Alex K.-Y. Jen

Hong Ma

Jingdong Luo

Comptes Rendus Chimie

Dendritic nanodevicesNanostructured functional dendrimers and polymers for photonicsJingdong Luo, Hong Ma, Alex K.-Y. Jen *Department of Materials Science & Engineering, Box 352120, University of Washington, Seattle, WA 98195–2120, USAReceived 2 April 2003; accepted 22 July 2003AbstractCompared to conventional molecular design for organic NLO materials, the concept of using nanoscale architectural controlto tailor the size, shape, conformation, and functionality of NLO chromophores and macromolecules provides a greatopportunity for simultaneous optimization of macroscopic electro-optic activity, thermal stability, and optical loss. All the resultsdiscussed in this review are extremely encouraging and this new approach may help to launch a new paradigm for the molecularengineering of next generation high-performance E–O materials. To cite this article: A. Dirksen, L. De Cola, C. R. Chimie 6(2003).© 2003 Published by Éditions scientifiques et médicales Elsevier SAS on behalf of Académie des sciences.1. IntroductionOrganic polymeric second-order nonlinear optical(NLO) materials have attracted considerable attentiondue to their potential for applications in photonic andopto-electronic devices, such as frequency convertersand ultrahigh-speed electro-optic (E–O) switches andmodulators [1]. One major obstacle that hinders therapid development of this technology is the difficultyto translate high molecular non-linearities (µb) intolarge macroscopic E–O activities (r33) in NLO poledpolymers [2]. In order to achieve this goal, it is neces-sary to develop an effective mechanism to improvepoling efficiency through minimization of the strongintermolecular electrostatic interactions between highdipole moment NLO chromophores.In a poled polymer, a NLO chromophore is usuallysurrounded by other chromophores and polymer chainsegments. During the poling process, the electric fieldneeds to overcome not only the intermolecular electro-static interactions between chromophores but also therestriction of chromophore reorientation imposed bypolymer chain entanglements. In order to improve pol-ing efficiency of NLO polymers, the chromophore inthe polymer matrix should be structurally shieldedfrom its neighboring chromophores to minimize thedipole-dipole interactions. Another approach is to cre-ate a well-defined macromolecular architecture thatwill provide a nanoscale ordering for reducing thehindrance to re-orient NLO chromophores. Previously,we have demonstrated the strategy of using shapemodification of NLO chromophores to improve polingefficiency [3]; however, the initial results obtainedfrom these studies have only shown an incrementalimprovement of the polymer’s E–O properties. Sincethe globular geometry of dendrimers is well-suited formodifying chromophores to the ideal spherical shape,we have recently explored the idea of using dendriticstructures to modify the shape of NLO chromophores* Corresponding author.E-mail address: ajen@u.washington.edu (A.K.-Y. Jen).C. R. Chimie 6 (2003) 895–902© 2003 Published by Éditions scientifiques et médicales Elsevier SAS on behalf of Académie des sciences.doi:10.1016/j.crci.2003.07.008and polymers and systematically investigate the role ofthe site isolation effect in determining the materialproperties [3–5].In the past two decades, unprecedented control ofcritical dendrimer design parameters, such as size,shape, surface chemistry, flexibility and topology, hasbeen rigorously explored to provide well-defined, den-dritic architectures. Our main efforts are focused ontuning the macromolecular architecture of E–O mate-rials ranging from the shape of flattened sphere, um-brella, to cylinder, and studying their effects on polingefficiency. Here, we highlight our latest progress in thedesign, synthesis, and properties of these NLO mate-rial systems. Their structures vary from the 3–Dshaped dendritic chromophore to the multi-functionaldendrimers with the center core connected to NLOchromophores and crosslinkable functional group atthe periphery, and to the side-chain dendronized NLOpolymers (Scheme 1). All the poling results from thesesystems have shown dramatically enhanced E–O prop-erties (a factor of ~2–3) compared to the conventionalNLO polymers.2. Dendritic NLO chromophore [6]Among various NLO polymer systems, fluorinatedmaterials are of particular interest because it has beendemonstrated that fluorinated polymers exhibit a com-bination of high thermal stability, chemical inertness,low dielectric constants, and optical transparency dueto the large electro-negativity of the fluorine atom. Inorder to explore the effect of fluoro-rich dendrons onthe microenvironments of dipolar NLO chromophore,we have designed and synthesized a highly fluorinateddendritic NLO chromophore 2 (Fig. 1). This dendritic-Scheme 1896 J. Luo et al. / C. R. Chimie 6 (2003) 895–902shaped chromophore was constructed by using aphenyl-tetracyanobutadienyl (Ph-TCBD) thiophene-stilbene-based NLO chromophore as the center coreand it was encapsulated with three highly fluorinatedaromatic dendrons.The resulting dendritic chromophore 2 exhibits ap-preciably different properties, such as a large blue-shifted absorption kmax (29–42 nm), a higher thermalstability (20 °C), and a much higher E–O activity (3 ×),compared to the pristine chromophore 1.Since both chromophores contain the same NLOactive moiety, the blue-shifted kmax is largely due to theencapsulation with fluoro-rich dendrons that have lowpolarizability and low dielectric constant. Moreover,the dendritic chromophore 2 also shows exactly thesame UV–Vis spectra when it was dispersed in either anon-fluorinated polycarbonate or a highly fluorinatedpolymer, two kinds of matrices that possess signifi-cantly different dielectric constants. From these re-sults, it indicates that the fluoro-rich dendrons domi-nate the microenvironments of the core chromophorein solid films.More importantly, this comparison provides a veryclear explanation about how the dendritic effect en-hances E–O activities. Under a similar condition, thepoled film of 2 exhibits an r33 value of 30 pm/V at1.3 µm, which is three times higher than that derivedfrom 1, indicating a significant improvement in polingefficiency. The encapsulation of 2 with fluorinated den-drons makes its shape globular-like, which is veryclose to the ideal molecular shape for achieving thepredicted optimum poling efficiency. Moreover, be-cause the fluorinated dendrons strongly affect the mi-croenvironments of the Ph-TCBD chromophore insolid state, the chromophores are site-isolated, therebythe intermolecular electrostatic interactions are signifi-cantly reduced. This, in turn, ameliorates the molecularorientation.The blue-shifted absorption and high C–F contentsin 2 also contribute to the material’s low optical loss atboth 1.3 µm and 1.55 µm which are the two majorwavelengths used in telecommunication. From thephoto-thermal deflection spectroscopy (PDS), the opti-cal loss of the materials induced by the absolute ab-sorption of 2 in polycarbonate are only 0.65 and0.85 dB cm–1 at 1.3 and 1.55 µm, respectively. Withregards to the relatively high C–H content in the poly-carbonate matrix, the major contribution of these lossFig. 1. Property comparison between dendritic chromophore and its pristine phenyl-tetracyanobutadienyl (Ph-TCBD) thiophene-stilbene-basedNLO chromophore. Td: initial decomposition temperature determined by seal-pan differential scanning calorimetry (DSC) at 10oC min–1 undernitrogen; APC: amorphous polycarbonate (APC) {poly[Bisphenol A carbonate–co–4,4'–(3,3,5–trimethylcyclohexylidene)diphenol carbonate};electro-optic coefficient (r33) obtained in APC matrix with 12 wt% of core chromophore density by contact poling.897J. Luo et al. / C. R. Chimie 6 (2003) 895–902values should be originated from the matrix itself in-stead of from 2. Since the intrinsic loss of 2 is lowerthan 1 dB cm–1, this type of highly fluorinated den-dritic chromophore should be very useful for photonicapplications.3. Crosslinkable NLO dendrimers [7]In order to create a stand-alone NLO material thatpossesses the ideal spherical shape and good alignmentthermal stability, we have constructed a chromophore-containing dendrimer with multiple NLO chro-mophore building blocks placed into a precisely de-signed molecular architecture with predeterminedchemical compositions.The crosslinkable NLO dendrimer 3 (Scheme 2)exhibits a very large optical non-linearity and excellentthermal stability. This NLO dendrimer was constructedthrough a double-end functionalization of NLO chro-mophore 1 as the center core and the crosslinkabletrifluorovinylether-containing dendrons as the exteriormoieties. Spatial isolation from the dendrimer shelldecreases chromophore-chromophore electrostatic in-teractions, and thus enhances macroscopic optical non-linearity. Through a sequential hardening/crosslinkingprocess during the electric field poling, a very largeE–O coefficient (r33 = 60 pm/V at 1.55 µm), andlong-term alignment stability (retaining > 90% of itsoriginal r33 value at 85 °C for more than 1000 h) wereachieved for the poled dendrimer.For comparison, E–O studies were performed on aguest/host system, in which the chromophore 1 with-out dendron modification (optimized loading level:30 wt%) was formulated into a high-temperaturepolyquinoline (PQ-100). This guest/host systemshowed a much smaller E–O coefficient (less than30 pm/V) and lower temporal stability (only re-tained < 65% of its original r33 value at 85 °C after1000 h). In addition, the attempt to corona pole thenon-trifluorovinyl ether functionalized dendrimer onlyshowed a very fast decay of E–O signal (< 10 pm/V)after the same sample was poled and measured at roomScheme 2898 J. Luo et al. / C. R. Chimie 6 (2003) 895–902temperature. This is due to its intrinsic low Tg (< 50oC)and very large free volume. On the basis of this system-atic study, the large r33 of the poled dendrimer is due tothe dendritic effect that allows the NLO dendrimer tobe efficiently aligned.4. Side-chain dendronized NLO polymers [8]To further optimize E–O material properties, a se-ries of hybrid crosslinkable dendritic NLO polymershave also been developed to combine the syntheticsimplicity and good processability offered by linearpolymers and the excellent site-isolation effect pro-vided by dendrimers. The first side-chain dendronizedpolymer 4 synthesized for this study is illustrated inScheme 3. The poled film of 4 (contact poling) showeda very large E–O coefficient (r33 = 81 pm/V) at 1.3 µm.In order to provide a systematic comparison, E–Oproperties derived from the conventional side-chainand guest/host polymers bearing a similar chro-mophore with the same loading level were also stud-ied.The E–O activity of side-chain dendronized poly-mer 4 is significantly higher than those obtained fromthe side-chain polymer 5 and the guest–host polymersystem (6/PMMA). The low E–O coefficients of thepoled films 5 and 6/PMMA indicate that strongchromophore-chromophore electrostatic interactionssignificantly limit the attainable E–O activities in ordi-nary NLO polymeric systems that do not have effectivesite isolation moieties. Although several other factors,such as conductivity of the samples and different poly-mer matrix may also affect the E–O activities of thematerials, the contribution of these factors can be lim-ited to a minor factor. For example, in both side-chainpolymers 4 and 5, the same polystyrene-based back-bone and poling electric fields were utilized; however,the r33 value of the side-chain dendronized NLO poly-mer 4 is ~2.5 times higher than that of the pristineside-chain NLO polymer 5. Therefore, high polingefficiency obtained in the poled film of 4 is mainly dueto better site isolation that allows the chromophores tobe spatially well distributed and efficiently oriented.This result is also quite consistent with the polingbehaviors obtained from both the E–O dendrimer andthe dendritic NLO chromophore doped polymer previ-ously described.More interestingly, the poled polymer 4 with such alarge free volume and without even crosslinking canstill retain more than 90% of its original r33 value afterScheme 3899J. Luo et al. / C. R. Chimie 6 (2003) 895–902several hundred hours at room temperature. This maybe because the bulky dendrons provide quite a largesteric hindrance and inhibit the free rotation of thepolymer chains.When this new molecular engineering concept wasapplied to a more efficient NLO molecule (a CLD-typechromophore), extremely large E–O coefficients(r33 = 97 and 111 pm/V at 1.3 µm) were achieved fromthe poled side-chain dendronized NLO polymer 7 and8, respectively. To our knowledge, these values areamong the highest one ever reported for side-chainNLO polymers (Fig. 2). These values are also superiorto those reported from the CLD-based guest-host E–Opolymer systems, resuming the high poling efficiencyof all the dendron-containing NLO materials men-tioned above.Polymer 8 also displays a very interesting macro-molecular structure. The densely packed bulky sidegroups may self assemble to cause the flexible polymermain chain to form a cylindrical rod shape (Fig. 3)[9–11]. The NLO chromophores are expected to bewell spatially isolated in the channels of such formedcylindrical structures. Moreover, due to the high rigid-ity of this type of polymers, the commonly observedchain entanglement in linear polymers should also besuppressed. This may provide a new paradigm to mo-lecular engineer the morphology and properties ofNLO materials to the next level.5. Crosslinkable low optical loss dendrimersFor fabricating integrated optical waveguide de-vices based on the above active dendrimers, the devel-opment of low optical loss crosslinkable passive den-drimers is quite important because they can provide amore compatible and better index-matched claddingmaterial. Crosslinkable fluorinated dendrimers canalso provide good processability (high solubility andlow viscosity to allow high solid content in solutionformulation) and low optical loss compared to linearpolymers. In the literature, there are many examplesdescribing the incorporation of fluorocarbons into theperiphery of dendrimers that leads to the formation ofamphiphilic fluorinated dendrimers with surfactant-like properties [12]. However, there is hardly any re-port discussing the use of crosslinkable fluorinateddendrimers in optical waveguide devices.Recently, we have developed a series of highly flu-orinated crosslinkable dendrimers such as macromol-ecule 9 that possesses most of the desirable propertiesneeded for the fabrication of optical waveguides [13](Scheme 4). The trifluorovinyl ether-containing den-Fig. 2. Property comparison between side-chain dendronized NLO polymers and guest-host system containing the similar CLD-type chromo-phore.900 J. Luo et al. / C. R. Chimie 6 (2003) 895–902drimer 9, with a Mw of 18 000 can still be formulatedinto non-polar solvent like mesitylene to afford a veryhigh solid content (50 wt%). Preliminary data from thePDS optical loss measurements have shown that theoptical loss for the crosslinked dendrimer 9 (cured at225 °C for 20 min) are as low as 0.36 and 0.52 dB cm–1at 1300 and 1550 nm, respectively. Since this samplecontains ~1 wt% DCM dye as dopant for proper PDScalibration, the intrinsic optical loss of the dendrimershould be even lower. This dendrimer also possessesvery good solvent resistance after curing that makes itsuitable for multilayer integration.Fig. 3. Site-isolation effect in cylinder-like dendronized NLO polymer.Scheme 4901J. Luo et al. / C. R. Chimie 6 (2003) 895–9026. Summary and outlookCompared to conventional molecular design for or-ganic NLO materials, the concept of using nanoscalearchitectural control to tailor the size, shape, confor-mation, and functionality of NLO chromophores andmacromolecules provides a great opportunity for si-multaneous optimization of macroscopic electro-opticactivity, thermal stability, and optical loss. All the re-sults discussed in this review are extremely encourag-ing and this new approach may help to launch a newparadigm for the molecular engineering of next gen-eration high-performance E–O materials.For the dendron-modified NLO chromophore/polymer systems, the interaction between chro-mophore and high Tg aromatic polymer should befurther investigated in addition to the chromophore-chromophore electrostatic interactions. For thecrosslinkable NLO dendrimers and side-chain den-dronized NLO polymers, the environment such as den-dron shape, size, dielectrics, hydrophilicity/hydro-phobility, rigidity and chirality, and the distribution ofchromophores inside the macromolecular environ-ments will play a very critical role for achieving themaximum realizable macroscopic NLO properties.With all these integrated efforts, the realization of thefull potential of organic NLO materials will be unprec-edented.AcknowledgementsFinancial supports from the National Science Foun-dation (NSF–NIRT and the NSF–STC Program underAgreement Number DMR-0120967) and the Air ForceOffice of Scientific Research (AFOSR) under theMURI Center on Polymeric Smart Skins are acknowl-edged. Alex K.-Y. Jen thanks the Boeing–JohnsonFoundation for its support.References[1] (a) J. Zyss (Ed.), Molecular Nonlinear Optics: Materials,Physics and Devices, Academic Press, New York, 1994; (b)S.R. Marder, B. Kippelen, A.K.-Y. Jen, N. 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Nanostructured functional dendrimers and polymers for photonics

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Nanostructured functional dendrimers and polymers for photonics

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