The photodegradation of the electro-optic chromophore 4N,N\u27-dimethylamino-4\u27nitrostilbene was investigated as a function of wavelength, temperature, local atmosphere, and local molecular environment, i.e., attached as a side chain to a polymer or dissolved as a guest in a host polymer

Canva, Michael

Galvan-Gonzalez, Adriana

Stegeman, George I.

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University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1999 Local and external factors affecting the photodegradation of 4N,N '-dimethylamino-4 ' nitrostilbene polymer films Adriana Galvan-Gonzalez University of Central Florida Michael Canva University of Central Florida George I. Stegeman University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Galvan-Gonzalez, Adriana; Canva, Michael; and Stegeman, George I., "Local and external factors affecting the photodegradation of 4N,N '-dimethylamino-4 ' nitrostilbene polymer films" (1999). Faculty Bibliography 1990s. 2634. https://stars.library.ucf.edu/facultybib1990/2634 Appl. Phys. Lett. 75, 3306 (1999); https://doi.org/10.1063/1.125333 75, 3306© 1999 American Institute of Physics.Local and external factors affectingthe photodegradation of -dimethylamino-4′nitrostilbene polymer filmsCite as: Appl. Phys. Lett. 75, 3306 (1999); https://doi.org/10.1063/1.125333Submitted: 26 July 1999 . Accepted: 24 September 1999 . Published Online: 16 November 1999Adriana Galvan-Gonzalez, Michael Canva, and George I. StegemanARTICLES YOU MAY BE INTERESTED INPhotodegradation of selected π-conjugated electro-optic chromophoresJournal of Applied Physics 94, 756 (2003); https://doi.org/10.1063/1.1578703Wavelength dependence of 4-dimethylamino- -nitrostilbene polymer thin filmphotodegradationApplied Physics Letters 73, 912 (1998); https://doi.org/10.1063/1.122035Local and external factors affecting the photodegradationof 4 N,N8-dimethylamino-4 8nitrostilbene polymer filmsAdriana Galvan-Gonzalez, Michael Canva,a) and George I. Stegemanb)School of Optics and Center for Research and Education in Optics and Lasers (CREOL),University of Central Florida, Orlando, Florida 32816-2700~Received 26 July 1999; accepted for publication 24 September 1999!The photodegradation of the electro-optic chromophore 4N,N8-dimethylamino-48nitrostilbene wasinvestigated as a function of wavelength, temperature, local atmosphere, and local molecularenvironment, i.e., attached as a side chain to a polymer or dissolved as a guest in a host polymer.© 1999 American Institute of Physics.@S0003-6951~99!01547-8#The performance of nonlinear polymers as electro-opticmodulators has reached record performance with.100 GHzbandwidths.1 The electro-optic active materials consist ofnoncentrosymmetric chromophores incorporated into a poly-mer and aligned to produce a macroscopic nonlinearity typi-cally by electric field poling.2 The resulting devices are ex-pected to operate under conditions of continuousillumination for periods of years under a diversity of envi-ronmental conditions. However, many organic materials areknown to be susceptible to a number of photoactivatedmechanisms which degrade the nonlinear activity of thepolymer.3–9 To date, photodegradation studies have focusedon the chromophore 4N,N8-dimethylamino-48nitrostilbene~DANS!, which has taken on the role of the prototype chro-mophore for degradation studies.3–6 They have establishedthat the photodegradation decreases by many orders of mag-nitude between 500 and 1064 nm, that oxygen complexationleading to an absorption feature at 1280 nm adversely affectsthe refractive index at 1320 nm and that the variation in thequantum efficiency associated with the degradation forl'500–600 nm suggests that there may be multiple degra-dation channels present.3–6 In this letter, we demonstrate thatthere are two charge transfer states which are photobleached,evaluate the quantum efficiency for both, investigate the tem-perature and environmental dependence of the photodegrada-tion and its dependence on whether the polymer is a sidechain or a guest host.As shown in the inset of Fig. 1, the molecular structureof DANS is that of a classical stilbene chromophore. Anelectron donor~amino! and an acceptor~nitro! group is con-nected via an electron conjugated bridge consisting of twobenzene rings connected by two carbons with a double bondbetween them. There has been some early discussion of thedominant degradation mechanism, trans-cis isomerizationleading to a bent molecular geometry or photo-oxidation ofthe central carbon double bond.3,4,10Both are initiated by theabsorption of a photon exciting the chromophore into one ofits two excited charge transfer states, and the subsequentconversion to another species with a quantum efficiencyB21. This results in a change in the absorption spectrumwith a reduction in the absorption in the deep blue and nearultraviolet ~UV!, and presumably the appearance of newspectral features in the UV. As discussed previously, ourtechnique consists of monitoring the transmission of thesample in the long wavelength tail of the charge transferabsorption peak as the film is illuminated with radiation ofdifferent wavelengths.6 The measurement of the transmissionvolution yields directly the ratioB/s wheres is the mo-lecular absorptivity. This is a key molecular parameter sincethe 1/e lifetime of the initial population of molecules withthe interesting charge transfer state is given byt5B/snpwherenp is the photon flux.The absorption spectrum shown in Fig. 1 for the side-chain polymer ~SCP, 43% weight fraction! exhibits twostrong peaks, both of which have been shown theoretically tohave strong charge transfer character by Bredas andco-workers.11 Illumination at 544 nm, in the tail of the domi-nant charge transfer spectral line, shows that both peaks arephotobleached with time. This shows conclusively that thereare at least two degradation pathways, one associated witheach peak. For a photon flux of 4.831021/cm2, the dominantphotodegradation effect is essentially finished in about 2 h.Further degradation occurs but at a much slower rate. It isthis initial ~faster! photodegradation that we study in thisletter. No further changes in the spectrum of the irradiatedmaterial were observed 150 h after the completion of thephotobleaching experiment showing that the photoproductsare stable.a!Permanent address: Laboratoire Charles Fabry de l’Institut d’Optique,UMR 8501 CNRS, Universite´ d’Orsay/Paris-XI, 91403 Orsay Cedex,France.b!Electronic mail: george@creol.ucf.eduFIG. 1. The absorption spectrum of the SCP DANS in air as a function ofillumination time with a 544 nm pump beam of 0.20 W/cm2. ~0, 80 min, 4,6, 20 h!.APPLIED PHYSICS LETTERS VOLUME 75, NUMBER 21 22 NOVEMBER 199933060003-6951/99/75(21)/3306/3/$15.00 © 1999 American Institute of PhysicsThe parameterB/s was measured from 442 to 1320 nmfor both the SCP and DANS as a guest in a polymethyl-methacrylate host~GHP! using the techniques outlined inRef. 6, see Fig. 2. In the guest–host case the chromophoresare simply dispersed in the polymer with doping concentra-tions of ,10% in order to avoid aggregation effects, 4%weight fraction in our case. Clearly the large change inB/sindicates that DANS is much more stable for wavelengths farfrom its dominant absorption peak. The trend of increasingB/s with wavelength is, however, broken at 1320 nm wheresignificant enhanced photodegradation occurs as can be seenin Fig. 2, probably due to the formation of oxygen com-plexes, as noted previously.4 We found here that the SCPwas roughly an order of magnitude more photostable thanthe guest–host polymer. Note that the side-chain approachshould therefore be used not only for its enhanced orienta-tional stability, but also for its superior photostability.The absorption spectrum of DANS is known for the SCPwhich allows B to be evaluated fromB/s over a limitedrange of wavelengths.12 A plot of B versus wavelength ex-hibits two plateaus whereB is independent of wavelengthgiving B>33104 in the blue-green visible andB>106 forl.0.65 mm.6 The photodegradation quantum efficiency ofthe chromophore in the guest–host system is about a factorof three higher that in the side chain. In both cases, the quan-tum efficiency for the photobleaching of the higher energycharge transfer state~centered at;300 nm! is 30 times largerthan that of the lower energy charge transfer state~centeredat ;430 nm!. The charge distributions associated with thesetwo excited states were calculated by Bredaset al. and Zooset al.11,13 The high energy peak involves larger bond-ordermodifications than the low energy one, and hence, more pro-nounced geometric deformations. Therefore, the 300 nmstate is characterized by a larger lattice distortion and there-fore molecules in this state are expected to be the more re-active of the two, in agreement with our results.The temperature dependence ofB/s can occur due tothe temperature dependence ofB, s, or both. Systematicmeasurements in both SCP (Tg5142 °C) and GHP (Tg5100 °C! systems were performed at different wavelengths~photon energies! between 442 and 780 nm, increasing thetemperature from 25 to 95 °C. Figure 3 shows representativeresults both inside and outside the absorption band. Increas-ing the temperature increases the photodegradation rate byless than an order of magnitude, with the side-chain systemdeteriorating faster than the guest host. We suspect that thetemperature dependence of the inhomogeneous broadeningmay play an important role.The effect of different atmospheres on the photodegra-dation was investigated to shed light on the dominant degra-dation mechanism and establish the sensitivity of the chargetransfer excited states to oxygen. The powder form of theside SCP was dissolved in the solvent cyclopentanone andthen either pure oxygen or nitrogen was bubbled vigorouslythrough the solution for 2 h. Both the spin-coating and opti-cal experiments were carried out in the respective atmo-spheres. A third control sample was prepared in air. Note thatalthough this procedure does not ensure that all of the oxy-gen is removed when purged with nitrogen, it does give anindication of the importance of oxygen in photodegradation.As in the previous case, both the side-chain and guest–host polymers were investigated at five wavelengths. Resultsat two wavelengths, 488 and 780 nm are shown in Fig. 4 forboth the side-chain and guest–host polymers. It is clear thatphoto-oxidation is an important photodegradation mecha-nism: The figure of meritB/s is very sensitive to oxygenconcentration. It increases in nitrogen and decreases in oxy-FIG. 2. The figure of meritB/s is plotted vs the photon energy for SCPDANS in air as a guest host~L! and in a side chain~X!.FIG. 3. Temperature dependence in air of the normalized~to ata at 25 °C!figure of meritB/s for the SCP~m, .! and GHP~n, ,! polymers at thetwo wavelengths 488~n, m! and 780 nm~,, m!.FIG. 4. Normalized~to data in air! figure of meritB/s for SCP~m, .! andGHP ~n, ,! films prepared and measured in three different atmospheres,nitrogen, air, and oxygen at the two wavelengths 488~n, m! and 780 nm~,, .!.3307Appl. Phys. Lett., Vol. 75, No. 21, 22 November 1999 Galvan-Gonzalez, Canva, and Stegemangen, differing by up to one order of magnitude between thosetwo limits. We found that the decrease inB/s from the ni-trogen case to the small amount of oxygen present in air islarger at 488 nm than at 780 nm, showing that photo-oxidation is stronger for the 300 nm charge transfer excitedstates than the 430 nm charge transfer excited state. Thiscould explain the values found forB for the two states, inqualitative agreement with theory.11The existence of multiple photodegradation pathwayswas confirmed by additional measurements of the photoin-duced change in the absorption spectrum of the SCP underdifferent atmospheres. In nitrogen, air, and oxygen, thecharge transfer peaks at 430 and 300 nm disappear rapidly.In nitrogen, Fig. 5, a new peak absorption peak at 330 nm,and a shoulder at about 250 nm appear, probably due toconformational changes such as cis-trans isomerization. Inoxygen~results not shown here!, on the same scale~30 min!the spectrum evolves directly to that obtained in Fig. 5 afterillumination in nitrogen for 120 min, followed by 10 min inoxygen. This shows that when oxygen is present, photo-oxidation determines the final state of the degradation pro-cess.In summary, many factors affect the photodegradation ofDANS. This work has shown that there are multiple degra-dation channels involving the two charge transfer states. Ofthese, the higher energy one has a much higher quantumefficiency for degradation than the low energy one. At longwavelength~.600 nm! this quantum efficiency is deter-mined by the dominant absorption feature and has the valueB21>1026. Although this seems small, it is large enough tomake DANS unacceptable for long term electro-optical ap-plications between 800 and 1320 nm, and possibly even at1550 nm where it has not been measured yet. For example,when the propagation of 1 mW in a DANS channel of 10mm2 cross section is assumed under ambient conditions, themaximum lifetime predicted is of the order of 3.5 days forwavelengths up to 1320 nm. Photo-oxidation was identifiedas the dominant but not sole degradation mechanism and itwas found that an oxygen atmosphere relative to a nitrogenone increased the degradation rate by up to an order of mag-nitude. It was also shown that increasing temperature fromroom temperature to 100 °C decreased the photostability byless than one order of magnitude. Overall, we conclude thatstilbenes like DANS are not good candidates for long life-time electro-optic devices.This research was supported by NSF. M.C. also ac-knowledges support from French DGA under Contract No.ERE96-1101 and NATO for Science Fellowship. This workis supported by a bilateral US/French NSF/CNRS collabora-tion. The authors also thank AKZO-NOBEL for supplyingthe DANS SCP.1For example, Y. Shi, W. Wang, J. H. Bechtel, A. Chen, S. Garner, S.Kalluri, W. H. Steier, D. Chen, H. R. Fetterman, L. R. Dalton, and L. Yu,IEEE J. Quantum Electron.2, 289 ~1996!.2K. D. Singer, M. G. Kuzyk, and J. E. Sohn, J. Opt. Soc. Am. B4, 968~1987!.3M. A. Mortazavi, H. N. Yoon, and C. C. Teng, J. Appl. Phys.74, 4871~1993!; M. A. Mortazavi, H. N. Yoon, and I. McCulloch, Polym. Prepr.~Am. Chem. Soc. Div. Polym. Chem.! 35, 198 ~1994!.4R. A. Norwood, D. R. Holcomb, and F. F. So, J. Nonlinear Opt. Phys.Mater.6, 193 ~1993!.5M. Cha, W. E. Torruellas, G. I. Stegeman, and W. H. G. Horsthuis, Appl.Phys. Lett.65, 2648~1994!.6Q. Zhang, M. Canva, and G. I. Stegeman, Appl. Phys. Lett.73, 912~1998!.7Y. Shi, W. Wang, W. Lin, D. J. Olson, and J. H. Bechtel, Appl. Phys. Lett.70, 1342~1997!.8Ph. Pretre, E. Sidlick, A. Knoesen, D. J. Dyer, and R. J. Twieg, ACSSymp. Ser.695, 328 ~1996!.9T. C. Kowalczyk, R. J. Twieg, and H. S. Lackritz, Extended Abstracts ofthe 4th International Conference on Organic Nonlinear Optics~ICONO4!,October 1998, p. 61.10For example, M. B. J. Diemeer, F. M. M. Suyten, E. S. Trommel, A.McDonahc, J. M. Copeland, L. W. Jenneskens, and W. H. G. Horsthuis,Electron. Lett.26, 379 ~1990!.11D. Beljonne, J.-L. Bredas, M. Cha, W. Torruellas, G. Stegeman, W. Horst-huis, and G. Mo¨hlmann, J. Chem. Phys.103, 7834~1995!.12A. Ottomo, M. Ja¨ger, G. Stegeman, W. Horsthuis, and G. Mollmann,Appl. Phys. Lett.65, 2648~1994!.13Z. Soos, S. Ramasesha, D. Galvao, and S. Etemad, Phys. Rev.47, 1742~1993!.FIG. 5. Evolution of the absorption spectrum for SCP–DANS illuminated ina nitrogen atmosphere at 488 nm at an intensity of@20 W/cm2 in the firsthalf hour then 101 W/cm2 for times up to 2 h#. After 2 h an oxygen atmo-sphere was used. Heret0 . . . t5 are 0, 30, 90, 120, 120110, and 120140min.3308 Appl. Phys. Lett., Vol. 75, No. 21, 22 November 1999 Galvan-Gonzalez, Canva, and Stegeman

Local and external factors affecting the photodegradation of 4N,N \u27-dimethylamino-4 \u27 nitrostilbene polymer films

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Local and external factors affecting the photodegradation of 4N,N \u27-dimethylamino-4 \u27 nitrostilbene polymer films

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