (b) dyes in the SiO2 matrix at different dye concentrations in the stock chloroform solution and different registration wavelengths: 1 – 10 6 M (λregDiO ¼515 nm and λregDNBI ¼ 530 nm); 2 – 10 4 M (λregDiO ¼ 560 nm and λregDNBI ¼575 nm); 3 – subtracted spectrum of the oblique dimer.
the DNBI decay times are shortened and the excimerization within the pores of SiO2 matrix was considered. Note, that SiO2 intrinsic luminescence contribution becomes smaller because of the DNBI luminescence intensity increasing for the larger dye concentration. To determine the DiO and DNBI excimer configuration the luminescence excitation spectra collected at the monomer and excimer bands (samples with low and high dye concentrations, respectively) were analyzed (Fig. 7). The monomer luminescence excitation spectra for both dyes were similar to the correspondent absorption spectra (Fig. 7, curve 1). Contrarily, the excimer luminescence excitation spectra differ significantly from the absorption spectra (Fig. 7, curve 2). Similarly to our previous observation for DiI excimers [27], in the DiO and DNBI excimer luminescence excitation spectra two bands can be distinguished. One band is bathochromically shifted with respect to the monomer excitation band (we will call it J-band similarly to J-aggregates [28]) with λJDiO ¼511 nm and λJDNBI ¼512 nm (Fig. 7, curve 2). Another band is hypsochromically shifted with respect to the monomer excitation band (we will call it H-band) with λHDiO ¼423.5 nm and
176
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
Fig. 8. Classical Kasha scheme of the oblique dimer configuration.
Table 1 Excitonic parameters of the dys excimers in nanoporous SiO2 matrices.
DiO DNBI
|M|2, D2
ΔE, cm 1
α, °
θ, °
R, Å
20.4 18.8
4060 6385
55 77
62.5 51.5
3.9 3.5
λHDNBI ¼377 nm (Fig. 5a, curve 2). Despite the similar excimer excitation spectral features for both dyes studied much larger contribution of the excimers comparing with that for the monomers is observed for DiO dye with respect to DNBI (Fig. 7) supposing more favorable excimerization process for the former dye. Subtracting the monomer band contributions from the excimer excitation spectra we obtain well-separated H- and J-bands (Fig. 7, curve 3). According to the Kasha exciton band splitting model [45], the observed two bands correspond to an oblique dimer configuration (Fig. 8). For the oblique dimers, the Kasha exciton model could be successfully applied to estimate the dimer geometrical parameters [46,47]. So, here we applied it for the excimer configuration evaluation also. According to [45,46] the angles α and θ (Fig. 8) can be calculated as follows: α f 180 3 α J ¼ ; θ¼ ð1Þ tg 2 2 fH 2 where fH and fJ are the oscillation strengths of the corresponding electronic transitions. In our case, the ratio fJ/fH is proportional to the ratio of areas under the correspondent bands in the excitation spectrum (Fig. 7). The center-to-center distance R (Fig. 8) can be found as [45,46]:
ΔE ¼
2jM j2 R3
ð cos α þ3 cos 2 θÞ;
ð2Þ
where ΔE is the exciton splitting energy and M is the transition moment for the singlet–singlet transition in the monomer. The latter can be calculated as [40]: Z 9n εðνÞ jM j2 ¼ 9:186 10 3 dν; ð3Þ ν ðn2 þ 2Þ2 R where ε(ν) is the molar extinction coefficient (with εðνÞdν is the area of the monomer absorption band) and n is the refractive index of the medium. So, using the H- and J-bands in the DiO and DNBI excitation spectra (Fig. 7, curve 3) and the monomer bands in the absorption spectra (Figs. 3b and 5) we estimated the excimer configuration for both dyes in the nanoporous SiO2 matrices (Table 1). As it could be seen from Table 1, molecules forming the excimers are located very close to each other at the distances for DiO 3.9 Å and DNBI 3.5 Å. Taking into account a quite large value of the dyes
Fig. 9. Modified scheme of the oblique dimer configuration proposed in [47]: a) side view, b) top view. Reprinted with permission from [47]. Copyright © 2006, John Wiley and Sons.
transition moments ( 4.5 D) and lengths of the chromophores (410 Å) we could suggested a twisted face-to-face arrangement of the molecules forming excimers (Fig. 9). Since we did not observe any changes in the absorption spectrum as a result of dye excimerization (Fig. 3b) and the same growing-in profiles of the time-resolved luminescence for the excimer and monomers bands (Figs. 4 and 6b) we assume a ground state dimer formation with a perpendicular dipole moment arrangement for both dyes [18,19]. Under excitation of one molecule of the ground state dimer, its dipole moment is reoriented forming angle α with the second molecule dipole moment (Fig. 9) and the excimer forms. Such excimer configuration is unique and was not found in a literature for typical excimer forming dyes. As a mean diameter of the pores in SiO2 matrices is only 2 nm while the studied dyes are quite large we consider that the spatial restriction in the pores is the main factor that provokes the dyes excimer formation with the oblique configuration.
4. Conclusions Using two different organic dye molecules belonging to cyanine and benzimidazole families, we have studied the features of their intermolecular interactions in nanoporous silica matrices. In both cases, increasing the dye amount loaded to the matrix causes the second red-shifted emission peak appearance and shortening luminescence lifetime, whereas the dyes absorption spectra did not change. Basing on these facts, dyes excimer formation was considered as the origin of the additional luminescence band. The excimer configuration for the both dyes was evaluated using the Kasha of exciton splitting model for oblique dimers. The short distance and large angles between molecules forming the excimers allow us to suggested ground dimers formation with twisted face-to-face configuration. It was considered that the excimer
A.V. Sorokin et al. / Journal of Luminescence 179 (2016) 171–177
formation is caused by spatial confinement in the nanoscale silica pores.
Acknowledgments We thank Dr. I.A. Borovoy (Institute for Scintillation Materials of NAS of Ukraine, Kharkiv) for providing DNBI used in this study. Authors are grateful to Prof. A.V. Ragulya (V.Ye. Lashkaryov Institute of Semiconductor Physics of NAS of Ukraine, Kyiv) for the help with porosity measurements and Dr. O.M. Vovk (Institute for Single Crystals of NAS of Ukraine, Kharkiv) for the help with TEM imaging.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2016.06. 025.
References [1] M.A. Aegerter, M. Mennig (Eds.), Sol-Gel Technologies for Glass Producers and Users, Springer Science þBusiness Media, New York, 2004. [2] G.Q. Lu, X.S. Zhao (Eds.), Nanoporous Materials: Science and Engineering, Imperial College Press, London, 2004. [3] R. Reisfeld, E. Yariv, H. Minti, Opt. Materials 8 (1997) 31. [4] J.K. Thomas, E.H. Ellison, Adv. Coll. Interf. Science 89-90 (2001) 195. [5] R. Reisfeld, J. Fluoresc. 12 (2002) 317. [6] T.V. Antropova, Yu.A. Gordeeva, B.D. Ryzhikov, A.M. Saletskii, J. Appl. Spectrosc. 72 (2005) 478. [7] J.A. Anta, E. Guillen, R. Tena-Zaera, J. Phys. Chem. C 116 (2012) 11413. [8] O.N. Bezkrovnaya, I.M. Pritula, A.G. Plaksyi, V.F. Tkachenko, ОМ Vovk, Yu. L. Slominskii, A.D. Kachkovskiy, Yu.A. Gurkalenko, S.N. Krivonogov, A.V. Lopin, J. Non-Cryst. Sol. 389 (2014) 11. [9] A. Dubois, M. Canva, A. Brun, F. Chaput, J.-P. Boilot, Appl. Optics 35 (1996) 3193. [10] E. Yariv, S. Schultheiss, Ts Saraidarov, R. Reisfeld, Opt. Mat 16 (2001) 29. [11] U. De Rossi, S. Daehne, R. Reisfeld, Chem. Phys. Lett. 251 (1996) 259. [12] B.A. Gnap, I.I. Bespalova, S.L. Yefimova, A.V. Sorokin, Funct. Mat. 18 (2011) 487. [13] S.L. Yefimova, A.M. Rekalo, B.A. Gnap, A.V. Sorokin, O.G. Viagin, Yu.V. Malyukin, Appl. Phys. A 116 (2014) 2131. [14] W. West, S. Pearce, J. Phys. Chem. 69 (1965) 1894. [15] Yu. A. Gordeeva, A.M. Saletskii, J. Appl. Spectrosc. 73 (2006) 924. [16] J.B. Birks, Rep. Prog. Phys. 38 (1975) 903. [17] K.A. Zachariasse, Excimer formation with pyrenes on silica surfaces, in: M. Anpo, T. Matsuura (Eds.), Photochemistry on Solid Surfaces, Elsevier Science Publishers B.V., Amsterdam, 1989, pp. 48–78.
177
[18] E. Wellner, M. Ottolenghi, D. Avnir, D. Huppert, Langmuir 2 (1986) 616. [19] T. Fujii, E. Shimizu, Chem. Phys. Lett. 137 (1987) 448. [20] T. Fujii, A. Ishii, N. Takusagawa, H. Yamashita, M. Anpo, J. Photochem. Photobiol. A 86 (1995) 219. [21] J.T. Barbas, M.E. Sigman, R. Arce, R. Dabestani, J. Photochem. Photobiol. A 109 (1997) 229. [22] C. Reyes, M.E. Sigman, R. Arce, J.T. Barbas, R. Dabestani, J. Photochem. Photobiol. 112 (1998) 277. [23] Yu Tozuka, E. Tashiro, E. Yonemochi, T. Oguchi, K. Yamamoto, J. Coll. Interf. Sci. 248 (2002) 239. [24] A. Thomas, S. Polarz, M. Antonietti, J. Phys. Chem. B 107 (2003) 5081. [25] R. Dabestani, M. Kidder, A.C. Buchanan III, J. Phys. Chem. C 112 (2008) 11468. [26] B.A. Gnap, I.I. Bespalova, S.L. Yefimova, A.V. Sorokin, Funct. Mat. 20 (2013) 407. [27] A.V. Sorokin, B.A. Gnap, I.I. Bespalova, S.L. Yefimova, Funct. Mat. 22 (2015) 207. [28] F. Wurthner, T.E. Kaiser, Ch.R. Saha-Muller, Angew. Chem. Int. Ed. 50 (2011) 3376. [29] F. Nüesch, J.E. Moser, V. Shklover, M. Grätzel, J. Am. Chem. Soc. 118 (1996) 5420. [30] N. Kato, K. Yamamoto, Y. Uesu, Jpn. J. Appl. Phys. 46 (2007) 5318. [31] I. Johnson, M.T.Z. Spence (Eds.), The Molecular Probes Handbook, A Guide to Fluorescent Probes and Labeling Technologies, 11th ed., Life Technologies Corporation, New York, 2010. [32] T.A. Blank, I.I. Ganina, Yu.V. Malyukin, L.P. Eksperiandova, Funct. Materials 16 (2009) 517. [33] V.V. Seminko, A.A. Masalov, O.G. Viagin, I.I. Bespalova, L.I. Voloshina, O. V. Zelenskaya, V.A. Tarasov, Yu. V. Malyukin, Development of nanocomposite alpha-detectors based on silica matrices and organic scintillators, in: P. Petkov, D. Tsiulyanu, W. Kulisch, C. Popov (Eds.), Nanoscience Advances in CBRN Agents Detection, Information and Energy Security, Springer Science þBusiness Media, Dordrecht, 2015, pp. 415–419. [34] K.S. Lam, X.-L. Zhu, D. Lo, Appl. Phys. B 68 (1999) 1151. [35] J.B. Condon, Surface Area and Porosity Determinations by Physisorption. Measurements and Theory, Elsevier, Amsterdam, 2006. [36] A.A. Ishchenko, Russ. Chem. Bull. 43 (1994) 1161. [37] S. Chakraborty, P. Debnath, D. Dey, D. Bhattacharjee, S.A. Hussain, J. Photochem. Photobiol. A 293 (2014) 57. [38] C. Reichardt, T. Welton, Solvents and Solvent Effects in Organic Chemistry, Fourth ed., Wiley-VCH Verlag, Weinheim, 2011. [39] A.V. Naumov, N.L. Naumova, I.S. Osad'ko, Opt. Spectrosc. 91 (2001) 704. [40] W.W. Parson, Modern Optical Spectroscopy, Springer-Verlag, Berlin, 2007. [41] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 3rd ed., Springer Science þBusiness Media, Singapore, 2006. [42] V. Pranculis, I. Šimkiene, M. Treideris, V. Gulbinas, Phys. Status Solidi A 210 (2013) 2617. [43] A. Mishra, R.K. Behera, P.K. Behera, B.K. Mishra, G.B. Behera, Chem. Rev. 100 (2000) 1973. [44] S.L. Efimova, Yu.V. Malyukin, Funct. Mat. 5 (1998) 349. [45] M. Kasha, H.R. Rawls, M. Ashraf El-Bayoumi, Pure Appl. Chem. 11 (1965) 371. [46] G. Grygorova, V. Klochkov, S. Yefimova, Yu Malyukin, Chem. Phys. Lett. 621 (2015) 46. [47] U. Rösch, Sh Yao, R. Wortmann, F. Würthner, Angew. Chem. Int. Ed. 45 (2006) 7026.