Squaraine Dye as an Exciton Trap for Cyanine J-Aggregates in a

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Squaraine Dye as an Exciton Trap for Cyanine J-Aggregates in a Solution Roman S. Grynyov, Alexander V. Sorokin,* Gleb Ya. Guralchuk, Svetlana L. Yefimova, Igor A. Borovoy, and Yuri V. Malyukin Institute for Scintillation Materials, NAS of Ukraine, 60 Lenin AVenue, 61001 KharkoV, Ukraine ReceiVed: May 7, 2008

The diindolenine derivative of squaraine dye Sq-2Me has been used as an exciton energy trap (an energy acceptor) for amphi-PIC J-aggregates in a solution. Using the modified Stern-Volmer equation, parameters of the energy transfer have been obtained. It has been revealed that 50% of J-aggregate luminescence is quenched in a binary DMF/water solution at the ratio trap/amphi-PIC ) 1:80 that corresponds to the exciton migration over 20 delocalization segments within the J-aggregate. Introduction A rapid development of nanotechnologies observed today requires a novel delivery systems which provide energy transport to the nanoscale reaction centers.1,2 Nature demonstrates us a prominent example of such delivery systems: light-harvesting complexes (LHCs), which provide extremely fast and efficient energy transport of the absorbed light to the photochemical reaction center of plants and photosynthetic bacteria.3,4 LHCs are molecular aggregates with a circular structure containing two rings of 8-18 molecules.3,4 Electronic excitations in LHCs are delocalized within the molecular ring that leads to the formation of excitonic states.5 The excitonic nature of photoexcitations plays a key role in unique energy transport properties of LHCs. One of the ways to create similar energy delivery nanosystems is to mimic LHCs by an artificial system, which should be less complex but provide a possibility to control its properties. Recently, J-aggregates of amphiphilic cyanine dyes have been shown to be the most promising artificial system to mimic LHCs.6 J-aggregates (or Scheibe aggregates) are luminescent well-ordered nanoensembles of noncovalently coupled luminophores, which are typically cyanines, porphyrins, or merocyanines.6–13 Due to high order degree of molecular packing in J-aggregates and strong dipole-dipole interaction between molecules forming the J-aggregate, an electronic excitation is delocalized within a molecular chain as in the case of LHCs. That leads to the formation of excitonic states (Frenkel excitons) and explains unique optical properties of J-aggregates: extremely narrow (for organic molecules) spectral width of the absorption band, resonant luminescent, giant oscillator strength, giant optical nonlinearities, and so forth.7–9 J-aggregates of amphiphilic pseudoisocyanine (amphi-PIC) dye (Chart 1) also reveal cylindrical geometry,12,13 which is still simpler than the double-cylindrical geometry described for J-aggregates of the TDBC family.6 So it is interesting to study energy transport parameters for amphi-PIC J-aggregates. There are a number of experimental works devoted to the study of exciton energy migration in J-aggregates.6,14–31 As a rule, two approaches are used to determinate the exciton transport characteristics: the analysis of time parameters of exciton luminescence decay14–17 and a direct observation of the exciton energy transfer to an energy or electron acceptor which is * To whom correspondence should be addressed. E-mail: sorokin@ isc.kharkov.com.

CHART 1: Structural Formulas of the Dyes Investigated: (a) amphi-PIC and (b) Sq-2Me

embedded into the J-aggregate chain and acts as an exciton trap.6,18–31 The data concerning the efficiency of exciton energy trapping differ strongly. For example, 50% of J-aggregate luminescence is reported to be quenched at the ratio of a trap molecule to molecules forming the J-aggregate of 1:100 in ref 26 or 1:250 in ref 30, whereas in refs 19 and 20 this ratio is reported to be 1:104. It should be noted that most of these researches were done on J-aggregates obtained in LangmuirBlodgett (LB) films,19,20,22,24,30 polymer layers,23,29,31 or even liposome surfaces.21 J-aggregates formed on such surfaces are supposed to possess a more perfect 2D structure described by the brick-stone lattice.7–9,20 That can explain the high efficiency of the exciton transport observed. Really, if the exciton trap was located in a layer other than the J-aggregate, the efficiency of the exciton migration is much less.19,29,30 When J-aggregates are formed in solutions, their structure is very complicated and a lot of various conditions can effect the exciton migration.6,12 We can refer to only two works that reported the incorporation of an exciton trap into J-aggregates self-organized in solutions.6,18 In refs 25–28, J-aggregates of cyanine dye pendant polymer (CDP) were obtained in a solution, but their formation was directed rather by polymerization of the polymer backbone than by self-assembly of cyanine dye. In the present paper, we report the investigation of the exciton energy transfer in amphi-PIC J-aggregates to the exciton trap, a zwitterionic diindolenine derivative of squaraine dye Sq-2Me (Chart 1). Similar to cyanines, squaraine dyes belong to the class of polymethine dyes and exhibit sharp and intense electronic absorption in the region from visible to near-IR, and sometimes

10.1021/jp809124m CCC: $40.75  2008 American Chemical Society Published on Web 11/24/2008

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show fluorescence emission properties.32–34 The intramolecular electronic resonance in the cyanine π-conjugation system is extended from the heterocyclic component at the one end to that at the other end.35 Contrary to cyanines, the squaraine molecule consists of the central cyclobutene moiety and heterocycles at both ends of the molecule, yielding an electron donor-acceptor-donor (D-A-D) charge transfer structure.32–34 Such a structure imparts hydrophobic properties to squaraine molecules.32–34 Experimental Section The dyes were obtained from the dye collection of Dr. I. A. Borovoy (Institute for Scintillation Materials, NAS of Ukraine) with the purity of the dyes controlled by thin layer chromatography. Sample solutions containing amphi-PIC J-aggregates with traps were prepared as follows. Sq-2Me and amphi-PIC were dissolved in dimethylformamide (DMF) to form a mixture at given ratio (from 1:1 to 1:1000, respectively), and then doubly distilled water was added to obtain a binary solution DMF/water with 90% water content. The concentration of amphi-PIC in all solutions was 5 × 10-5 M. Luminescence spectra were recorded using a spectrofluorimeter on the base of two grating monochromators MDR-23 and a xenon lamp. One of the monochromators was used to select a required wavelength (FWHM ∼ 0.5 nm), and the other one was used for the luminescence collection. In all experiments, the excitation wavelength was 530 nm. For absorption spectra registration, the spectrofluorimeter was supplied with an incandescent lamp. Results and Discussion Squaraine dye Sq-2Me dissolved in DMF reveals quite narrow and intense absorption and luminescence bands with maxima λabs ) 636 nm and λlum ) 649 nm (Figure 1a). For amphi-PIC J-aggregate preparation, a DMF/water (1:9) binary solution is required.12,13,18 In such a binary DMF/water solution, absorption and luminescence bands of Sq-2Me appeared to be considerably blue-shifted (λabs ) 623 nm and λlum ) 637 nm, respectively) as a result of the solvatochromic effect36 and their intensities strongly decrease (Figure 1b). However, when Sq-2Me was added to the binary DMF/water solution containing amphi-PIC J-aggregates (λabs ) 580 nm and λlum ) 585 nm), its luminescence and absorption maxima were revealed to be strongly redshifted (λabs ) 665 nm and λlum ) 671 nm) (Figure 2) as compared to the spectra in both DMF and DMF/water solutions (Figure 1). That is the evidence of Sq-2Me coupling with amphiPIC J-aggregates. Moreover, in this case, the bands, which are associated with the absorption and luminescence of uncoupled Sq-2Me (Figure 1b), are not observed that points to the complete coupling of squaraine molecules with J-aggregates. When the water content in the binary solution is reduced to 75% (DMF/ water ) 1:3), absorption and luminescence bands of uncoupled Sq-2Me (λabs ) 623 nm and λlum ) 637 nm) appear as shoulders (Figure 3). Note that, in the binary solution DMF/water ) 1:3, the J-aggregate absorption band (J-band) becomes wider and less intense due to distortion of J-aggregates13 and increasing disorder degree in J-aggregate chains.37 Further reduction of the water content in the binary solution leads to the increase of the uncoupled Sq-2Me spectral band intensity. So, we can conclude that the driving force of coupling squaraine molecules with J-aggregates is the hydrophobic interaction. Since the structure of amphi-PIC J-aggregates13 is similar to that of surfactant micelles,38 hydrophobic Sq-2Me molecules can be solubilized by J-aggregate “micelles”.

Figure 1. Absorption (solid line) and luminescence (dashed line) spectra of Sq-2Me dye in (a) DMF and (b) DMF/water (1:9) solutions.

Figure 2. Absorption (solid line) and luminescence (dashed line) spectra of amphi-PIC J-aggregates with Sq-2Me (amphi-PIC/Sq-2Me ) 20:1) in a DMF/water (1:9) solution.

Sq-2Me incorporation into amphi-PIC J-aggregates is followed by the enhancement of its luminescence, while the intensity of J-aggregates luminescence decreases as compared to the one component solution. One of the plausible reasons for the intensity redistribution could be the exciton energy transfer from the J-aggregate to Sq-2Me. To check this possibility, the concentration of Sq-2Me was varied (the ratio amphi-PIC/Sq-2Me was changed from 1000:1 to 1:1). The increase of the squaraine dye portion leads to the redistribution of J-aggregate and squaraine luminescence bands that points to the exciton energy transfer from amphi-PIC J-aggregates to the squaraine dye (Figure 4). It was found that at high Sq-2Me concentration (the ratio amphi-PIC/Sq-2Me ) 10:1) the J-

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Figure 3. Absorption (solid line) and luminescence (dashed line) spectra of amphi-PIC J-aggregates with Sq-2Me (amphi-PIC/Sq-2Me ) 20:1) in a DMF/water (1:3) solution.

Figure 4. Luminescence spectra of the J-aggregate with the trap at different amphi-PIC/Sq-2Me ratios: 1000:1, 500:1, 200:1, 100:1, 50:1, and 20:1.

aggregate absorption band (J-band) begins to decrease that indicates the J-aggregate distortion. Thereby, solutions with smaller concentrations of the trap Sq-2Me were chosen for the analysis. As one can see, the most efficient exciton energy transfer is achieved at the ratio amphi-PIC/Sq-2Me ) 20:1 (Figure 4). Since J-aggregate luminescence quenching and sensitized squaraine dye luminescence are observed at a very small ratio of amphi-PIC/Sq-2Me ) 1000:1 (Figure 4), we consider the dye Sq-2Me to be a very effective exciton trap. To estimate the efficiency of the exciton migration to the trap, J-aggregate luminescence quenching was analyzed using the well-known Stern-Volmer equation:36

F0 ⁄ F ) 1 + KSV[Q]

(1)

where F0 and F are the J-aggregate luminescence intensities in the absence and presence of the trap, respectively, [Q] is the quencher (trap) concentration, and KSV is the Stern-Volmer constant. The value 1/KSV gives us the concentration of a trap that quenches 50% of the J-aggregate luminescence.36 The plot of F0/F against [Q] does not follow the linear law and shows downward curvature toward the X-axis (Figure 5a). A similar result was observed in refs 6, 19, and 30, but it was analyzed in detail only in ref 19. Such a behavior of the SternVolmer plot is a characteristic feature of two fluorophore populations, one of which is not accessible to a quencher.36 In the case of J-aggregates, this means that some of the excitons do not reach the trap and relax with a photon emission due to

Figure 5. (a) Stern-Volmer and (b) modified Stern-Volmer plots for amphi-PIC J-aggregate luminescence quenching by Sq-2Me trap.

a very short radiative lifetime of J-aggregates (hundreds of picoseconds and less).7,8 Indeed, for amphi-PIC J-aggregates, the luminescence decay time is 300 ps at room temperature.18 In such a case, the modified Stern-Volmer equation should be used:36

F0 1 1 ) + F0 - F fqKSV[Q] fq

(2)

where fq is the fraction of the initial fluorescence which is accessible to a quencher. The plot of F0/(F0 - F) versus 1/[Q] for amphi-PIC J-aggregate luminescence quenching by Sq-2Me is linear and yields 1/fq as the intercept and 1/(fqKSV) as the slope (Figure 5b). A y-intercept 1/fq may be understood intuitively.36 The intercept represents the extrapolation to the infinite quencher concentration (1/[Q] ) 0). The value of F0/ (F0 - F) at this quencher concentration represents the reciprocal of the fluorescence which was quenched. At the highest (infinite) quencher concentration, only fluorophores which are inaccessible for a quencher will be fluorescent. How can we interpret this parameter for excitons in J-aggregates? There is a competition between two processes: exciton radiative relaxation and exciton trapping,18 and even if an exciton forms near the trap, there is a possibility that it deactivates radiatively before trapping. From the plot (Figure 5b), we can obtain fq ) 0.8 and KSV ) 1.65 ×106 M-1. The results obtained allow us to make two conclusions. First, 20% of excitons in the J-aggregate do not achieve the trap and relax radiatively. That is why the luminescence of J-aggregates cannot be totally quenched

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even at a very high concentration of the trap (at the ratio 10:1). Second, taking into account that the concentration of amphi-PIC in the binary solution is 5 × 10-5 M and taking 1/KSV ) 6.06 × 10-7 M, we obtain that 1 trap molecule quenches luminescence of 80 amphi-PIC molecules forming the J-aggregate. The value of KSV obtained is much smaller than that reported in refs 6 and 18–31. To clarify this feature, we have to take into consideration a large disorder degree observed in amphi-PIC J-aggregates37 that is manifested as J-band broadening. Indeed, amphi-PIC J-aggregates are characterized by the broadest absorption band among the whole J-aggregate family (in our case, FWHM ) 650 cm-1).7–9 Due to a large disorder degree, different localization effects can influence the exciton dynamics in amphi-PIC J-aggregates.12,37,39 Optical properties of J-aggregates are known to be governed by the exciton delocalization length Ndel rather than the physical length of the J-aggregate.7–9 To estimate Ndel, the following equation was used:40

Ndel )

mon 3(∆νFWHM )2 J 2(∆νFWHM )2

-1

(3)

J where ∆νmon FWHM and ∆νFWHM are the full width at half-maximum (FWHM) of the monomer and J-aggregate absorption bands, mon ) 1200 cm-1 from ref 37 and respectively. Taking ∆νFWHM -1 J ∆νFWHM ) 650 cm for amphi-PIC J-aggregates, we obtain Ndel ) 4 monomer molecules (at room temperature). So, to achieve the trap (Sq-2Me molecule) embedded in the amphiPIC J-aggregate, an exciton has to overcome about 20 delocalization segments by the hopping mechanism.8 It would be interesting to compare our results on the efficiency of exciton transport in amphi-PIC J-aggregates with those obtained for other J-aggregates. However, such a comparison will be incorrect, because, in most works reported, the exciton transport was studied in J-aggregates formed in LB films or polymer layers,19–24,29–31 where electronic excitation is described within the frame of the 2D model of Frenkel excitons.20 In those cases, to describe the exciton migration, instead of a delocalization (coherence) length a coherent exciton domain is introduced.20 Thereby, we chose for the analysis works where authors examine exciton energy transport to an energy trap in CDP Jaggregates formed in a solution from polymer chains containing dye JC-1.25–28 Using data from ref 26, we can estimate mon ) 655 cm-1 and Ndel for such J-aggregates. Taking ∆νFWHM J -1 ∆νFWHM ) 250 cm for monomers and the most perfect J-aggregates (at polymer repeat units (PRU) ) 906), respectively, we obtain Ndel ) 9 monomer molecules. As follows from ref 26, 1 trap molecule quenches 50% luminescence of 104 JC-1 molecules which built the J-aggregate, so to reach the trap an exciton has to overcome about 11 delocalization segments. Thus, we can conclude that, despite the significant disorder degree observed in amphi-PIC J-aggregates,37 they reveal efficient exciton migration and confirm their suitability as artificial light-harvesting systems.

Conclusions In this paper, squaraine dye Sq-2Me has been examined as a trap of excitation energy in amphi-PIC J-aggregates in a binary DMF/water solution. In the binary solution with a high water content, a strong interaction between Sq-2Me and J-aggregates has been revealed. Even a small amount of Sq2Me (the ratio amphi-PIC/Sq-2Me ) 1000:1) causes J-

aggregate luminescence quenching and the appearance of Sq2Me sensitized luminescence. The efficiency of J-aggregate luminescence quenching by the trap has been analyzed using the modified Stern-Volmer equation. It has been found that only 80% of excitons reach the trap, whereas the others relax radiatively due to the small lifetime of excitons in Jaggregates. Fifty percent of the J-aggregate luminescence is quenched at the ratio of 1 Sq-2Me molecule per 80 amphiPIC monomer ones. To achieve the trap, an exciton has to overcome about 20 delocalization segments that is the evidence of efficient exciton transport. References and Notes (1) Balzani, V.; Credi, A.; Venmri, M. Molecular DeVices and Machines - A Journey into the Nano World; Wiley-VCH Verlag: Weinheim, 2003. (2) Kelsall, R. W.; Hamley, I. W.; Geoghegan, M., Eds. Nanoscale science and technology; John Wiley & Sons: Chichester, 2005. (3) McDermott, G.; Prince, S. M.; Freer, A. A.; HawthornthwaiteLawless, A.; Papiz, M.; Cogdell, R.; Isaacs, N. Nature 1995, 374, 517. (4) van Oijen, A. M.; Ketelaars, M.; Ko¨hler, J.; Aartsma, T. J.; Schmidt, J. Science 1999, 285, 400. (5) Leupold, D.; Stiel, H.; Teuchner, K.; Nowak, F.; Sandner, W.; ¨ cker, B.; Scheer, H. Phys. ReV. Lett. 1996, 77, 4675. U (6) Kirstein, S.; Da¨hne, S. Int. J. Photoenergy 2006, 20363. (7) Mobius, D. AdV. Matter. 1995, 7, 437. (8) Kobayashi, T., Ed. J-Aggregates; World Scientific Publishing: Singapore, New Jersey, London, Hong Kong, 1996. (9) Shapiro, B. I. Russ. Chem. ReV. 2006, 75, 433. (10) Maiti, N. C.; Mazumdar, Sh.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (11) Nu¨esch, F.; Moser, J. E.; Shklover, V.; Gra¨tzel, M. J. Am. Chem. Soc. 1996, 118, 5420. (12) Lang, E.; Sorokin, A.; Drechsler, M.; Malyukin, Y. V.; Ko¨hler, J. Nano Lett. 2005, 5, 2635. (13) Malyukin, Yu. V.; Efimova, S. L.; Sorokin, A. V.; Ratner, A. M. Funct. Mater. 2003, 10, 715. (14) Sundstro¨m, V.; Gillbro, T.; Gadonas, R. A.; Piskarskas, A. J. Chem. Phys. 1988, 89, 2754. (15) Spitz, C.; Daehne, S. Int. J. Photoenergy 2006, 84950. (16) De Rossi, U.; Da¨hne, S.; Gomez, U.; Port, H. Chem. Phys. Lett. 1998, 287, 395. (17) Scheblykin, I. G.; Sliusarenko, O. Yu.; Lepnev, L. S.; Vitukhnovsky, A. G.; Van der Auweraer, M. J. Phys. Chem. B 2001, 105, 4636. (18) Malyukin, Yu. V.; Tovmachenko, O. G.; Katrich, G. S.; Efimova, S. L.; Kemnitz, K. Mol. Cryst. Liq. Cryst. 1998, 324, 267. (19) Mo¨bius, D.; Kuhn, H. J. Appl. Phys. 1988, 64, 5138. (20) Tuszyn˜ski, J. A.; Jørgensen, M. F.; Mo¨bius, D. Phys. ReV. E 1999, 59, 4374. (21) Sato, T.; Kurahashi, M.; Yonezawa, Y. Langmuir 1993, 9, 3395. (22) Ishizawa, H.; Sato, T.; Sluch, M. I.; Vitukhnovsky, A. G.; Yonezawa, Y. Thin Solid Films 1996, 284-285, 134. (23) Nakajima, H.; Kometani, N.; Asami, K.; Yonezawa, Y. J. Photochem. Photobiol., A 2001, 143, 161. (24) Yonezawa, Y.; Yamaguchi, A.; Kometani, N. Phys. Status Solidi B 2005, 242, 803. (25) Jones, R. M.; Lu, L.; Helgeson, R.; Bergstedt, T. S.; McBranch, D. W.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 14769. (26) Lu, L.; Helgeson, R.; Jones, R. M.; McBranch, D. W.; Whitten, D. G. J. Am. Chem. Soc. 2002, 124, 483. (27) Achyuthan, K. E.; Bergstedt, T. S.; Chen, L.; et al. J. Mater. Chem. 2005, 15, 2648. (28) Whitten, D. G.; Achyuthan, K. E.; Lopez, G. P.; Kim, O.-K. Pure Appl. Chem. 2006, 78, 2313. (29) Dai, Z.; Da¨hne, L.; Donath, E.; Mo¨hwald, H. Langmuir 2002, 18, 4553. (30) Sakomura, M.; Takagi, T.; Nakayama, H.; Sawada, R.; Fujihira, M. Colloids Surf., A 2002, 198-200, 769. (31) Kobayashi, T.; Taneichi, T.; Takasaka, S. J. Chem. Phys. 2007, 126, 194705. (32) Ajayaghosh, A. Acc. Chem. Res. 2005, 38, 449. (33) Kim, S.-H., Ed. Functional Dyes; Elsevier: Amsterdam, 2006. (34) Sreejith, S.; Carol, P.; Chithra, P.; Ajayaghosh, A. J. Mater. Chem. 2008, 18, 264.

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