J. Chem. Sci. Vol. 128, No. 1, January 2016, pp. 119–132. c© Indian Academy of Sciences.

DOI 10.1007/s12039-015-1009-5

Effects of temperature and CO2 pressure on the emission ofN,N′-dialkylated perylene diimides in poly(alkyl methacrylate) films.Are guest-host alkyl group interactions important?

KIZHMURI P DIVYAa,b, MICHAEL J BERTOCCHIa and RICHARD G WEISSa,∗aDepartment of Chemistry, Georgetown University, Washington, DC 20057-1227, USAbPSMO College, Tirurangadi, Malappuram, Kerala 676 306, Indiae-mail: weissr@georgetown.edu

MS received 29 July 2015; revised 20 October 2015; accepted 14 November 2015

Abstract. Static and dynamic fluorescence measurements have been made on four N,N ′−dialkylated pery-lene diimides in films of poly(alkyl methacrylate)s (PAMAs) with 5 different alkyl groups and in a ‘modelsolvent’, n-butyl acetate, over wide temperature ranges. The results indicate that the excited singlet states ofthe perylene guest molecules are controlled primarily by chain relaxations rather than hole free volumes inthe polymer matrixes. The short singlet lifetimes of the perylene molecules require that the guest moleculesrespond primarily to the environments experienced by their ground states within the PAMA matrixes; each ofthe PAMAs offers slightly different locations in which the guest molecules can reside as a result of interactionsbetween the N−alkyl substituents on the imide groups of the perylenes and the alkyl groups on the PAMAside chains. PAMAs with branched side chains were found to have a larger influence than PAMAs with linearside chains on the fluorescence properties of the guest molecules. The results are compared to those employingpyrenyl derivatives (with much longer excited singlet lifetimes) in the same PAMA films. The overall resultsindicate that the perylenes can be used as a complementary probe of local polymer chain dynamics, but they areless sensitive to their environments than are pyrenyl groups. However, they offer some distinct advantages: (1)a much wider range of N,N ′−disubstituted perylene diimides can be synthesized easily; (2) those substituentscan be designed to allow a greater or lesser interaction with an anisotropic host matrix. Also, rapid conforma-tional changes of a bis-perylene derivative appear to be restricted in the polymer matrixes. Those restrictionsappear reduced when the polymer films are placed under high pressures of the plasticizing gas, CO2, but notwhen they are under equal pressures of a much less intervening gas, N2.

Keywords. Perylene Diimides; fluorescence; conformational changes; polymer matrixes; excited singletstates.

1. Introduction

Here, we compare the dependence of the photophysicalproperties of four N,N ′−dialkylated perylene diimides(referred to as ‘perylenes’ here for convenience;figure 1) to those of pyrene in films of 5 poly(alkylmethacrylate)s (PAMAs). Such a study is importantbecause a wide range of dialkylated perylenes is mucheasier to synthesize than the corresponding pyrenes,and many perylenes exhibit near-unity fluorescencequantum yields, high stability to UV radiation andheat, strong electron-accepting ability, and a propensityto π-type aggregation.1–3 As well, we were interestedto study the intramolecular electron transfer of thediamino derivative of perylene, (N,N ′-di(2-N ′′, N ′′-dimethylamino)ethylperylene-3,4:9,10-tetracarboxylic

∗For correspondence

diimide) in n−butyl acetate and PAMA films. How-ever, it exhibited an excited state lifetime, ∼4 ns,comparable to those of the 3 mono perylenes, inn−butyl acetate in spite of a very low fluorescencequantum yield. (Similar observations were reportedby Wu et al.4). A detailed analysis leads us to suggestthat a small amount of impurity is responsible for thefluorescence detected; because our efforts to separatethe impurity from the desired diamino derivative wereunsuccessful, no further studies of the latter in solutionor in the PAMA films were conducted. In addition,because the pyrenyl excited singlet state lifetimesare ∼50-100 times longer than those of the perylenediimides, the two sets of results yield information aboutrelaxation processes in different time domains. Threeof the perylenes have a single core and N−alkyl groupsthat vary from short to long alkyl chains (i.e., butyl(PERBUT) and 8-pentadecyl (PERPDA)) or bulkier,more rigid cyclohexyl groups (PERCYA); one has a

119

120 Kizhmuri P Divya et al.

Figure 1. Structures of perylene diimide guest molecules.

bis-perylene core (TP; also with 8-pentadecyl groupsattached to the imide functionalities) in which theoptimal 90◦ angle between the aromatic groups can bechanged rather easily in its ground and excited singletstates.

Neat PAMAs are anisotropic on the micrometerscale because they consist of regions enriched in morepolar ester groups and less polar alkyl groups. Both thebulk and microscopic properties of PAMA films aredependent on the size and branching of the alkyl sidegroups attached to the carboxyl moieties, as well asthe tacticity of the main chains.5,6 PAMAs with shortor branched alkyl groups are amorphous materials,whereas those with long (12 or more carbon atoms)n-alkyl ester groups contain micro-crystalline domains atlower temperatures.7 As the length of alkyl side groupsincreases, the main chains are moved farther apart,which decreases the energy needed for their movementin the rubbery state. Also, the preferred locations,conformations, and orientations of guest moleculesinside the polymer matrixes depend on the magnitudesof intimate probe−polymer and polymer−polymerinteractions.8,9 Others10–12 and we13–15 have observedinteresting properties of different PAMAs based onthe comportment of photochemical and photophysicalprobes in ensemble average measurements; a cartoonrepresentation of the most probable site locationswithin the PAMAs is presented in figure 3 of ref 14.These studies differ from single molecule excitations inwhich stochastic measurements can be made multiple

times to generate ensemble averages. An interest-ing example of this technique has been reported inthe glassy phase of poly(methyl methacrylate) usingseveral perylene derivatives as the probe.16,17 In ourprevious photophysical and photochemical investiga-tions with 5 different PAMAs, it was found that theycan be placed in three different categories, dependingupon the nature of the dynamic and structural charac-teristics of the alkyl side groups:13–15 (i) in PAMAs withshort, linear alkyl chains, such as poly(ethyl methacry-late) (PEMA) and poly(butyl methacrylate) (PBMA),the sizes and shapes of the alkyl groups allow guestmolecules to approach closely the ester functionalitiesand the main chains; (ii) In PAMAs with more rigid andbulkier side-chains, such as poly(isobutyl methacry-late) (PIBMA) and poly(cyclohexyl methacrylate)(PCHMA), the nature of the alkyl groups forces guestmolecules to reside farther from the ester functional-ities and main chains of the polymer; (iii) in PAMAswith long n-alkyl side chains, such as poly(hexadecylmethacrylate) (PHDMA), guest molecules areexcluded from the volume segments of the micro-crystallites, but can enter those regions after chainmelting.

Photophysical probes, such as pyrene, are known tobe sensitive to the polarity,18–21 correlated motions22,23

and chemical nature24,25 of their microenvironments. Inthat regard, we have shown that the size and lengthof side-chain alkyl groups in PAMAs play an impor-tant role in determining the rates of inter- and intra-molecular photophysical processes of pyrenyl guestmolecules.13,14 It was concluded that the dynamics ofthe pyrenyl-guest molecules in these media are con-trolled primarily by side chain relaxation rates of thepolymer chains. However, based on both photophysi-cal and photochemical studies, the most important fac-tor in polyethylene films was found to be ‘hole’ freevolume.15

The subtle changes in the fluorescence propertiesof the perylenes in the PAMA films, over temperatureranges that include the glass (or crystalline)-to-rubberytransitions, have been analyzed carefully to discern howthe interplay between the natures of the alkyl chains onthe perylenes and on the PAMAs alter the average loca-tions and the ease of movement of the guest moleculeswithin the polymer films. In addition, we have exploredthe effect of CO2 and N2 pressure on the luminescentproperties of the perylenes in the PAMA films; theswelling caused by imbibed CO2 has the same effectas increasing temperature (but manifested isothermally)on the ability of the polymer chains to undergo specificmotions that affect the perylene excited singlet statedynamics.

N,N′-dialkylated perylene diimides in acrylate polymers 121

2. Experimental

2.1 Materials and Methods

Poly(butyl methacrylate), poly(hexadecyl methacry-late), poly(ethyl methacrylate), and poly(cyclohexylmethacrylate) were purchased from Scientific Poly-mer Products, Inc. Poly(isobutyl methacrylate) wasobtained from Aldrich. Some of their characteristics arereported in Supporting Information. They were puri-fied as reported in the literature.13 Perylene-3,4,9,10-tetracarboxylic dianhydride (Aldrich, 97%), cyclo-hexylamine (Aldrich, ≥99%), 1-butylamine (Aldrich,99.5%), anhydrous toluene (EMD Chemicals, Inc.99.8%), and methanol (Aldrich, 99.8%) were used asreceived. Anhydrous dichloromethane (Aldrich, 99.8%)was placed overnight over anhyd. CaCl2, decanted intoCaH2, and distilled onto dried molecular sieves (Type3A) under nitrogen, where it was stored in a brown bot-tle until being used.26 All flattened capillaries were fromVitro Dynamics, Inc.

2.2 Synthesis

Detailed procedures for the syntheses and character-ization of PERBUT and PERCYA are described inSupporting Information. Syntheses of TP and PER-PDA are reported elsewhere.27,28 They were >99% pureaccording to HPLC analyses.

2.3 Preparation of Doped Films

All doped films (except those of PHDMA) wereprepared by dissolving PERBUT, PERCYA, PER-PDA or TP and a PAMA polymer in anhydrousdichloromethane and pouring the solution onto a Teflonplate. The initial concentrations in the dichloromethanesolutions were adjusted so that the ultimate pery-lene concentrations in the films were ∼10−6 mol/kgof PAMA. After most of the solvent had evaporatedat room temperature, the films were washed withmethanol and then dried under vacuum (0.25 torr) for10 h. They were cut into pieces of desired sizes, andflame-sealed in flattened, 4 mm pathlength, Pyrex cap-illaries under vacuum (0.19 torr) on a mercury-free vac-uum line. The film thicknesses were determined to be0.4-0.9 mm using a Mitutoyo Vernier.

Doped PHDMA films were prepared by addingappropriate amounts of a perylene and polymer indichloromethane solutions into flattened Pyrex capillar-ies of ∼0.6 mm pathlength. A drying tube was affixedand the solvent was removed by placing the capillaries

first in a water bath at 343 K and then under vacuum(0.25 torr) for 24 h. Thereafter, the Pyrex capillarieswere flame-sealed under vacuum (0.19 torr) using amercury-free vacuum line.

2.4 Instrumentation and Procedures

1H NMR spectra were recorded on a Varian 400 MHzspectrometer in either CDCl3 (TMS as the internalstandard) or CF3CO2D (residual proton peak at ∼11ppm as standard) with 64 scans. MestReNova v5.2.4-3924 software by Mestrelab Research was used to ana-lyze the spectra. Elemental analyses were performedon a Perkin-Elmer Model 2400 Elemental Analyzer.The purities of the perylenes were determined by high-performance liquid chromatography (HPLC) using anAgilent Technologies (Hewlett Packard Series1100)liquid chromatograph with a Phenomenex silica col-umn (250X4.60 mm, 5 micron) using CHCl3 as eluent.UV/Vis absorption spectra were recorded on a VarianUV-visible (Cary 300 Bio) spectrophotometer.

Steady-state emission and excitation spectra wererecorded on a Photon Technology International fluo-rimeter (SYS 2459) with Felix 32 software for dataanalysis (linked to a personal computer) and a 150 Whigh-pressure xenon lamp with a Quantumwest tem-perature controller and an Omega temperature probe.Quantum yield measurements were performed on solu-tions in 1.0 cm pathlength quartz cuvettes that werepurged with N2 for 30 min and closed with rubber stop-pers. The fluorescence quantum yields were calculatedusing eq 1.29,30

�f = �r (ArFs/AsFr)(η2

s /η2r

)(1)

As and Ar are the absorbances of the sample and ref-erence solutions, respectively, at the same excitationwavelength, Fs and Fr are the corresponding areas ofthe fluorescence spectra (intensity versus wavelength),and η is the refractive index of the solvents. Rhodamine6G (�f = 0.95 in ethanol)31 was used as the referencecompound.

Fluorescence decay histograms were obtained withan Edinburgh Analytical Instruments single photoncounting system (model FL900) using H2 as the lampgas. An “instrument response function” was determinedusing Ludox as scatterer. Data were collected in 1023channels. Deconvolution was performed by nonlinearleast-squares routines that minimize χ 2 using soft-ware supplied by Edinburgh. The solution samples fordynamic decay measurements were placed in 0.4 mmthickness flattened glass capillaries and were degassedby ≥ 5 freeze (liquid nitrogen)-pump-thaw cycles at∼0.15 torr and flame-sealed.

122 Kizhmuri P Divya et al.

For solution phase studies, the temperature probe anda flattened Pyrex capillary (7 mm (length) × 4 mm(width) × (0.4 or 3 mm (i.d.)) were placed inside a 1cm cuvette filled with decane. For studies with films,spectra were recorded front-face at an angle of ∼45◦

with respect to the incident beam, and the emission wascollected at 90◦ with respect to the excitation source.For CO2 and N2 pressure-dependent fluorescence stud-ies, polymer films were affixed to a side of a triangu-lar quartz cuvette that was placed inside a high pres-sure chamber32 which, in turn, was placed in the samplecompartment of the fluorimeter.

2.5 Quantum calculations

The ground state geometry of the N,N ′−dimethyl ana-logue of TP was optimized using the M06/6-31(d) pro-gram in the Gaussian09 suite33 with the dielectric con-stant of butyl acetate, 5.0, as the ‘solvent’, as modeledby the polarizable continuum model (PCM) function.Potential energy surfaces for rotation about the cen-tral N—N bond of the optimized structure (figure S1 inSupplementary Information) in the ground and excitedsinglet states were constructed at increments of 5 degreeangles of twist using the same program.

3. Results and Discussion

3.1 Solution state absorption and emission studies

At room temperature, absorption spectra of the 3 monoperylenes (10−6 M) in butyl acetate, a solvent which canbe viewed as a monomeric, low-viscosity analogue ofthe PAMAs, showed a vibronic progression of peaks at518, 482 and 452 nm (figure S2a, in SI) that is charac-teristic of the S0 →S1 electronic transitions.34,35 Peaksat 527, 567 and 614 nm were observed in the emis-sion spectra for the same solutions (figure S2b in SI).Not surprisingly, given the coupling between the two

chromophoric groups of TP,36–38 its absorption andemission spectra were shifted bathochromically by ∼8nm compared to the other perylenes. Also, the inten-sity ratio of the 0→0 to 0→1 peaks (I0−0/I0−1 = 2.4)in TP was larger than in the other perylenes (1.6), andthe corresponding emission ratios were 3.5 and 2.4(table 1). The I0−0/I0−1 ratios in absorbance or fluo-rescence are known to be a monitor of aggregation39

and local environments of perylene dimide derivatives40

when the changes are pronounced.41,42 Also, the fluo-rescence quantum yields for the 4 perylenes were nearunity and their excited singlet-state lifetimes were ∼4ns (table 1 and figures S18–S20 in SI).

3.2 Effect of temperature on fluorescence intensitiesand decay times in butyl acetate

A small (but perceptible) decrease in fluorescenceintensity with increasing temperature, probably due toincreased rates of non-radiative decay, was observedfor 10−6 M PERBUT, PERCYA and PERPDA in butylacetate (figure S3a in SI). However, the intensity of flu-orescence from TP increased and then decreased withincreasing temperature (figure S3b in SI). Unlike theother probe molecules investigated here, TP is able toundergo conformational changes about a central N—Nbond (twisting and/or bending motions) that can affectprofoundly the degree of interaction between the twoperylene groups.35–37 The initial increase in emissionintensity with increasing temperature is probably linkedto conformational changes, and the decrease at highertemperatures to a greater importance of non-radiativedecay processes. However, within the resolution lim-its of our measurements, the fluorescence decay ratesshowed no discernible changes over the temperatureranges investigated. In this regard, the perylenes are lesssensitive probes than pyrenes.

In both its ground and excited singlet states, deforma-tion of the lowest energy conformation of the dimethyl

Table 1. Photophysical characteristics of perylene guest molecules in n−butyl acetate and PIBMA (∼10−6 mol/kg ofperylene in PIBMA) at 295 K. Fluorescence quantum yields (�f) and excited singlet state lifetimes (τ) of the perylenes arein n−butyl acetate (λem = 530 nm).

n- Butyl acetate PIBMAPerylene diimide λabs (nm) λem (nm)a λabs (nm) λem (nm) �b

f τ (ns)c I0−0/I0−1 (abs) I0−0/I0−1 (em)b

PERBUT 518, 483 527, 566 523, 486 529, 569 0.97 ± 0.03 4.0 1.6d 1.2e 2.4d 2.3e

PERCYA 519, 482 527, 566 522, 485 530, 570 0.96 ± 0.03 4.0 1.6d 1.2e 2.4d 1.8e

PERPDA 520, 484 528, 569 523, 486 529, 569 0.97 ± 0.03 4.4 1.6d 1.2e 2.3d 2.3e

TP 527, 490 534, 574 532, 494 537, 578 0.99 ± 0.01 3.4 2.4d 1.7e 3.5d 3.0e

aλex =480 nm; bλex =489 nm; cχ2 ≤1.2 and residual plots exhibited no systematic deviation from zero; d in n−butyl acetate;ein PIBMA.

N,N′-dialkylated perylene diimides in acrylate polymers 123

80 60 40 20 0 -20 -40 -60 -800

50

100

150

200

250

300

350

400

Ground State Excited State (1S)

Ene

rgy

(kca

l/mol

)

Dihedral Angle

Figure 2. Ground and excited singlet state potentialenergy surfaces from single point calculations of twistedN,N ′−dimethyl bisperylene diimide starting from the opti-mized ground state geometry.

analogue of TP, where its two aromatic moieties are per-pendicular to each other, requires only 1.5 kcal mol−1 totwist by 15◦ (i.e., from 90◦ to 75◦); a 30◦ twist increasesthe energies by ca. 7 kcal mol−1 (figure 2 and table S2in SI). The energies of TP conformers must be virtuallythe same as those of the model compound.

3.3 Dynamic emission studies in PAMA films

Fluorescence intensity decay histograms from time-correlated single photon counting experiments of thethree mono perylenes in the PAMAs at 10−6 mol/kgwere mono-exponential. At 298 K, the lifetimes, ∼4 ns,were comparable to those in butyl acetate (table S3 inSI), and they remained virtually unchanged over rangesof temperatures from below to above the glass or melt-ing transition temperatures. Representative decay his-tograms for the samples reported in table 1, with resid-ual plots that demonstrate the lack of deviation of thecurves to best fits, are shown in figures 3 and 4. Filmsof TP in the PAMAs also yielded mono-exponentialdecays with lifetimes ∼3 ns throughout the temperatureranges investigated. Thus, the dynamic measurementsoffer very little insights into the intimate perylene inter-actions with its polymer hosts (or in solution with butylacetate).

3.4 Spectroscopic properties of the perylenesin PAMA films

At room temperature, PEMA, PCHMA and PIBMA arein their glassy state, and PBMA and PHDMA are in

their rubbery state (table 2). The initiation or cessationof main chain motions (i.e., α-relaxations) occur closeto the glass transition or melting temperatures. Theabsorption, excitation and emission spectra of perylenesin the PAMA films were similar in shape and positionto the corresponding spectra in butyl acetate, and theemission spectra were independent of excitation wave-length. However, the absorption and emission spectrawere red-shifted by ∼5 and ∼3 nm, respectively, inthe PAMAs with respect to those in butyl acetate. Noaggregation of the perylene moieties was expected atthe very low concentrations employed, and the spectraldata are consistent with that being the case (figure 5).The I0−0/I0−1 ratios from the perylene absorption spec-tra were 1.2 and 1.7 in the PAMAs, and in butyl acetate,respectively. Except in PHDMA, where the emissionratios of the mono perylenes, 2.5, were similar to thosein butyl acetate, and the ratio for TP, 4.0, was higherthan in butyl acetate, the emission ratios in the PAMAswere also slightly lower—1.8 for PERCYA and 2.3 forPERBUT and PERPDA (table 1).

We interpret these differences as a consequence ofPERCYA being located in a slightly different envi-ronment than PERBUT and PERPDA in the polymermatrixes (vide infra).

3.5 Effect of temperature on fluorescence intensitiesin the PAMA films

The fluorescence intensity changes of the peryleneswere compared in temperature ranges from below toabove the glass transition/melting temperatures. Due tothe more rapid relaxation processes above the transitiontemperatures, movement of guest molecules betweenand within sites is enhanced in films of the PAMAs.Fluorescence intensities of PERBUT and PERPDAdecreased as temperature was increased in all of thePAMAs except PHDMA. Also, no large changes in flu-orescence intensity were observed near the glass tran-sition temperatures of the 3 glassy PAMAs, but a largechange was observed near Tm of PHDMA, due to achange in the refractive index of the polymer mediumrather than a photophysical effect experienced by theprobe.

Cooling the films to their initial temperature afterheating above their Tg or Tm reestablished the orig-inal fluorescence intensity; the polymer systems arereversible and the perylenes are thermally stablewithin the temperature range investigated. Contraryto the other two mono perylenes, the fluorescenceintensity of PERCYA, the perylene derivative withthe bulkiest N,N ′−substituents, cyclohexyl, increased

124 Kizhmuri P Divya et al.

Figure 3. Decay histograms (�) of ∼10−6 M PERBUT (a), PERCYA (b), PERPDA (c) andTP (d) with best fit lines (—) in butyl acetate at 295 K and lamp profiles (�); (λex = 480,λem = 530 nm). The residual plots are below each decay panel and χ2 values are reported intable 1.

with increasing temperature in the four glass-formingPAMAs. The different comportment in the fluorescencechanges can be ascribed to the PERCYA moleculesresiding preferentially in a location whose polarityand chain mobility are somewhat different from thoseexperienced by PERBUT and PERPDA.

The response of the fluorescence intensity of TP toincreasing temperature in the PAMA films, a decrease,was very different from that found in butyl acetate (arise followed by a fall; figure S3b in SI). As men-tioned, the angle between the planes defined by its twoperylene groups determines the degree and nature oftheir interactions. Although the angle experienced byTP molecules in their ground state may be very similar

in the polymer matrixes and in butyl acetate (based onthe absorption spectra), its ability to change during itsexcited singlet state lifetime can be vastly different as aresult of the very high microviscosity in the PAMAs andthe very low one in butyl acetate: very little structuralchange is expected of TP in the films during its veryshort excited singlet-state lifetime (vide infra). As ameans to compare the general behavior of fluorescencechanges with temperature, the normalized fluorescenceintensities {[I(T )-I(Tg)]/I(Tg)} were plotted as a func-tion of reduced temperature (T -Tg) (figure 6). The plotscan be divided into regions below (T -Tg <0; i.e., theglassy state) and above the glass transition (T -Tg >0;i.e., the rubbery state). As can be seen, the temperature

N,N′-dialkylated perylene diimides in acrylate polymers 125

Figure 4. Decay histograms (�) of ∼10−6 M PERBUT (a), PERCYA (b), PERPDA (c)and TP (d) with best fit lines (—) in PIBMA at 295 K and lamp profiles (�); (λex = 480,λem = 530 nm). The residual plots are below each decay panel and χ2 values are reported intable 1.

induced changes are larger in the branched polymers,PIBMA and PCHMA, than in the unbranched ones withshort alkyl chains, PBMA and PEMA, where the pery-lenes are expected to reside more closely on average tothe polymer backbones.

Table 2. Transition temperatures of PAMAs.a

Glass (g) or melting (m)Polymer temperature (K) Tα(K)a

PHDMA 281 (m) 330PBMA 290 (g) 322PIBMA 322 (g) 330PEMA 342 (g) 290PCHMA 368 (g) 280

aData from ref.43

3.6 Effects of CO2 and N2 pressure

The influence of high pressures of CO2 on the emissioncharacteristics of the perylenes in the PAMA films wasalso investigated. At a particular temperature, increas-ing the CO2 pressure may conceptually have two coun-teracting effects on the mechanical properties of thepolymer films: (1) dissolved gas may plasticize thefilms44,45 and, thus, lower glass transition temperatures;(2) increased hydrostatic pressure on the polymers mayincrease glass transition temperatures as a result ofdecreased free volume.46–48

As shown in figure 7a, increasing the pressure of CO2

decreased the fluorescence intensities of the perylenesin PIBMA films at 295 K. The changes were compara-ble for PERCYA, PERBUT and PERPDA, and smaller,

126 Kizhmuri P Divya et al.

(a) (b)

Figure 5. Normalized (a) excitation (λem = 570 nm) and (b) emission (λex =480 nm) spectra of ca. 10−6 mol perylene/kg PIBMA at 293 K: PERBUT(——), PERCYA (- - - -), PERPDA (.....), and TP (-·-·-).

but discernible, for TP. At 324 K (i.e., slightly aboveTg), the onset pressure where decreases in the intensityoccur, are much lower (figure 7b).

Comparison of changes in the fluorescence intensityof PERCYA in various PAMAs at 295 K (figure S16 inSI) demonstrates that polymers with higher Tg’s requirea higher pressure to initiate the fluorescence intensitychanges. Although the optical density and spectral fea-tures of PERBUT in a PBMA film at 295 K (Tg = 290K) were indistinguishable at 1 and 40 atm of CO2, adecrease in fluorescence intensity was noted at >10 atmpressure. However, significant changes were noted only

above ∼30 atm CO2 pressure in PIBMA (Tg = 322 K)and PEMA (Tg = 342 K). These data and those infigure S16 demonstrate clearly that the degree to whichCO2 pressure affects the polymer matrix depends on theproximity of the temperature to Tg(i.e., side and mainchain polymer mobility), and that the decreases in flu-orescence intensity are related to photophysical aspectsof perylene-polymer interactions.

At 295 K, the PIBMA film expanded visually upto 40 atm of CO2; above that pressure, the film brokeinto pieces. At 324 K, 30 atm was the maximum pres-sure at which no deformation of the PIBMA films was

Figure 6. Fluorescence intensity changes at emission maxima of (a) PER-BUT, (b) PERCYA, (c) PERPDA and (d) TP in the PAMA films (∼10−6

mol/kg; λex = 480 nm) in four of the PAMA films as a function of reducedtemperature (◦C).

N,N′-dialkylated perylene diimides in acrylate polymers 127

Figure 7. Emission intensities at the emission maxima ofperylenes in PIBMA films (∼10−6 mol/kg): PERBUT (•),PERCYA (�), PERPDA (�), and TP (�) as a function ofCO2 pressure at (a) 295 K and (b) 324 K (λex = 480 nm).Note that Tg = 322 K for PIBMA at one atm of pressure.

detectable. Contrary to the effect of CO2, even 40 atmof N2 did not lead to noticeable changes in the fluores-cence intensity of PERCYA in PBMA at 295 K (figureS17 in SI).

3.7 What controls PAMA host-guest interactions?

The temperatures at which the fluorescence intensitieswere recorded are far above the onset of the γ relax-ation (involving rotations of side chains; Tγ = 120-160K) and β relaxation (involving rotation of the ester sidegroups; Tβ = 220-270 K) processes for all the PAMApolymers,15,44,45,49,50 but they span a range that includesthe α-relaxation processes (involving movement of thepolymer backbones), which are near to and associatedwith the glass transition temperatures, Tg.44,45,51,52 Boththe β- and α-relaxations are strongly coupled in acry-lates like the ones employed here.53 Also, motionalchanges of the polymer segments associated with thesetwo relaxation processes may couple with transitionsof the singlet excited states of the perylenes.51,52 Whenthat occurs, both the quantum yield and intensity of the

perylenes can decrease when fundamental vibrationsand harmonics of their excited singlet states are cou-pled with vibrational modes of the PAMAs.54,55 Thus,the observed decreases in the fluorescence intensity ofPERBUT and PERPDA with increasing temperaturemay be related to the larger segmental motions of thePAMAs that enhance non-radiative deactivations or toeffects induced in the ground states by the alkyl chainsof PERBUT and PERDPA.

Perylene guest molecules can experience lower-polarity (near the alkyl groups) or higher-polarity (nearto the ester functionalities) local environments in thePAMAs that are mediated by van der Waals inter-actions. In that regard, carboxy groups of polyacry-lates are known to interact with the π-electrons ofaromatic56,57 and carbonyl groups58 of guest molecules.The interaction energies between carbonyl groupsof polyacrylates and polycyclic aromatic hydrocar-bons like perylenes are consequential to determiningwhere the perylenes prefer to reside within the PAMAmatrixes:59 the calculated binding energy betweenformaldehyde and benzene is 1.86 kcal mol−1.60

Although the corresponding magnitudes of the interac-tion energies between carbonyl groups and excited sin-glet states of aromatic molecules are unknown, it is rea-sonable to assume that they will be stronger in the more-polarizable, excited singlet states of the perylenes thanin their ground states. Furthermore, because the inter-actions must be orientationally selective and sensitiveto the intermolecular separation distances, any static ordynamic change of the side chains within the PAMAfilms, caused by a phase transition or even differenttemperatures within a phase, must affect the degree ofcarbonyl-perylene excited state interactions (and, thus,the fluorescence properties). Therefore, from the struc-tural features of the guest molecules in the presentstudy, it is reasonable to expect that the perylenes preferto reside near the main chains of the PAMAs, as well,and will do so to the extent that steric factors permit.

The fluorescence from pyrene-based probes has alsobeen used to report on the size, shape, and flexibil-ity of the cavity walls of the guest sites in polyethy-lene and poly(alkyl methacrylate) films.8,9 Althoughchanges in the fluorescence properties of the perylenesare less influenced by the polarity of the medium thanpyrenyl guests (because the perylene core is less sen-sitive to the polarity of its environment61), the fluo-rescence intensity ratios can still be used to differen-tiate among the environments offered by the PAMAsbased primarily on their proximity to sites of host relax-ations. The similarity among the I0−0/I0−1fluorescenceintensity ratios indicate that PERBUT, PERPDA, andPERCYA reside in similar locations within the polymer

128 Kizhmuri P Divya et al.

Table 3. I0−0/I0−1 emission ratios for the perylenes ca. 10◦C below and above the glass or melting temperatures of thePAMAs.

PEMA PIBMA PBMA PCHMA PHDMA(Tg =342 K) (Tg =322 K) (Tg =290 K) (Tg =368 K) (Tm =281 K)

333K 353K 313K 333K 283K 303K 363K 373K 273K 293KPERBUT 2.0 2.0 2.3 2.3 2.2 2.2 2.0 2.0 2.6 2.7PERCYA 1.9 1.9 1.8 1.8 2.0 2.0 1.8 1.7 2.6 2.7PERPDA 2.0 2.2 2.3 2.3 2.2 2.2 2.3 2.3 2.4 2.5TP 3.1 3.1 3.0 3.0 3.1 3.1 2.9 2.9 4.2 4.0

matrixes of all of the glass-forming PAMAs (i.e.,excluding PHDMA), and that those locations do notchange appreciably in the glassy and rubbery states(table 3). Although these ratios are similar to thosefound in n−butyl acetate (table 1), there are small andsystematic differences that are consistent with theseperylenes occupying slightly different average locationswithin each of the glass-forming PAMAs. Specifically,the I0−0/I0−1 ratios for PERCYA, the perylene with thebulkiest N−alkyl group, cyclohexyl, are consistentlylower than those of PERBUT and PERPDA within eachPAMA. In that regard, both the intensity ratios and theaforementioned increase in overall fluorescence inten-sity of PERCYA with increasing temperature may beascribed to the steric effect of its rather rigid and bulkycyclohexyl groups, which impede its ability to resideas near the polymer main chains as the other monoperylenes.

As noted, guest molecules are excluded from themicrocrystalline regions of the hexadecyl chains belowthe melting temperature, Tm, of PHDMA, but they stillcan and do reside within low polarity regions that areconstituted by the non-crystalline portions of the chains.Above Tm, the low-polarity, highly-viscosity region isexpanded to include all of the region of the meltedhexadecyl chains.5,6,62 As a result, the major changesin the observed absolute intensities occur for opticalrather than physical reasons, and the values below andabove Tm are not useful indicators of perylene loca-tions. However, the I0−0/I0−1 ratios do remain physi-cally useful indicators. For the mono-perylenes (includ-ing PERCYA) in PHDMA those ratios, ∼2.4-2.7, arealmost the same both above and below Tm, but aresignificantly higher than in the glass-forming PAMAs;the perylenes remain in locations that appear physi-cally to be very similar throughout. Of special note arethe very high ratios for TP in PHDMA: the I0−0/I0−1

ratios are ∼3 in all of the glass-forming PAMAs and∼4 in PHDMA. Whereas the ratios from the otherperylenes are only slightly higher in PHDMA than inglass-forming PAMAs, the difference for TP is muchlarger, and it reflects its greater sensitivity to local

environment. The temperature dependence on the flu-orescence intensity of the mono perylenes is largerin the PAMAs than in butyl acetate. Below the glasstransition temperatures, the guest molecules are ableto move only very slowly between sites. The rate oftheir movement and their redistribution between differ-ent site types is facilitated by increasing temperature,especially when the increases involve a transition fromglassy to rubbery states. However, whether the inten-sities increased or decreased with increasing tempera-ture depended on the specific structures of the N−alkylgroups on the perylenes and the PAMAs. Thus, thedecrease in the emission intensity from PERBUT withincreasing temperature in the other PAMAs follows theorder, PIBMA>PCHMA>PBMA>PEMA, and that ofPERCYA, containing 2 cyclohexyl groups, increased inthe order, PIBMA>PCHMA> PEMA> PBMA. Inter-estingly, and indicative of the relationship between thenature of the alkyl chains on the perylenes and the sidechains on the PAMAs, PERPDA and TP, both with 8-pentadecyl chains, showed similar trends in their emis-sion intensities that differed from those of PERBUT orPERCYA (i.e., (PCHMA>PIBMA>PBMA>PEMA).

Furthermore, comparisons among the reduced tem-perature plots in figure 6 show that the 3 monoperylenes exhibit larger changes in their fluorescenceintensities with temperature in PCHMA and PIBMA(i.e., the polymers with bulkier ester groups) than inPEMA and PBMA (i.e., the polymers with less bulkyester groups). The small side chains of PEMA andPBMA attenuate movements of segments of the mainchains, leading to higher activation energies for α-relaxations.7 As a result, the perylene molecules areless mobile in PEMA and PBMA than in the compa-rable phases of PCHMA and PIBMA at comparabletemperatures. The long alkyl side chains of PHDMAincrease the average distance between neighboringmain chains, allow the formation of ‘voids’,60 andfacilitate short-range translational mobility, even in thesolid state. The side groups of PIBMA and PCHMArestrict guest mobility63 in the glassy states, but theirguest sites are sufficiently flexible to permit short-range

N,N′-dialkylated perylene diimides in acrylate polymers 129

Scheme 1. Cartoon representations of possible preferred locations of pery-lene solutes (green objects) in the different PAMA matrixes: (a) PERBUTand PERPDA in PEMA or PBMA. (b) PERBUT and PERPDA in PIBMAor PCHMA. (c) PERCYA in PEMA or PBMA. (d) PERCYA in PIBMA orPCHMA.

conformational changes, especially in the rubberystates. Accordingly, the PAMAs can be placed into thesame three categories that were based upon results withpyrene reporter molecules.13,14 These considerationsand concepts are illustrated in scheme 1 as cartoon rep-resentations of the predicted, preferred solubilizationsites of the different perylenes in the 5 PAMAs.

Given that both the microviscosities of the polymersand the average locations of the perylenes change con-tinuously with temperature, nonlinear slopes like thosefound in figure 6 were expected. Note also that thereare no abrupt discontinuities in the fluorescence inten-sities at the α- transition temperatures; they would beexpected if void volume changes were responsible forthe observed changes in the fluorescence intensitiesrather than relaxation phenomena.

The primary factor influencing the fluorescence prop-erties of probe molecules in polyethylene films appearsto be hole free volume. Guest molecules are restrictedto reside in the non-crystalline (amorphous) and interfa-cial (i.e., along the surfaces of microcrystallites) withinthese matrixes.64 During the long excited singlet-statelifetimes of the pyrene molecules (∼200 ns), the dis-tance traversed in polyethylene is estimated to be <1Å. However, as noted above, the results presented hereand in previous studies with PAMAs12–14 indicate thatthe fluorescence properties of aromatic guest moleculesdepend much more on chain segment relaxation rates

and micro-diffusion than on hole free-volume. Thus,the nature of the side chains in the PAMAs is a veryimportant parameter in understanding the behaviors ofthe different perylenes.

As mentioned, plasticizing and hydrostatic effectscan act in potentially opposite ways on the properties ofthe fluorescence of the perylene guest molecules whenPAMA films are placed under high pressures of gasessuch as CO2 and N2.50,65 The results indicate that thelowering of Tg, caused by large amounts of CO2 (butnot N2) dissolved within the PAMA films, plays themore important role here; the observed decreases influorescence intensity of the perylenes in the PAMAfilms above a particular pressure of CO2 under isother-mal conditions is most easily attributed to relaxationof motional constraints on the guest molecules by theirhosts. In essence, the ‘walls’ constituting the local hostcavities are softened. The solubility of the less polargas, N2, is known to be much lower than that of CO2

in poly(methyl methacrylate), PEMA and, presumably,the other PAMAs employed here.63,66 Whereas bothmolecules have zero dipoles, the quadrupole momentof CO2 ((− 14·27 ± 0·61)x 10−40 C m2) is morethan 3 times as large as that of N2 ((− 4·65±0·08)x10−40 C m2).67 Thus, N2 pressure is sensed primar-ily as a hydrostatic force on the PAMA films, and itexerts a much less effective influence on the peryleneemissions.

130 Kizhmuri P Divya et al.

4. Conclusions

A detailed examination of the influences of struc-tural changes in four N,N ′−dialkylated perylenediimides on their fluorescence properties in 5 poly(alkylmethacrylate)s has been explored, and the data are com-pared with those in a model liquid solvent, n−butylacetate. The temperature ranges over which the mea-surements have been made include either the glass-to-rubber or crystalline-to-melt (rubber) transitions ofthe polymers. In addition, the influence of high pres-sures of a polar, more soluble gas (CO2) and a lesspolar, less soluble gas (N2) on the emission character-istics of the perylenes in the polymer films has beeninvestigated. From the data, we conclude that the sub-tle changes in the average locations of the peryleneswithin the polymers as a function of temperature, pery-lene alkyl substituents, and polymer side chains areresponsible for the observed changes in the intensi-ties and intensity ratios of the fluorescence from thefour perylene guest molecules. Although similar con-clusions have been reached from studies employingthe fluorescence of pyrene-based probes in the same 5PAMAs, the nature of the coupling between the excitedstates of the perylene and pyrenyl guests are differentand the former can be modulated in subtle and syn-thetically easier ways by changing the type of alkylgroups attached to the nitrogen atoms of the imide func-tionalities; for example, the distinctly different tem-perature dependences of the I0−0/I0−1 emission inten-sity ratios from PERCYA and either PERBUT or PER-PDA demonstrate that the nature of the N−alkyl groupson the imide ends of the perylene cores has a dis-cernible effect on the positions of the guest moleculesin the PAMA matrixes. Also, the much shorter excitedsinglet lifetimes of the perylene molecules requiresthat their emissions be much more dependent ontheir ground-state environments within the PAMAmatrixes.

Finally, the results employing TP have allowed us toexamine the influence of the polymer matrixes on theability of the two perylene units to twist with respect toeach other, and the manifestations of that twisting areclearly manifested, especially in a comparison of theI0−0/I0−1 emission ratios in the glass-forming PAMAsand in PHDMA. The dissimilar behavior of TP in thePAMAs and in n-butyl acetate demonstrates that thehigh micro-viscosity of the polymer matrixes restrictsrapid conformational changes of the linked ring sys-tem. Those restrictions are decreased when the polymerfilms are placed under high pressures of the placticizinggas, CO2, but not when they are under equal pressuresof a much less intervening gas, N2.

In summary, the results presented here develop auseful set of probes to investigate detailed interac-tions occurring in the nanometer distance scale withinpolymer matrixes, and to follow the changes that theyundergo as a result of various macroscopically appliedstimuli. They offer complementary information to thatfrom pyrenyl probes. In principle (and in practice), therange of substituents that can be placed easily on a pery-lene diimide far exceeds that for pyrene. For this rea-son, the ability to tune structural interactions between aperylene diimide and a host matrix is greater than withpyrene. At the same time, the electronic interactionsare generally more sensitive between the excited singletstate of a substituted pyrene and its local environmentthan those of a perylene diimide. Thus, each systemhas advantages and disadvantages. We emphasize thatthe approach and probes employed here are applicabledirectly to many other types of polymer matrixes.

Supplementary Information (SI)

Electronic Supplementary Information (ESI) avail-able: Details of synthetic procedures of PERBUTand PERCYA, characterizations, purification methodsfor PAMAs, and instrumentation details and proce-dures. Energies of conformations from single point andoptimized geometry calculations of N,N ′−dimethylbisperylene diimide, fluorescence decay curves andexcited state lifetimes and additional fluorescence dataof PERCYA, PERBUT, PERPDA and TP, florescenceintensities versus temperature plots, and fluorescenceintensity versus pressure plots of films. These data arealso available at www.ias.ac.in/chemsci.

Acknowledgements

Prof. Russell Schmehl of Tulane University and Prof.Sridhar Rajaram of the Jawaharlal Nehru Centre forAdvanced Scientific Research in Bangalore are thankedfor stimulating discussions. Also, we are extremelygrateful to Prof. Rajaram for generously supplying twoof the perylene diimides used in these studies. Prof.Miklos Kertesz is thanked for his help with the quantumcalculations. The US National Science Foundation isgratefully acknowledged for it financial support throughgrants CHE-1147353 and -1502856.

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