Bonding and molecular environment effects on near-infrared optical absorption behavior in nonlinear optical monoazo chromophore-polymer materials

Article abstract of DOI:10.1021/ma060033k

The mono-azo dyes Disperse Red 1 and Disperse Red 19 have been studied for several years as chromophores in polymers for nonlinear optical properties. These materials are examined here for the effects of dye donor group esterification in guest-host system

Full text of DOI:10.1021/ma060033k

7566
Macromolecules 2006, 39, 7566-7577
Bonding and Molecular Environment Effects on Near-Infrared Optical
Absorption Behavior in Nonlinear Optical Monoazo
Chromophore-Polymer Materials
Richard R. Barto*
Department of Materials Science and Engineering, Stanford UniVersity, Stanford, California
Curtis W. Frank
Department of Chemical Engineering, Stanford UniVersity, Stanford, California
Peter V. Bedworth, Rebecca E. Taylor, William W. Anderson, and Susan Ermer
Lockheed Martin Space Systems Company, AdVanced Technology Center, Palo Alto, California
Alex K.-Y. Jen, J. D. Luo, and Hong Ma
Department of Materials Science and Engineering, UniVersity of Washington, Seattle, Washington
Hong-Zhi Tang†
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina
Michael Lee‡
APIC Corporation, CulVer City, California
Albert S. Ren
Arena Pharmaceuticals, Inc., 6166 Nancy Ridge DriVe, San Diego, California 92121
ReceiVed January 5, 2006; ReVised Manuscript ReceiVed August 5, 2006
ABSTRACT: The mono-azo dyes Disperse Red 1 and Disperse Red 19 have been studied for several years as
chromophores in polymers for nonlinear optical properties. These materials are examined here for the effects of
dye donor group esterification in guest-host systems and by dye-polymer covalent attachment on near-infrared
absorption behavior. The dye-polymer systems DR1-acrylate, DR1-poly-4-vinylphenol, and DR19-aliphatic
epoxy are characterized by UV-vis absorption spectroscopy and photothermal deflection spectroscopy as guest-
host systems with and without esterification of the DR1 and DR19 ethanol -OH substituents in guest-host
systems, and as covalently attached dye copolymers, as a function of dye concentration up to 1700 µmol per
gram of polymer. Esterification is shown to be an effective route to reducing near-IR loss in DR1/acrylate guest-
host materials at high concentrations and in DR19/epoxy guest-host materials at most concentrations. Dye-
polymer covalent attachment is shown to be highly effective for minimizing near-IR loss in the two DR1-
polymer materials at all concentrations and is shown to be effective for DR19-epoxy materials at higher
concentrations. Charge-transfer formation and the distributions of dye-polymer and dye-dye orientation states
play crucial roles in the loss behavior of these materials.
Introduction
tail of the main dye absorption peak and concomitant effects
on near-IR loss. In the previous two studies,1,2 we reported on
near-IR loss behavior in dye-polymer guest-host materials
based on FTC-like NLO dyes, showing effects of dye spacer
length and polymer structure on solvatochromism and inhomo-
geneous broadening of the main dye electronic transition peak.
These processes were shown to correlate with near-IR loss
versus dye concentration behavior.
We report on a systematic study of Disperse Red monoazo
dyes incorporated into three unrelated aliphatic polymers, under
three dye incorporation schemes: unmodified guest-host
systems, esterified dye guest-host systems, and covalently
attached dye-polymer systems. As was reported on for two
previous studies of highly active nonlinear optical dye/polymer
guest-host materials,1,2 we systematically vary dye concentra-
tion under each of these schemes and qualitatively assess the
effects of polymer structure and dye incorporation scheme on
molecular interaction mechanisms that influence the low energy
The number density of the highly polar chromophores
incorporated into the polymer system is a key determinant of
the E-O device sensitivity, which can be constrained by E-O
attenuation due to dipole interactions on close approach3 or by
dye solubility limits in the polymer, exacerbated by polarity
mismatch between dye and polymer. To increase chromophore
loading and alignment stability, researchers have investigated
covalent attachment of the chromophore to the polymer as either
* Corresponding author. E-mail: rick.barto@lmco.com.
† Current address: The Molecular Foundry, Materials Science Division,
Lawrence Berkeley National Laboratory, Berkeley, CA 94720.
‡ Current address: Quintessence Photonics Corporation, 15632 Roxford
Street, Sylmar, CA 91342-1265.
10.1021/ma060033k CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/07/2006

Macromolecules, Vol. 39, No. 22, 2006
Absorption Behavior in Chromophore-Polymer Materials 7567
a pendant group4-29 or a comonomer,30-44 cross-linking the
active moiety into the polymer system,45-51 or constructing
covalently self-assembled superlattice structures containing
oriented chromophores.52-57 These approaches have had mixed
success in improving chromophore loading and have frequently
led to higher device insertion loss.
The near-IR spectral structure can provide a rich description
of the relative contributions to absorption loss. One such class
of contributions arises from weak sub-electronic bands due to
donor-acceptor,58-63 acid-base,64,65 or charge-transfer com-
plexes66-68 or defect states.69-73 Complexes associated with
these states can arise from specific interactions between
neighboring molecules, and the strength and breadth of these
bands gives an indication of the distribution and relative
population of such states. The shape, position, and magnitude
of these states will have direct consequences on loss in the near-
IR.74 Another significant contribution to near-IR loss is Urbach
tail broadening,75 associated with sub-gap absorption in amor-
phous materials exhibiting an exponential band tail distribution
given by
For each of the dye-polymer combinations discussed above
for studies of covalently attached dyes, analogous guest-host
materials are studied at comparable molar concentrations of dye,
both for unmodified dye and with the -OH groups protected
by esterification. For DR1/acrylate and DR19/epoxy systems,
the -OH group is substituted with acetate (OAc), while for the
DR1/PVP system, the -OH group is substituted with benzoate
(OC6H5). The choice of -OH protecting groups in each dye-
polymer system is intended to more closely mimic the molecular
surroundings of the dye in the covalently attached state.
We show the benefits of covalent dye-polymer attachment
in reducing near-IR loss in all of the dye-polymer systems,
and effectiveness of dye donor group ester protection for
reducing near-IR loss in DR1/acrylate and DR19/epoxy guest-
host systems. Charge transfer is shown to be an important effect
in DR1-PVP and, to a lesser extent, in DR19-epoxy systems.
Urbach tail broadening is important for all systems studied.
Experimental Section
Materials Synthesis. Poly(Disperse Red 1 methacrylate-co-
methyl methacrylate) at a Disperse Red 1 (DR1) doping level of
1010 µmol per gram of polymer (10 mer%) was received from
IBM Almaden Research Labs and used as-received (CAS No.
119989-05-8, Figure 1a, with R ) methyl, Mw ) 160 000,
polydispersity 5, “DR1-acrylate”). The structure of unmodified poly-
(methyl methacrylate) is shown in Figure 2a, with R ) -CH2-.
Poly(DR1 methacrylate-co-ethyl methacrylate) at DR1 doping
levels of 560 and 1710 µmol per gram of polymer (Figure 1a, with
R ) ethyl, “DR1-acrylate”) was prepared as follows. To the solution
of DR1 (6.00 g, 18.1 mmol) in dry methylene chloride (20 mL)
were added triethylamine (2.7 mL) and methacryloyl chloride (1.77
mL, 1.90 g, 18.1 mmol). The resulting mixture was stirred at room
temperature for 4 h. The methylene chloride solution was extracted
with water to remove water-soluble impurities. The crude DR1-
acrylate monomer product was purified by silica gel chromatog-
raphy eluting with hexane/methylene chloride (1:1 to 1:3) to afford
R(E) ) R0eE/E
(1)
u
where R(E) is a constant, R(E) is the absorption coefficient at
energy E, and Eu is referred to as the Urbach energy and
indicates the width of the exponential distribution. The Urbach
width has a thermal contribution of magnitude ∆kT, as well as
contributions from temperature-independent disorder and dis-
tribution of defects in the material.76
In this study, we investigate three structural strategies for
incorporation of an NLO dye system into polymers for spectral
near-IR absorption loss behavior: (1) covalent attachment of
the dye to the polymer as either a comonomer or a pendant
group; (2) solution mixing of the unmodified dye as a guest in
the polymer host; and (3) solution mixing of the dye as a guest
in the polymer host, with substitution of potentially charge-
transfer forming -OH groups on the dye by ester groups. The
well-characterized monazo dyes 4-[ethyl(2-hydroxyethyl)amino]-
4′-nitroazobenzene (Disperse Red 1, CAS 2872-52-8, Figure
1a) and 4′-[(N,N-dihydroxyethyl)amino]-4-nitroazobenzene (Dis-
perse Red 19, CAS 2734-52-3, Figure 1b) copolymerized as a
pendant group and as a comonomer, respectively. These dyes
are close analogues of each other, differing only in the number
of donor ethyl -OH groups, with Disperse Red 1 (DR1)
possessing one -OH group and Disperse Red 19 (DR19)
possessing two -OH groups. Their structures fall within the
merocyanine class of NLO dyes, and their commercial avail-
ability makes them convenient for studies of optical loss
behavior associated with different molecular bonding and
polymer host environments.
DR1 is incorporated into two different polymers as a pendant
group, poly(R-methacrylate) and poly(4-vinylphenol) (Figure
1i and j, respectively). The copolymer poly(DR1-methyl meth-
acrylate) at 10 mer% DR1 has been widely studied and shown
to exhibit relatively low absorption loss (<0.5 dB/cm77).
Preliminary UV-vis measurements indicated that poly(4-
vinylphenol) (PVP) has high transparency in the visible to near-
IR, and the acidic pendant -OH group lends itself to pendant
dye attachment. DR19 is incorporated as a comonomer into a
linear aliphatic Bisphenol A epoxy derivative, Bisphenol A
N-hydroxyethyl ether-N′-ether-co-N,N-bis(2-hydroxypropyla-
niline (“epoxy”, Figure 1h). High transparency in the visible to
near-IR was found for the neat epoxy derivative in initial UV-
vis measurements. Each of these copolymers is investigated at
three dye loading levels.
1
a red solid (4.85 g, 70%). H NMR (300 MHz, CDCl3, ppm) δ:
1.27 (t, J ) 7.2 Hz, 3H), 1.94-1.95 (m, 3H), 3.56 (q, J ) 7.5 Hz,
2H), 3.74 (t, J ) 6.3 Hz, 2H), 4.36 (t, J ) 6.3 Hz, 2H), 5.60 (m,
1H), 6.11 (s, 1H), 6.83 (d, J ) 9.3 Hz, 2H), 7.90 (d, J ) 6.6 Hz,
2H), 7.93 (d, J ) 6.3 Hz, 2H), 8.33 (d, J ) 9.3 Hz, 2H).
Synthesis of DR1-Containing Poly(ethyl methacrylate)s. Po-
lymerization of the monomer to form poly(ethyl metharylate)
(Figure 1i, R ) -CH2CH2-) was carried out in dimethylformamide
(DMF) solution under nitrogen atmosphere at 65 °C in the presence
of 1 wt % 2,2′-azobisisobutyronitrile (AIBN) for 24 h. The resulting
polymer solution was cooled and poured into methanol to precipitate
the polymer. The precipitated polymer was filtered, redissolved and
reprecipitated, filtered, and finally dried at 60 °C under reduced
pressure overnight. This procedure yielded a copolymer containing
560 µmol of DR1 per gram of acrylate polymer (Mw ) 88 700,
1
polydispersity 2.46). H NMR (300 MHz, CDCl3, ppm) δ: 0.87
(br), 1.03 (br), 1.24 (br), 1.81 (br), 1.91 (br), 3.53 (br), 3.70 (br),
4.02 (br), 4.14 (br), 6.81 (br), 7.89 (br), 8.31 (br). A second DR1-
acrylate copolymer was prepared by this procedure at a dye
concentration of 1710 µmol of DR1 per gram of acrylate polymer
1
(Mw ) 74 200, polydispersity 2.50). H NMR (300 MHz, CDCl3,
ppm) δ: 0.88 (br), 1.04 (br), 1.25 (br), 1.82 (br), 2.00 (br), 3.56
(br), 3.70 (br), 4.03 (br), 4.04 (br), 4.15 (br), 6.84 (br), 7.91 (br),
8.32 (br).
Poly(DR1 styrene-co-4-vinylphenol) (Figure 1j, “DR1-PVP”)
was prepared at DR1 doping levels of 560, 1180, and 2120 µmol
per gram of polymer via a Mitsunobu condensation between the
pendant hydroxyl group on DR1 chromophore and the phenol group
on poly-4-vinylphenol (PVP, Figure 1g). The loading level of the
1
DR1 chromophore was estimated from the integration of the H
NMR spectra of the materials. All of the chemicals were purchased
from Aldrich. For each composition (560, 1180, and 2120 µmol
DR1 per gram of polymer), DR1 was recrystallized from acetone,

7568 Barto et al.
Macromolecules, Vol. 39, No. 22, 2006
Figure 1. Monoazo guest dyes (a-e), host polymers (f-h), and azo dye copolymers (i-k) evaluated for this study.

Macromolecules, Vol. 39, No. 22, 2006
Absorption Behavior in Chromophore-Polymer Materials 7569
Scheme 1
(7.1 mL) and 28.6 mL of anhydrous pyridine. The mixture was
stirred overnight under an argon blanket, and then rotovapped at
40 °C for 1 h. The red crystalline crude product was washed with
ethanol, and then heated into solution at 90 °C and filtered through
a fritted filter funnel. The filtrate was dissolved in chloroform and
0.2 µm membrane filtered, followed by recrystallization in ethanol
to form the purified product.
Figure 2. Schematic of PDS experimental configuration.
and tetrahydrofuran (THF) was distilled under nitrogen from sodium
benzophenone ketyl prior to use. All of the other chemicals were
used as received unless otherwise mentioned. To the THF solution
of poly(4-vinylphenol) (1.0 g, 8.32 mmol), DR1 at appropriate
molar quantities (0.636, 1.66, and 2.50 mmol, respectively, for 560,
1180, and 2120 µmol/gram), and triphenylphosphine (1.7:1 molar
ratio to DR1) was added dropwise diethyl azodicarboxylate (DEAD,
1:1 molar ratio to triphenylphosphine). The reaction mixture was
allowed to stir under nitrogen at room temperature for 36 h. The
filtered solution was then added dropwise into diethyl ether. The
collected red precipitation was further purified by Soxhlet extraction
with methylene dichloride for 72 h and dried at 50 °C under vacuum
overnight to afford the product as a red solid.
1
OAc-DR1. H NMR (200 MHz, CDCl3) δ: 8.33 (d, J ) 9.0
Hz, 2H), 7.95-7.88 (m, 4H), 6.8 (d, J ) 9.0 Hz, 2H), 4.30 (t, J )
6.2 Hz, 2H), 3.68 (t, J ) 6.0 Hz, 2H), 3.50 (q, J ) 7.0 Hz, 2H),
2.67 (s, 3H), 1.26 (t, J ) 7.0 Hz, 3H).
1
Bis-OAc-DR19. H NMR (200 MHz, DMSO-d6) δ: 8.34 (d, J
) 8.8 Hz, 2H), 7.92 (d, J ) 8.8 Hz, 2H), 7.83 (d, J ) 8.8 Hz, 2H),
6.97 (d, J ) 9 Hz, 2H), 4.20 (br t, J ) 5.8 Hz, 4H), 3.74 (br t, J
) 5.2 Hz, 4H), 2.47 (br s, 6H).
For evaluation of benzoyl-protected DR1/PVP guest-host
materials, the ethanolamine of DR1 was esterified with benzoate
to form DR1-benzoate (Scheme 1). DR1 was received from Aldrich
and recrystallized in acetone. Dichloromethane was distilled over
phosphorus pentoxide, and THF was distilled over sodium/
benzophenone prior to use. To the mixture of 1.0 g of DR1 (3.18
mmol) and 0.466 g of benzoic acid (Aldrich, 3.816 mmol) in
dichloromethane/THF (2:1, v/v, 50 mL) were added 0.224 g of
DPTS (4-(dimethylamino)-pyridinium 4-toluenesulfonate, 0.763
mmol) and then 0.866 g of DCC (1,3-dicyclohexylcarbodiimide,
Aldrich, 4.197 mmol). The reaction mixture was allowed to stir at
room temperature overnight. After filtration to remove the white
precipitation of urea, the crude product (from the filtrate) was
purified by column chromatography using dichloromethane/hexane
1
DR1-PVP (560 µmol DR1/g polymer). H NMR (300 MHz,
acetone-d6, ppm) δ: 8.27-8.42 (br s), 7.82-8.18 (br m), 6.2-
7.13 (br, m), 4.06-4.33 (br s), 3.78-3.98 (br s), 3.53-3.74 (br s),
0.59-2.40 (br m). Molecular weight: Mw ) 27 950, polydispersity
2.05. UV-vis λmax (in acetone): 480 nm.
DR1-PVP (1180 µmol DR1/g polymer). 1H NMR (300 MHz,
acetone-d6, ppm) δ: 8.23-8.46 (br s), 7.77-8.20 (br m), 6.16-
7.17 (br, m), 4.01-4.31 (br s), 3.72-3.96 (br s), 3.43-3.71 (br s),
0.44-2.39 (br m). Molecular weight: Mw ) 23 500, polydispersity
1.87. UV-vis λmax (in acetone): 483 nm.
DR1-PVP (2120 µmol DR1/g polymer). 1H NMR (300 MHz,
acetone-d6, ppm) δ: 8.15-8.40 (br s), 7.72-8.10 (br m), 6.17-
7.09 (br m), 3.95-4.28 (br s), 3.36-3.93 (br m), 0.44-2.30 (br
m). UV-vis λmax (in acetone): 484 nm.
1
as eluent (30:1, v/v) to afford 1.2 g of product (compound 1). H
NMR (200 MHz, CDCl3, ppm) δ: 8.32 (d, J ) 9.15 Hz, 2H), 7.95-
8.07 (m), 7.93 (d, J ) 8.78 Hz, 2H), 7.92 (d, J ) 9.16 Hz, 2H),
7.38-7.64 (m), 6.87 (d, J ) 9.16 Hz, 2H), 4.55 (t, J ) 6.23 Hz,
2H), 3.83 (t, J ) 6.22 Hz, 2H), 3.60 (q, J ) 6.96 Hz, 2H), 1.28 (t,
J ) 6.96 Hz, 3H).
Guest-Host Mixture Preparation. All solvents used in guest-
host solutions were dried with molecular sieve. DR1 and DR19
from Aldrich were 1X recrystallized in acetone. With the exception
of DR1/PVP and benzoyl-DR1/PVP guest-host solutions, all
guest-host solutions were prepared as dilute solutions (low polymer
solids contents in solvent), due to the generally poor solubility of
the azo dyes. An exhaustive dye/polymer solubility screening study
was performed in a wide range of solvents for each azo dye-
polymer pair to determine solvent and concentrations at which stable
solutions could be prepared.
Poly(Disperse Red 19 Bisphenol A N-hydroxyethyl ether-N′-
ether-co-N,N-bis(2-hydroxypropylaniline) (Figure 1k, “DR19-
epoxy”) was prepared at DR19 doping levels of 530, 1010, and
1630 µmol/g as follows. Epoxide functionalized Disperse Red 19
(DR19) chromophore was synthesized from the azo coupling
reaction of N,N-bis(2,3-epoxypropyl)aniline and p-nitroaniline.
Carefully matching the stoichiometry of epoxy groups, from
epoxide-DR19 and Bisphenol A-diglycydyl ether, with the amine
(from anilines), all monomers were dissolved in anhydrous dioxane
in a drybox. The solution was heated to 90 °C and stirred for 30
min, and then heated to 110 °C for 36 h to complete polymerization
in the drybox to form a linear epoxy polymer with loading of DR19
at 530, 1010, and 1630 µmol/g (Figure 1k). Next, the solution was
pumped down to remove the dioxane, the polymer was titrated with
methanol 3X (until the methanol from the titration was clear), and
the polymer product was thoroughly dried. The undoped linear
Bisphenol A epoxy backbone polymer is depicted in Figure 1h.
The weight-average molecular weight Mw of the epoxy backbone
polymer was determined to be 10 300 by GPC against a polystyrene
standard.
DR1/PMMA guest-host solutions were prepared by adding DR1
to a dilute stock solution of PMMA (Scientific Polymer Products,
CAS 9011-14-7, Mw ) 75 000) of 6.7 wt % polymer solids in
N-methylpyrrolidone, at DR1 doping levels of 560, 1060, and 1710
µmol per gram of polymer.
DR1/PVP guest-host solutions were prepared by adding DR1
to a stock solution of PVP (Aldrich, CAS 24979-70-2, Mw
)
For evaluation of protected DR1/acrylate and protected DR19/
epoxy guest-host materials, the ethanolamine moieties of DR1 and
DR19 were esterified to form acetate-protected DR1 (Figure 1c)
and DR19 analogues (Figure 1d). DR1 was used as received from
Aldrich. DR19 was 1X recrystallized in acetone. To 1.5 mmol of
DR1 or DR19 (0.5 g) were added 75.7 mmol of acetic anhydride
20 000) of 24.0 wt % polymer solids in a 3:1 by weight mixture of
dimethylacetamide and cyclopentanone, at DR1 doping levels of
560, 1060, and 1710 µmol per gram of polymer.
DR19/epoxy guest-host solutions were prepared by adding
DR19 to a dilute stock solution of the epoxy backbone polymer
(Figure 1h, Mw ) 10 300) of 8.4 wt % polymer solids in

7570 Barto et al.
Macromolecules, Vol. 39, No. 22, 2006
N-methylpyrrolidone, at DR19 doping levels of 530, 1010, and 1630
µmol per gram of polymer.
spectra as long as the film morphology remains amorphous, defect
free, and free of gross surface undulations. These conditions are
generally met by films <20 µm thick with reasonable near-IR
transparency. For this study, the majority of the films prepared and
characterized were <8 µm thick and all were held to <18 µm thick.
Film thickness variation within a sample contributes to the
systematic uncertainty in the PDS optical absorption coefficient
measurement. The measured variation in film thickness was within
the error limits of the PDS measurements shown in the error bars
in the Results.
In a few special cases, the films were deposited in two or three
successive sequences of spinning or manually spreading followed
by baking to remove solvent to prevent crystallization of the dye
out of solution with the polymer. The benzoylated-DR1/PVP guest-
host solution doped at 1720 µmol dye per gram of polymer was
highly crystallized when spun and baked using the 14.8 wt %
polymer solids solution. A crystal-free film of this dye/polymer
composition was prepared by diluting to 7 wt % polymer solids,
spreading one drop (filtered at 0.2 µm) across the surface manually,
baking per the schedule shown in Table 2 for this composition,
and repeating the drop-spread-bake sequence twice. Bis-OAcDR19/
epoxy guest-host films doped at 540 and 1000 µmol dye per gram
of polymer were prepared by spinning and baking 2-4 drops of
the 4.9 wt % solids solution (filtered at 0.2 µm) under conditions
shown in Tables 1 and 2 of the Supporting Information, followed
by manually spreading an additional 3-4 drops of solution (filtered
at 0.2 µm) across the spun film, and baking under conditions shown
in Table 1 of the Supporting Information.
While we did not perform surface analyses to investigate film
forming kinetics under this successive maunal deposition scheme,
we speculate that the first deposition to form an ultrathin film on
the substrate served to denature the substrate surface of defects
that nucleate dye crystallites, and rendered the surface more
hydrophobic for improved wetting in subsequent depositions. This
first ultrathin layer (or first and second layers in the cases of three
successive depositions) was likely disrupted by the subsequent
deposition, but we assert that these one or two initial layers were
very thin and acted as a sacrificial quasi-surfactant layer, such that
the disruption in film thickness was comparable to natural film
thickness variations.
Thicknesses of the films prepared by manual spreading of dilute
solution were determined after PDS and UV-vis characterization
using a Tencor Instruments Alpha-Step profilometer over at least
two razor scratches in the area characterized by PDS, with a
minimum of three thickness measurements taken for each sample.
All films accepted for PDS characterization exhibited visual
thickness and color uniformity, and lacked obvious microstructure
(crystals or distinct phases). Thickness variation within a sample
(standard deviation) for DR1/PMMA films ranged from 1.2% to
10.8%, with all but one sample having variation <5.4%. For
OAcDR1/PMMA films, thickness variation within a sample ranged
from 1.6% to 4.6%. DR19/epoxy films exhibited sample thickness
variations ranging from 4.6% to 11.3%. Bis-OAcDR19/epoxy films
had sample thickness variations ranging from 0.3% to 11.3%, with
all but one sample showing variation <8%. Benzoyl-DR1/PVP
films at 1720 µmol of dye per gram of PVP had sample thickness
variations of 4.0% and 6.4%.
OAcDR1/PMMA guest-host solutions were prepared by adding
OAcDR1 (Figure 1c) to a dilute stock solution of PMMA (Scientific
Polymer Products, CAS 9011-14-7, Mw ) 75 000) of 6.7 wt %
polymer solids in a 1:1 by weight mixture of dimethylformamide
and cyclopentanone, at OAcDR1 doping levels of 560, 1060, and
1710 µmol per gram of polymer.
Benzoyl-DR1/PVP guest-host solutions were prepared by adding
benzoyl-DR1 (compound 1) to a stock solution of PVP (Aldrich,
CAS 24979-70-2, Mw ) 20 000) of 14.8 wt % polymer solids in a
11:7 by weight mixture of cyclopentanone and N-methylpyrrolidone,
at benzoyl-DR1 doping levels of 420, 560, 1060, and 1720 µmol
per gram of polymer. The 1720 µmol/gram solution was diluted to
7 wt % polymer solids with 11:7 cyclopentanone:N-methylpyrroli-
done and boiled for 5 s.
Bis-OAcDR19/epoxy guest-host solutions were prepared by
adding bis-OAcDR19 (Figure 1d) to a dilute stock solution of the
epoxy backbone polymer (Figure 1h, Mw ) 10 300) of 4.9 wt %
polymer solids in N-methylpyrrolidone, at bis-OAcDR19 doping
levels of 430, 540, 800, 1000, and 1300 µmol per gram of polymer.
Specimen Preparation. DR1-co-acrylate polymer spin solutions
were prepared by dissolving the copolymers in a 3:1 by weight
mixture of cyclopentanone and diglyme at 22 wt % polymer solids
content. DR1-co-PVP polymer spin solutions were prepared by
dissolving the copolymers in cyclopentanone at polymer solids
contents of 22.1, 21.1, 20.1, and 18.1 wt % for attached DR1 loading
levels of zero (undoped), 560, 1180, and 2120 µmol per gram of
polymer, respectively. DR19-co-epoxy polymer spin solutions were
prepared by dissolving the copolymers in cyclopentanone at polymer
solids contents of 21.7, 34.1, 25.0, and 25.1 wt % for attached DR19
loading levels of zero (undoped), 530, 1010, and 1630 µmol per
gram of polymer, respectively.
Film samples for PDS spectral characterization were prepared
on 1” diameter × 0.020” thick ultralow OH (<1 ppm) optical
quality fused silica substrates. The substrates were pre-cleaned using
a Piranha (H2SO4:H2O2) process. All dilute guest-host solutions
(DR1/PMMA, OAcDR1/PMMA, DR19/epoxy, and bis-OAcDR19/
epoxy at all dye concentrations, and benzoyl-DR1/PVP at a dye
concentration of 1720 µmol per gram of polymer) were filtered in
place at 0.2 µm, and cast by manual spreading 1-5 drops over the
substrate surface. Each of these films was baked under flowing
dry nitrogen in a dark box to remove the solvent immediately after
spreading the solution, using a bake schedule involving a lower
hold temperature of 100-130 °C for 1-2 min, ramping to an upper
hold temperature in 2-4 min, dwelling at the upper temperature
for 5-7 min, and then cooling to 50 °C in <3 min. The lower and
upper bake temperatures, ramp times, and hold times are given for
each dilute solution guest-host mixture in Table 1 of the Supporting
Information.
This manually spreading deposition technique was developed in
response to initial spin trials on silicon wafers, in which spun and
baked Disperse Red/polymer guest-host films exhibited a com-
bination of crystallites and thinning or poor wetting in the central
regions of the film. The primary objective of the manual technique
was to achieve a wetted film area of at least 8 mm × 4 mm (slightly
greater than the PDS pump beam projected image on the sample)
or reasonable uniformity (<15% variation), and free of crystallites,
voids, or significant “orange peel” surface texture. Film thickness
was not tightly controlled by this process, other than holding it to
<17 µm to maintain the condition of an optically and thermally
thin film within the PDS thermooptical model of Rosencwaig.77 It
was found through experimental trial and error that the manual
spreading technique for these dilute guest-host solutions was better
suited than conventional spin deposition to meet these film property
objectives.
All remaining solutions were spun onto the substrate by filtering
4-8 drops in place at 0.2-0.45 µm, and spinning using a Headway
Research 2-step spinning apparatus with a 2 s spread followed by
a higher speed spin ranging from 10 to 60 s depending on the
solution viscosity and wetting characteristics. Each of these films
was baked under flowing dry nitrogen in a dark box to remove the
solvent immediately after spreading the solution, using a bake
schedule involving a lower hold temperature of 100-130 °C for 5
min, ramping to an upper hold temperature in 5 min, dwelling at
the upper temperature for 5 min, then cooling to 50 °C in <3 min.
The spread, spin speeds, and spin times used for the dye-polymer
systems prepared by this procedure are summarized in Supporting
Information Table 2, and lower and upper bake temperatures and
times are summarized in Supporting Information Table 1.
As an explanatory note, we have found experimentally over the
course of over 200 spectral PDS measurements spanning a wide
range of NLO dye polymer systems that for films greater than 0.2
µm in thickness and within the optically and thermally thin limit,
differences in film thickness are inconsequential to the resultant

Macromolecules, Vol. 39, No. 22, 2006
Absorption Behavior in Chromophore-Polymer Materials 7571
Figure 3. PDS spectral concentration series measured for azo dye-polymer guest host materials: (a) DR1/poly(methyl methacrylate); (b) DR1/
poly(4-vinyl phenol); (c) DR109/linear aliphatic Bisphenol A epoxy.
All film samples were examined visually and at 50-200×, and
judged to be of acceptable quality for PDS characterization if they
exhibited thickness and color uniformity, and lacked obvious
microstructure (crystals or distinct phases). Thicknesses of all spun
samples were measured prior to PDS characterization by Alpha-
Step over small razor scratches in two representative locations on
each sample away from the center. Thicknesses of manually spread
films ranged from 1.7 to 10 µm, while thicknesses of spun films
ranged from 2 to 3.6 µm.
PDS scans were performed by measuring deflection signal, input
power, and phase lag at each wavelength, scanning from low energy
to high energy in 10 nm increments. Low energy scans were
performed first from 1830 nm to a minimum of 870 nm to reduce
the possibility of photobleaching near the absorption maximum. A
second, high energy scan was performed from 1830 to 600 nm for
the dual purpose of finding the PDS saturated signal (strong
absorption limit) and checking for any photobleaching effects from
previous scans. A UV-vis absorption spectrum was collected in
transmission mode on the same sample using a Cary 5E spectro-
photometer, scanning from 1100 to 300 nm, to complete the optical
absorption spectrum through the regions of overlap between PDS
and Cary transmission data and the absorption maximum.
Spectroscopy. The principles of PDS were described in complete
detail previously.1 The PDS test bed used a 1 kW Hg(Xe) dc short
arc lamp as the broadband illumination source at 12.6 Hz chopping
frequency, with a 1/8 m dual-grating monochromator. The source
beam image is a vertical ellipse, which is focused through vertical
entrance slits to the monochromator at a divergence nearly matching
that of the monochromator (f/3.7). To increase the length of
interaction between the sample illuminated region and the probe
beam, and thereby improve signal-to-noise, a Dove prism was used
in the beam path (see Figure 2) to rotate the beam image by 90°,
rendering it horizontal when focused onto the sample. A portion
of the pump beam was split off and focused onto a pyroelectric
detector to monitor optical power input at each wavelength via a
lock-in amplifier. The probe beam was a 10 mW CW HeNe laser,
attenuated to ∼1.5 mW. The sample disk was mounted inside a
fixture comprised of a back plate that slightly bends the vertical
centerline of the sample into the probe beam. The fixtured sample
was held inside a fused silica cuvette and filled with 3M Fluorinert
Liquid FC-72 as the index fluid. The cuvette was closed with an
O-ring sealed lid to prevent fluid evaporation. The cuvette was
placed in a custom holder on a rotational translation stage attached
to a triple axis translation stage, permitting precise alignment of
the sample surface to the probe beam for maximized signal. Probe
beam deflection was detected using a pn photodiode with an op-
amp drive circuit providing gain and zero offset control. A
blackened razor was used in front of the detector to shadow half
of the probe beam. Half of the probe beam was split to a replicate
reference detector prior to the sample. The primary and reference
detector dc signals were conditioned with a preamplifier, with the
difference signal forwarded to a second lock-in amplifier. This
scheme provided cancellation of probe beam pointing noise.
Results
DR1 and DR19 Polymer Guest-Host Mixtures. Repre-
sentative UV-vis-PDS absorption spectra for the DR1/PMMA,
DR1/PVP, and DR19/epoxy guest-host concentration series are
shown in Figure 3a-c, respectively.
As seen in Figure 3a, extremely well-defined, sharp overtone
peaks and very low near-IR loss values are seen for undoped
PMMA (0.42, 0.51, and 0.94 dB/cm at 1060, 1300, and 1550
nm, respectively). The overtone peaks for DR1/PMMA at 560
µmol dye per gram of PMMA are equally sharp, with slight
increases in loss (<0.6 dB/cm) at each wavelength. The overtone
peaks are strongly attenuated at 1060 µmol/g and are completely
overwhelmed at 1710 µmol/g by extreme Urbach tail broaden-
ing.
For DR1/PVP guest-host mixtures (Figure 3b), a distinct
charge-transfer peak centered at 1140 nm is seen for all dye
concentrations that is absent for undoped PVP. The shape of
this charge-transfer (CT) band is invariant with concentration.
This CT band controls near-IR loss at all concentrations at 1060
and 1300 nm, but at 1550 nm, loss appears to be controlled by
a combination of the CT band and a strong, broad ν0,2 O-H
overtone peak at 1440 nm seen for undoped PVP.
In Figure 3c, sharp overtone peaks are seen for undoped
epoxy, with a moderate ν0,2 O-H overtone peak at 1440 nm

7572 Barto et al.
Macromolecules, Vol. 39, No. 22, 2006
Figure 4. PDS spectral concentration series measured for acylated azo-dye-polymer guest host materials: (a) DR1-acetate/poly(methyl methacrylate);
(b) DR1-benzoate/poly(4-vinyl phenol); (c) DR19-bis-acetate/linear aliphatic Bisphenol A epoxy.
Figure 5. PDS spectral concentration series measured for azo-dye copolymers: (a) DR1-co-alkyl methacrylate; (b) DR1-co-4-vinyl phenol; (c)
DR19-co-linear aliphatic Bisphenol A epoxy.
responsible for increased loss at 1550 nm relative to undoped
PMMA, which is free of -OH. A broad CT band centered at
∼1020 nm convolved with a broad Urbach tail is responsible
for significant loss at 1060 and 1300 nm, and combines with
the epoxy ν0,2 O-H overtone peak at 1440 nm to give larger
loss at 1550 nm. Loss at 1060 and 1300 nm scales with the CT
intensity (highest at 1630 µmol/g, lowest at 530 µmol/g).
Esterified-DR1 and -DR19 Guest-Host Mixtures. Repre-
sentative UV-vis-PDS absorption spectra for the acylated-
DR1/PMMA, benzoylated-DR1/PVP, bis-acylated-DR19/epoxy,
and bis-acylated-DR19/PFC guest-host concentration series are
shown in Figure 4a-d, respectively.

Macromolecules, Vol. 39, No. 22, 2006
Absorption Behavior in Chromophore-Polymer Materials 7573
Figure 6. Disperse Red 1-acrylate polymer loss versus concentration trends at wavelengths of (a) 1060 nm; (b) 1300 nm; and (c) 1550 nm. [
DR1-co-acrylate; O DR1/PMMA guest-host; 2 OAcDR1/PMMA guest-host.
For acylated-DR1/PMMA (Figure 4a), the sharp overtone
peak structure and low near-IR loss for undoped PMMA is
nearly replicated for DR1-acetate doping at 560 µmol/g. Near-
IR loss at all concentrations is significantly lower than that of
DR1/PMMA. Distinct Urbach tail broadening is seen at the
intermediate concentration (1060 µmol/g), yielding near-IR loss
greater than that seen at the highest concentration (1710 µmol/
g).
consequence relative to DR1 ethanol -OH substitution or
covalent attachment for purposes of evaluation.
Spectra for all DR1-PVP dye concentrations (Figure 5b) are
free of CT bands, in contrast with DR1/PVP and DR1-acetate/
PVP guest-host systems. Overtones for all DR1-PVP spectra
are less well defined than for DR1-PMMA, and the main peak
for DR1-PVP is red shifted by ∼24 nm relative to DR1-
PMMA at 560 µmol/g. This strong bathochromism, combined
with a broader Gaussian main peak, is responsible for the higher
observed loss at 1060 and 1300 nm in DR1-PVP relative to
DR1-PMMA. Note that the DR1-PVP copolymers have
equivalent or lower loss than undoped PVP. Loss at 1550 nm
appears to be due to a combination of the strong ν0,2 O-H
overtone peak at 1440 nm and solvatochromism in the main
absorption peak. The DR1-PVP concentration series shows a
downward progression of the near-IR loss minima at 1300 and
1550 nm with dye concentration. This appears to be due in part
to a hypsochromic shift of the main absorption peak with
increasing dye concentration. All of the broadening of the main
absorption peak with increasing concentration occurs on the blue
edge. The reduction in loss at 1550 nm with dye concentration
appears to be due to a progressively less intense ν0,2 O-H
overtone peak, which indicates a net reduction in the concentra-
tion of -OH moieties as DR1 concentration increases.
The structure of the spectra for DR1-benzoate/PVP (Figure
4b) is nearly identical to that of DR1/PVP at all concentrations,
indicating no apparent effect of benzoylation on charge-transfer
behavior for the DR1/PVP guest-host systems.
For DR19-bis-acetate/epoxy guest-host materials (Figure 4c),
a CT band is convolved with an Urbach tail in all cases. The
sharpest CT band occurs at the two lowest concentrations (430
and 540 µmol/g), yielding slightly higher loss at 1300 nm, but
not at 1550 nm. Spectral structure and loss magnitudes at 1300
and 1550 nm are seen to be relatively insensitive to concentra-
tion for DR19-bis-acetate/epoxy.
Dye-Attached Polymers. Representative UV-vis-PDS ab-
sorption spectra for covalently attached DR1-acrylate, DR1-
PVP, and DR19-epoxy concentration series are shown in Figure
7a-d, respectively.
Spectra for DR1-acrylate copolymers (Figure 5a) show sharp
overtone peaks at all concentrations, with loss at each near-IR
minimum (1060, 1300, 1550 nm) increasing progressively with
concentration. With the exception of acylated DR1/PMMA at
560 µmol/g, the DR1-acrylate copolymer spectra exhibit the
lowest near-IR loss values among all of the azo dye polymer
systems studied. The main absorption peak shows no solvato-
chromism with increased concentration or with changing acrylate
alkyl substituent length (R) and shows progressive broadening
with concentration. For these reasons, the effect of acrylate
substituent length on loss mechanisms is judged to be of little
A prominent CT band is seen at ∼1000 nm for DR19-epoxy
at 530 µmol/g (Figure 5c), which diminishes and red shifts to
∼1110 nm at 1010 µmol/g, and completely disappears at 1630
µmol/g. Loss at all near-IR wavelengths (1060, 1300, 1550 nm)
scales with the strength of the CT band and, as such, decreases
with dye concentration from 530 to 1630 µmol/g.
Discussion
Loss versus concentration trends for each of the three
molecular environments (guest-host, esterified guest-host, and

7574 Barto et al.
Macromolecules, Vol. 39, No. 22, 2006
Figure 7. Disperse Red 1-PVP loss versus concentration trends at wavelengths of (a) 1060 nm; (b) 1300 nm; and (c) 1550 nm. [ DR1-co-PVP;
O DR1/PVP guest-host; 2 benzoyl-DR1/PVP guest-host.
Figure 8. Disperse Red 19-epoxy loss versus concentration trends at wavelengths of (a) 1060 nm; (b) 1300 nm; and (c) 1550 nm. [ DR19-co-
epoxy; O DR19/epoxy guest-host; 2 bis-OAcDR19/epoxy guest-host. Curved lines are for visual guidance only.
covalently attached dye-polymer) are compared for each dye-
polymer system in Figures 6-8 at wavelengths of 1060, 1300,
and 1550 nm.
DR1-Acrylate Materials. For the DR1-acrylate system
(Figure 6a-c, thick solid), covalent attachment is seen to yield
exceptionally low loss over all concentrations at each wave-
length. The acylated DR1/PMMA guest-host materials exhibit
loss values nearly equal to those of the covalently attached
DR1-acrylate materials, except at intermediate concentration
(1060 µmol/g), while the DR1/PMMA guest-host system shows

Macromolecules, Vol. 39, No. 22, 2006
Absorption Behavior in Chromophore-Polymer Materials 7575
an exponential increase in loss with concentration, at each
wavelength (Figure 6a-c, dashed).
two wavelengths. Higher near-IR loss ratio of DR1/PVP to
benzoylated-DR1/PVP seen at 1550 nm suggests that loss for
DR1/PVP at this transmission band has a greater contribution
from the intensity of the ν0,2 O-H overtone peak at 1440 nm
than that contributed by the CT band.
The acylated DR1/PMMA system shows marked Urbach tail
broadening at 1060 µmol/g (Figure 4a), and this is seen to result
in a slight increase in near-IR loss at this dye concentration. A
proposed mechanism for the Urbach broadening at this con-
centration is dipole-dipole interactions between neighboring
dye molecules that depend strongly on relative orientation. At
the intermediate dye concentration, the acylated dye has
sufficiently close average dye-dye spacing for dipole interac-
tions to be important, with sufficient orientational freedom to
take on a large distribution of dipole-dipole states. At higher
concentration, the DR1-acetate molecules take on fewer
orientations due to more localized ordering between the rigid
molecules.
The CT band in both guest-host systems occurs at the same
energy (∼1140 nm), but is ∼3 times more intense for the
benzoylated-DR1/PVP system. The occurrence of the CT band
maximum at the same intermediate molar concentration in both
systems suggests that the PVP phenol -OH group is responsible
for the CT interaction with an electron-donating moiety on the
dye. Steric hindrance to preferred orientation for CT is suggested
for higher dye concentrations. Dye-polymer CT interaction
appears to be significantly enhanced by the increased planarity
of the dye donor group in the benzoylated-DR1/PVP system,
as seen by the more intense CT band as compared to that of
nonbenzoylated DR1/PVP, indicating a hydrogen bonding
between the DR1 ethyl donor group (ethyl benzoate or ethanol)
and the PVP phenol -OH in both guest-host systems.
Quenching of the CT band seen in the covalently attached DR1-
PVP system is most likely due to suppression of dye-polymer
hydrogen bonding in this interaction scheme.
The exponential loss-concentration dependence observed for
DR1/PMMA guest-host materials (Figure 6a-c, solid) is
attributed to a broader distribution of dipole-induced dipole
interactions between dye and polymer as concentration increases.
Hydrogen bonding is ruled out in this system due to a lack of
charge-transfer band structure in the spectra. The large reduction
in loss at higher concentrations with both acylation and covalent
attachment of DR1 provides evidence that the ethanol -OH on
DR1 is responsible for a large broadening of the distribution of
states between dye and polymer in the DR1/PMMA system,
consistent with the progressive increase in Urbach tail width
with concentration (Figure 3a). Acylation of DR1 as a means
of reducing loss in this type of system is synthetically simpler
than covalent attachment.
DR19-Epoxy Materials. The DR19-epoxy system shows
advantages of both covalent incorporation of the dye into the
polymer backbone and ester protection of the dye -OH groups
in guest-host materials at all three near-IR transmission
wavelengths, as compared to the nonacylated guest-host system
(Figure 8). At concentrations from 430 to 1300 µmol/g, the bis-
acylated-DR19/epoxy guest-host system exhibits loss decreas-
ing linearly with concentration at 1060 and 1300 nm, but is
nearly independent with concentration at 1550 nm. As seen in
Figure 4c, this inverse linear loss-concentration dependence
coincides with the relative strength of a broad CT band centered
at ∼1030 nm, convolved with a concentration-independent
Urbach tail. The CT band intensity steadily decreases with
concentration. This gradual reduction in loss and CT band
strength with increasing dye concentration can be explained by
a dye-polymer CT equilibrium that is first order in polymer
concentration, that is, linearly inhibited by the addition of dye
above 430 µmol/g. A likely dye-polymer interaction is CT
between the epoxy alkyl -OH groups and the bis-ethyl acetate
donor group of the dye. In this equilibrium, the dipole-dipole
interaction between dye molecules increases linearly with dye
concentration, proportionally decreasing the number of dye
molecules available to the dye-polymer CT interaction.
DR1-PVP Materials. For the DR1-PVP system, covalent
attachment is clearly favored, showing a dramatic reduction in
near-IR loss over the guest-host systems at each wavelength
at all concentrations (Figure 7a-c, thick solid). Loss for the
covalently attached materials has no concentration dependence
at 1060 nm (Figure 7a, thick solid) and decreases linearly with
concentration at 1300 and 1550 nm (Figure 7b and c, thick
solid). More complex loss-concentration dependence is seen
for both DR1/PVP and benzoylated-DR1/PVP guest-host
systems, with an apparent loss maximum at ∼1350 µmol/g for
both systems at each wavelength (Figure 7a-c). Benzoyl
substitution of the DR1 ethanol -OH group in the guest-host
system results in the largest values of loss at all concentrations
among the DR1-PVP materials investigated, by orders of
magnitude over DR1-co-PVP at 1060 and 1300 nm. Comparing
the loss behavior of DR1/PVP guest-host systems (unprotected
and ester protected) to that of DR1/acrylate, distinct charge-
transfer (CT) behavior is seen in DR1/PVP while no CT is
exhibited in DR1/acrylate. The carboxyl moiety of the acrylate
backbone structure lacks sufficient electron-donating character
to support a CT interaction with either DR1 or acylated DR1,
while the acidic character of the phenol group in PVP gives
rise to strong CT character with DR1 and DR1-benzoate. This
CT character is greatly enhanced by the coplanarity of the DR1-
benzoate and PVP phenol substituents. In contrast, esterification
of DR1 reduces loss in the DR1/acrylate guest-host systems
by sterically hindering the dye-polymer and dye-dye orienta-
tion states.
The covalently attached DR19-epoxy system exhibits loss
decreasing linearly with concentration at all three near-IR
wavelengths over the concentration range 530-1630 µmol/g,
and loss that is equivalent to or less than that measured for the
bis-acylated-DR19/epoxy system at all wavelengths over most
of the concentration range. As for the bis-acylated system, CT
band intensity in attached DR19-epoxy decreases with con-
centration, and this dominates the net loss versus concentration
trend.
These results show that CT in DR19-epoxy systems occurs
in the absence of ethanol -OH groups (consistent with the CT
bands observed in bis-acylated-DR19/epoxy) and is disrupted
by the addition of dye (also consistent with the bis-acylated-
DR19/epoxy results). At low dye concentration in attached
DR19-epoxy, the dye has sufficient rotational freedom to form
the dye-polymer charge-transfer interaction in attached DR19-
epoxy. As concentration increases, the dye is more sterically
hindered by other attached dye molecules to form the dye-
polymer CT interaction, leading finally to CT quenching by
Losses measured for DR1/PVP guest-host materials are ∼1/4
to 2/3 the values measured for benzoylated-DR1 at all concen-
trations at each wavelength. The measured loss ratios between
DR1/PVP and benzoylated-DR1/PVP guest-host systems at
1060 and 1300 nm are in agreement with ratios of charge-
transfer band intensities at each concentration, in support of
charge-transfer dominated near-IR loss for these systems at these

7576 Barto et al.
Macromolecules, Vol. 39, No. 22, 2006
steric hindrance at high concentration (1630 µmol/g). Slight red-
shifting of the CT band position with increased dye concentra-
tion suggests the CT interaction occurs in a more locally polar
environment. Unlike bis-acylated-DR19/epoxy, covalent attach-
ment of DR19 to the linear epoxy eliminates Urbach tail
broadening, and at the highest concentration (1630 µmol/g),
DR19-epoxy attachment eliminates charge transfer.
decreasing Urbach tail widths, and by quenching charge transfer
in PVP. Covalent attachment was less effective in a DR19-
epoxy system, due to less steric hindrance in doubly attached
DR19 on the epoxy versus that by phenyl attachment in DR1-
PVP, and due to a less homogeneous backbone structure in the
epoxy versus acrylate and PVP.
Azo dye donor group esterification was effective in reducing
near-IR loss in DR1/acrylate guest-host systems at high
concentration, and in DR19/epoxy guest-host systems at most
concentrations, due to steric hindrance of rotational and
translational motion of the dye, restricting the number of dye-
polymer and dye-dye orientational states and reducing Urbach
tail width. Esterification of the DR1 donor group by benzoate
in the DR1/PVP guest-host system sharply increased near-IR
loss, due to charge-transfer enhancement by dye donor-polymer
acceptor substituent coplanarity.
The unmodified DR19/epoxy guest-host system shows loss
increasing linearly with concentration at 1060 and 1300 nm,
opposite to the trends observed for copolymerized and acylated
guest-host systems, while at 1550 nm, loss is relatively
insensitive to concentration. As seen in Figure 3c, this loss-
concentration dependence is due to a weak CT band centered
at ∼1020 nm that grows with increased concentration and that
is convolved with broad Urbach tails. The CT band dominates
loss magnitude at 1060 and 1300 nm, while at 1550 nm, the
net loss is seen to be balanced between the CT band intentsity
and that of the ν0,2 O-H overtone peak at 1440 nm, which
decreases with increasing DR19 concentration. The O-H
overtone intensity decay with dye concentration shows that the
number density of -OH moieties is controlled by the linear
epoxy concentration. The concentration dependence of CT band
strength and loss at 1060 nm and 1300 follows a CT equilibrium
that is first order in dye concentration. Hydrogen bonding
between the bis-ethanol -OH groups of DR19 and the epoxy
alkyl -OH groups is limited by the relative abundance of more
mobile DR19 -OH groups in this CT equilibrium.
The higher loss observed for the nonacylated DR19/epoxy
guest-host system relative to attached- and acylated-DR19/
epoxy at all concentrations (see Figures 3c vs 4c) is seen to
derive from greater Urbach tail broadening due the smaller size
of the unattached molecule, allowing it to take on more dye-
dye and dye-polymer orientational states. DR19 acylation is
therefore more effective in reducing loss in the guest-host
epoxy as a result of increased steric hindrance to rotational and
translational motion of the dye to reduce Urbach tail broadening.
Covalent attachment of DR19 to epoxy has an even greater
effect on reducing near-IR loss at high concentration, due to
CT band quenching.
Characterization of spectral absorption behavior as a function
of dye concentration is shown to be a valuable technique for
gaining insights into the mechanisms responsible for near-IR
loss in NLO azo-dye polymer materials.
Acknowledgment. R.R.B. gratefully acknowledges Lock-
heed Martin Corp. for financial support through the Lockheed
Martin-Stanford Honors Co-op Program and Lockheed Martin
independent research and development funding. R.R.B. would
like to acknowledge Lawrence J. Dries, Dexter G. Girton, and
Wendell D. Eades from Lockheed Martin, Andrew J. Skumanich
from Applied Materials, and Carlton H. Seager from Sandia
National Laboratories for significant technical guidance and
many helpful discussions. Finally, we express sincere gratitude
to Dr. Dan Dawson of IBM Almaden Research Labs for
providing the 24% DR1-PMMA copolymer material, and to
Prof. Shahin Maaref of Norfolk State University for providing
recrystallized Disperse Red 19.
Supporting Information Available: Bake and spin conditions
used for PDS film preparation. This material is available free of
charge via the Internet at http://pubs.acs.org.
References and Notes
Comparing the loss behavior of DR19/epoxy systems to that
of the two DR1/polymer guest-host systems (Figures 3-5), a
less distinct CT band forms in DR19-epoxy in all bonding
environments (guest-host, acylated guest-host, attached) and
with less Urbach tail broadening. This band behavior suggests
more restricted rotational and translational motion by the larger
DR19 and DR19-acetate molecules relative to DR1.
Covalent attachment is shown to be less effective in quench-
ing CT in DR19-epoxy as compared to that by DR1-PVP
(Figure 5c and b, respectively). This is attributed to less
restriction to rotational motion by double attachment in DR19-
epoxy as compared to that by the phenyl group upon DR1
attachment to PVP.
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