Solvent-Assisted Enhanced Emission of Cationic Perylene Diimide

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Solvent-Assisted Enhanced Emission of Cationic Perylene Diimide Supramolecular Assembly in Water: A Perspective from Experiment and Simulation Kausik Bag,† Ritaban Halder,‡ Biman Jana,*,‡ and Sudip Malik*,† †

School of Applied and Interdisciplinary Sciences and ‡School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India

J. Phys. Chem. C Downloaded from pubs.acs.org by WEBSTER UNIV on 03/04/19. For personal use only.

S Supporting Information *

ABSTRACT: We report a solvent-assisted increase in the emission of a cationic perylene diimide derivative, which is water-soluble in nature. Favorably, all the emission-assisting solvents used in all the experiments are water-miscible such as ethanol, tetrahydrofuran (THF), or dimethyl sulfoxide (DMSO). The preliminary assumption of associated fluorescent assembly of perylene stacks (via spectroscopic analysis) has been further explored with dynamic light scattering (DLS), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and time-correlated single photon counting (TCSPC) techniques. Spectroscopically observed drastic enhancement in the emission intensity seems to be born out of a major change in the assembly pattern of the interacting hydrophobic surfaces of perylene derivatives. These changes further have a macroscopic impact on the DLS or powder XRD outcomes. Furthermore, molecular dynamics simulations have been performed to interpret the nature of aggregation of perylene oligomers in water and in an ethanol/water binary mixture. The aggregation propensity (H-type dimer) of the dye molecules is found to decrease with increase in the ethanol concentration. It has been revealed that in water, the association of perylene-based oligomers is entropic in nature. In contrast, the association is driven via enthalpy in ethanol. Moreover, if we extend the oligomer size, the enthalpic stabilization for this unfamiliar supramolecular assembly of perylene moiety seems to be enhanced greatly. These results along with the calculated molar extinction coefficient of monomer and dimer provide the molecular rationale behind the observed solvent-assisted increase of emission intensity. ionic functionality with the nonpolar segment of the π-surfacebased system.17−19 However, achieving better solubility and analyzing the supramolecular assembly only in aqueous medium still remain as a fresh field to be nurtured.20,21 Unfortunately, in most cases, the fluorescence properties of the perylene-based derivatives are decreased because of aggregation as they come into contact with polar water molecules.7,11 Considering it as a problem-solving tool, many groups have targeted to prepare a highly soluble perylene derivative with good quantum yield. Apart from previously mentioned functional modifications at the imide position of PDIs, aggregation of the hydrophobic π-surface is prohibited and it eventually provides a good fluorescence quantum yield also in an aqueous environment.22−26 Aggregation in aqueous medium can also be directly hampered by the introduction of core twisting of perylene at the molecular level.27 The assembly study of perylene-based molecules in aqueous

1. INTRODUCTION In the arena of supramolecular self-assembly, perylene diimides (PDIs) have cultivated extensive interest because of its primitive structure−property relationship which enables scientists to use it as a basic functional building block.1−3 The outstanding thermal and photophysical properties make this π-surfaced system unique in spite of the limitations in solubility in different solvents.1 Such a self-assembled π-system has multidimensional applications ranging from basic organic electronics to regulating as well as monitoring of biological process.4,5 Because of its limited solubility in hydrophilic solvents, the supramolecular self-assemblies of PDIs are mainly studied in organic medium. In a nonaqueous solvent, there are already different ways of supramolecular engineering to have simple to complex architecture utilizing noncovalent interactions such as π−π staking and hydrogen bonding.6−12 The problem of poor solubility of PDI chromophores in polar solvents has been addressed with different modifications such as inserting hydrophilic dendrimer, long peptide chain, or glycol at imide positions.13−16 As an extension of mimicking the cell membrane architecture via a supramolecular approach, the design of bola-amphiles has been adapted by attaching © XXXX American Chemical Society

Received: November 14, 2018 Revised: February 18, 2019

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DOI: 10.1021/acs.jpcc.8b11054 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

2.3. Synthesis of PMI. Perylene-3,4,9,10-tetracarboxylic dianhydride (0.50 g, 1.28 mmol) was taken in a 5 mL 1:1 water/isopropanol mixture and was degassed with nitrogen three times. N,N-dimethylethylenediamine (0.35 mL, 3.2 mmol) was added to it and the mixture was heated at 80 °C overnight. The resulting red precipitate was washed with 1 N NaOH solution to discard the unreacted residue and then with methanol. The crude product was recrystallized from 2:1 DCM/MeOH to get a small needle-shaped crystal of PMI. Finally, these crystals were dissolved in 2 N HCl and dried to get the final product.

medium and their diverse applications can generate a plethora of advantages mainly in the biological arena.28−30 How alcohols and aqueous alcoholic mixtures alter the extent of hydrophobic interactions of a hydrophobic system is a topic of interest in recent times.31−34 Often, molecular dynamics simulations have been utilized to elucidate the influence of aqueous binary mixtures on pair hydrophobicity,35,36 polymer structure, and protein folding.31,37 To demonstrate this, we have rationally designed a completely water-soluble cationic perylene analogue [bis-(N,Ndimethylaminoethyl)perylene-3,4,9,10-tetracarboxylic diimide dihydrochloride (PMI)] that provides fluorescence enhancement of self-assembled systems in mixed solvent environments such as ethanol/water as well as in other water-miscible solvents. Molecular dynamics simulations have been used to find out the effect of water/ethanol on the aggregation process of PMI molecules as evidenced by light-scattering studies. To the best of our knowledge, there is no report where this type of phenomenon of induction of hydrophilic solvents has been used to create a cationic fluorescent self-assembled nanostructure from a completely water-soluble system.

2.4. Molecular Dynamics Simulation Study. Molecular dynamics simulations of PMI oligomers in water, ethanol, and water−ethanol binary mixture were carried out by using Gromacs4.6.5 package. The structure of the PMI monomer was modeled using automated topology builder38 software according to the GROMOS53a6 force field.39 Next, we have prepared different oligomer configurations (dimer, trimer, tetramer, and pentamer) using visual molecular dynamics software.40 To solvate the PMI oligomer in water, an extended simple point charge (SPC/E) water model41 was used, and to solvate the PMI oligomer in ethanol, the united atom force field of ethanol was considered. The standard protocols of molecular dynamics simulations were followed by performing a steepest descent method of energy minimization and then the system was set in an NPT ensemble (N, P, and T represent the number of molecules, pressure, and temperature, respectively). The system was sufficiently equilibrated, and the temperature and pressure were maintained by a Nose−Hoover thermostat42,43 and a Parrinello−Rahman barostat.44 Periodic boundary conditions were applied and electrostatic interactions were handled by a particle mesh Ewald method. To calculate the potential of mean force (PMF), the umbrella sampling method was utilized.45 First, the PMF profile of the PMI dimer in the water or ethanol or water/ ethanol binary mixture was estimated and the distance between them was varied from 0.3 to 2.5 nm at an interval of 0.1 nm. A small interval was chosen in order to ensure the sufficient overlap between each window with its neighbor. A harmonic force of 1000 kJ/mol/nm2 was used to maintain the particular distance at every window. An umbrella sampling simulation of 3 ns long was conducted at each and every window. The weighted histogram analysis method46 was used to construct the free-energy profile. The PMF profile was obtained from the free-energy profile after performing the entropy correction. Similar protocols were followed for trimer, tetramer, and pentamer. In the case of a trimer, the most stable conformation of the PMI dimer was chosen and the distance between the dimer and a PMI monomer was varied. In a tetramer, the distance between a PMI trimer and a PMI monomer was varied. In the case of a pentamer, the distance between the PMI tetramer and a PMI monomer was considered. The enthalpy and entropy calculations were performed by using the finite difference method. Details about the calculation protocol were followed from earlier studies.47,48

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Perylene-3,4,9,10-tetracarboxylic dianhydride and N,N-dimethylethylenediamine were purchased from Aldrich Chemical. Co., USA, and were used without further purification unless otherwise mentioned. Solvents ethanol (EtOH), chloroform (CHCl3), and dichloromethane (DCM) were obtained from Merck India and used after distillation. All other chemicals [sodium hydroxide (NaOH)] were purchased from Merck India and used as received. For spectroscopic studies, water (18 MΩ) obtained from a Millipore Milli-Q system was used. All experiments were performed at room temperature at about 25 °C. 2.2. Instruments and Techniques. 1H NMR spectra were recorded at 25 °C on 500/400 MHz with DPX spectrometers (Bruker). 1H NMR chemical shifts (δ) were reported in parts per million (ppm). Splitting patterns have been designated as s (singlet), d (doublet), and br (broad). UV−vis spectra of the solution were recorded with an Agilent (model 8453) UV−vis spectrophotometer with a quartz cell (path length 1 cm). Fluorescence studies of solutions were carried out with a Horiba Jobin Yvon Fluoromax 3 spectrometer. Powder X-ray diffraction (XRD) analysis was performed by using a Bruker AXS diffractometer using Cu Kα radiation (λ = 1.542 Å), scanned from 2° to 40° with a rate of 0.5 s/step. A field emission scanning electron microscopy (FESEM) instrument (JEOL, JSM 6700F) operating at 5 KV was used to understand the morphology of assembly, and samples were platinum-coated prior to scanning the image. Fluorescence lifetimes were measured by using a timecorrelated single photon counting fluorometer (Fluorecule, Horiba JobinYvon). The system was excited with a 375 nm NanoLED from Horiba Jobin Yvon having λmax at 440 nm. All solutions were prepared at room temperature (25 °C), and the solutions were allowed to equilibrate for 2 h before performing fluorescence anisotropy studies. Dynamic light-scattering (DLS) experiments were carried out in a Malvern instrument after filtering the solution with a Millipore syringe filter. The atomic force microscopy (AFM) topologies of the assembled species were recorded with an anatomic force microscope (Veeco, model AP0100) in noncontact mode after spreading the solution over a freshly cleaved mica surface. B


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Figure 1. (a) Absorption profile of PMI solution (3.2 × 10−5 M) with different % (v/v) of ethanol addition (inset: A0−0/A0−1 with ethanol percentage); (b) emission spectra of the same solution (excited at 480 nm, slit = 0.5/0.5, inset indicating I/I0 of the same). The inset at the bottom shows the picture of PMI solution under UV light in water only (left) and in a 40% ethanol/water mixture (right).

Figure 2. (a) DLS measurements in only water (black bars), in a 40% ethanol/water mixture (green bars), and in a 40% THF/water mixture (red bars); (b,e,d) AFM pictures of PMI in only water, in a 40% ethanol/water mixture, and in a 40% THF/water mixture, respectively; (c,f) FESEM images of PMI in only water and in a 40% ethanol/water mixture; and (g) CLSM image from a PMI 40% ethanol/water mixture.

3. RESULTS AND DISCUSSION

(THF), N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or glycerine. The broad absorption spectrum of PMI in aqueous medium has a maximum around 501 nm along with a shoulder at 538 nm (Figures 1a, S1, and S2). This can be considered as the signature of extended dimeric H-type assembly of the perylene core. The absorption band in water

3.1. Photophysical Study. PMI molecule is completely water-soluble because of its cationic nature in the di-imide positions. Except water, amphiphilic PMI has very limited or no solubility in other polar protic or aprotic solvents such as methanol, ethanol, acetonitrile, acetone, tetrahydrofuran C


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in the ethanol/water mixture. With time and with the evaporation of the solvent medium, those smaller assemblies are probably coinciding with the neighboring assembly to form bigger aggregates that can be visualized with sharp contrast in the FESEM image (Figure 2f). Interestingly, the higher-order spherically aggregated species have a diameter around 500− 600 nm associated with the emissive property that is nicely observed under the confocal microscope (Figure 2g). Therefore, the inherent nature of formation of fluorescent assembly interacting with other solvent molecules has been extended in the bulk assembly. The initial construction of a modified selfassembled structure which already has started in solution (proved via DLS in the solution phase) can be manifested up to the bulk macroscopic state. This stable state is capable of responding to XRD through a solid−solution interfacial support (proved via confocal microscopy). 3.3. XRD Study. X-ray powder diffraction measurement of the bulk powder sample is performed from 4° to 40° to monitor the packing pattern of the assembled structures received from the ethanol/water mixture. The XRD powder pattern of the PMI molecule has been previously developed by our group.61 The well-resolved five ordered reflection peaks (Figure 3) at 1.22, 0.63, 0.42, 0.30, and 0.25 nm corresponding

has a Franck−Condon ratio of 0.5 favoring the S 0−1 transition.49−51 The corresponding fluorescence spectrum shown in Figure 1b directs the presence of perylene monomers with a well-resolved vibronic structure. Therefore, in preliminary impression, we can assume that in water, PMIfree molecules are in dynamic equilibrium with their H-type assembled oligomeric species.52,53 However, with the addition of other water-miscible solvents (here, ethanol), the preassembled state seems to be modified and it allows the S0−0 transition at 528 nm over S0−1.at 493 nm. With the increase of the percentage of ethanol in the mixture, the emission intensity of the bright yellow solution is gradually increased and finally gets saturated with the proportion of the 50% (v/v) ethanol/water mixture. The amplification of the fluorescent quantum yield is about 940 times than that of the only aqueous solution of PMI (from 0.05 to 47%). The highly emissive nature of PMI in the ethanol/water mixture is initially presumed to be due to the disruption of cofacial π−π stacking of the H-type assembly. The calculated Stokes shift has also been decreased from 44 to 19 nm and it suggests the change in the nature of assembly. Generally, gradual addition of a poor solvent facilitates the formation of aggregated species, which may improve the emissive nature governed by an aggregationinduced enhanced emission process avoiding aggregationcaused quenching.54−57 In this situation, it has been hypothesized that induction of a new type of assembly is favored in the mixture of solvents; as a result, the enhancement of fluorescence intensity occurs. Moreover, well-structured emission spectra along with a mirror image Stokes profile for the disrupted fluorescent state indicate that the later state in the ethanol/water mixture is quite different from a conventional J- or H-type aggregation.58 3.2. DLS and Morphology. The surveillance of alternation in absorption maxima along with the concomitant visual change of the solution color from deep red in water to bright yellow in the mixture suggests that there is modulation in the packing arrangements in the assembly of PMI chromophores. To get further insights into the PMI self-assembly process, DLS measurement has been performed and morphological images of PMI solution in water and mixed solvents have been obtained.59,60 In an aqueous system, a single assembled state is present as observed in the absorbance profile which is essentially supported by a sharp DLS distribution maximizing around 160 nm (Figure 2a). It is already reported that the cationic PMI molecules generate the assembled structures, which are also responsible for showing liquid-crystalline property in this type of molecules.49 Nevertheless, the corresponding AFM (Figure 2b) and FESEM (Figure 2c) pictures, which have been taken by air drying from PMI aqueous solution (3.2 × 10−5 M), suggest some higher sort of ordering. It may happen because of the higher concentration sample during evaporation of water from it. Again in the DLS experiment, we observe that with the addition of ethanol, a broader distribution of bigger assemblies with a mean hydrodynamic diameter around 300−450 nm was reported. This variation in the size of assembly may serve as a direct evidence of the existence of another assembled species in mixed solution.4 This experiment has been repeated with the THF (another water-miscible solvent) addition to confirm the reproducibility of nano-self-assembly in the solution and it as usual supports the formation of ordered nanostructures dried from the dilute solution. The corresponding AFM images (Figure 2d,e) reveal the quite different mode of self-assembly

Figure 3. XRD of PMI powder received from water only (red line) and assembled structures received from the ethanol/water mixture (black line). Drying was initially performed under a pool of air and finally in vacuum.

to d, d/2, d/3, d/4, and d/5, respectively, apparently suggest the lamellar packing in the preassembled powder form. The distance corresponding to the (0, 1, 0) lattice plane is 1.22 nm, which is much shorter than the calculated molecular length of 1.94 nm, substantiating the extreme tilted packing of the πaromatic core. Therefore, the calculated breadth also requires to be increased to 0.78 nm (from a breadth of 0.48 nm), which also has a higher-order reflection at 0.39 nm. The threedimensional lamellar structure appears to be completely reframed in the different self-assembled state, which has been created via drop-casting from the ethanol/water mixture of the PMI molecule. The (0, 1, 0) and (0, 0, 2) planes have induced a shift to 1.55 and 1.01 nm, respectively, along with a broad range of amorphous belts, revealing an extended amorphous domain of π−π stacking.62 Such a contrasting change indicates D


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experiments. In the case of xEtOH = 0.07 (20% v/v), the freeenergy value of the PMI dimer at CM is ∼−43 kJ/mol, and for xEtOH = 0.26 (51.4% v/v), it is found to be −25 kJ/mol. Therefore, as the ethanol concentration in the ethanol/water binary mixture increases, the contact minima gradually destabilizes, which indicates that the PMI dimer forming tendency is relatively less favored at higher ethanol concentration. Therefore, as the concentration of ethanol in the ethanol/water binary mixture increases, the PMI molecules prefer to stay as monomers rather than dimeric assembled structures. In the inset of Figure 4, the representative structure of the dimer is shown, which is a H-type dimer. We have also found that although the association of the PMI dimer in water is entropic in nature, it is enthalpic in the ethanol (Figure S5). Therefore, our free-energy calculations also indicate that with the increase of ethanol concentration, the ratio between monomer and H-type dimer increases and it is in good agreement with experimental findings. 3.4.2. Calculation of the Theoretical Molar Extinction Coefficient of Monomeric and Dimeric PMI Molecules. To further correlate our theoretical results with the observed absorption spectra, we have calculated the molar extinction coefficient of the PMI monomer and dimer using the semiempirical ZINDO method (Figure 5). The calculated

that the process of self-assembly has a strong impact on the alteration of lamellar packing among the structural units.63,64 3.4. Molecular Dynamics Simulation Study. In order to perceive a comprehensive understanding of the spectroscopic signature of the aggregation process of this cationic perylene analogue (PMI) in water and in different compositions of the water/ethanol binary mixture, molecular dynamics simulation studies have been performed. 3.4.1. Variation of PMF as a Function of Distance between Two Monomers of PMI in Water and in an Aqueous Ethanol Mixture. In general, the modulation of absorption spectra of aggregation-prone dye molecules in an aqueous binary mixture with varying cosolvent composition was explained in terms of varying ratios of dimer to monomer in solution at equilibrium. In our case, the experimental absorption spectra have hinted at the possibility of H-type dimer formation in solution, and the relative content of dimer with respect to monomer gradually decreases with increasing ethanol concentration. To probe the relative ratio of dimer to monomer with varying composition of ethanol in an ethanol/ water binary mixture, we have calculated the distancedependent PMF of the PMI dimer in water and in an ethanol/water binary mixture. The PMF measures the freeenergy cost of bringing two hydrophobes from infinite separation to a particular distance. One often encounters (as here also) a deep minimum when two hydrophobes are in close proximity. This minimum is referred to as contact minimum (CM). Stabilization of contact minima shows that the molecules prefer to stay in close contact rather than staying apart from each other. In the present study, we have solvated two PMI molecules in water and in different concentrations of the ethanol/water binary mixture, and finally the PMF values of these have been calculated by varying the distance between the two PMI monomers. One can note that the CM of the PMI dimer in water is more stable as compared to that in the ethanol/water binary mixture. It indicates that the PMI dimer is more stable in water than in the water/ethanol binary mixture. At the CM, the free-energy value of the PMI dimer in water is ∼−57 kJ/mol (Figure 4). In the case of PMF calculation in the ethanol/water binary mixture, we have considered two different concentration regimes of the ethanol/ water binary mixture [xEtOH = 0.07 (20% v/v) and xEtOH = 0.26 (51.4% v/v)] in accordance with our above photophysical

Figure 5. Theoretical molar extinction coefficients of the monomeric and dimeric PMI molecule calculated by the semiempirical ZINDO method. Note that in the case of a monomer, there is a shift of the maxima toward higher wavelength compared to a dimer.

molar extinction coefficient for the PMI monomer has a peak at ∼488 nm and the same for the PMI dimer is found to be ∼460 nm. Our experimental results show that moving from water only to ethanol/water mixture, the intensity of higherwavelength peak increases. Therefore, in the mixture, the PMI monomer is favored rather than the PMI assembly, giving rise to a gradual increment of intensity at a higher-wavelength peak. Thus, it is safe to state that our theoretical results are in good agreement with the experimental findings. Additionally, to understand the stability of higher-order aggregates in pure water and pure ethanol, we have also performed thermodynamic analysis for PMI trimer, tetramer, and pentamer (results are provided in the Supporting Information, Figures S3−S6). 3.5. NMR Titration with DMSO-d6. Monitoring the nature of changes observed in nuclear magnetic resonance (NMR) signals, we can elucidate the deportment of different assembled species by creating diverse polarity of the solvent mixture.65 The observed increase of fluorescence intensity in the solvent mixture is initially stipulated by the deaggregation

Figure 4. PMF profiles of the PMI dimer as a function of distance in water and in ethanol/water binary mixtures. The structure of the PMI dimer in CM has also been shown in the plot. The errors of PMF shown here were calculated by the bootstrapping method. E


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The Journal of Physical Chemistry C of perylene cores from strong π-stacking. In 1H NMR of PMI in D2O, both the Hb and Ha protons have a very broad signal at δ = ∼7.48 and ∼6.99 ppm, respectively, because of the presence of a severe aggregated state of the perylene cores (Figures 6, S7, and S8). However, with the addition of other

the fact of the presence of severe aggregated state.26,66 The original NMR spectra can also be compared to understand the impact of addition of DMSO in the aqueous solution of PMI (Figures S7 and S8). 3.6. Time-Correlated Single Photon Counting. Finally, to understand the enhancement of emission intensity of PMI in the ethanol/water mixture with respect to aqueous solution, the time-correlated single photon counting (TCSPC) studies of 10−5 M PMI solution were performed with picosecond excitation at 440 nm and the emission was monitored at 548 nm, and the results were fitted to a biexponential decay profile in both cases (Figure 7a). In water, there are two exponential decays in the lifetime profile, entailing the presence of two components which are a short-lived (0.28 ns) and a long-lived (4.2 ns) species. Addition of other water-miscible solvents (such as THF and ethanol) may not help to increase the average lifetime much. A slight increase in the fluorescence lifetime of both short-lived (0.36 ns) and long-lived (4.9 ns) species is observed in the case of adding 40% THF. Species having short lifetime are ascribed to the aggregated moieties and long lifetime is for monomeric species.67,68 Therefore, it can be reasonably assumed that the increase in the fluorescent intensity is not only due to the autoconversion of the aggregated state to the free molecule of PMI. It has been further rectified by looking at the fluorescence anisotropy profiles of those systems (Figure 7b). The experimental rotational correlation time parameter is 0.27 ns for the assembly in water only, and in a 40% ethanol/water mixture, it elevates to 0.54 ns, which is almost double the value. In a lowviscosity solvent, this value should be near to zero, indicating the free rotation of monomers.69 However, upon the formation of an assembly structure, rotational diffusion should be slowed down by increasing the anisotropy decay time. By looking at the observed rotational correlation time, we can conclude the presence of aggregates both in water only and in water/ethanol mixtures. The rotational correlation time is almost doubled, indicating the presence of another distinct assembled state other than the previous state existed in aqueous condition only. It is worth mentioning that if there is only transformation from aggregated to monomeric PMI molecules, there might have much more increase in the fluorescence lifetime.67,70 Therefore, from the resulted similar fluorescent lifetime, we can strongly support the presence of assembly along with the free rotating molecule before and after the introduction of water-

Figure 6. 1H NMR titration experiments with different amounts of DMSO-d6 addition at the same concentration of 8 mg/mL.

water-miscible solvents such as DMSO-d6 in the range of 0 to 40% (v/v) (which is completely compatible in D2O), the broad signal of the Hb peak shifts to δ = 8.03 ppm and Ha to 7.81 ppm. The significant downfield shifts of signals of aromatic protons indicate the deshielding behavior of the same protons upon gradual addition of DMSO-d6. It has also been noticed (Figure S9) that DMSO also have a similar visual outcome (in the presence of UV light). Hence, in the event of enhancement of fluorescent intensity in a mixed solvent system from the preassembled nonfluorescent state of water, these NMR titration experiments have correctly pointed out the intermolecular interaction of PMI molecules in the course of reassembly process that enhances the overall fluorescence in the solution. Moreover, the residual broadness in the NMR peaks in both stages before and after d-solvent addition proves

Figure 7. (a) TCSPC lifetime of PMI in only water (black) and in a 40% water/ethanol mixture (red line) and (b) fluorescence anisotropy decay profile for PMI in only water (red line) and in a 40% water/ethanol mixture (green line). Both fitted and raw data are presented. F



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miscible solvent. However, the mode of assembly has different signatures as already detected in the photophysical profile where there is strong fluorescence amplification. 3.7. Probable Mechanism of Supramolecular Assembly. In the aqueous solution of PMI, there is already an established channel of dynamic equilibrium between H-type stacked dyes and free-cationic PMI molecules.27,51 The poor fluorescence property in pure water is due to the presence of self-assembled structures of H-type assembly, which is eventually responsible for the quenching of fluorescence. This statement is further manifested by the prevailed signatures observed in the absorbance spectroscopy and DLS results. However, with the addition of previously mentioned water-miscible solvents, the fluorescence intensity increases quite rapidly. From molecular dynamics simulation, it has been shown that as the size of the PMI oligomer assembly is increased in ethanol the more it is governed by enthalpic contribution. On the contrary, this type of aggregation-forming tendency decreases in water with the increase of oligomer assembly. Though experimentally it has also been seen that PMI is not quite soluble in ethanol, the abrupt increase of the fluorescence intensity in the mixture as well as finer vibronic structure in the absorption profile indicates either an increase of the free PMI molecules in the mixture or change in the assembly pattern from H type. However, the combined DLS and confocal laser scanning microscopy (CLSM) studies at this point have proved the existence of specific self-assembled PMI architectures, which are yellow emissive in character. A noteworthy feature is that the values of fluorescence lifetime are more or less fixed. It does mean that free PMI molecules are present in smaller extent and are in equilibrium with the self-assembled PMI architectures in the mixture. In addition, fluorescence anisotropy value is almost doubled for a mixed solvent system with respect to an aqueous solution of PMI and it is a result of higher-ordered emissive assembled species in mixed solution. Subsequent theoretical results also support the presence of bigger assembly where enthalpic contribution is dominant. Hence, we suspect that there is a definite change in the mode of assembly of PMI because of which there is an enhancement of the fluorescence intensity in the mixed solvent system.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b11054. Calculation of PMF, NMR spectra of PMI in D2O and DMSO-d6/D2O, absorption and emission studies in water and THF/water mixture, and so forth (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.J.). *E-mail: [email protected] (S.M.). ORCID

Biman Jana: 0000-0001-7684-1963 Sudip Malik: 0000-0001-5358-5653 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS SM acknowledges DST-SERB, India (file number: EMR/ 2016/005767), for the support. We thank the IACS central computing facility for providing computational support. R.H. is thankful to IACS for the fellowship. K.B. thanks Dr. S. Mondal and B. Biswas for TCSPC discussion. We are also thankful to S. Chakraborty and R. Manna for helpful discussion.

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REFERENCES

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4. CONCLUSIONS In summary, we have successfully demonstrated a simple and practical approach for fluorescent enhancement with amphiphilic solvent addition in water. Dramatic emissive assembly formation is hereby achieved in mixed solution by little modulation in the preassociated H-aggregated PMI system. PDI chromophores remain in the assembled stack and show absorption and emission signature as a highly fluorescent free monomer. At the molecular level, the difference in PMI assembly in mixed solvent is proved by NMR along with DLS, which is further supported via drastic change in the XRD. TCSPC also helps to understand the nature of species involved in both kinds of assembly. The molecular dynamics simulation unveils that PMI aggregation forming tendency in the ethanol/ water mixture is favored by enthalpic contribution. Therefore, we have tried to demonstrate a plausible mechanism of fluorescence enhancement associated with a change in the mode of assembly in the presence of other water-miscible solvents for the water-soluble PDI derivative. G


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Solvent-Assisted Enhanced Emission of Cationic Perylene Diimide

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