Reactive & Functional Polymers 70 (2010) 951–960

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Reactive & Functional Polymers

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Association phenomena of a chiral perylene derivative in solutionand in poly(ethylene) dispersion

Andrea Pucci a,b,⇑, Filippo Donati a, Samuele Nazzi c, Gloria Uccello Barretta a, Gennaro Pescitelli a,Lorenzo Di Bari a, Giacomo Ruggeri a,b

a Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italyb INSTM, U.d.R di Pisa, via Risorgimento 35, 56126 Pisa, Italyc Istituto per i Processi Chimico-Fisici – CNR, Area della Ricerca, via G. Moruzzi 1, 56124 Pisa, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 July 2010Received in revised form 30 August 2010Accepted 5 September 2010Available online 21 September 2010

Keywords:Chiral perylene derivativeSupramolecular stacked assembliesOptical propertiesPolyethylene dispersionsOptical indicators to thermal stress

1381-5148/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.reactfunctpolym.2010.09.001

⇑ Corresponding author at: Department of ChemisUniversity of Pisa, via Risorgimento 35, I-56126 Pisa,fax: +39 0502219320.

E-mail address: apucci@ns.dcci.unipi.it (A. Pucci).

In this work, the aggregation behaviour of a chiral perylene derivative, that is, N,N0-bis-(R)-(10-phenyl-ethyl)-perylene-3,4,9,10-tetracarboxyldiimide (R-Pery), was studied by means of 1H NMR (includingDOSY techniques), circular dichroism, UV–Vis and fluorescence spectroscopies in various conditions.The acquired data demonstrate that R-Pery tends to aggregate in pure solvents at high concentrations(>10�5 M in DMSO and >10�3 M in CHCl3), whereas aggregation occurred at lower concentrations by add-ing a poor solvent, such as water (to DMSO) or CH3CN (to chloroform). Interesting results were obtainedstudying the optical behaviour of R-Pery when dispersed into a linear low-density polyethylene matrix(LLDPE). In particular, the aggregation extent of R-Pery into LLDPE was investigated by means of UV–Vis, fluorescence and CD spectroscopies as a function of dye concentration as well as thermal stimuli.The optical responsiveness of R-Pery (both in absorption and in emission) versus moderate temperaturechanges (i.e., for T > 35 �C) suggests applications of polymer dispersions as smart and reversible indicatorsto thermal solicitations.

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1. Introduction recently reported the possibility to transfer the optical responsive-

Nowadays perylene derivatives represent an extremely impor-tant class of organic dyes which are employed in several applica-tions ranging from fluorescence standards [1–3], thin filmtransistors [4–6], liquid crystals [7–9], light emitting diodes [10],to photovoltaic devices [4,10–13].

Perylene consists of five fused benzene rings, yielding an ex-tended p-conjugated planar structure which is reported to favourthe formation of complex functional supramolecular architecturesas recently reviewed by Wuerthner [14,15]. Depending on theperipheral functionalities, perylene dyes can originate H- or J-typesupramolecular aggregates which are characterised by well-de-fined and different optical properties both in absorption and inemission [14–16]. Interestingly, the equilibrium between the iso-lated non-interacting form and the aggregated form of perylenechromophores is demonstrated to be easily perturbed by severalstimuli, triggered for example by chemical or temperature changesof the environment [17]. In connection with these findings we have

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ness of perylene derivatives to solid state materials: the aggrega-tion–disaggregation process for dispersed perylene dyes intopolymers appeared promising for the preparation of highly effi-cient optical indicators sensitive to different external stimuli[18–20].

A possible fruitful way to engineer even more efficient opticalplastic devices is represented by the use of chiral perylene deriva-tives, whose optical response may result more sensitive to confor-mational variations induced by the physicochemical solicitations.It is well-known that optical activity is very sensitive to structuralvariations. Moreover, chirality can be transferred from low-molec-ular weight chromophores into supramolecular assemblies whichoften show well distinct optical features and larger optical activitythan the isolated chromophores [21,22]. Moderate external pertur-bations can therefore easily perturb the chiral stacked structures ofdyes and provide modification of the macroscopic optical responseof the dye-doped material.

In this work, our interest was indeed focused on a chiralperylene bisimide derivative, N,N0-bis-(R)-(10-phenylethyl) pery-lene-3,4,9,10-tetracarboxyldiimide (R-Pery), already reported inliterature for the preparation of devices such as molecular chirop-tical switches [23,24]. Accordingly, R-Pery was obtained through acondensation reaction of perylene tetracarboxylic acid bisanhy-dride (PTCDA) with enantiopure (R)-methyl benzylamine. The

952 A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960

optical properties of the dye were first characterised in solutionupon dissolving R-Pery in different solvents like chloroform,DMSO, and in their mixtures with poor solvents such as waterand acetonitrile. Moreover, the ability of R-Pery to promote theaggregate formation was investigated by NMR, taking also advan-tage of bidimensional techniques like Diffusion Ordered Spectros-copY (DOSY), while the influence of the chirality on the stackingbehaviour was investigated with circular dichroism (CD)spectroscopy.

The optical properties of R-Pery were then transferred to poly-ethylene films (i.e., linear low-density polyethylene, LLDPE) by dis-persing the dye into the polymer matrix by thermo-mechanicalprocessing. The composite films containing different concentrationsof R-Pery (0.01–0.1% by wt.) were studied by absorption and emis-sion spectroscopies and the results are discussed in terms of theirapplication as plastic optical indicators to thermal solicitations.

2. Experimental

2.1. Materials

Perylene-3,4,9,10-tetracarboxy-dianhydride and imidazole weresupplied by Aldrich and used without further purification. (R)-1-phenylethylamine and benzylamine (Aldrich) were distilled beforeuse. Linear low-density polyethylene (LLDPE, Dowlex SC 2107, meltflow index 190 �C/2.16 kg 2.3 g/10 min, d = 0.917 g/cm3, supplied byDow Plastics, USA) was used as the polymer host matrix.

2.2. Synthesis of N,N0-bis-(R)-(10-phenylethyl)perylene-3,4,9,10-tetracarboxyldiimide

(R-Pery): (0.80 g, 2.0 mmol) of perylene tetracarboxylic dianhy-dride were dissolved in imidazole (18.0 g) at 90 �C. Then (0.56 mL,4.4 mmol) of (R)-1-phenylethylamine were added to the solutionand the mixture was warmed up to 180 �C and stirred for 4 h. Aftercooling to room temperature the solution was treated with water(10 mL), and then with 70 mL of HCl 2 N. The mixture was stirredfor 12 h and the resulting dark-red solid was filtered off, washed thor-oughly with distilled water until the pH of washings turned to be neu-tral and dried. The crude product was purified by chromatography onsilica gel using chloroform/ethyl acetate 20:1 by vol. as eluent; 0.99 g(1.6 mmol, yield: 83.2%) of a deep red solid were recovered.

1H NMR (CDCl3, 25 �C): d = 8.3 (d, J(H,H) = 7.9 Hz 4H; CH2); 8.0(d, J(H,H) = 8.1 Hz 4H; CH2); 7.3 (m, 5H; CH2); 6.5 (q,J(H,H) = 6.9 Hz 2H; CH), 2.0 (d, J(H,H) = 7.1 Hz 6H; CH3,) ppm.13C NMR (CDCl3, 25 �C) = 163.0 (CO), 140.0 (2C; benzene), 134.4(4C; perylene), 130.9 (4C; perylene), 128.5 (4C; benzene + 8C;perylene), 126.7 (2C; perylene + 6C; benzene), 123.5 (2C; pery-lene), 51.2 (CHaliph), 17.7 (CH3) ppm.FTIR (KBr): t = 3093 (tCH arom.), 2964, 2938 (tCH aliph), 1697,1657, 1593, 1577 (tCO imide), 1503, 1438, 1395, 1340 (tCCring) cm�1.UV–Vis (CHCl3): kmax = 527 nm (e = 80,640 L cm�1 mol�1).Emission (CHCl3, kexc = 300 nm): kem,max = 539.5 nm.Elemental analysis: calcd (%) for C40H26N2O4 (598): C 80.25, H4.38, N 4.68; Found C 79.3, H 3.9, N 4.0.Specific rotation: ½a�25

D ¼ 4:1�ðdioxaneÞ.

2.3. Synthesis of N,N0-dibenzyl-perylene-3,4,9,10-tetracarboxyldiimide(Pery-Bz)

(0.47 g, 1.2 mmol) 3,4,9,10-Perylenetetracarboxylic acid dian-hydride and benzylamine (1 mL, 0.019 mol), were dissolved in

20 mL of quinoline under nitrogen atmosphere and heated at180 �C for 5 h. After cooling to room temperature the solutionwas treated several times with methanol and with 20 mL KOHmethanolic 10% solution to remove the unreacted anhydride. Final-ly, the solution was thoroughly washed with distilled water untilthe pH of washings turned to be neutral and dried. The crude prod-uct was purified by dissolution in H2SO4 conc. and precipitationinto water. 0.43 g (0.75 mmol, yield: 63%) of a deep red solid wererecovered.

1H NMR (CDCl3, 25 �C): d = 8.2 (d, J(H,H) = 7.90 Hz 4H; CH2); 8.1(d, J(H,H) = 8.00 Hz 4H; CH2); 7.2–7.3 (m, 5H; CH2); 4.7 (m, 4H;CH2) ppm.IR (KBr): t = 1697, 1656, 1593, 1576 (tCO imide) 1494, 1404,1336 (tCC ring) cm�1.UV–Vis (CHCl3): kmax = 526 nm (e = 56,000 L cm�1 mol�1).Emission (CHCl3, kexc = 300 nm): kem,max = 543 nm.

2.4. LLDPE/dye dispersions

LLDPE blends were prepared in a Brabender plastograph mixer(mod. OHG47055, 30 cc) under nitrogen atmosphere by introduc-ing about 20 g of the polymer and 0.01–0.1 wt.% of the dye in themixer at 180 �C with a rotor speed of 50 rpm. After 10 min, themixing was stopped and the recovered materials was grinded atroom temperature by using an IKA MF10 analytical mill. The pow-der obtained was successively moulded between two aluminiumfoils under compression in a Collin-mod.200M press at 180 �C for5 min. After removal from the press, the films were allowed toreach room temperature slowly (�5 �C min�1). The films were ana-lysed after 2–3 days. The thickness of the obtained films was in therange of 80–150 lm. Samples were named by listing guest mole-cule (abbreviated as R-Pery), polymer (abbreviated as LLDPE), con-centration (in wt.%): e.g., LLDPE_R-Pery_0.02.

2.5. Apparatus and methods

1H and 13C NMR spectra were recorded with a Varian Gemini300 MHz on 5–10% CDCl3 (Aldrich, 99.8 atom% D) solutions. NMRspectra were registered at 25 �C and the chemical shifts were as-signed in ppm using the solvent signal as reference. FT-IR spectrawere recorded with a Perkin–Elmer Spectrum One spectrometeron dispersions in KBr. 1H NMR aggregation measurements wereperformed on a Varian INOVA 600 MHz spectrometer. The temper-ature was controlled to ±0.1 �C. 1H NMR chemical shifts are refer-enced to TMS as external standard. The 2D NMR spectra wereobtained by using standard sequences. DOSY experiments werecarried out by using a stimulated echo sequence with self-compen-sating gradient schemes, a spectral width of 8000 Hz and 64 K datapoints. Typically, a value of 500 ms was used for D, 1.2 ms for d,and g was varied in 30 steps (16 transients each) to obtain anapproximately 90% decrease in the resonance intensity at the larg-est gradient amplitudes. The baselines of all arrayed spectra werecorrected prior to processing the data. After data acquisition, eachFID was apodized with 1.0 Hz line broadening and Fourier-trans-formed. The data were processed with the DOSY Varian macro(involving the determination of the resonance heights of all thesignals above a pre-established threshold and the fitting of the de-cay curve for each resonance to a Gaussian function) to obtainpseudo two-dimensional spectra with NMR chemical shifts alongone axis and calculated diffusion coefficients along the other. CDspectra of solutions and polymer films were measured with a JascoJ-710 spectropolarimeter using quartz cells of various lengths tokeep maximum absorption below 1.5 units. UV–Vis absorption

A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960 953

spectra of solutions and polymer films were measured with a Per-kin–Elmer Lambda 650 spectrophotometer. Steady-state fluores-cence spectra of solutions and polymer films were acquiredunder isotropic excitation with a Perkin Elmer luminescence spec-trometer LS55 controlled by FL Winlab software and equipped withthe front-surface accessory. Both instruments are equipped withjacketed cell holders, the temperature control being within±0.1 �C. In the analysis of the absorption and emission data, thescattering contribution was corrected by the use of appropriatebaselines. Origin 7.5, software by Microcal Origin�, was used inthe analysis of the absorption and emission data. Elementary anal-yses were executed at the microanalysis laboratory at the Facultyof Pharmacy, University of Pisa. Thermogravimetric scans were ob-tained by means of a Mettler-Toledo Star-system TGA-SDTA-851under nitrogen flux, at a scan rate of 20 �C/min. The thermal behav-iour of a LLDPE film was evaluated by differential scanningcalorimetry (DSC) under nitrogen atmosphere by using a Mettler-Toledo StarE System, equipped with a DSC822c module. Opticalmicroscopy measurements were obtained with a Reichert Polyvaroptical microscope with crossed polarizers, equipped with a pro-grammable Mettler FP 52 hot stage. Solid-state drawings of thebinary films were performed at room temperature. The draw ratio(Dr), defined as the ratio between the final and the initial length ofthe sample, was determined by measuring the displacement of ink-marks printed onto the films before stretching.

Fig. 1. UV–Vis absorption (a) and fluorescence (kexc = 300 nm) (b) spectra of R-Peryin CHCl3 at different concentration and dependence of absorption maxima at490 nm with dye concentration (inset of panel 1a).

3. Results and discussion

N,N0-bis-(R)-(10-phenylethyl)-perylene-3,4,9,10-tetracarboxyl-diimide (R-Pery, Scheme 1) was obtained as a dark-red solid in agood yield (83.2%) through a condensation reaction of perylenetetracarboxylic acid bisanhydride (PTCDA) with (R)-1-phenyleth-ylamine.

The dye showed high melting point (higher than 300 �C) anddegradation temperatures of 401 �C (loss of the imide group) and530 �C, as measured by thermogravimetric analysis (TGA) in air.No thermal transitions attributed to conformational changes ofthe neat R-Pery dye were recorded by differential scanning calo-rimetry and optical microscopy investigations up to 200 �C.

3.1. Optical characterisation in solution

R-Pery dissolved in CHCl3 showed three pronounced absorptionpeaks (in the range 450–550 nm) and a shoulder at around 425 nm,which correspond respectively to the 0–0, 0–1, 0–2, and 0–3 vib-ronic components of the first p–p� transition [20,25]. A plot ofthe absorption maximum of R-Pery at 525 nm as a function of con-centration showed the typical linear Lambert–Beer behaviour(Fig. 1a) up to 50 lM concentration. The fluorescence spectra re-ported in Fig. 1b show the same peak structure in a mirror im-age-fashion to the absorption, with emission maxima at about543 and 576 nm. In particular, the red-shift of the 0–0 radiativetransition band by about 10 nm (from 534 nm to 543 nm) occur-ring with increasing R-Pery concentration was attributed to

Scheme 1. N,N0-bis-(R)-(10-phenylethyl)-perylene-3,4,9,10-tetrac

auto-absorption phenomenon [20]. These results suggest that inboth absorption and emission spectra no bands attributed to theformation of aggregates were detected in the range of concentra-tion investigated.

The UV–Vis absorption of R-Pery in DMSO appeared similar tothat in CHCl3, with absorption maxima in the range between 460and 530 nm (Fig. S1a). However, a deviation from the linearity ofthe plot of Abs492 versus concentration seemed to indicate theoccurrence of aggregation phenomena at high molarity, even ifartefacts associated with the high extinction cannot be excluded.The fluorescence spectra in DMSO (Fig. S1b) showed two emissionmaxima at about 553 and 581 nm. A red-shift of the 0–0 radiativetransition band of about 10 nm (from 544 nm to 553 nm) is alsoobserved for DMSO solutions upon increasing concentration.

arboxyldiimide (R-Pery) synthesis and protons assignment.

Fig. 2. UV–Vis absorption (a), fluorescence (kexc = 300 nm) (b) and CD (c) spectra ofR-Pery (2 � 10�3 M) in pure CHCl3 and in different%v of CH3CN; (cell = 0.01 cm).

1 For interpretation of color in Figs. 3 and 10, the reader is referred to the webversion of this article.

954 A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960

The ability of R-Pery to undergo aggregation as a consequenceof p–p stacking interactions among perylene chromophores wasinstead observed in solution obtained by dissolving the dye intoa mixture of solvent/poor solvent. In particular, when CH3CN (poorsolvent) was added to CHCl3 solutions (2 � 10�3 M), the absorptionfeatures of the dye progressively changed (Fig. 2a). The intensity ofthe 0–0, 0–1 and 0–2 transitions decreased, flanked by a progres-sive increase of the absorption tail at wavelengths above 550 nm.

Moreover, quenching of the emission of R-Pery (Fig. 2b) was ob-served in CH3CN-rich mixtures with chloroform suggesting that inthese conditions the perylene derivative molecules tend to aggre-gate due to the lower solubility in such a solvent mixture.

The effects of aggregation are more evident in the CD spectra(Fig. 2c), which evolve from the series of weak negative bands

observed in CHCl3 toward a more complex pattern. In particular,R-Pery dissolved in pure CHCl3 displays only a weak negative CDband between 400 and 550 nm, with distinct vibronic components,in analogy with the absorption spectrum. The g-factor (DA/A or De/e) is around 10�5, that is, as small as predictable for a strong elec-tric dipole-allowed transition perturbed by remote chirality ele-ments. The exciton coupling [26,27] occurring between theperylene and the phenyl p–p� transitions (below 260 nm) is ex-pected to give very weak signals in the 400–550 nm region, fortwo reasons: (1) the large energy difference between the two tran-sitions; (2) the unfavourable arrangement between the two transi-tion dipoles which are almost coplanar. On the contrary, in thesolution richest in CH3CN, a series of bands of alternating signs isapparent, the strongest of which attains a g value of 5 � 10�5.The spectrum may be interpreted as the superposition of the weaknegative CD of the isolated molecules to a moderate positive,vibronically structured, exciton couplet due to the aggregatescomposed of perylene units with a twisted arrangement. Exciton-coupled CD due to aggregate species is usually much stronger thanthe inherent CD of chiral perylene derivatives like R-Pery; such adistinctive chiroptical response renders CD a useful diagnostic toolfor the detection and characterisation of perylene aggregates[21,22,28–31].

A more extensive aggregation of molecules of R-Pery mediatedby p�p stacking interactions was promoted by the use of a solventmixture composed of DMSO and water as the poor solvent. Asshown in Fig. 3a, the dispersion of R-Pery into a mixture ofwater/DMSO 60:40 v/v, caused a dramatic change of the absorp-tion spectrum: two new absorption bands emerged at about 480and 540 nm, devoid of a clear vibronic structure, suggesting theformation of molecular dye aggregates caused by effective p�pstacking co-facial interactions among perylene nuclei. The twobands may be in fact assigned to the transitions from the groundstate to two (or more) exciton-split excited states [32–34]. Accord-ingly, a pronounced quenching of the emission of R-Pery (Fig. 3b)was observed in water-rich mixtures with DMSO. A progressive in-crease of water content in DMSO from 0% to 60% by vol. led to agradual transformation of isolated R-Pery molecules into theaggregate form, as shown by the corresponding increase in theemission quenching. It is interesting to observe that from 0% to20% water, the intensity of emission increased. In parallel, theemission colour detected from the mixture (Fig. 3c), under excita-tion with a long-range UV lamp, gradually moved from yellow1 (forthe free dispersed molecules, 0% water), to a brighter yellow (20%water), and eventually to a dull red (for the molecular aggregate,60% water).

In the case of DMSO/water mixtures, the water-promotedaggregation gave rise to a striking CD response as well (Fig. 4a).The very weak CD in pure DMSO evolved, from 30% water on, intoa clear-cut positive CD couplet with residual vibronic structure.Correspondingly, the g value increased by a factor 40, from�10�5 to 4 � 10�4. Interestingly enough, the CD of aggregate spe-cies appears with its maximum intensity already at 30–40% watercontent and does not increase upon further water addition, in ac-cord with UV–Vis absorption spectra of the same samples. Thissuggests that in the current conditions a single mode of aggrega-tion is viable.

In addition to solvent/non-solvent proportion, the aggregationof perylene units may be also triggered by temperature variation.Temperature-dependent CD spectra of R-Pery dissolved in 30%and 40% water/DMSO mixtures were therefore recorded (these lat-ter are shown in Fig. 4b). Upon heating, the CD couplet first

Fig. 3. UV–Vis absorption (a) and fluorescence (kexc = 300 nm) (b) spectra of R-Pery(1 � 10�5 M) in pure DMSO and in different% by volume of water. (c) Picture of thesame dispersions taken under irradiation at 366 nm for a series of DMSO/waterbinary solvents containing (from left to right) 0%, 10%, 20%, 30%, 40%, 60% of water.

Fig. 4. (a) CD spectra of R-Pery (1 � 10�5 M) in pure DMSO and in different% byvolume of water. (b) Variable-temperature CD spectra of R-Pery (1 � 10�5 M) in 40%water/DMSO (not baseline-corrected).

A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960 955

progressively decreased in intensity while retaining the sameshape as at ambient temperature, then it evolved into a new fea-ture indicative of a different aggregation mode; the transition be-tween the two sets of spectra occurred at 40 �C for 30% water/DMSO mixtures, and at 65 �C for 40% water/DMSO mixtures. Forthe samples in 40% water/DMSO below the critical temperature,plots of the CD intensities at three different wavelengths revealregular trends, which lend themselves to a quantitative treatment.Thus, we could verify the hypothesis that the aggregates present inwater-rich solutions consist of dimers. In fact, Van’t Hoff-type plotsfit well a dimerization model (Eq. (1) below). In particular, thesample 10�5 M in 40% water/DMSO led to the following parame-ters for the dimerization equilibrium at 298 K: DH0 = �13 ± 2 kcal/mol, DS0 = �35 ± 5 cal/mol K, DG0 � �2.6 kcal/mol, KD � 82 M�1. Itis noteworthy that an achiral analogue of R-Pery (N-benzyl substi-tuted) also tends to form dimeric aggregates in DMSO [35].

3.2. NMR characterisation of R-Pery in solution

NMR spectroscopy gave us the opportunity to analyze thebehaviour of more concentrated solutions (i.e., up to 20 mM) thanthose investigated by optical spectroscopies. 1H NMR (600 MHz,CDCl3, 25 �C) chemical shifts of R-Pery dissolved in deuteratedchloroform showed a remarkable dependence on the solution con-centration. In fact, perylene protons H-a and H-b (Scheme 1)underwent significant low-frequency shifts (Table 1) as the conse-quence of concentration increase from 0.1 mM to 20.5 mM, whichcould be attributed to reciprocal anisotropy effects produced byaromatic nuclei probably associated in a face-to-face stackingmode. Increased concentration affects the position of phenyl pro-tons H-c and H-d to a lower extent and caused an opposite effect,i.e., a high-frequency shift (Table 1).

Quantitative analysis of auto-association phenomena of R-Peryin solution was performed on considering that, at least in dilutedsolutions, aggregation could involve mainly fast equilibrating di-meric (D) and monomeric (M) species, as already discussed for CDexperiments (Eq. (1)). The dimerization constant (KD) depends onthe concentration C0 of the solution and on the molar fractions XM

and XD of the monomer and dimer, respectively, as showed in Eq. (2).

2M�D ð1Þ

KD ¼XD

2C0X2M

¼ XD

2C0ð1� XDÞ2ð2Þ

Table 1Dependence of 1H NMR (600 MHz, CDCl3, 25 �C)chemical shifts (d, ppm) of R-Pery on concentra-tion (from 20.5 mM to 0.1 mM).

Proton d (20.5 mM)–d (0.1 mM)

H-a �0.35H-b �0.19H-c 0.04H-d 0.03

Fig. 5. 1H NMR (600 MHz, 25 �C) spectra of R-Pery in (a) CDCl3 and (b) in themixture CDCl3/CD3CN (3:1).

956 A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960

In the fast-exchange conditions, the measured chemical shifts(dobs, Eq. (3)) are the average of the monomer (dM) and dimer(dD) limit chemical shifts, weighted for the respective molar frac-tions XM and XD.

dobs ¼ XMdM þ XDdD ð3Þ

which was obtained by combining Eqs. (2) and (3), was employed tofit chemical shifts dependence on concentration gradients of CDCl3

solutions of R-Pery progressively diluted from 20.5 mM to 0.1 mM.

C0 ¼1

2KD

ðdobs � dMÞðdD � dMÞðdobs � dDÞ2

ð4Þ

In this way a dimerization constant of KD = 21 ± 1 M�1 was ob-tained. Therefore a 35% maximum percentage of dimer is presentat the highest concentration studied (20.5 mM), whereas below1 mM concentration negligible amounts of dimer are present (lessthan 4%, Table 2).

This result was in keeping with the spectroscopic evidenceshown before, confirming that negligible aggregation occurredfor R-Pery in CHCl3 for concentration less than 10�3–10�4 M.

Diffusion Ordered SpectroscopY (DOSY) NMR technique hasconsiderable potential in the analysis of aggregation phenomenasince it provides a way to separate the different compounds pres-ent in solution based on the differing translational diffusion coeffi-cients [36,37]. In fact, self-aggregation phenomena, which give riseto an increase of molecular sizes, are expected to produce a de-crease of observed diffusion coefficients. Thus, DOSY maps of R-Pery in CDCl3 were analysed at two different concentrations,1 mM and 20.5 mM. In the diluted solution a diffusion coefficientof 6.7 � 10�10 m2 s�1 was measured, which decreased to the valueof 5.8 � 10�10 m2 s�1 in the 20.5 mM solution, reflecting the occur-rence of self-aggregation of R-Pery in highly concentrated CHCl3

solution.Self-aggregation phenomena were further enhanced by addi-

tion of increasing aliquots of CD3CN (Table S1) to the CDCl3 solu-tion of R-Pery (20 mM). As a matter of fact, very large chemicalshifts variations were detected (Fig. 5, Table S1).

Addition of CD3CN produced low-frequency shifts of all protons,which were once again in accord with a face-to-face aggregationmode. Increase of dimer amount consequent to the addition of0.25 mL of CD3CN was detected by DOSY analysis: a corrected dif-fusion coefficient of 4.8 � 10�10 m2 s�1 was obtained, lower thanthe value of 5.8 � 10�10 m2 s�1 measured for the CDCl3 solutionof R-Pery. The correction of diffusion coefficient value for the vis-cosity change produced by addition of CD3CN to the CDCl3 solution

Table 2Dimer molar fractions of progres-sively diluted solutions of R-Pery.

C0 (mM) XD

20.5 0.3510 0.265 0.151 0.040.1 0

was performed by comparing the diffusion coefficient of TMS, usedas reference, in CDCl3 and in the mixtures CDCl3/CD3CN [38].

Finally, the DOSY maps of R-Pery in pure DMSO (0.1 mM) and ina 50:50 DMSO/D2O mixture (Fig. S2) were analysed, again by usingTMS as a reference. The two corrected values for the diffusion coef-ficient of 2.3 � 10�10 m2 s�1 and 1.2 � 10�10 m2 s�1 were mea-sured, respectively. The two values were about the first twice thesecond, demonstrating that self-aggregation processes occur forDMSO/D2O mixture even for diluted R-Pery solutions (0.1 mM),similarly to what reported above for spectroscopic investigations.In addition, it is noteworthy that the aggregation process inDMSO/D2O mixtures caused dramatic changes in the 1H NMR sig-nals (Fig. 6).

3.3. Optical properties of LLDPE based films

Linear low-density polyethylene (LLDPE) films with a thicknessof about 100–150 lm and containing different concentrations of R-Pery (0.01–0.1 wt.% dye) were prepared by compression mouldingof the respective LLDPE/dye mixtures, obtained by blending thecomponents at 190 �C for 10 min in a Brabender type mixer. Thedispersion degree of R-Pery into LLDPE was evaluated by scanningelectron microscopy (SEM) (Fig. S3, a micrograph of a LLDPE_R-Pery_0.1 film, containing the highest R-Pery concentration), whichevidenced the absence of macro-sized aggregates of the guestmolecules.

3.4. Effect of dye concentration

Considering the apolar nature of the polyethylene matrix andthe relative poor solubility of R-Pery in heptane, it was not surpris-ing that LLDPE_R-Pery composite films displayed absorption fea-tures similar to those observed for R-Pery in solvent–non-solventmixtures, even at low dye concentration (0.01 wt.% of R-Pery). Be-sides the typical absorption peaks of the isolated non-interactingchromophores at 480 and 520 nm, an unstructured band centredat 549 nm emerged upon increasing the concentration, possiblyattributed to the formation of nano/micro-sized dye aggregates[20] (Fig. 7a).

The emission of LLDPE films containing the 0.01 wt.% of R-Perydisplayed the typical luminescence features of non-interactingdyes at about 525 and 565 nm and an additional band located athigher wavelengths (�620 nm), attributed to the presence of ameasurable amount of dye aggregates even at low concentration(Fig. 7b) [20]. In particular, by increasing R-Pery concentration inthe LLDPE matrix, the electronic coupling between perylene chro-mophores that come in close contact with each other, induced theprogressive quenching of the 0–0 and 0–1 emission bands, whereas

Fig. 6. 1H NMR (600 MHz, 25 �C) spectra of R-Pery in (a) DMSO and (b) in themixture DMSO/D2O.

Fig. 7. Absorption (a) and fluorescence (kexc = 300 nm) (b) spectra of LLDPE_R-Peryfilms at different dye concentration.

Fig. 8. Fluorescence spectra (kexc = 300 nm) of a LLDPE film containing the 0.1 wt.%of R-Pery before (pristine film) and after uniaxial deformation at draw ratios (Dr) of5 and 10.

A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960 957

the relative intensity of the aggregation band with respect to themonomer contribution was strongly enhanced.

3.5. Effect of uniaxial deformation

As reported by our group and others [18,20,39–43] the mechan-ical deformation of a semi-crystalline polymer matrix containing‘‘aggregachromic” chromophores is able to unfold the macromo-lecular chains leading to microfibrils in oriented crystalline andamorphous regions and to promote the breakup of the dye aggre-gates and their molecular mixing within the polymer matrix.The colour of composite films changed according to the increaseof the monomer-to-aggregate ratio caused by progressive film

drawing. In the present case, a LLDPE_R-Pery_0.1 film containing0.1 wt.% of R-Pery dye was uniaxially oriented at room tempera-ture at different draw ratios (Dr, defined as the ratio between thefinal and the initial length of the sample, respectively). Afterstretching, the luminescence contribution at 620 nm, attributedto dye aggregates, strongly decreased already at Dr = 5, but itwas not flanked by a significant recovery of the fluorescence fromthe isolated chromophores at 530 nm (Fig. 8), as occurred insteadfor LLDPE films containing alkyl-functionalised perylene bisimides[20].

This phenomenon could suggest that the mechanical stretchingapplied to the LLDPE matrix is not sufficient to destroy most of theinterchromophoric interactions among R-Pery dyes. Analogous re-sults were observed for LLDPE films containing 0.05 wt.% of R-Pery.

3.6. Effect of temperature

The equilibrium between the aggregate form and the molecu-larly dissolved form of a dispersed dye into a polymer can beperturbed by applying a thermal solicitation above a certain tem-perature, as recently reported for different classes of dyes[44,45,42,46–48]. Analogously, LLDPE_R-Pery_0.1 films containing0.1 wt.% of R-Pery were thermally stressed at temperatures rangingfrom about 30 �C to 80 �C, by placing them in contact with a ther-mostatically controlled (±0.1 �C) metal surface. The effect providedby temperature changes on the equilibrium between the aggre-gated form and the isolated form of R-Pery dispersed into LLDPEwas evaluated by means of absorption and fluorescence spectros-copies. All the spectra were collected 15 s after the temperature in-crease to ensure that the relaxation to a new thermal equilibriumhad been achieved.

Upon heating a LLDPE_R-Pery_0.1 film, the absorption featuresprogressively assumed the typical vibronic behaviour which char-acterises the isolated and non-interacting R-Pery chromophores(Fig. 9). This was also flanked by a significant reduction of theabsorption at wavelengths longer than 530 nm, suggesting a stea-dy reduction of the R-Pery aggregation extent.

Even more interestingly, the luminescent response of LLDPE_R-Pery_0.1 appeared strongly affected by temperature changes. At22 �C the film was characterised by the prevalence of the emissionband at about 620 nm attributed to the presence of a significantamount of R-Pery aggregates (Figs. 7b and 10a).

Upon heating, the 0–0 and 0–1 transitions at about 530 and570 nm started to be predominant already at annealing tempera-tures as high as 35 �C, together with the progressive recovery ofthe overall fluorescence intensity.

Fig. 9. UV–Vis absorption spectra of a LLDPE film containing the 0.1 wt.% of R-Peryat different temperatures.

Fig. 10. (a) Fluorescence spectra (kexc = 300 nm) of a LLDPE film containing the0.1 wt.% of R-Pery at different temperatures; (b) plot of the (I � I0)/I0 ratiocalculated for the peak at 530 nm as a function of temperature (open circle duringheating, open squares during cooling); (c) digital image of the same film takenunder irradiation at 366 nm at different annealing temperatures.

958 A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960

During heating, the different solubility of R-Pery in the polymerand the increased mobility of the macromolecular structure ofLLDPE resulted in temporarily breaking aggregates among dyesor, more likely, keeping them at a distance at which they did notinteract with each other.

Moreover, the plot reported in Fig. 10b for the (I � I0)/I0 ratiocalculated for the peak at 530 nm as a function of temperature,could suggest that the film results more sensitive to temperaturesolicitations comprised between 40 and 70 �C where the interpola-tion between data points increases its slope. Above this tempera-ture regime, R-Pery appeared to be mostly distributed within thepolymer matrix as non-interacting chromophores profiting alsoby the non-negligible reduction of the LLDPE crystallinity occurredat T > 80 �C (Fig. S4). Above this temperature, dye molecules aredispersed in the amorphous phase of polymers and any decreaseof the crystallinity content of the polymer matrix favours theirmolecular distribution [45,49].

The progressive increase of the LLDPE_R-Pery_0.1 emission dueto the recovery of the 0–0 and 0–1 transition intensities recordedduring thermal stimuli was also detected by exposing the film toexcitation by long-range UV light at 366 nm (Fig. 10c). The tapesgradually changed their emission colour from violet to yellow–green by increasing temperature from 25 to 65 �C.

Interestingly, once the heating was removed the film restoredrapidly the original optical properties (both in absorption and inemission) previously recorded at 22 �C, suggesting a completereversibility of the phenomenon (Fig. 10b). The rapid exchange be-tween the two forms of R-Pery (isolated and aggregated) is likelyattributed to a twofold effect: first, the large mobility of dye mol-ecules within the viscoelastic amorphous phase of LLDPE(Tg < �50 �C) [50]; second, the increased solubility of R-Pery inLLDPE with temperature. In the temperature range comprised be-tween 25 and 80 �C, we can exclude that fluorescence colourchanges may be attributed to thermal transitions of neat R-Pery,since these latter are reported for perylene derivatives functional-ised with short alkyl or aryl peripheral moieties only at tempera-tures higher than 300–350 �C [20,35,51] (see Fig. 11).

In the case of LLDPE_R-Pery_0.05 and LLDPE_R-Pery_0.02 films,the exposition at temperatures higher than 35–40 �C reduced thequenching of the luminescence, providing the films with typicalemission of isolated R-Pery chromophores even if the phenomenonappeared less evident than for the films containing the highest R-Pery concentration.

The sensitivity of the LLDPE_R-Pery system towards thermalsolicitations was finally compared to that of LLDPE films containingan achiral analogue of R-Pery endowed with N-benzyl groups, i.e.,N,N0-dibenzyl-perylene-3,4,9,10-tetracarboxyldiimide (Pery-Bz),which also generates dimeric aggregates in solution [35]. LLDPE_P-ery-Bz films showed weak emission bands centred at about 540and 580 nm which appeared to be largely quenched already forthe film containing the 0.06 wt.% of dye.

When the thermal stimulus was applied to the LLDPE_R-Perysystem, a moderate recovery of the luminescence appeared onlyat 80 �C. These results unequivocally suggest that the supramolec-ular structure generated by the assembly of chiral perylene dyesinto the polyethylene matrix results extremely more sensitive totemperature solicitations with respect to chromophoric assembliesobtained from analogue achiral dyes.

4. Conclusions

The chiral perylene derivative N,N0-bis-(R)-(10-phenyl-ethyl)perylene-3,4,9,10-tetracarboxyldiimide (R-Pery) was foundto generate supramolecular aggregates in solution caused byp�p stacking interactions among perylene nuclei, as shown by

Fig. 11. Fluorescence spectra (kexc = 300 nm) of LLDPE films containing the 0.01 and the 0.06 wt.% of Pery-Bz at 25 �C and for this last concentration at 80 �C.

A. Pucci et al. / Reactive & Functional Polymers 70 (2010) 951–960 959

several spectroscopic techniques (UV–Vis, fluorescence, circulardichroism, and NMR). In detail, R-Pery generated aggregates inchloroform solutions only at concentrations above 10�3 M, as re-ported by 1H NMR and confirmed by DOSY technique, whereasstrong aggregation phenomena occurred by dissolving the dye intomixtures of solvent/poor solvent (i.e. CHCl3/CH3CN or DMSO/H2O)for concentrations as high as 10�5 M, as well evidenced by CDexperiments.

The tendency of the chiral R-Pery molecule to promote the for-mation of supramolecular assemblies was even more evidencedwhen the dye was dispersed into a completely apolar polyethylenematrix (linear low-density polyethylene, LLDPE) at moderate con-centrations (0.01–0.1 wt.%), as reported by UV–Vis, fluorescenceand CD spectroscopies.

The equilibrium between the isolated non-interacting form andthe aggregated form of R-Pery appeared to be easily perturbed inthe solid state by means of thermal solicitations. Temperaturestress above 35 �C caused the progressive disruption of the R-Peryaggregates favouring the molecular dispersion of the dye withinthe LLDPE matrix together with a striking change of the optical fea-tures of the composite film. It is worth noting that similar thermalresponsiveness was not achieved by LLDPE films containing anachiral analogue of R-Pery, confirming that chiral derivatives gen-erate supramolecular assemblies more sensitive to physicochemi-cal solicitations.

The steady and reversible optical responsiveness of theLLDPE_R-Pery system suggests applications as smart optical indi-cators to thermal stimuli.

Acknowledgements

The authors wish to express their thanks to Mr. Piero Narduccifor his help in electron microscopy. G.P. and L.D.B. thank MIUR forfinancial support (PRIN 2007PBWN44). G.R. thanks MIUR for finan-cial support (PRIN 2008ALLB79).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.reactfunctpolym.2010.09.001.

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