Host-Dependent Optical Dephasing of Dye Molecules

Volume 194, number 4,5,6 CHEMICAL PHYSICS LETTERS 3 July 1992

Host-dependent optical dephasing of dye molecules doped in cross-linked polyvinyl alcohols

M. Kawase a,b, S. Fujiwara a, S. Nakanishi a and H. Itoh a a Department of Physics, Faculty ofEducation,Kagawa University, Takamatsu City, Kagawa 760, Japan b Department of Chemistry, Faculty of Education,Kagawa University, Takamatsu City, Kagawa 760, Japan

Received I 1 February 1992; in final form 24 March I992

Host-dependent optical dephasing of the zero-phonon line of two organic dye molecules doped in several cross-linked polyvinyl alcohol (PVA) derivatives was studied by using an incoherent photon echo technique. It was found that the optical dephasing time of the zero-phonon line increases with increasing the length of the cross-link introduced to the PVA backbone. Our results indicate that, by the introduction of the cross-link, the effect of the two-level tunneling system in PVA on optical dephasing of a doped dye is greatly reduced and the dephasing time of the dye becomes longer than that in the PVA without the cross-link. The decrease of the optical dephasing can be interpreted by assuming a void space in PVA, which is created near to an introduced cross-linker.

1. Introduction

The optical dephasing of organic dye molecules doped in polymer host has been extensively inves- tigated since it shows a strong difference in magni- tude and temperature dependence compared with those embedded in crystalline hosts [ 1,2 ] # . The dephasing in the dye-doped polymer systems has been studied by nonlinear spectroscopic techniques such as photon echoes and persistent spectral hole-burning. In particular, the dephasing characteristics of the zero-phonon line have been a focus of interest, since the zero-phonon line usually shows much longer dephasing time than the vibronic lines [ 4-71 and is very sensitive to the interaction with host polymer. It has been demonstrated that the dephasing of the zero- phonon line and its superlinear temperature dependence can be interpreted from a model of the two- level tunneling system (TLS) and pseudolocal modes in amorphous host [ 8- 10 1. The photophysical hole- burning phenomena in the dye-doped polymer are

Correspondence to: M. Kawase, Department of Physics, Fac- ulty of Education, Kagawa University, Takamatsu City, Kagawa 760, Japan. li For a review of hole-burning studies see ref. [ 3 1.

believed to occur by the persistent configurational change of nearby TLSs upon the optical excitation and manifest an important property for potential ap- plication to the frequency domain optical memory. A study of Fourier-transform spectroscopy for photon echoes, combined with the persistent hole-burning, has shown that the photon echo decay includes all information obtained in the hole-burning experiment [ 111. Therefore, the investigation of the dephasing by the photon echo technique provides important information to elucidate the dephasing mechanism in the dye-doped polymer system and exploit a functional optical memory. It is also demonstrated that the phonon sideband spectrum is a useful measure of the electron-phonon coupling in the dye-doped polymer system [ 12,13 1.

In this Letter, we report on the host-dependent optical dephasing of the zero-phonon line for two organic dye molecules, rhodamine 640 (Rh640) and nile blue (NB), doped in several cross-linked polyvinyl alcohol (PVA) derivatives, measured by an incoherent photon echo technique. In these PVAs the cross-link was introduced to the PVA backbone by means of a photochemical or chemical reaction. We show that the photon echo decay of the zero-phonon line for the two dyes was approximately exponential

268 0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.


and that the dephasing time obtained from the echo decay increased with increasing the length of the cross-link. In addition, the absorption curve of the dyes exhibited a red-shift when doped in a PVA with an ionic cross-link. These findings indicate that the introduction of the cross-link to the PVA backbone reduces the effect of TLS in PVA on the dephasing of a doped dye, and consequently gives the longer dephasing time. Therefore, we consider that our results add a useful knowledge about the dephasing properties in the dye-doped polymer system. It is emphasized that the homogeneous linewidth of a dye in PVA was readily reduced by a factor of more than 2 with introducing the cross-link. The effect of the cross-link on the dephasing mechanism in the PVAs is discussed based on a conventional model of TLS, since the TLS model has often been successful in ex- plaining the dephasing properties of an optical active center doped in an amorphous host [ 1,8-lo].

2. Experimental

We used Rh640 and NB as a probe of optical dephasing in the cross-linked PVAs. The structure of two dyes employed is shown in fig. la. The cross- linkers were introduced to PVA backbone by using acetal formation reaction. The cross-linkers introduced were glutaraldehyde (1,5_pentanedial), acro- lein (propenal) and stilbazolium (SbQ) group, whose chemical structures are shown in fig. lb. Hereafter, these host polymers are referred to as PVA- G, PVA-A and PVA-SbQ, respectively. PVA-SbQ was provided by Ichimura [ 141. The other two host polymers were synthesized by a similar method of Ichimura and Watanabe [ 151. Briefly, 5.0 mmol of cross-linker was dissolved into the solution of 1.5 g PVA in 15 ml deionized water, followed by adding hydrochloric acid as catalyst ( final concentration, 10 mM ). After stirring the solution at ambient temperature for more than 18 h, purification of the polymer was performed following the method in ref. [ 15 1. The solution of purified polymer, in which a dye (NB or Rh640) was doped, was casted on a sapphire plate. To cross-link the PVA backbone, the near-UV and visible light were irradiated to the casted solutions of PVA-A and PVA-SbQ, respectively. In the case of PVA-G, the irradiation was not necessary because

OH b OH

OH

OH

OH

PVA

Fig. 1. (a) Molecular structures of Rh640 and NB. (b) Molecu- lar structures of PVA-G, PVA-A and PVA-SbQ. (c) Estimated molecular structures of PVA-SiOH and PVA-SiPr. Typical value of n was evaluated to be 2 from data of IR and XPS. etc.

the cross-link occurred chemically. The content of the cross-linker, measured by elemental analysis, was 1.6, 2.6 and 1.2 mol% for PVA-A, PVA-G and PVA- SbQ, respectively.

Two PVAs cross-linked by organosilanes were also used as host polymers. These polymers are considered to have the structures as shown in fig. lc, and were synthesized by the same way mentioned above. Organosilane (tetraethoxysilane or propyltriethoxy- silane) of 5.0 mmol was dissolved into 15 ml of PVA solution (lo%), and hydrochloric acid (final concentration 10 mM) was added to form a G-0-Si bond. The reaction was carried out at room temperature for more than 24 h. These polymers are referred to as PVA-SiOH and PVA-SiPr, respectively.. Doping of dye and casting were also performed as described above. Typical value of n in fig. lc was

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evaluated to be 2 from data of IR and XPS, etc. The experimental apparatus used to measure the

dephasing of the zero-phonon line for NB and Rh640 doped in the cross-linked PVA was the same as that in our previous study [ 6,7 1. A dye laser pumped by the second harmonics of YAG laser produced a broadband 8 ns pulse with a bandwidth of about 1.6 nm. Two laser dyes, rhodamine B and DCM, were used to excite the long-wavelength tail of absorption for Rh640 and NB, respectively. The output of the dye laser was split into two beams and one beam was delayed on the picosecond time scale with respect to the other beam. Both beams were focused on the sample to generate incoherent photon echoes in a phase matched direction. The sample was cooled to 10 K by a temperature-variable cryostat. In the incoherent photon echo experiment, the time resolu- tion is determined by the inverse of the laser bandwidth [ 16,17 1, and it was estimated, from a photon echo signal at room temperature, to be about 0.5 ps for both dye lasers. Incoherent photon echo signal was accumulated by a boxcar integrator as a function of the delay time 7 between two beams and the dephasing time was obtained from the echo decay curve.

3. Results and discussion

The incoherent photon echo decays at 10 K of NB and Rh640 doped in four host polymers are displayed in fig. 2 on a logarithmic scale. The usual PVA without the cross-link, referred to as PVA, is in- cluded to use the dephasing time in PVA as a standard in evaluating the effect of the cross-link. We obtained these photon echo decays by exciting the individual dye-doped polymer system at the long- wavelength tail of absorption ( = 670 nm for NB and x 590 nm for Rh640). One can see that the photon echo decay consists of the initial fast decay around 7~0 ps followed by an approximately exponential decay. As is well established in a previous work [ 111, the former is ascribed to the contribution from the phonon sideband and the latter originates from the zero-phonon line on which our interest is focused in the present study. We notice that the intensity ratio of the zero-phonon line to the total echo intensity for Rh640 is smaller than that for NB doped in the same host polymer, which leads to the worse signal-to-noise

270

Delay Time ( ps )

Fig. 2. (a) Incoherent photon echo decays observed at 10 K for NB doped in the cross-linked PVAs. The host polymers are PVA (A), PVA-A (B), PVA-G (C) and PVA-SbQ (D). (b) Incoher- ent photon echo decays observed at IO K for Rh640 doped in the same PVAs as in (a). The echo decay in PVA (A) is represented by small solid circles.

ratio in photon echo decay signal for Rh640. The intensity ratio is predominated by the electron-phonon coupling strength, if other conditions, such as the transition dipole moment of the zero-phonon line, excitation power and excitation bandwidth, are sup- posed to be similar. This implies the stronger electron-phonon coupling for Rh640. In addition, it should be emphasized that the intensity ratio for the two dyes increases when the host polymer is changed from PVA to PVA-SbQ, indicating that the coupling between a dye and phonons in PVA-SbQ is smaller than that in PVA. We deduced the dephasing time T2 of the zero-phonon line from the exponential decay part in fig. 2 with a least square fitting. The dephasing times at 10 K measured for NB and Rh640 doped in the cross-linked PVAs are listed in table 1. It should be noted that, as is seen in table 1, the dephasing time for NB increases in the order of PVA, PVA-A, PVA-SiOH ( PVA-SiPr ), PVA-G and PVA- SbQ, which corresponds to the order of the length of the cross-link introduced to the PVA backbone. The dephasing time of NB in PVA-SbQ, for instance, is longer than that in PVA by a factor of more than 2.


Table 1 Optical dephasing times at 10 K in the cross-linked PVAs

Host polymer Tz (ps) a) Length of cross-link b,

NB Rh640 (A)

PVA 24.3 24.8 PVA-A 28.4 30.3 5.6 PVA-G 33.8 32.1 6.5 PVA-SbQ 67.4 51.5 14.3 PVA-SiOH 33.7 n.t. ) 6.1 d, PVA-SiPr 32.3 n.t. c, 6.1 d

a) Dephasing times were determined within the error limits of & 3% and f 5% for NB and Rh640, respectively.

b, Estimated from a standard bond length within the error limit of f 0.2 (A) based on the standard molecular models corre- sponding to the structures in figs. lb and lc.

) Not tested. d, Estimated from several data such as IR and XPS spectra, etc.

The host dependence of the dephasing time for Rh640 is essentially similar to NB, while we have not yet measured the dephasing time of Rh640 in PVA- SiOH and PVA-SiPr. These results decisively sug- gest that the presence of the cross-link in PVA reduces the interaction between a doped dye and TLS, and then the longer dephasing time is observed in the cross-linked PVA.

To understand the host dependence of the dephasing time, we employ a density matrix formalism incorporated with the model of TLS that has been applied to the dephasing of a chromophore doped in the amorphous hosts. As we previously presented [ 18 1, the third-order density matrix, which corresponds to the incoherent photon echoes generated in the phase-matched direction with the delay time r between two exciting beams, is described in the form

( > 3

p3(r)-,4 & I2 - Ifi

ccl m

x dt, s s d&E(t-t, -?)G(t, -7) exp(--y2tl) 0 0

x[exp(-y,fz)+exp(-y,fz)lD(tl,t2,tl). (1)

Here, y, and yz denote the population relaxation rate of the optical ground 1 1 > and excited ( 2) state for the zero-phonon line, respectively, and G(t) repre-

sents the field correlation function of laser light, defined by (E*(t+t)E(t)). p,2 denotes the transition dipole moment between I 1) and ) 2 ) . D( t,, t2, t,) is the four-point correlation function defined as Wtr, t2, tl)=(C(t,, tz, tr)) with

12 +I20

=exp i ( s

d(r31dr3-ij!d(r3Jdr3), (2) 12+11 0

where d(t) denotes the time-dependent phase ex- perienced by the induced polarization in the dye molecule. The phase A( t ) is assumed to be a sum of the phase fluctuations, A(t) = Cj @,( t), where g,(t) is the phase fluctuation caused by thejth TLS through the dipolar interaction (q/r3) and obeys a Gauss- Markov stochastic process, (6@j( t)S@j( t ) ) = (q/r3)2exp(-R(t-t(). The angle brackets mean an ensemble average taken over the distribution of the spatial position r, fluctuation rate R and energy splitting E of TLS. The theoretical analysis of photon echo decay and hole-burning spectrum based on this four-point correlation function has been extensively developed by Bai and Fayer [ 19,201, and it is demonstrated that the t2 dependence of D( t,, f2, 2, ) makes the measured dephasing rate larger by a factor of more than three than the homogeneous dephasing rate which is obtained in the two-pulse photon echo experiment. This effect of the tz dependence originates from the fluctuations of TLSs occurring on the long time scale and is referred to as the spectral diffusion. The effect is experimentally confirmed in both photon echo and hole-burning experiments in chromophore-doped glasses [ 2 1-23 1. Therefore, our incoherent photon echo decay inevitably includes the spectral diffusion effect during the pulse duration of 8 ns. Nevertheless, we believe that our results on the host dependence of dephasing measured by the incoherent photon echoes definitely reflects the variation of the dye-TLS interaction in the cross-linked PVA because, in the TLS model, both the homogeneous dephasing and spectral diffusion are assumed to result from the fluctuation of TLS.

Our model to explain the host-dependent dephasing assumes a large void space which is created around the introduced cross-link in PVA. We consider that most of the dye molecules are preferen-

271


tially entrapped in the void spaces, because the dye gets into the void space more easily. The cross-link is considered to suppress the flip-flop of TLSs near to the void space, and then the dye in the void space sees a different local distribution of effective TLS from the averaged distribution of TLS over the polymer. This model is similar to the two-domain model exploited by Pack et al. in the study of solvation shell effects on glass dynamics [ 241. In our case one domain is modeled to be a sphere of radius r, including a void space at the center, where the distribution of effective TLS is altered locally by the void space. The other domain is the outer bulk region of the sphere. Following ref. [ 241, the four-point correlation function can be divided into two parts as

D(ti, tz, t,)=exp -4xpGl

h

x drr2(1-exp[-(rll~3)2~~Q(~~,~~,~,~)l~~,~ s 0

m

-4rrpo, drr2( 1 I h

-exp[ - (v/r) 2tiQ(b, b, R 0 1 >R,E >

, (3)

where Q(t,, t2, R, E)=sech*(E/ZkT)f 2(Rt,, Rt2) and f (Rt,, Rt2) is defined by

f(Rt,,Rf2)= I $Rt +[exp(-Rt )-I] I 1

-exp[-R(t,+t,)][cosh(Rt,)-11). (4)

As photon echoes primarily detect the phase fluctuation on the fast time scale and the dipolar interaction between dye and a TLS falls off rapidly with the distance r between them, the first exponent in eq. (3) dominates in photon echo decay, if r, is appro- priately chosen. Therefore, we ascribe the observed host-dependent dephasing to the decrease of the local density of TLS near the void space, pGI. The fact that thedephasing time increases with the length of cross-link is interpreted by the assumption that the volume of the void space becomes larger with increasing the length of the cross-link, which probably

272

results in the smaller ho,. The effect of the cross-link on optical dephasing is slightly smaller for Rh640, and this would be related to the larger molecular size of Rh640 than NB.

If we assume a uniform density of TLS, Pot =po2 the four-point correlation function results in the form

D(t,, tz, tl)

=exp{ -BpG(kT) +Pt,[@+ln(t,/t,)]}. (5)

Here, B and 8 are a collection of unimportant con- stants and a constant equal to 3.66, respectively, and t, is defined by t,=min( t, + t2, 1 /R,i) with R,,, being the minimum fluctuation rate of TLS. The distribution of TLS energy splitting E is assumed to obey a power law dependence,- P(E) aE@. Though this four-point correlation function does not give an exponential decay, the actual echo decay calculated by using eqs. ( 1) and (5 ) can be approximated by an exponential decay as demonstrated in ref. [ 19 1. In this case it may be possible to explain the host-dependent dephasing as a result of the variation of the uniform TLS density, PG. It means that po in PVA- SbQ is smaller by a factor of 2.3 than in PVA-A. However, since the content of the cross-linking group in PVA-SbQ is approximately equal to that in PVA- A, we do not think that such smaller po in PVA-SbQ than in PVA-A is likely. Consequently, we conclude that the change of PG due to introducing the cross- link cannot provide a comprehensive explanation for our host-dependent dephasing.

Fig. 3 shows the absorption spectrum at room temperature of NB and Rh640 doped in the cross- linked PVAs. For instance, the absorption of NB in PVA, PVA-A and PVA-G had nearly the same spectrum, but the absorption in PVA SbQ showed a pronounced red-shift of about 8 nm. Note that the absorption in PVA-SbQ for the wavelength shorter than 450 nm does not result from NB but from the absorption of PVA-SbQ itself. Absorption of Rh640 also displayed a similar host dependence. Taking account of the fact that the absorption spectrum of the dye in solution is generally shifted to longer wavelength by a more polar solvent, the red-shift observed in PVA-SbQ host can be understood as the effect of a positive charge involved in SbQ cross-linker, that is distinct from the other two cross-linkers. This may


C--T--] trapped in a void space near to the cross-linker.

400 500 600 700 Wavelength ( nm )

Fig. 3. (a) Absorption spectrum at room temperature ofNB doped in the cross-linked PVAs. The host polymers are PVA (-), PVA-A (- - - - -), PVA-G (- - -) andPVA-SbQ (- - -). (b) Ab- sorption spectrum at room temperature of Rh640 doped in the same PVAs as in (a).

imply the stronger dye-host interaction in PVA-SbQ compared to other cross-linked PVAs. However, as mentioned above, it is concluded from the photon echo results in fig. 2 that the dye-host interactions in PVA-SbQ, such as dye-phonon and dye-TLS interaction, are smaller than other PVAs. This dis- crepancy can be explained as follows. It is the elec- trostatic coupling between a dye and SbQ cross-linker with the positive charge that is actually increased in PVA-SbQ host, causing the pronounced red-shift of dye absorption spectrum. But it is likely to be static and does not contribute to the optical dephasing of the dye molecule, which is justified from the smaller contribution of the phonon sideband in PVA-SbQ to photon echo signal. In contrast, the coupling between the dye and TLSs; which probably predomi- nates the optical dephasing in polymer host, is much reduced because of the large void space created by the long SbQ cross-linker, as explained above. As a consequence, we observe much longer dephasing time in PVA-SbQ. We conclude that the red-shift observed in PVA-SbQ ensures that the doped dye is en-

4. Conclusion

We have presented a host-dependent dephasing of NB and Rh640 doped in the cross-linked PVAs. Our finding that the dephasing time increases with increasing the length of the introduced cross-link, is ascribed to the void space which entraps the dye molecule and reduces the effective TLS density around the void space. The resulting smaller local density of TLS gives a longer dephasing time of the zero-phonon line in the cross-linked PVA. The pronounced red- shift of absorption spectrum of dye molecule in PVA- SbQ provides an indirect evidence of the dye entrapped in the void space. Our model of the void space has to be confirmed by further investigations of temperature dependence of the dephasing time and hole-burning in the dye-doped cross-linked PVA systems.

Acknowledgement

We would like to acknowledge Professor Ichimura, Tokyo Institute of Technology for providing us the PVA-SbQ polymer.

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Documents

Host-Dependent Optical Dephasing of Dye Molecules