Intermolecular energy transfer after vibrational excitation of a perylene dye in solution,in polymer binder, and in a side-chain copolymerJohannes Baier, Peter Pösch, Gert Jungmann, Hans-Werner Schmidt, and Alois Seilmeier Citation: The Journal of Chemical Physics 114, 6739 (2001); doi: 10.1063/1.1358868 View online: http://dx.doi.org/10.1063/1.1358868 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/114/15?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High resolution IR diode laser study of collisional energy transfer between highly vibrationally excitedmonofluorobenzene and CO2: The effect of donor fluorination on strong collision energy transfer J. Chem. Phys. 141, 234306 (2014); 10.1063/1.4903252 Photoinduced intermolecular electron transfer from aromatic amines to coumarin dyes in sodium dodecylsulphate micellar solutions J. Chem. Phys. 119, 388 (2003); 10.1063/1.1578059 Intramolecular vibrational energy redistribution and intermolecular energy transfer in the (d,d) excited state ofnickel octaethylporphyrin J. Chem. Phys. 111, 8950 (1999); 10.1063/1.480253 The intermolecular vibrations of the NO dimer J. Chem. Phys. 109, 4378 (1998); 10.1063/1.477040 Anharmonic and harmonic intermolecular vibrational modes of the DNA base pairs J. Chem. Phys. 106, 1472 (1997); 10.1063/1.473296

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Intermolecular energy transfer after vibrational excitation of a perylenedye in solution, in polymer binder, and in a side-chain copolymer

Johannes BaierPhysikalisches Institut, Universita¨t Bayreuth, 95440 Bayreuth, Germany

Peter Posch, Gert Jungmann, and Hans-Werner SchmidtMakromolekulare Chemie I and Bayreuther Institut fu¨r Makromoleku¨lforschung (BIMF),Universitat Bayreuth, 95440 Bayreuth, Germany

Alois Seilmeiera)

Physikalisches Institut, Universita¨t Bayreuth, 95440 Bayreuth, Germany

~Received 25 September 2000; accepted 5 February 2001!

Skeletal modes between 1550 cm21 and 1750 cm21 of the perylene chromophor are vibrationallyexcited in different surroundings via resonant absorption of ultrashort IR pulses. The vibrationalenergy transfer is monitored via electronic transitions to theS1 state with the help of a second,delayed visible pulse. In liquid chloroform solution of the dye an intermolecular energy transfertime of te5(1561) ps was determined. In the solid dye/PMMA blend the intermolecular energytransfer time was withte5(10,561) ps substantially smaller. In the copolymer a similar timeconstant ofte5(961) ps is found. Obviously the spacer decouples in the copolymer not only theelectronic systems but also the vibrational manifolds. ©2001 American Institute of Physics.@DOI: 10.1063/1.1358868#

I. INTRODUCTION

It is well known that large molecules bound to polymerscan be decoupled electronically by aliphatic spacers. Opticalproperties of dye molecules like electronic absorption andfluorescence, which are connected with thep-electron sys-tem, are not influenced by the covalent binding via a suffi-ciently long aliphatic chain. Obviously there is only a weakinteraction between the electronic wave functions of the dyemolecules and the polymer backbone.

Less information is available on the vibrational energytransfer in such systems. Investigations on the intramolecularenergy redistribution in large molecular systems have givenindications for an equilibration of vibrational energy on apico- or subpicosecond time scale. Vibrational excitation atthe CH3 end group of aliphatic chains of dye molecules isfollowed by an energy redistribution on a time scale in theorder of a picosecond.1 The question arises, whether a rapidtransfer of vibrational energy via the spacer takes place ornot. In contrast to the electronic states, there exist severalvibrational states in the spacer with energies close to reso-nance with the vibrational modes of the dye molecules andthe polymer chain. Collision induced energy transfer ortransfer via anharmonic coupling may be possible.

So far studies on vibrational relaxation of large mol-ecules in condensed matter concentrate on liquid surround-ings. There exists a series of papers, which show that a firstintramolecular redistribution process generates a vibrationalstate which can be described by an increased vibrationaltemperature.2–9 This process is followed by an intermolecu-

lar energy transfer proceeding on a time scale in the order ofa few tens of picoseconds.7,9,10 On the other hand, work onperylene dyes has presentedT1 times up to a few 100 pswhich strongly depended on the surrounding solvent and onthe mode investigated.11,12 These observations were inter-preted as long-range resonancev –v energy transfer.

In this paper the question, whether a rapid transfer ofvibrational energy from a dye molecule to a polymer via aspacer takes place or not, is addressed experimentally. Wereport on experimental data on the vibrational relaxationand intermolecular energy transfer of a perylene dye inliquid solution, in a solid polymer binder and covalentlyattached to a polymer backbone in a copolymer. As mono-meric perylene dyeI an unsymmetrical 1,6,7,12-tetrachloro-perylenedicarboximide chromophor was used~Fig. 1!. Thedye contains on one side an aliphaticn-butyl group and onthe other side a spacer consisting of six methylene units anda polymerizable acrylate group. This dye was dissolved inchloroform and in a polymethylmethacrylate~PMMA! IImatrix. The monomeric perylene dyeI was copolymerizedby free radical polymerization with methlymethacrylate, re-sulting in a copolymerIII where the dye is covalently at-tached via a flexible spacer to the polymer backbone. Thedye content in this copolymer was 1% by weight. The ex-periments are performed by directly exciting the perylenedye within the skeletal and CvO stretching modes situatedbetween 1550 cm21 and 1750 cm21. Generally, a very fastintramolecular energy redistribution signal is found which isspecific for the excited vibrational mode. Subsequently anintermolecular energy transfer to the surrounding takesplace.

a!Author to whom correspondence should be addressed. Tel: 49 921 553162;Fax: 149 921 553172; electronic mail: alois.seilmeier@uni-bayreuth.de

JOURNAL OF CHEMICAL PHYSICS VOLUME 114, NUMBER 15 15 APRIL 2001

67390021-9606/2001/114(15)/6739/5/$18.00 © 2001 American Institute of Physics

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II. EXPERIMENT

A. Materials

The unsymmetrical 1,6,7,12-tetrachloro-perylenedi-carboximide monomerI was supplied by Dr. K.-H. Etzbach~BASF AG, Ludwigshafen!. Polymethylmethacrylate~PMMA! II with an weight average molecular weight of120 000 g/mol was purchased from Aldrich and was used asreceived. The reported glass transition temperature was114 °C.

The perylene dye containing PMMA-copolymerIII witha dye content of 1% by weight was prepared by the follow-ing procedure:13

In a 10 ml Schlenk tube 30.18 mg (4.0831025 mol!perylene dye monomerI and 3.0 g ~0.03 mol! methyl-methacrylate were dissolved in absolute dioxane~dried overpotassium sodium alloy!. 49.3 mg (3.031024 mol! azo-bisisobutyronitrile~AIBN ! were added via stock solution ofAIBN in dioxane ~300 mg/50 ml!. Prior to the polymeriza-tion the reaction mixture was degassed with nitrogen for 15min. The polymerization was done at 70 °C for 48 h. Aftercompletion the reaction mixture was precipitated with 200ml of methanol. The copolymer was filtered, washed withmethanol, and dried in vacuum at 70 °C. In order to separatethe nonpolymerized dye monomer the copolymer was puri-fied by preparative GPC~Merkogel 6000, eluent THF!. Theyield of copolymerIII was 92%. The copolymer was char-acterized by UV/Vis-, FT-IR spectroscopy, and elementalanalysis.

B. Film preparation

For the optical measurements films of the blend and thecopolymer were prepared from chloroform solution. About5% by weight of blend or copolymer were dissolved infreshly distilled chloroform. The polymer solution wascasted via knife coater technique~knife gap 150mm! on abariumfluoride substrate. The wet films were dried at roomtemperature. The resulting film thickness was about 3mm.

C. Methods

The experimental method we use is the two-pulse exci-tation technique2 which is appropriate for the investigation ofhighly diluted systems like large molecules in liquid andsolid solution. A first ultrashort laser pulse in the mid-

infrared range with frequencyn IR resonantly excites a vibra-tional mode. The excess population is probed by a secondvisible pulse of a frequencynpr,n00 which promotes onlyvibrationally excited dye molecules to the electronicS1 state.The subsequently emitted fluorescence light reflects the in-duced transient population of vibrational states in the elec-tronic ground stateS0 at frequencies aroundn00–npr withfairly large Franck–Condon factors. Details on the modesobserved are discussed later on.

This method requires tunable infrared picosecond pulsesfor the resonant excitation of the vibrational dye modes andtunable picosecond pulses in the visible which serve as probepulses. The experimental system is based on a flashlamp-pumped mode-locked Nd:YLF laser system with a repetitionrate of 10 Hz. The frequency doubled pulse train of the laserpumps two KTP optical parametric oscillators.14 The firstone operates between 1.1mm and 1.4mm. Difference fre-quency mixing with a part of an amplified single pulse of theNd:YLF laser in a AgGaS2 crystal generates pulses tunablebetween 4mm and 10mm with an energy of up to 4mJ forthe resonant excitation of vibrational modes. The second op-tical parametric oscillator provides pulses tunable between1.0 mm and 1.6mm. Probe pulses are obtained by sum fre-quency generation of these pulses with another part of theamplified single pulses of the Nd:YLF laser. The tuningrange covers the red and green part of the visible spectrumbetween 510 nm and 630 nm. The duration of pump andprobe pulses amounts to;2–3 ps.

The fluorescence light of the sample is detected under anangle of;115° relative to the probe beam within an accep-tance angle of;0.5 sr by anf 525 cm spectrometer and aphotomultiplier. The stray light suppression of the probewavelength is improved by suitable interference filters.Weak parts of the pump and probe pulses are separated forthe simultaneous measurement of their cross correlationcurve in a thin LiIO3 crystal. This curve is shown togetherwith the time-resolved data in Fig. 3 byL and the dashed–dotted line.

Generally the probe pulse generates a weak fluorescencesignal F0 without IR excitation which originates from thethermal occupation of the vibrational manifold of the elec-tronic ground state. This signal is subtracted in the time-resolved data and the relative increase of the fluorescencesignal due to the vibrational excitationDF(t)/F05@F(t)2F0#/F0 is discussed in the following.

III. RESULTS AND DISCUSSION

The perylene dye used in the investigations is 1,6,7,12-tetrachloro-3,4,9,10-perylenedicarboximide with an acrylategroup bound via a C6 spacer. The structural formula isshown in Fig. 1/I . Figure 2 shows the infrared spectrum ofthe monomeric perylene dye between 1500 cm21 and 1800cm21 taken in a KBr pellet. The vibrational bands of the dyeclearly show up and are assigned to an in-plane peryleneCvC stretching vibration at 1590 cm21, the antisymmetricCvO stretching vibration at 1666 cm21, and the symmetricCvO stretching vibration of the perylene chromophore at

FIG. 1. Chemical structural formula of the investigated monomeric perylenedye I , PMMA II , and the perylen dye containing copolymerIII .

6740 J. Chem. Phys., Vol. 114, No. 15, 15 April 2001 Baier et al.

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1704 cm21.15 The latter band shows a shoulder which origi-nates from the CvO stretching mode of the acrylate grouparound 1724 cm21.16

The vibrational energy transfer is investigated in threedifferent surroundings, in a liquid solution of CHCl3 , in asolid solution of PMMA, and covalently bound to thePMMA chains. We start with a discussion of the data takenin a solution of the perylene dye in CHCl3 with a concentra-tion of c5231022 M and a sample thickness of 10mm. Thefrequency of the pure electronic transition of the perylenedye was estimated from the electronic absorption and fluo-rescence spectra to ben00;18 850 cm21. The frequency ofthe visible probe pulse was tuned tonpr;17 350 cm21. InFig. 3 the fluorescence signal as a function of time is shownafter resonant excitation of the in-plane CvC stretchingmode of the perylene dye at 1590 cm21. The signal consistsof two components with different time constants shown bybroken lines. The first one represents the lifetime of the di-rectly excited vibrational mode which has obviously a suffi-ciently high Franck–Condon factor giving rise to a largefluorescence signal. A detailed numerical analysis provides alifetime of (1.560.5) ps for this vibrational mode. Thisvalue is longer than the time resolution of our experimentalsystem which is measured simultaneously~see L and

dashed–dotted line in Fig. 3! and does not depend on thesurrounding of the dye molecule in general.17

The depopulation of the initially excited mode is a com-plicated matter. In several papers3,7,9,18it has been found thatthe excess energy is redistributed over the vibrational mani-fold resulting in an energy distribution which can be de-scribed approximately by an increased vibrational tempera-ture T* . Particularly in large, nonsymmetric molecules,where a coupling between all vibrational states is possible,such a picture appears to be reasonable.

At longer delay times, the transfer of the excess energyfrom the excited dye molecules to the surrounding solvent isobserved. For a detailed interpretation of the fluorescencesignal, the nearly exponentially falling long wavelength edgeof the electronicS1 absorption has to be analyzed. The fluo-rescence signalF is proportional to the absorptiona~npr!which can be written in a simplified model as a sum over thefew modesm of frequencyn00–nm.0 with a large Franck–Condon factorxm,0 ,

a~npr!5C(m

Pmxm,0nprg~npr2nm,0!.

Pm are the~thermal! population of the modesm and g(n2nm,0) are the line profiles of the modesm with temperaturedependent widths. Low frequency modes and modes withsmall Franck–Condon factors are lumped together and con-tribute to the linewidth. The constantC contains the elec-tronic transition moment. Details are discussed in Ref. 19. Ifn00–npr is large, the frequency dependence of the signal isdominated by the Boltzmann populationsPm which are de-termined by the vibrational temperature in quasiequilibriumstates. The large widths of the line profilesF give rise to asmooth, exponentially falling long wavelength edge of theS1

band.At longer delay times the signal in our time resolved

experiments, in whichn00–npr is large, is governed by theinstantaneous populationsPm which are assumed to be de-termined by an increased instantaneous vibrational tempera-tureT* (t). The observed signalDF/F051 in the maximumof the second component in Fig. 3 corresponds to a tempera-ture increase of;30 K which nicely compares with an esti-mate from the energy supplied to the molecule and the spe-cific heat. Consequently the picture of an increasedvibrational temperatureT* appears to be a reasonable model.

The experimental data concerning the intermolecular en-ergy transfer are described in a simplified picture by an ex-ponential decay of the molecular vibrational excess energyE* 2E0 with a time constantte according to

E~ t !5E01~E* 2E0!exp~2t/te!,

where E0 is the vibrational energy without excitation andE* 5E01hn IR the vibrational energy immediately after ex-citation. The relationship between excess energy and vibra-tional excess temperatureT* is calculated in a simplifiedharmonic model in which the vibrational manifold of the dyeis approximately represented according to Ref. 20.

Within the described model the temporal evolution ofthe second signal component in Fig. 3 provides a time con-stant ofte5(1561) ps for the intermolecular energy trans-

FIG. 2. Infrared absorption spectrum of the investigated perylene dye in aKBr sample. See text for the assignment of the most intense lines.

FIG. 3. Vibrational relaxation after excitation of the perylene dye dissolvedin CHCl3 via the CvC stretching mode at 1590 cm21. The solid line is thesum of the two contributions to the signal, the depopulation of the initiallyexcited vibrational state~dotted curve!, and the subsequent intermolecularenergy transfer~dashed curve!. L and the dashed–dotted line represent thesimultaneously measured cross correlation curve.

6741J. Chem. Phys., Vol. 114, No. 15, 15 April 2001 Intermolecular energy transfer

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fer from the excited dye molecule to the surrounding solvent.The same value for the intermolecular energy transfer time isalso found after excitation of both CvO stretching modes at1666 cm21 and 1704 cm21. It does not depend on the origi-nally excited vibrational mode. Furthermore the observed en-ergy transfer time nicely compares with the correspondingvalue found for the dye coumarin 6 in the same solvent.There, a value ofte5(1462) ps was reported in contrast tofaster energy relaxation times ofte5(761) ps in acetoneand alcohols~methanol, ethanol!.17 An increased number ofhalogen atoms in the solvent molecules was discussed to beresponsible for longer transfer times.

Next we discuss experimental data on the systemPerylene in PMMA. Two different samples have been inves-tigated. In the first sample the perylene dye was dissolved inthe PMMA matrix with a mass concentration of 1%. Thesample was prepared by dissolving the perylene dye and thePMMA in CHCl3 . Subsequently the solution was carefullyspread over a BaF2 substrate with a thickness of;150 mm.After drying a film of;3 mm thickness resulted. The secondsample was a copolymer in which the perylene dye was co-valently bound to the PMMA matrix via its acrylate group.Thus the influence of the chemical bond on the intermolecu-lar energy transfer could be studied.

The electronic absorption and fluorescence spectra of thedye were found to be unchanged by the chemical bond. Forboth samples the frequency of the pure electronic transitionof the perylene dye was estimated to ben00;18 980 cm21.The frequency of the visible probe pulse was set tonpr517 480 cm21.

Figure 4 shows two measurements in which the antisym-metric CvO stretching vibration of the perylene dye atn51666 cm21 was excited. The lower curve~full data points!represents data taken on the sample in which the dye is dis-solved, the upper curve~open data points! contains datataken on the sample with the covalently bound dye. Gener-ally after excitation atn51666 cm21, only one exponen-tially relaxing component is observed~also in liquid solu-tion!. Obviously, the small Franck–Condon factor of theresonantly excited CvO stretching vibration does not allow

to observe the directly excited mode. So the excess fluores-cence signal is governed exclusively by the signal compo-nent representing the intermolecular energy transfer. In thesample containing the dissolved perylene dye we find a timeconstant ofte5(10,561) ps for the intermolecular energytransfer. This value is a little bit smaller than the value re-ceived for the dye solution in CHCl3 . Such a behavior isexpected for a surrounding containing no halogen atoms.17

So there seems to be no substantial difference in the effi-ciency of intermolecular energy transfer in low molecularsolvents and polymer matrices.

Of special interest are the experimental data taken onthe sample in which the perylene dye is chemically bound tothe PMMA matrix via the acrylate group~circles in Fig. 4!.Here a weak but clearly detectable acceleration of the inter-molecular energy transfer is observed. The corresponding en-ergy transfer time amounts tote5(1961) ps. The largerfluorescence signals are due to an increased intensity of thepump pulses which do not influence the energy transfer time.The measurements were repeated with different pump inten-sities and the time constants given above were reproduced.The energy transfer times measured after initial excitationof the other two intense vibrational absorption bands at 1590cm21 and 1704 cm21 ~see Fig. 2! turned out to be same asin Fig. 4.

Two conclusions can be drawn from these results. Firstof all, we observe nearly the same energy transfer time fromthe dye molecule to the surrounding in liquid and solid so-lutions and even in a copolymer where the dye molecule iscovalently bound. Obviously the most efficient channel is thedirect intermolecular transfer to neighboring molecules andnot a transfer along the polymer chains. The additional re-laxation channel along the C6 spacer and the acrylate groupto the PMMA chain in the covalently bound sample givesonly a minor contribution to the intermolecular energy trans-fer. From the difference between the energy transfer times ofthe two curves in Fig. 4 one can roughly estimate that theefficiency of this additional relaxation channel is about anorder of magnitude smaller than that via intermolecular in-teraction with the not bound surrounding. A transfer time inthe order of 100 ps can be estimated for this transfer channel.

IV. SUMMARY

In this paper the vibrational energy transfer via spacersin covalently bound dye/PMMA copolymers is investigated.It is found that a C6 spacer decouples the vibrational mani-folds of a perylene dye and PMMA quite well. We find nobasic difference in a liquid solution, in a dye/polymer blendand in a copolymer. Of interest is now the question how longthe spacer has to be for an efficient separation of the vibra-tional manifolds of the dye and the polymer.

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FIG. 4. Intermolecular energy transfer of the perylene dye in a PMMApolymer matrix taken on two different samples. In the first sample~d, lowercurve! the dye molecules are dissolved in the polymer, in the second sample~s, upper curve! the perylene dye is covalently bound to the PMMA matrix.A weak acceleration of the intermolecular energy transfer due to the chemi-cal bond is observed.

6742 J. Chem. Phys., Vol. 114, No. 15, 15 April 2001 Baier et al.

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