& Solvent Effects

Charge-Transfer Complex Formation in Gelation: The Role ofSolvent Molecules with Different Electron-Donating Capacities

Shibaji Basak,[a] Sumantra Bhattacharya,[b] Ayan Datta,[b] and Arindam Banerjee*[a]

Abstract: A naphthalenediimide (NDI)-based synthetic pep-tide molecule forms gels in a particular solvent mixture(chloroform/aromatic hydrocarbon, 4:1) through charge-transfer (CT) complex formation; this is evident from the cor-responding absorbance and fluorescence spectra at roomtemperature. Various aromatic hydrocarbon based solvents,

including benzene, toluene, xylene (ortho, meta and para)and mesitylene, have been used for the formation of the CTcomplex. The role of different solvent molecules with vary-ing electron-donation capacities in the formation of CT com-plexes has been established through spectroscopic andcomputational studies.

Introduction

The association of electron-rich and -deficient aromatic moiet-ies is an important tool for utilising molecular assembly in sol-ution[1a–h] and in the gel phase[1i–n] to make soft materials. Thesolid-state assembly of aromatic electron-donor and -acceptorsystems has been known for several decades. For example, thecrystal structure of benzene and hexachlorobenzene was re-ported as early the 1960s.[1d] Charge-transfer (CT) complex[1]

formation, involving electron-donor and -acceptor aromaticmolecules, can offer interesting applications in organic photo-voltaics.[2] Low-molecular-weight gels (LMWGs) are importantsoft materials[3] with various interesting applications in photon-ics,[3g–i] nanoparticle synthesis,[3j,k] hybrid systems,[3l–n] nanoclus-ter formation in the gel phase,[3] oil-spill recovery,[3p] amongothers.[3q–u] There are several examples of two-componentgels.[4] However, electron-donor–acceptor-mediated gels[4e–g]

belong to a special class of two-component gel, in which a CTcomplex is formed through partial CT from an electron-richdonor to the electron-deficient acceptor molecule. Such typesof CT complex formation leads to a visible colour changeduring complex formation. Donor–acceptor CT complexes sta-bilise the orientation of the functional molecular componentsto form a gel-phase material. NDI is a brilliant n-type semicon-ducting molecule with appealing properties, including molecu-

lar planarity, electron acceptability and characteristic redox be-haviour.[5] NDIs have potential applications in organic field-effect transistors (OFETs), photovoltaic devices, and flexible dis-plays.[6] NDI-based molecules can be easily assembled intoa gel[5a] or aggregate through p–p stacking interactions; thismakes NDIs interesting candidates for electron- and energy-transfer systems. The electronic properties of NDI are depen-dent on the nature of the interactions between constituentmolecules. Examples of NDI-based CT complex formation in or-ganogelation are rare.[7] However, the involvement of solventmolecules in CT complex based gel formation has not yet beenreported, to the best of our knowledge. It is interesting to ad-dress the issue of the involvement of a co-solvent in CT com-plex formation and gelation, and also to probe the role ofthese solvent molecules with varying electron-donation capaci-ties in CT complex based gel formation to examine which sol-vent system forms the strongest CT complex and which formsa weak CT complex in molecular self-association and gelation.In the course of our exploration of the self-assembly of NDI-based compounds,[8] we have encountered an interesting find-ing of the formation of a CT complex based gel of an NDI-con-taining peptide in chloroform/aromatic hydrocarbon based sol-vent(s), including toluene, xylenes and mesitylene. Thestrength of the formation of the CT complex (involving a NDI-containing, peptide-based acceptor and aromatic hydrocarbonbased donor solvent) depends on the electron-donating ca-pacity of the corresponding co-solvent molecules. Benzeneforms a weak complex with the NDI-based acceptor, whereasmesitylene forms the strongest CT complex.

Results and Discussion

NDI-containing peptide 1 (Figure 1 a) was first dissolved inchloroform at a concentration of 3 mm. After mixing aromatichydrocarbon solvents, such as toluene or xylenes (o-, m- andp-), into this NDI-appended peptide at a ratio 4:1 (chloroform/aromatic hydrocarbon), a CT complex forms as a light-yellow

[a] S. Basak, Prof. A. BanerjeeDepartment of Biological ChemistryIndian Association for the Cultivation of ScienceJadavpur, Kolkata 700032, IndiaFax: (+ 91) 332473-2805E-mail : bcab@iacs.res.in

[b] S. Bhattacharya, Dr. A. DattaDepartment of SpectroscopyIndian Association for the Cultivation of ScienceJadavpur, Kolkata 700032, IndiaE-mail : spad@iacs.res.in

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201303889.

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solution. These aggregated species in the solvent mixtureslowly form a yellow-coloured gel in toluene, o-xylene, m-xylene, p-xylene and mesitylene. However, it fails to formeither a yellow solution or any kind of gel in a mixture of ben-zene/chloroform. Interestingly, irradiation with UV light(365 nm) before gel formation intensifies the colour of the so-lution to deep yellow. The minimum gelation concentrationsare in the range of 2.7–3 mm for all of these gels (Table S1 inthe Supporting Information). These gels are thermo-reversiblein nature and the gel melting temperatures (Tgel) are in therange of 59–69 8C (Table S1 in the Supporting Information).The value of Tgel is the highest for the mesitylene gel andlowest for the o-xylene gel. The mixture of chloroform/toluene(4:1) slowly turns into a gel after one day; this indicates theformation of the CT complex involved in gel formation andprompted us to study systematically the donor–acceptor CTcomplex formation in various aromatic hydrocarbon solvents,including benzene, o-xylene, m-xylene, p-xylene and mesity-lene, with different electron-donation capacities. Although theNDI moiety is a very good charge acceptor, the electron-donat-ing abilities of these aromatic solvents, including benzene, o-xylene, m-xylene, p-xylene and mesitylene, have yet not beenstudied well. Generally, the donor molecule interacts with anacceptor through CT. Remarkably, in this study, solvent mole-cules interact with the gelator molecules through the forma-tion of CT.

To examine CT in gel formation in a mixture of solvents (tol-uene and chloroform), the gelator 1 (3 mm) was first dissolvedin chloroform and toluene was gradually added to this solutionin chloroform until a 4:1 ratio of chloroform/toluene wasreached. After mixing, the solution was colourless (Figure 1 c).The solution was heated to 80 8C for a few minutes in a waterbath and it was slowly allowed to come to room temperatureto check whether it formed a gel. The solution was kept atroom temperature for 6 h; a light-yellow colour was devel-oped, but there was no gel formation. However, after 24 h, the

yellow-coloured gel formed (Figure 1 b). Interestingly, thisyellow colour was intensified after UV irradiation at 365 nm for10 min (Figure 1 c). These observations support the formationof a CT complex involving the NDI-based gelator as an accept-or and solvent as a donor. UV radiation and/or application ofheat accelerated the CT process. Various aromatic co-solvents,such as benzene, o-xylene and mesitylene, were studied to in-vestigate the role of aromatic solvents in CT complex forma-tion (Figure 1 c) with different electron-donation capacities andalso to examine which solvent formed a strong CT complexwith the NDI-based gelator and which formed moderate orweak CT complexes with the gelator.

A light-yellow colour was instantly developed after mixingmesitylene into the solution of NDI-based gelator in chloro-form in a 4:1 ratio (chloroform/mesitylene). However, other co-solvents, including benzene, o-xylene, m-xylene and p-xylene,did not cause any instant colour change upon the addition ofsolvent to the solution of the gelator in chloroform in a 1:4ratio of hydrocarbon/chloroform. This indicates the presenceof a very strong interaction in CT complex formation betweenthe NDI-based gelator and mesitylene. After heating for a fewminutes and keeping the solution for several hours, the ben-zene-containing solution did not show any colour change.However, other aromatic solvent (o-xylene, m-xylene and p-xylene) containing solutions showed the development of a sim-ilar yellow colour change to that with toluene (Figure 1 c). In-terestingly, the intensity of the light-yellow colour developedin the solution in mesitylene initially remained the same afterheating, and even after storage for more than one day afterheating. The colour of these solutions in benzene and mesity-lene was not intensified after UV irradiation (Figure 1 c). TheTEM image of the gel obtained from a mixture of chloroformand toluene showed a nanofibriller network structure (Fig-ure 1 d). Gels obtained from other solvent systems (such asmixtures of chloroform/o-xylene, chloroform/mesitylene andchloroform/p-xylene) also show similar three-dimensionalnanostructures (Figure S1 in the Supporting Information). All ofthese nanofibres are several micrometers long and 10–30 nmwide.

The soft solid-like gel formation is also supported by a rheo-logical study (Figure 2 and Figure S2 in the Supporting Infor-mation). These figures clearly show that the storage modulus(G’) is higher than the loss modulus (G’’) for all of these gelsobtained from different mixtures of aromatic hydrocarbon sol-vents and chloroform (1:4). No cross-over point is observedwithin the experimental frequency region (10–100 rad s�1) ; thisindicates the dominance of the elastic property in the gel-phase material.[9] The stiffness of these gels is similar ; this isevident from the rheological study. The highest stiffness (Fig-ure S2 in the Supporting Information) is observed for the gelobtained from a mixture of o-xylene and chloroform (1:4).

UV/Vis spectroscopy was used to investigate the CT phe-nomenon. Figure 3 clearly shows the appearance of the CTbands of gelator 1 in different solvent mixtures in a ratio of 1:4after heating and storage at room temperature for severalhours. Interestingly, the UV spectra of these CT complexes areremarkably different in the gel state than in their correspond-

Figure 1. a) The chemical structure of gelator 1. b) Photographs of 1 in purechloroform (i) and the CT gel formed after aging for 1 day in a mixture ofchloroform/toluene (ii). c) Solutions of 1 in chloroform and different aromaticsolvent mixtures (4:1). d) The TEM image obtained from the gel of 1 in a mix-ture of chloroform and toluene (4:1).

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ing initial solution states. In a mixture of toluene/chloroform(1:4) of the NDI-based gelator 1, the UV/Vis spectrum showsthe appearance of a broad band at around 450 nm in the visi-ble region (Figure 3 b). This is a characteristic band of CT com-plex formation between electron-donor and -acceptor mole-cules. The intensity of the CT band increases sharply with time,which indicates an increase in the number of CT species withtime. Similar UV/Vis experiments were performed by usingother aromatic co-solvents, such as o-xylene, m-xylene and p-xylene (Figure 3 c and Figure S3 a–b in the Supporting Informa-tion). In the benzene-containing solution, no CT band formed(Figure 3 a). In the case of the mesitylene-containing solution,no characteristic CT band was observed in the UV/Vis spectrum(Figure 3 d). The intensity of the CT band was highest for o-xylene compared with other aromatic hydrocarbon solvents at

the same concentration (3 mm) of the NDI-based gelator after24 h. The intensity ratio of the CT band after 24 h follows theorder benzene<mesitylene<p-xylene<m-xylene< toluene<o-xylene (Table 1) at the same concentration of NDI-based ge-lator 1. An increase in the concentration of 1 causes a consider-able reduction in the gelation time. A 6 mm solution of thepeptide gelator in a mixture of o-xylene and chloroform (1:4)gels within 14 h. The UV/Vis spectra of the gelator at differentconcentrations show an increase in the intensity of the CTband with an increase in the gelator’s concentration (Figure S4in the Supporting Information).

To find out the effective ratio of gelator to aromatic solventin chloroform for CT complex formation as well as gelation, theamount of toluene was increased gradually in the 3 mm solu-tion of gelator 1 in chloroform. The titration curve obtained byplotting the change in absorption with respect to the increasein molar equivalents of toluene was monitored at 450 nm. Thetitration curve shows that the 1:1 mixture of gelator and tolu-ene forms a very weak CT complex (Figure S5 in the Support-ing Information). The addition of more toluene (1–300 equiv)results in a very sharp increase of the CT band. This indicatesthe quantitative formation of the non-covalent CT complexwith no gelation (Figure S5 in the Supporting Information).After the addition of 600 equivalents of toluene (1:4, toluene:chloroform), gel formation occurs and a change in the slope ofthe curve is observed. This observation clearly indicates thattoluene acts not only as an aromatic donor for the formationof a CT complex, but also serves as a solvent required for gela-tion.

To gain more insights into the association of gelator–solvent,a circular dichroism (CD) study was performed. Just aftermixing aromatic hydrocarbon solvents into the solution of 1 inchloroform, the gelator forms a weak CT complex, which is evi-dent from the characteristic absorption feature and the pres-ence of weak CD signals (Figure S6 in the Supporting Informa-tion) at around 360 and 380 nm. However, after one day,a strong CT complex is formed in different mixtures of aromatichydrocarbon solvent–chloroform and corresponding CD signalsare enhanced with positive maxima at 360 and 380 nm. TheCD spectrum of 1 in a mixture of mesitylene/chloroform showsnegative maxima at 360 and 380 nm after CT complex forma-tion.

The stability of complex formation is the highest for theNDI-based gelator/mesitylene pair due to the presence ofthree electron-donating methyl groups. An instant colour

Figure 2. Plot of storage modulus (G’) and loss modulus (G’’) versus angularfrequency of the gel obtained from 1 in a mixture of chloroform/toluene(4:1) at 25 8C.

Figure 3. UV/Vis spectra and CT bands of the donor–acceptor complexes of1 (3 mm) in a) benzene, b) toluene, c) o-xylene, and d) mesitylene at a 1:4ratio of solvent/chloroform.

Table 1. Absorption at a fixed wavelength (450 nm) and calculated molarextinction co-efficient of CT complexes formed in different aromatic hy-drocarbon solvents after UV exposure.

Solvent Absorbance e [L mol�1 cm�1]

benzene 0.062 24.8toluene 0.728 291.2o-xylene 1.117 446.8m-xylene 0.606 242.4p-xylene 0.585 234.0mesitylene 0.139 55.6

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change was also observed for the formation of this type ofcomplex. However, this colour did not intensify with time,even after UV irradiation. This observation is supported by theunchanged UV spectrum with respect to time. This is oppositeto the Mulliken CT theory, which predicts that the stability ofthe CT complex is directly proportional to the extinction coeffi-cient in UV/Vis spectroscopy studies.[10a] However, this type ofobservation can be explained in terms of a contact CT spec-trum observed by previous research groups.[10] Benzene didnot form any CT complex with the NDI-based gelator. Tolueneand xylenes (o-, m- and p-) form the CT complex easily andwith moderate stability. Among the o-, m- and p-xylenes, o-xylene shows maximum absorption due to its high dipolemoment compared with its meta and para isomers.

The solution of the NDI-containing gelator in chloroformshows a very feeble fluorescence;[8] however, a mixture of1 (3 mm) in chloroform and toluene shows a significant fluo-rescence band at 495 nm (Figure 4 b). The intensity of this

peak changes slightly with respect to time. After 24 h, thispeak red-shifted to 515 nm with the appearance of a newpeak at 540 nm. The presence of these two new peaks indi-cates the formation of a CT complex involving toluene and theNDI moiety. Previous studies also demonstrated the presenceof a fluorescence peak for CT complex formation between thecorresponding donor and acceptor chromophore.[1d,e] A similarchange in the photoluminescence spectra was observed in thepresence of o-xylene/m-xylene; this suggested the formationof a CT complex. In a mixture of o-xylene/chloroform (1:4), twopeaks appeared at 510 and 540 nm (Figure 4 c). The peak at510 nm gradually diminished in intensity over time and a red-shift of the peak at 540 nm to 550 nm took place. Similar re-sults were observed for a mixture in m-xylene/chloroform (seeFigure S7 a in the Supporting Information). However, in ben-

zene, a fluorescence peak appeared at 445 nm with a shoulderat 548 nm (Figure 4 a). Neither of these peaks change withtime. In the presence of p-xylene (Figure S7 b in the SupportingInformation) and mesitylene (Figure 4 d), initially the majorpeak appeared at 548 nm with a shoulder at 510 nm. In thecase of p-xylene, the major peak slightly shifted to longerwavelength with a decrease in intensity over time. However, inmesitylene, the intensity of the major peak remained the samewith the disappearance of the shoulder.

A FTIR study was performed to examine the role of hydro-gen bonding in CT complex formation (see Figure S8 in theSupporting Information). In a mixture of toluene and chloro-form (1:4), two peaks were observed at 3304 and 3404 cm�1

after aging for 1 day followed by UV irradiation (365 nm). Thefirst peak at 3304 cm�1 corresponds to the hydrogen-bondedN�H stretching frequency, whereas the other one (3404 cm�1)is attributed to the non-hydrogen-bonded N�H stretching fre-quency. The simultaneous existence of these two peaks clearly

indicates that all amide N�H groups present in theNDI-based gelator are not hydrogen bonded amongthemselves. The presence of non-hydrogen-bondedN�H groups indicates the interaction of aromatic hy-drocarbon solvents with the gelator to form the CTcomplex. In other solvent systems, including o-xylene, m-xylene, p-xylene and mesitylene, similar ob-servations were found; this indicated the presence ofboth hydrogen-bonded and non-hydrogen-bondedN�H groups. However, in benzene, all N�H groupsare hydrogen bonded, which indicates the absenceof a CT complex.

To understand the nature of the interaction, andhence, CT between the solvents and the molecule,we performed DFT[11] calculations on these systems.Because first-principles calculations of substitutedNDI are computationally expensive, we chose pristineNDI as a model for the substituted compound. Weused the Gaussian 09 suite of programs[12] for all ofour calculations. We applied DFT[11] at the meta-GGAhybrid M06-2X[13] level. The M06-2X functional hasbeen a method of choice for the study of aromaticstacked systems, in which middle-range interactionsare important.[14] The Pople’s Gaussian 6–31 G + (d, p)

basis set[15] was used for all molecules. The structures of the in-dividual molecules (NDI, benzene, toluene, o-xylene, m-xylene,p-xylene and mesitylene) along with their dimers (NDI···ben-zene, NDI···toluene, NDI···o-xylene, NDI···m-xylene, NDI···p-xylene and NDI···mesitylene) were optimised. Additional fre-quency calculations were performed to ensure that thesestructures were local minima and not saddle points. Basis setsuperposition errors (BSSEs) were eliminated by the CounterPoise correction scheme.[16] In Table 2, the binding energy (inkcal mol�1) and centre···centre distances (in �) between NDIand the solvent molecule, and the Mulliken charge distributionin the systems are shown. From the results given in Table 2, itis clear that the binding energy of NDI and benzene is lowest(�21.0 kcal mol�1) and highest for NDI–mesitylene (30.0 kcalmol�1). The centre···centre distance between NDI and mesity-

Figure 4. Photoluminescence spectra of 1 in a) benzene, b) toluene, c) o-xylene, andd) mesitylene at a ratio of 1:4. Concentration of 1 maintained at 3 mm.

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lene is 3.25 �, which is smaller than that for the NDI and ben-zene complex (3.32 �). Mulliken charge analyses show that, forevery case, there is ring to ring CT. Figure 5 shows the opti-mised structures of NDI–benzene and NDI–mesitylene. The NDIunit behaves as an acceptor with a net negative charge (dueto the presence of the four electron-withdrawing carbonylgroups) and the solvent molecules act as the donor. Theextent of charge transfer (qCT) in NDI–benzene is low (j0.181 j),but high in NDI–mesitylene (j0.225 j) ; this is indicative of effi-cient CT within the aromatic rings through strong p–p-stackinginteractions for the NDI···mesitylene complex.

Computational studies have vividly shown that the interac-tion between NDI and mesitylene forms the strongest CT pair,whereas NDI–benzene forms the weakest CT complex. Thestrength of the CT complex formation is in the following order:benzene< toluene<m-xylene<p-xylene = o-xylene<mesity-lene. It is evident that experimental results match well with thecomputational studies.

Conclusion

An NDI-based peptide gelator forms CT complexes with differ-ent aromatic hydrocarbon based solvents, including toluene,o-xylene, m-xylene, p-xylene and mesitylene, in a 4:1 mixtureof chloroform/aromatic hydrocarbon solvent. CT complex for-mation ultimately results in supramolecular gelation. Herein,a unique example of the involvement of solvent molecules inCT complex formation with gelator molecules has been report-ed for the first time, to the best of our knowledge. Spectro-scopic (UV/Vis and fluorescence spectroscopy) and computa-tional studies were used to characterise the formation of these

CT complexes. Interestingly, different electron-dona-tion capacities of these solvent molecules play an im-portant role in CT complex formation with differentstrengths; this is evident from spectroscopic andcomputational studies. This study holds future prom-ise for making new gel-based soft materials throughthe modulation of the strength of CT complex forma-tion.

Experimental Section

Transmission electron microscopy (TEM)

TEM images were recorded on a JEM 2010 electron mi-croscope at an accelerating voltage of 200 kV. A drop of dilute so-lution of the gel-phase material was placed on carbon-coatedcopper grids (300 mesh) and dried by slow evaporation. Each gridwas then allowed to dry in a vacuum for 2 days before the imageswere taken.

UV/Vis spectroscopy

UV/Vis absorption spectra were recorded on a Hewlett-Packard(model 8453) UV/Vis spectrophotometer (Varian carry 50.bio).

PL spectroscopy

Fluorescence studies of the hydrogel in quartz cuvette were per-formed on a PerkinElmer LS55 Fluorescence Spectrometer instru-ment. The gel sample in a quartz cell of 1 cm path length was ex-cited at l= 340 nm and emission scans were recorded from l=350 to 750 nm.

FTIR spectroscopy

All FTIR spectra of dried gels were recorded as KBr pellets ona Nicolet 380 FTIR spectrophotometer (Thermo Scientific).

Rheology

The rheology experiments were performed by using an Anton PaarModular Compact Rheometer MCR 302 at 25 8C. Gels were pre-pared in a mixture of chloroform/aromatic hydrocarbon solvents(3 mm).

Determination of Tgel

Determination of the gel melting temperature was performed byheating gels in a thermostatically controlled water bath at a heatingrate of 2 8C/5 min until the gel melted. The calculated error rangein Tgel determination was �1 8C.

Acknowledgements

S.B. acknowledges CSIR and DST, India, for financial assistance.A.D. thanks DST, CSIR-EMR and INSA for partial funding. Com-putational time on the IBM-P7 machine is duly acknowledged.A.B. acknowledges the unit of Nano-Science at IACS.

Keywords: charge transfer · gels · self-assembly ·semiconductors · solvent effects

Table 2. Binding energies, equilibrium distances and Mulliken charge analysis of NDIand solvent.

NDI–Solvent Binding energy Equilibrium Mulliken charge analysis[kcal mol�1] (BSSE corrected) distance [�] NDI Solvent molecule

NDI–benzene �21.0 (�19.8) 3.32 �0.181 + 0.181NDI–toluene �23.6 (�22.3) 3.33 �0.171 + 0.171NDI–o-xylene �27.4 (�26.0) 3.17 �0.192 + 0.192NDI–m-xylene �26.3 (�24.9) 3.71 �0.235 + 0.235NDI–p-xylene �27.5 (�26.0) 3.29 �0.238 + 0.238NDI–mesitylene �30.0 (�50.5) 3.25 �0.225 + 0.225

Figure 5. Optimised structures of a) NDI–benzene and b) NDI–mesitylene.

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Received: October 4, 2013

Revised: January 28, 2014

Published online on && &&, 0000

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& Solvent Effects

S. Basak, S. Bhattacharya, A. Datta,A. Banerjee*

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Charge-Transfer Complex Formation inGelation: The Role of SolventMolecules with Different Electron-Donating Capacities

To gel or not to gel : A naphthalenedi-imide-based peptide molecule formsa charge-transfer complex with differentaromatic hydrocarbon solvents, includ-ing toluene, xylenes, and mesitylene, ina particular mixture (chloroform/aromat-ic hydrocarbon, 4:1). The role of sol-vents with different electron-donatingcapacities in the formation of CT com-plex with different strengths has beenestablished (see figure).

Chem. Eur. J. 2014, 20, 1 – 7 www.chemeurj.org � 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7 &&

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