optical study on relaxation of excitons and initiation of...

Vol. 1 INTERNATIONAL JOURNAL OF PHOTOENERGY 1999

Optical study on relaxation of excitons and initiationof photopolymerization in diacetylene

crystals at low temperatures

Chihiro Itoh

Department of Physics, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8602, Japan

Present address: Department of Material Science and Chemistry, Wakayama University,

930 Sakaedani, Wakayama 640-8510, Japan

Abstract. Luminescence and optical absorption induced by irradiating with a pulse of 266 nm laser beamhave been measured in 5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (DA-TCDU). In addition to the re-combination luminescence of the lowest triplet state, luminescence from the excited single state is identified.Irradiation with a pulse of 266 nm laser beam having the fluence above 4.4 mJcm−2 leads to formation ofseries of optical absorption bands at 3.05 eV, 2.52 eV and 2.18 eV, which are ascribed to the optical transitiondue to dimer diradicals, trimer diradicals and the tetramer diradicals, respectively. The laser-fluence depen-dence of the integral intensity of the recombination luminescence of the singlet and the triplet exciton andthe absorption strength of the 3.05 eV band have been measured. The nature of the triplet exciton and itsrelation to the initiation of the photopolymerization are discussed.

1. INTRODUCTION

Photoexcitation of organic crystals often leads tochange of the molecular configuration or chemical re-actions, which generally resemble those of moleculesin solutions. However, it is known that there are exoticcrystals which exhibit photochemical reactions distinctfrom that found in solutions. Photopolymerization ofcrystalline diacetylenes (R−C≡C−C≡C−R′) is a typicalexample of such reaction [1, 2]. Irradiation of singlecrystals of diacetylenes with UV light or gamma ray trig-gers solid-state polymerization, and the monomer crys-tals transform into polydiacetylene crystals with nearlythe same dimensions as the monomer crystals. ThisPhotopolymerization has attracted considerable inter-est, and extensive studies have been carried out for re-vealing its mechanism [3]. In recent years, there hasbeen renewal of interest in this photopolymerizationas an example of photo-induced cooperative phenom-ena, and theoretical studies have been made in termsof photo-induce phase transition in solids [4]. On thebasis of the insights accumulated so far, photopoly-merization is initiated by formation of dimer diradi-cals, and the formation of stable polymer chain followssuccessive addition reaction of reaction intermediatesalong the stacking axis. Although the configuration ofthe reaction intermediates formed during the polymer-ization have been studied in details, only limited in-sight on the dimer-diradical formation has been accu-mulated. The important finding on dimer-diradical for-mation have been made by the Neumann and Sixl [5].They showed that the EPR signal originated from thereaction intermediates induced by irradiating with UVlight depends nonlinearly on the illumination dose. Be-cause of the result, they proposed that the dimer dirad-ical is formed as the result of bimolecular reaction ofthe lowest triplet states [5, 6]. However, this suggestion

has not been validated experimentally. This is partly be-cause only limited knowledge on the lowest triplet statein diacetylene crystals has been accumulated. Thus, elu-cidation of the characteristics of the lowest triplet stateincluding the relation to the dimer-diradical formationare required for understanding the mechanism of thedimer-diradical formation.

Recently, Itoh, Kondoh and Tanimura [7] have foundthat a broad luminescence band having a peak at2.57 eV is induced by irradiation of single crystal of5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (DA-TCDU; R=R′=(CH2)4O(CO)NHC6H5) with a pulse of1 MeV electron beam at 10 K. They found that the2.57 eV luminescence decays single exponentially witha time constant of 113 ms at 10 K. The authors at-tributed the 2.57 eV luminescence to the radiative re-combination of the lowest triplet state because of itslong decay.

In this paper, the experimental results on the mea-surements of the luminescence and the optical absorp-tion induced in DA-TCDU are presented. The natureof excited states induced by optical excitation and themechanism of the dimer-diradical formation inducedare discussed.

2. MATERIALS AND METHODS

DA-TCDU powder obtained from Dojindo, co. LTD.was purified by repeated recrystalization from acetonesolutions. Single crystals of DA-TCDU with typical di-mensions of 3×3×1 mm3 were obtained from acetonesolutions by slow evaporation at 25 centigrade. Thecrystals thus obtained were transparent and showed nodetectable optical absorption in the wavelength rangefrom 800 nm to 285 nm at room temperature. Speci-men, which was prepared by cleaving from the as-growncrystal, was attached at cold finger of a conventional

2 Chihiro Itoh Vol. 1

cryostat, and it cooled down to 8 K. For exciting thespecimens, 266 nm laser pulses generated by laser sys-tem comprised from a pulsed YAG : Nd laser (Coher-ent Infinity 40) operated at a repetition of 10 Hz anda BBO crystal for converting second harmonic of theYAG laser beam into fourth harmonic was employed.The time duration of the laser pulse is 5 ns, and therepetition ratio of the excitation was controlled in therange from 10 Hz to 1 Hz using a mechanical shutterplaced in the excitation path. The luminescence spectrainduced by irradiating with laser pulses were measuredby the multichannel detection system, which consistsof a monochrometer (Jobin Yvon HR-320) and a gatablemultichannel detector (Princeton IRY-1024). For mea-suring the integral intensity of the luminescence, theluminescence from the specimen was guided to the op-tical detection system comprised from a monochrom-eter (Jobin Yvon CP-200) and a CCD detecor (PrincetonTE/CCD-1100PF). Optical absorption spectra inducedfollowing the laser irradiation were measured using aflush Xe lamp (Hamamatsu L2358) as the light sourceand the detection system which was the same as thatused for measuring the integral intensity of the lumi-nescence.

3. EXPERIMENTAL RESULTS

Luminescence spectra induced by irradiating with apulse of 266 nm laser beam having the fluence lowerthan 0.5 mJcm−2 at 8 K are shown in Figure 1. The lu-minescence band at 4.26 eV was observed prominentlyat the time delay of 2 ns with a gate width of 1 ns. Abroad luminescence band having a peak at 2.57 eV wasmeasured at 10 ms with a gate width of 8 ms. By theluminescence-intensity measurements at various delaytime from the excitation with the fixed gate width, the4.26 eV and the 2.57 eV luminescence were found todecay with a time constant of about 5 ns and about100 ms, respectively. The spectrum of the 2.57 eV lumi-nescence is identical with that induced by irradiationwith a 1 MeV electron pulse, and the luminescence in-tensity was reported to decay single exponentially witha time constant of 113 ms [7].

Optical absorption spectra of DA-TCDU crystal havemeasured after 60 s following the irradiation with apulse of 266 nm laser beam having various fluences,and the results are shown in Figure 2. Each spectrumshown in the figure was measured by changing the spotat every irradiation. At the fluence of 4.4 mJcm−2, weakabsorption exhibiting a peak at 3.05 eV was induced.Irradiation with larger fluence gives rise to growth ofseries of absorption bands at 2.52 eV and 2.18 eV inaddition to the 3.05 eV absorption. Similar series ofthe sharp absorption bands have been reported in sev-eral diacetylene crystals, and the bands have been as-signed to optical transition of the reaction intermedi-ates having diradical form [8]. As rising the sampletemperatures, reduction in the absorption strength ofthe 3.05 eV band and increase of the 2.52 eV band wereobserved. This clearly shown that the 2.52 eV band is

(a)1.0

0.5

0.0

td = 2 nstw = 1 ns

(b)1.0

0.5

0.0

td = 10 mstw = 8 ms

1 2 3 4 5

inte

nsi

ty(a

rb.u

nit

s)

photon energy (eV)

Figure 1. Luminescence spectra of DA-TCDU crystals mea-sured at the delay time (td) of 5 ns with the gate width (tw)of 2 ns (a) and at td = 10 ms with tw = 8 ms (b) followingthe irradiation with a pulse of 266 nm laser beam at 8 K.

DA-TCDUT=8 K

λex = 265 nm 0.01

Φ = 20.47 mJcm−2

Φ = 12.7 mJcm−2

Φ = 10.6 mJcm−2

Φ = 8.0 mJcm−2

Φ = 4.4 mJcm−2

1.5 2.0 2.5 3.0 3.5

photon energy (eV)

op

tica

ld

ensi

ty

Figure 2. Optical absorption spectra of DA-TCDU crystalsmeasured after 60 s following the 266 nm laser excitationwith various fluences at 8 K.

Vol. 1 Optical study on relaxation of excitons . . . 3

formed at the expense of the 3.05 eV band. The 3.05 eV,the 2.52 eV and the 2.18 eV absorption are, therefore,attributed to optical transition due to dimer diradicals,trimer diradicals and the tetramer diradicals, respec-tively.

In order to get detailed information on the dimer-diradical formation, the fluence-dependence of the in-tegral intensity of the 4.26 eV and the 2.57 eV lumines-cence have been measured together with that of thepeak strength of the 3.05 eV absorption induced follow-ing the excitation. The results are plotted as a functionof laser fluence in Figure 3. It is clear that the absorp-tion strength of the 3.05 eV band increases nonlinearlywith increasing the laser fluence. This result was consis-tent with the previous report [5]. On the other hand, theintegral intensity of the 4.26 eV and the 2.57 eV lumi-nescence show distinct behavior in this fluence region.While the integral intensity of the 4.26 eV luminescenceis in proportion with the laser, the 2.57 eV lumines-cence exhibits saturation at higher fluences. Deviationfrom the linear increase of the 2.57 eV luminescencerises in the fluence region where the 3.05 eV absorp-tion shows marked increase. The result implies that thedimer-diradical formation is in coompetition with theemission of the 2.57 eV luminescence.

(a)

0

5

2.57 eV luminescence

4.26 eV luminescencelum

ines

cen

cein

ten

sity

(arb

.un

its)

(b)

0.04

0.02

0.000 10 20 30

3.05 eV absorption

laser fluence (mJcm−2)

op

tica

ld

ensi

ty

Figure 3. Laser-fluence dependence of the integral inten-sity of the 2.57 eV and the 4.26 eV luminescence (a) and thestrength of the 3.05 eV absorption band (b) induced by ir-radiating with a pulse of 266 nm laser beam in DA-TCDUcrystal at 8 K. Solid and broken curves shown in the figureare the results of the parameter fitting (see the text).

4. DISCUSSION

Excitation of DA-TCDU crystal with a pulse of 266 nmlaser beam induces the 4.26 eV and the 2.57 eV lumines-cence bands. In terms of the decay time, it is reason-able to ascribe these bands to radiative transition fromthe singlet and the triplet state, respectively. Recently,the excitation spectra for the 4.26 eV and the 2.57 eVluminescence have been measured [9], and the resultsdemonstrate that these luminescence are induced byiluminating with UV phonons having an energy abovethe onset of the fundamental absorption of DA-TCDU.On the basis of this result, these luminescence are at-tributed to radiative recombination of the intrinsic ex-cited states. The 4.26 eV and the 2.57 eV luminescencebands are, therefore, ascribed to radiative recombina-tion of the singlet and the triplet excitons, respectively.

It is notable that the 2.57 eV luminescence shows sin-gle exponential decay. In organic crystals in which ex-citon has large mobility, like anthracene, decay of therecombination luminescence is governed by collisionof excitons but not by monomolecular deexcitation. Insuch crystals, the triplet exciton generally decays non-exponentially because of this bimolecular process. Thesingle exponential decay of the triplet-exciton lumines-cence is a manifestation that the deexcitation of thetriplet excitons in DA-TCDU crystals is governed bymonomolecular process. This implies that the tripletexciton in DA-TCDU crystal has limited mobility. Lowmobility of the triplet exciton is also suggested by an-other characteristics of the 2.57 eV luminescence: thebroad and unstructured spectrum and the large peakshift of 1.71 eV from the onset of the fundamental ab-sorption. These features imply that the initial state ofthis luminescence is accompanied with large lattice re-laxation. The existence of the large lattice relaxationhas been also elucidated by the measurement of lumi-nescence polarization [7]. Large lattice relaxation ac-companied by the lowest triplet state leads to local-ization of the lowest triplet state because of breakingof the transnational symmetry in the crystal. Conse-quently, the 2.57 eV luminescence is conceivably origi-nated from the localized triplet center with large latticerelaxation formed by the relaxation of intrinsic excita-tion. Such a metastable center is known as self-trappedexciton, which is well established in inorganic and or-ganic crystals [10].

The nonlinear increase of the yield of dimer diradicalimplies that its formation involves interaction of the ex-cited species, as suggested previously, but not simplemonomolecular relaxation of the excited state. In thefluence region in which the dimer diradical is formedefficiently, the 2.57 eV luminescence shows underlin-ear increase, while the integral intensity of the 4.26 eVluminescence is in proportion with the laser fluence.Based on the results, it is clear that the dimer-diradicalformation is in competition with radiative recombina-tion of the self-trapped excions. The model that dimer-diradical formation involves bimolecular reaction of thelowest triplet state may not conflict with the experimen-

4 Chihiro Itoh Vol. 1

tal result obtained in this study. However, the possi-bility of the formation of a pair of the lowest tripletstates at adjacent sites is conceivably zero at low tem-perature. This is because, as discussed above, the low-est triplet state in DA-TCDU crystal is regarded as theself-trapped excitons, whose mobility is vanished at lowtemperature due to large lattice relaxation. Therefore,the model involving bimolecular reaction of the self-trapped excitons is not appropriate for explaining thedimer-diradical formation.

The alternative model has been suggested by Itoh,Kondoh and Tanimura very recently [11]. In this model,the dimer-diradical formation involves collision of un-relaxed excitons with the self-trapped excitons. For ex-amining the model, the experimental results are ana-lyzed in terms of the kinetic equation model as the fol-lowings. Irradiation of a laser pulse having flux of φgenerates unrelaxed excitons at efficiency of σ . The ex-citon thus generated relaxes into the excited singlet andthe lowest triplet state by the rate of γi(i= S or T) re-spectively. The dimer diradicals are formed at efficiencyof η by collision of unrelaxed excitons with the lowesttriplets. The kinetics of this system are described byfollowing equations:

dNEX

dt= σφ−

∑iγiNEX−ηNTNEX, (1)

dNTdt

= γTNEX−λiNT −ηNTNEX, (2)

dNSdt

= γSNEX−λSNS, (3)

dNPdt

= ηNTNEX, (4)

wher Ni(i = EX, S, T or D) represents the populationof the unrelaxed excitons, the singlet excitons, the self-trapped excitons and dimer diradicals, respectively. Inthese equations, λi denotes a monomolecular deexcita-tion rate of the excited state labeled i. In the experimen-tal condition employed in this study, relaxation of theexciton generated by optical excitation is consideredmuch faster than the generation given by the rate ofσφ. Typical decay time of the unrelaxed exciton in or-ganic crystals is considered several pico second, whichis much shorter than the time duration of the excita-tion pulse employed in this study. In this condition,the population of the unrelaxed excitons is conceiv-able in steady state within the duration of the excitationpulse. In the low laser-fluence regime, it is conceivablethat relaxation of the unrelaxed excitons is conceivablyfaster than the generation of dimer diradicals. Thus,for simplifying the kinetic equations, the case where∑i γi� ηNT is considered. Then the populations of the

unrelaxed exciton, the singlet exciton, the triplet self-trapped exciton and the dimer diradicals induced afterthe excitation are given by

Nex = σφ∑iγi, (5)

NT = ωTσφλT +ωDσφ

[1−exp

{(λT +ωDσφ)τ

}], (6)

Ns =ωSσφ[1−exp(−λSτ)], (7)

ND =ωDωT

(σφ

λT +ωDσφ

)2

·[(λT +ωDσφ)τ+exp{(λT +ωDσφ)τ

}−1], (8)

ωj =γj∑i γi

(j = T ,S), ωD = η∑i γi

,

where τ is the time duration of the laser pulse. Theintegral intensity of the recombination luminescenceof the triplet self-trapped exciton and the singlet ex-citon are in proportion with their population given bythe equations (6) and (7). The equations (6) and (7) werefitted to the data points shown in Figure 3(a) first. Thesolid and broken curves shown in Figure 3(a) are the re-sults of the parameter fitting of the equations (6) and (7)and the data points. The curves well explain the laser-fluence dependence of the intensity of the 4.26 eV lu-minescence and the 2.57 eV luminescence. It is notablethat the equation (8) with the same parameter set ob-tained by this fitting can reproduce the laser-fluencedependence of the dimer-diradical yield with a param-eter for adjusting the magnitude as shown by the solidcurve in Figure 3(b). The good agreement of the fit-ting curves to the experimental points implies that themodel proposed here is appropriate for explaining thephoto-induced dimer-diradical formation. The fitting,however, gives only qualitative interpretation on theexperimental results, because the parameters requiredfor quantitative discussion, like oscillation strength ofthe 3.05 eV absorption, have not been obtained yet. Forelucidating the microscopic mechanism of the photo-induced dimer-diradical formation, further studies onthe relaxation dynamics at excited states in diacetylenecrystals with higher time-resolution are required.

ACKNOWLEDGEMENTS

The author is grateful to Prof. K. Tanimura and Mr. T.Kondoh for many valuable discussions.

REFERENCES

[1] G. Wegner, Makromol. Chem. 145 (1971), 85.[2] G. Wegner, Pure Appl. Chem. 154 (1972), 35.[3] H. J. Catlow (ed.), Advances in polymer science,

vol. 63, Polydiacetylenes Springer-Verlag, NewYork, 1984.

[4] E. Hanamura, Solid State Science, vol. 124,Springer-Verlag, New York, 1996, Relaxation of Ex-cited State and Photo-induced Phase Transitions,K. Nasu (ed.).

[5] W. Neumann and H. Sixl, Chem. Phys. 50 (1980),273.

[6] W. Neumann and H. Sixl, Chem. Phys. 58 (1980),273.

[7] C. Itoh, T. Kondoh, and K. Tanimura, Chem. Phys.Lett. 261 (1996), 191.

[8] H. Gross and H. Sixl, Chem. Phys. Lett. 91 (1982),262.

[9] C. Itoh, T. Kondoh, and K. Tanimura, to be pub-lished.

Vol. 1 Optical study on relaxation of excitons . . . 5

[10] K. S. Song and R. T. Williams (eds.), Solid State Sci-ence, vol. 105, Springer-Verlag, New York, 1992,Self-trapped excitons.

[11] C. Itoh, T. Kondoh, and K. Tanimura, J. Phys. Soc.Jpn (1999), in press.

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