ARTICLE pubs.acs.org/Langmuir
Contrasting Behavior of Classical Salts versus Ionic Liquids toward Aqueous Phase J-Aggregate Dissociation of a Cyanine Dye Vinod Kumar,§ Gary A. Baker,† Shubha Pandey,† Sheila N. Baker,*,‡ and Siddharth Pandey*,§,† § †
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Department of Chemistry and ‡Department of Chemical Engineering, University of Missouri-Columbia, Columbia, Missouri 65211, United States
bS Supporting Information ABSTRACT: The effect of addition of ionic liquids (ILs) on the aggregation behavior of a cyanine dye, 5,50 ,6,60 -tetrachloro-1, 10 -diethyl-3,30 -di(4-sulfobutyl)-benzimidazolocarbocyanine (TDBC), was investigated. In basic aqueous buffer solutions (pH g 10), TDBC preferably exists in its J-aggregated form. Addition of hydrophilic ILs > 5 wt % is observed to disrupt the TDBC J-aggregates, converting them to monomer form most likely because of the interaction between bulky IL cation and the J-aggregates in a time-dependent fashion. This is evidenced by the observed increase in monomer band absorbance at the expense of the absorbance band due to J-aggregates over time. Inorganic salts at similar molar concentrations do not cause this phenomenon but instead induce TDBC precipitation. At low concentrations (5 wt % IL. The unique and dual behavior of ILs as an additive toward affecting cyanine dye aggregation is demonstrated.
’ INTRODUCTION The aggregation or self-association of cyanine dyes in solution or at solidliquid interfaces due to strong intermolecular van der Waals-like forces is an extensively studied phenomenon in chemistry.1 Often, a very narrow, intense absorption band (J-band), bathochromically shifted from the monomer absorption (M-band), arises from the formation of staircase-type head to tail arrangements (end-to-end stacking) known as J-aggregates. H-aggregates arising from plane-to-plane stacking into a sandwich-type arrangement are characterized by a broader hypsochromically shifted absorption band (H-band).2 For Jand H-aggregates, transition dipoles are aligned parallel and perpendicular, respectively, to the line connecting neighboring molecules in the aggregate.3 More precisely, J- and H-aggregates can be distinguished on the basis of slippage angle (α), i.e., the angle between the line of centers of a column of dye molecules and the long axis of any one of the parallel molecules. Large molecular slippage (α < 32°) and small slippage (α > 32°) results in a J-aggregation and H-aggregation formation, respectively.4 These aggregates are known to play an important role in many technological applications such as, photography, sensors, photoconductors, biology, medicine, nanotechnology, and for future light harvesting systems.5 Dye aggregates exhibit a strong coherent excitation phenomenon resulting in their ultrafast and high r 2011 American Chemical Society
nonlinear optical responses.6 For this reason, they are considered as potential nonlinear optical materials and have shown great promise for use in the field of opto-electronics.7 Further, their unique optical properties, such as fast electron transfer and long distance excitonic energy propagation processes, have led to their applications as optical switches, serial-to-parallel pulse converters, and heterojunction photovoltaic devices.8 Dye aggregates are able to store light energy and are able to release it essentially “on-demand”.8 The efficiencies of such energy and electron processes are known to be sensitive to the specific aggregation state of the dye.9 Recently, a class of fluorescent organic nanoparticles, termed GUMBOS (group of uniform materials based on organic salts), were developed based on a heptamethine cyanine dye.10 The spectral properties of these nanoparticles were based on the J/H aggregate ratio which the researchers were able to control based on the counterion used. Cyanine dye self-association has a complex dependency upon the dye (structure and concentration) and the environment (solvent polarity, pH, ionic strength, temperature, etc.).510 Water is one of the most favorable solvents for aggregation because water’s Received: July 26, 2011 Revised: September 16, 2011 Published: September 20, 2011 12884
dx.doi.org/10.1021/la203317t | Langmuir 2011, 27, 12884–12890
Langmuir Scheme 1. Structures of ILs and Dyes Used in This Study and the Abbreviations Used in the Manuscript Text
high dielectric constant (ε) reduces the repulsive forces between the charged dye molecules.11 The strong dispersion forces between the cyanine molecules in solution are due to the high polarizability of the π-electrons along the polymethine groups (at least three times as high as the polarizability of polyenes of comparable size due to the strong alternation of charge density).12 These dispersion forces between the polymethine chains serve as the main attractive forces for formation of extended aggregates of cyanines.13 Ionic liquids (ILs) have recently been demonstrated as a new medium for the process of dye aggregation.14 Toward this end, we have shown that cyanine dyes triggered to form fluorescent H-aggregates in IL containing BF4 anion, whereas J-aggregates were observed to form in ILs with other anions as 2 wt % 1 M aqueous NaOH was added; the difference in hydrolytic properties of [BF4] ILs over other ILs was hypothesized to be the reason.14a Due to their complete ionic structure and unique properties, ILs may offer advantages over conventional additives and molecular solvents in many aspects of chemistry.15 Though the environmentally benign nature of ILs is yet to be fully established, it is supposed that the main development in the field of ILs will come from the distinctive molecular architecture of these neoteric materials.16 In this paper, we demonstrate the unique role of ILs as additives in the conversion of J-aggregates of a cyanine dye into its monomers in aqueous milieu. Specifically, we show that three common water-miscible ILs efficiently convert preformed J-aggregates into monomers within basic buffer solutions. Addition of similar conventional (inorganic) salts, on the other hand, results in dye precipitation only. A dual role for ILs is observed where at low concentrations (99%, halides 0.98 in all cases) that the monomer growth and the J-aggregate decay, in the presence of the ILs at 20 wt %, can be considered to follow pseudo-first-order kinetics. Both the rate constants, kM,apparent and kJ,apparent for monomer growth and J-aggregate decay, respectively, are found to have a maximum value when the IL present was [bmim][BF4] and a minimum value when the IL [bmpyrr][OTf] was used. This suggests that, while the aromaticity of the IL cation or the possibility of ππ interaction between the IL cation and the J-aggregates may not be the essential factor for conversion to monomers, it appears to expedite this process. This is in line with similar observations reported earlier for J-aggregation of tetrakis(4-sulfonatophenyl)porphyrin.14 Next, the effect of the IL [bmim][PF6], which has low-water miscibility,24 was investigated up to the maximum [bmim][PF6] solubility limit (2.0 wt %). The absorbance spectra of TDBC in pH 12.0 PB with [bmim][PF6] indicates that, with increasing [bmim][PF6] wt%, AJ decreases, but AM remains almost constant (Figure 6, panel A). This result is similar to that of up to 5 wt % [bmim][BF4] where J-aggregate absorbance declines with no recovery of the monomer units. As discussed earlier, this may be due to the reduced solubility of TDBC as a consequence of the cation exchange from Na+ to [bmim]+.15b The TDBC behavior induced by these ILs at low concentrations (
