The Influence of Solubilizing Chain Stereochemistry on Small Molecule Photovoltaics

Article abstract of DOI:10.1002/adfm.201401030

Three stereochemically pure isomers and two isomeric mixtures of a solution-processable diketopyrrolopyrrole-containing oligothiophene (SMDPPEH) have been used to study the effect of 2-ethylhexyl solubilizing group stereochemistry on the film morphology and bulk heterojunction (BHJ) solar cell characteristics of small molecule organic photovoltaics. The different SMDPPEH stereoisomer compositions exhibit nearly identical optoelectronic properties in the molecularly dissolved state, as well as in amorphous films blended with PCBM. However, for films in which SMDPPEH crystallization is induced by thermal annealing, significant differences in molecular packing between the different stereoisomer formulations are observed. These differences are borne out in photovoltaic device characteristics for which unannealed devices show very similar behavior, while after annealing the RR- and SS-SMDPPEH enantiomers show blue-shifted peak EQE relative to the SMDPPEH isomer mixtures. Unannealed devices made from the most crystalline stereoisomer, meso RS-SMDPPEH, are not completely amorphous, and show improved photocurrent generation as a result. Unlike the other compounds, after thermal annealing the RS-SMDPPEH devices show reduced device performance. The results reveal that the chirality of commonly used 2-ethylhexyl solubilizing chains can have a significant effect on the morphology, absorption, and optimum processing conditions of small molecule organic thin films used as photovoltaic device active layers.

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The Influence of Solubilizing Chain Stereochemistry
on Small Molecule Photovoltaics
Raghida Bou Zerdan, Nathan T. Shewmon, Yu Zhu, John P. Mudrick, Kyle J. Chesney,
Jiangeng Xue,* and Ronald K. Castellano*
vacuum deposition or solution processing
Three stereochemically pure isomers and two isomeric mixtures of a solution-
processable diketopyrrolopyrrole-containing oligothiophene (SMDPPEH) have
been used to study the effect of 2-ethylhexyl solubilizing group stereochem-
istry on the film morphology and bulk heterojunction (BHJ) solar cell char-
acteristics of small molecule organic photovoltaics. The different SMDPPEH
stereoisomer compositions exhibit nearly identical optoelectronic properties
in the molecularly dissolved state, as well as in amorphous films blended
with PCBM. However, for films in which SMDPPEH crystallization is induced
by thermal annealing, significant differences in molecular packing between
the different stereoisomer formulations are observed. These differences are
borne out in photovoltaic device characteristics for which unannealed devices
show very similar behavior, while after annealing the RR- and SS-SMDPPEH
enantiomers show blue-shifted peak EQE relative to the SMDPPEH isomer
mixtures. Unannealed devices made from the most crystalline stereoisomer,
meso RS-SMDPPEH, are not completely amorphous, and show improved
photocurrent generation as a result. Unlike the other compounds, after
thermal annealing the RS-SMDPPEH devices show reduced device perfor-
mance. The results reveal that the chirality of commonly used 2-ethylhexyl
solubilizing chains can have a significant effect on the morphology, absorp-
tion, and optimum processing conditions of small molecule organic thin films
used as photovoltaic device active layers.
of the active materials; while the former
offers highly controlled thickness and
uniformity as well as readily available
multilayer structures, the latter is prom-
ising for large scale, low cost, and low
[
1]
temperature production.
Dissolution
of otherwise aggressively aggregating
π-conjugated materials for thin film prepa-
ration from solution requires judicious
[
2,3]
choice of pendent solubilizing groups,
typically saturated hydrocarbon chains
that promote favorable van der Waals
interactions with polarizable organic sol-
vents and weaken intermolecular π–π
[
3]
interactions. For applications that mutu-
ally rely on good solution processability,
closely packed π-surfaces, and appropriate
active layer morphology (e.g., field-effect
transistors or FETs, photovoltaics, etc.),
solubilizing groups emerge as impor-
tant structural components for study and
material/device optimization. Indeed, for
both oligomers and polymers, side chain
[
4–13]
[14–28]
[19,22,29–34]
type,
length,
branching,
[
35–39]
[40–49]
placement,
and stereochemistry
have independently been shown to influ-
ence thin film morphology, charge carrier
mobility, and device performance.[50–52]
Among the most popular solubilizing groups in the
organic materials community is “2-ethylhexyl” (EtHx),
1
. Introduction
[
53–63]
π-Conjugated organic semiconductors remain actively pur-
sued for diverse solid state photonic, electronic, and optoelec-
tronic device applications. Device fabrication involves either
and it comes pre-installed on many commercially-available
building blocks for π-conjugated oligomer/polymer prepara-
tion. Given the substituent’s asymmetric carbon atom, mix-
tures of stereoisomers are formed in syntheses that begin
from racemic starting materials. For molecules bearing two
or more EtHx side chains, mixtures include enantiomers and
diastereomers, although the isomeric complexity is gener-
ally ignored. While the stereoirregularity presumably reduces
R. Bou Zerdan,[+] Y. Zhu, K. J. Chesney,
Prof. R. K. Castellano
Department of Chemistry
P.O. Box 117200
University of Florida
Gainesville, FL, USA
E-mail: castellano@chem.ufl.edu
N. T. Shewmon,[+] J. P. Mudrick, Prof. J. Xue
Department of Materials Science and Engineering
P.O. Box 116400
University of Florida
Gainesville, FL, USA
E-mail: jxue@mse.ufl.edu
[+]R.B.Z. and N.T.S. contributed equally to this work.
[
40]
crystallinity and improves solubility,
the ramifications for
otherwise carefully performed structure-property-function
studies involving organic π-systems, particularly small mol-
[
40]
ecules, have only recently come to light. The motivation of
the current work is to understand to what extent it is reason-
able to ignore the stereochemistry of the EtHx solubilizing
group, and the isomeric complexity it creates, for small
π-conjugated molecule morphology and bulk heterojunction
photovoltaic devices.
DOI: 10.1002/adfm.201401030
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DOI: 10.1002/adfm.201401030
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and photovoltaic properties of SMDPPEH
[
55,65,66]
(
Figure 1d), a well-studied
and com-
mercially-available organic semiconductor.
The requisite stereoisomers of the material
(
RR- , SS- , and RS-SMDPPEH) have been
independently synthesized and then char-
acterized together with as-synthesized (syn-
SMDPPEH) and commercially-available
(
com-SMDPPEH) samples that are stereoi-
someric mixtures. Our results show that side
chain stereochemistry and stereoisomeric
composition have measurable consequences
on the morphology, charge transport charac-
teristics, and photophysical properties of the
active layer and the shape of the absorption
spectra of the neat materials, but have a lim-
ited effect on the overall photovoltaic perfor-
mance in bulk heterojunction devices.
2
. Results and Discussion
Figure 1. π-Conjugated polymers (a,b) and small molecules (c,d) prepared in stereocontrolled
fashion with respect to their 2-ethylhexyl side chains. SMDPPEH has been studied in the cur-
rent work.
2
.1. Synthesis
Five
different
SMDPPEH
composi-
tions were employed in this study: com-
For polymeric π-systems, the consequences of stereocon-
trol with respect to the EtHx side chain have been modestly
evaluated. For example, (R)-2-ethylhexyl side chains biased
the helical backbone conformation of polyfluorene homopoly-
mers (R)-PF2/6 (Figure 1a),[ leading to enhanced chiroptical
SMDPPEH (sublimed, 97% (HPLC), commercially-obtained
from Sigma-Aldrich and used without further purification),
syn-SMDPPEH (prepared from racemic reagents in our
laboratory), and the pure stereoisomers (RR- , SS- and RS-
SMDPPEH) available through stepwise synthesis involving
the independently prepared enantiomers of 2-ethylhexyl bro-
mide (3-(bromomethyl)heptane) 1R and 1S (Scheme 1). Our
approach to 1 recognized that 2-ethylhexanol 2, a suitable pre-
cursor, has been prepared in enantiomerically enriched form
64]
properties in the solid state. For PProDOT-((2S)-ethylhexyl)
(
2
Figure 1b), prepared by Reynolds and coworkers,[ the chiral
41]
side chains further encouraged the formation of chiral aggre-
gates in solution. Interestingly, the solid state electronic (i.e.,
[
41,67]
electrochemical potentials, conductivity) and optical (i.e., λmax
,
through chemical synthesis
matic reduction of 2-ethylidenehexan-1-ol). We made small
(and more recently via enzy-
[
42]
optical gap) properties of the enantiomerically pure polymer
[
67]
were essentially identical to PProDOT-(ethylhexyl) (not shown)
adjustments to a synthesis of 2
that began from Evans’
2
prepared from racemic EtHx starting materials.[ One conclu-
sion, although not put forth by the authors, is that the solu-
bilizing chain stereochemical details are washed out in dis-
perse polymer environments with respect to bulk thin film
properties.
41]
chiral oxazolidinone as shown for 1R (Scheme 1). Acylation
of (R)-4-benzyl-2-oxazolidinone 3R with the in situ gener-
ated mixed anhydride derivative of hexanoic acid afforded
[68]
In contrast, Nguyen and coworkers have recently reported a
significant influence of EtHx stereoisomerism on π-conjugated
oligomer crystallization behavior and consequently thin film
morphology and FET characteristics.[
40]
Independent char-
acterization of the three stereoisomers (RR, SS, and RS) of
DPP(TBFu) (isolated by chiral HPLC separation) (Figure 1c)
2
as well as the as-synthesized material (a roughly statistical
mixture of stereoisomers) revealed greater planarity, smaller
interplanar (π–π) spacing, red-shifted absorption, and several
fold higher hole mobility for the centrosymmetric RS isomer
versus the RR/SS isomers and the isomeric mixture.[ To the
best of our knowledge, a comparable analysis in the context of
small molecule bulk heterojunction photovoltaics has not been
performed.
40]
Reported here are the functional consequences of 2-ethyl-
hexyl stereochemistry on the optical, electrical, morphological,
Scheme 1. Synthesis of enantiomerically enriched 2-ethylhexyl bromide
1R and 1S.
2
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4
R.[69] Deprotonation with sodium bis(trimethylsilyl)amide
2.2. Isomeric Composition
followed by addition of ethyliodide gave 5R (in moderate
yield but excellent diastereopurity),[ and subsequent reduc-
67]
The stereoisomeric purity of RR- , SS- and RS-SMDPPEH, as
well as the isomeric composition of syn-SMDPPEH and com-
SMDPPEH, were evaluated by chiral HPLC using the condi-
[
67]
tion using lithium borohydride yielded 2-ethylhexanol 2R.
Spectroscopic and analytical data for 2R, including optical
[
40]
rotation, are identical to those reported in the literature (see
tions established by Nguyen and coworkers (Figures S1–S3).
the Supporting Information for details).[
41,42,70]
Direct conver-
Analysis of the independently synthesized isomers reveals just
one major peak in each case confirming their excellent iso-
meric purity (∼ 97%); the technique also provides the individual
isomer retention times (SS = 50 ± ±1 min; RS = 58 ± ±1 min; and
sion to the bromide proved troublesome under a variety of
conditions.[
71–73]
Replacement of the hydroxyl group with a
bromine, via the Appel reaction, generated a mixture of 2-eth-
ylhexyl bromide 1R and 1,2-dibromooctane, for example. The
desired product was difficult to isolate by column chromatog-
raphy. Fortunately, conversion of 2R fi rst to the corresponding
tosylate 6R,[ followed by treatment with lithium bromide
gave 1R cleanly. Preparation of 1S from 3S was performed
similarly (see Scheme S2). The high enantiomeric excess of
[
40]
RR = 69 ± ±1 min). Analogous to the synthesis of DPP(TBFu)2,
preparation of SMDPPEH from (± )-2-ethylhexyl bromide
should generate the SS- , RS- and RR- stereoisomers in a statis-
tically predicted 1:2:1 (25%, 50%, 25%) ratio, respectively. Three
peaks are indeed detected for com-SMDPPEH in a nearly statis-
tical area ratio of 24:48:28. For syn-SMDPPEH, the component
ratios (Figures S4 and S5) deviate slightly from the commercial
material (Figure S6) depending on the synthetic batch and final
target purification protocol (e.g., batch #1 (one chromatography
column and two recrystallizations): 28:42:30; batch #2 (one
chromatography column and one recrystallization): 26:41:33).
While it is difficult to say whether the differences are statisti-
cally meaningful, the results should give pause to practitioners
who assume that oligomer synthesis necessarily provides batch-
to-batch reproducibility. The stereoisomeric ratios for π-systems
containing two or more EtHx substituents can easily vary
depending on synthetic preparation and purification.
74]
1
R and 1S ( >95%) was established by chiral HPLC of subse-
quent products (vide infra).
Synthesis of the SMDPPEH materials then followed the
general approach pioneered by Nguyen and coworkers begin-
ning from readily prepared dihydropyrrolo[3,4-c]pyrrole-
75,76]
1
,4-dione 7 (Scheme 2).[
syn-SMDPPEH, RR-SMDPPEH,
and SS-SMDPPEH were prepared using (± )-2-ethylhexyl bro-
mide, 1R, and 1S, respectively, as N-alkylating agents (Schemes
S3, S4, and S6). Only the synthesis of RS-SMDPPEH is shown
(
Scheme 2) as it required a stepwise N-alkylation procedure.
Reaction of 1R with a 10-fold excess of 7 generated the mon-
oalkylated product 8R, which was further alkylated with 1S
to afford 9RS. Bromination of the terminal thiophenes of
9
RS with N-bromosuccinimide (NBS) and subsequent palla-
2.3. Molecular Properties
dium-catalyzed Suzuki coupling with commercially available
boronic ester 11 generated RS-SMDPPEH in amounts suit-
able for solution-phase and thin film investigation. All five
compounds were successfully analyzed by H NMR, C NMR,
HRMS, and elemental analysis. Worth noting, syn- and com-
SMDPPEH showed no evidence by NMR of existing as iso-
meric mixtures.
The optical properties of the SMDPPEH formulations were first
−6
−6
evaluated in dilute solution (CHCl ; 2.5 ×±10 M – 30 ×±10 M).
3
1
13
All five share identical UV-Vis absorption spectra (Figure 2 and
Table 1), confirming that the chiral side chain exerts no influ-
ence on the intrinsic electronic properties of the chromophores
or their solution-phase conformations. Linear Beer-Lambert
plots additionally confirmed that no aggre-
gation occurs at the considered concentra-
tions (Figures S7–S16). Three maxima are
observable in the UV-visible region. The high
energy absorption band (λ = 384 nm) cor-
responds to a π–π* transition for both the
thiophene and the central diketopyrrolopyr-
[
66,77]
role (DPP) units,
in the visible region (maxima at 616 and
46 nm) is assigned as an intramolecular
while the absorption
6
charge transfer transition. The extinction
coefficients for these materials only vary
slightly, and all show expectedly low optical
gaps of 1.77 eV consistent with their strong
[
78]
donor-acceptor character (Table 1). Finally,
fully consistent with the absorption data, rep-
resentative SMDPPEH compositions have
nearly identical electrochemical potentials
(
based on CV/DPV measurements) in dichlo-
romethane (Table S1 and Figures S17–S19).
The HOMO energies (average = –5.36 eV),
LUMO energies (average = –3.54 eV), and
Scheme 2. Synthesis of RS-SMDPPEH.
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Table 2. SMDPPEH thermal properties.
Material
5% Weight Loss
°C]
Tm
Tc
[°C]
a)
b)
[
[°C]
158
159
syn-SMDPPEH
com-SMDPPEH
SS-SMDPPEH
RR-SMDPPEH
RS-SMDPPEH
342
341
343
343
343
126
133
156/160
155/160
180
132b
127b
154
a)The 2nd and 3rd heating and cooling scan cycles were employed to determine the
thermal transition temperatures, at a scan rate of 10 °C/min, under N2 atmos-
b)
phere; Although two crystallization peaks were expected for the enantiomers, they
could not be resolved even upon changing the heating/cooling rate.
studied isomers.[40] RS-SMDPPEH is accordingly also the least
soluble of the SMDPPEH compositions in our hands.
Figure 2. Normalized absorbance spectra of SMDPPEH in solution
X-ray diffraction (XRD) was used to study the crystallinity
of neat and blended (with PCBM) spin-coated SMDPPEH thin
films prepared on silicon substrates (precoated with 25 nm
thick PEDOT:PSS films to allow a direct comparison with
films incorporated in photovoltaic devices, vide infra). For
neat, unannealed films (Figure 3a) a single, relatively weak dif-
fraction peak is observed for all five materials. The peaks for
syn-SMDPPEH (2θ = 6.20°; d = 14.2 Å), com-SMDPPEH (2θ =
6.16°; d = 14.3 Å), and RS-SMDPPEH (2θ = 6.21°; d = 14.2 Å)
share similar 2θ values. Of these three the RS-SMDPPEH peak
is most intense, consistent with the compound’s intrinsically
higher degree of crystallization (vide supra). Enantiomers SS-
SMDPPEH (2θ = 6.66°; d = 13.3 Å) and RR-SMDPPEH (2θ =
6.63°; d = 13.3 Å) show expectedly nearly identical XRD spectra
but a slightly tighter (by ∼ 1 Å) molecular packing than the
other three materials. After annealing (at 100 °C for 5 min),
a 2–3 fold increase in diffraction intensity (and increase in
degree of crystallization) is observed for all of the SMDPPEH
materials (Figure 3b). syn-SMDPPEH and com-SMDPPEH
continue to exhibit identical XRD spectra, with a single peak
centered at 2θ = 6.00° (d = 14.7 Å), indicating that the differ-
ences in isomer composition between these two compounds
have little effect on neat film crystallinity. Neat, annealed films
of SS-SMDPPEH and RR-SMDPPEH again show expectedly
similar behavior, but now with two peaks (2θ = 5.54˚ (d = 15.9 Å)
and 2θ = 6.36° (d = 13.9 Å)) observed for each film. Implied
are co-existing crystal phases, a result consistent with the DSC
data (vide supra). The difference in relative intensity for the two
peaks between the SS- and RR-SMDPPEH samples indicates
a different fractional film coverage for the two phases, with
both materials preferentially forming in the d = 15.9 Å phase
while RR-SMDPPEH forms slightly more of the d = 13.9 Å
−6
(
CHCl ; 20±×±10 M).
3
HOMO–LUMO gaps (average = 1.8 eV) estimated from the oxi-
dation and reduction peak potentials are also consistent with
the optical data.
2
.4. Characterization of the SMDPPEH Compositions in the
Solid State
The thermal properties of the SMDPPEH samples were studied
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) in order to understand the effect of isomeric
composition and EtHx stereochemistry on the bulk behavior
of the materials. All compounds show similar thermal stability
with loss of 5% of their original weight at high temperature
(
341–343 °C; Figure S20). DSC (Table 2 and Figure S21) reveals
a melting transition (T ) of ∼ 160 °C for four of the materials;
m
this is increased significantly to 180 °C for the meso compound
RS-SMDPPEH. An additional melting transition is found for
the enantiomers RR- and SS-SMDPPEH (at 156 and 155 °C,
respectively) implicating the co-existence of two different
crystal phases. With the exception of RS-SMDPPEH, the mate-
rials all crystallize upon cooling (T ) at ∼ 130 °C. The higher T
c
c
for the meso compound (154 °C), taken together with its higher
Tm, speak to a greater crystallization tendency for this isomer
formulation. This result is consistent with what was reported
for RS-DPP(TBFu) that showed the highest thermal transition
2
temperatures and lowest solubility among its independently
−
6
Table 1. Optical properties of SMDPPEH in CHCl (20±×±10 M).
[79]
3
phase. Precedent leads one to suspect that such differences
might arise from the clockwise substrate rotation during spin
coating that was used here, which could affect the chiral mate-
rials differently. At any rate, it appears that even for enanti-
omers the film crystallinity can vary depending on subtle dif-
ferences in film preparation. Interestingly, the RS-SMDPPEH
XRD peak shifts about half an angstrom to a smaller d-spacing
4
Material
λmax
λonset
[nm]
ε ×±10
ΔEopt
[eV]
[M ·cm−1]
6.2 ± 0.1
7.3 ± 0.2
6.5 ± 0.2
6.2 ± 0.1
6.6 ± 0.1
−
1
[
nm]
syn-SMDPPEH
385/616/646
700
699
699
699
699
1.77
1.77
1.77
1.77
1.77
com-SMDPPEH 384/616/646
SS-SMDPPEH
RR-SMDPPEH
RS-SMDPPEH
384/616/647
384/615/646
385/615/646
(
2θ = 6.46°; d = 13.7 Å) upon annealing, while the XRD peaks
of the other four films shift toward slightly higher d-spacings.
Again, the RS-SMDPPEH material shows the strongest peak
4
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Figure 3. XRD spectra for spin-coated films of (a,b) neat SMDPPEH or (c,d) SMDPPEH:PC BM. Films (a,c) were not annealed, whereas films (b,d)
61
were annealed at 100 °C for 5 minutes.
intensity, indicating a higher degree of crystallization in the
film.
isomer composition between the syn- and com-SMDPPEH
materials. For SS- and RR-SMDPPEH, a single peak for each is
observed at 2θ = 6.79° (d = 13.0 Å) and 2θ = 6.84° (d = 12.9 Å),
respectively. Again, we would expect identical behavior between
the enantiomers, however small processing differences, for
example the clockwise rotation of the spin coater, may con-
tribute to the differences observed in peak intensity. Overall,
the XRD patterns for these annealed blended films (Figure 3d)
exhibit strong resemblance to those for the neat SMDPPEH
films prior to annealing (Figure 3a), but are considerably dif-
ferent from those for the annealed neat films (Figure 3b). Com-
paring the annealed neat films to the annealed films blended
with PCBM, it is clear that PCBM can have a strong effect
on the phase adopted by the different SMDPPEH isomers.
Of the five materials, the neat crystal structure appears to be
maintained only by RS-SMDPEH after blending with PCBM,
while the other four materials show a significant shift toward
shorter d-spacing upon PCBM addition. Again, Scherrer grain
sizes were calculated from the FWHM of the blend film XRD
peaks, with a size of 7 nm determined for the unannealed RS-
SMDPPEH fi lm (no peaks were observed for the other mate-
rials under these conditions). After annealing, grain sizes of 8,
12, 17, 25 and 14 nm were found for syn-, com-, SS- , RR- and
RS-SMDPPEH, respectively.
The full width at half maximum (FWHM) of the XRD peaks
observed for neat films (Figures 3a,b) can be related to the size
[
80]
of the contributing crystallites using the Scherrer equation.
Crystallite sizes, calculated using this method, are observed to
increase 2–3 fold for all five neat films after thermal annealing.
Before thermal annealing, Scherrer grain sizes of 8, 9, 13, 11
and 15 nm (with a typical error of 2 nm) were found for syn-,
com-, SS- , RR- and RS-SMDPPEH, respectively; after thermal
annealing, the Scherrer grain sizes were increased to 25, 25,
2
7, 25 and 30 nm for syn-, com-, SS- , RR- and RS-SMDPPEH,
respectively.
When blended in a 1:1 weight ratio with PCBM in solution,
the spin coated films of SMDPPEH are completely amorphous
with the notable exception of the RS-SMDPPEH:PCBM film,
which displays a weak, broad peak centered at 2θ = 6.11° (d =
1
4.4 Å) (Figure 3c). The presence of PCBM along with rapid
[
81,82]
drying of the CHCl solvent inhibits crystallization,
sug-
3
gesting the effectiveness of the PCBM acceptor to frustrate the
SMDPPEH molecular packing. Unlike the other four mate-
rials, crystallization of the RS-SMDPPEH:PCBM film is not
completely suppressed. However, after annealing at 100 °C
for 5 minutes a single XRD peak is present for each material
(
Figure 3d). The peaks for com-SMDPPEH and RS-SMDPPEH
The thin film surface morphologies of the five SMDPPEH
materials were investigated using atomic force micros-
copy (AFM). Neat, unannealed films (Figure S22) all show
a random pattern of approximately 3 nm deep, 80–120 nm
are both centered at 2θ = 6.53° (d = 13.5 Å), while a broader
peak is centered at 2θ = 6.36° (d = 13.9 Å) for syn-SMDPPEH.
The spectral differences here may be related to differences in
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Figure 4. AFM images of spin-coated films of (a,f) syn-SMDPPEH, (b,g) com-SMDPPEH, (c,h) RS-SMDPPEH, (d,i) SS-SMDPPEH and (e,j) RR-
SMDPPEH. All films were annealed at 100 °C for 5 minutes (for unannealed films see Figure S22). (a–e) are neat films while (f–j) are blended 1:1 by
weight with PCBM. The full height scales (see the color bar shown at the far right of this figure) are 80 nm for (a–e), 10 nm for (f,g,j) and 40 nm for
(h,i). The scanning area is 5 ×±5 µm for all images.
wide indentations on an otherwise flat surface. The root-
mean-square (RMS) roughness for all five films is in the
range of 1.0–1.3 nm. Upon annealing, large crystal formation
is observed (Figures 4a–e). The surfaces of the annealed neat
films of syn-SMDPPEH and com-SMDPPEH (Figures 4a,b)
appear nearly identical, covered with randomly oriented crystal-
lites that are approximately 100–200 nm across and 20–40 nm
tall. The annealed RR- and SS-SMDPPEH fi lms (Figures 4d,e)
both show noticeably larger domain features than the syn-
SMDPPEH and com-SMDPPEH fi lms, and contain a majority
phase composed of flat, terrace structures with dimensions up
to approximately 1 µm, as well as a minority phase composed
of protruding crystals with sizes up to 500 nm. Based on the
XRD patterns shown in Figure 3b, we assign the former to the
d = 15.9 Å XRD peak and the latter to the d = 13.7 Å XRD peak.
As indicated by the XRD peak intensities, the 13.7 Å phase is
more prevalent on the RR-SMDPPEH surface (Figure 4e). The
annealed neat film of RS-SMDPPEH (Figure 4c) is significantly
flatter than the others, with an RMS roughness of 3.3 nm. The
syn-SMDPPEH, com-SMDPPEH, SS-SMDPPEH, and RR-
SMDPPEH fi lms show RMS roughnesses of 9.3, 8.1, 6.6, and
isomers; however, upon thermal annealing significant differ-
ences are observed. The observed differences agree well with
XRD data, and the techniques taken together suggest a number
of different phases form depending on the isomer type and pro-
cessing conditions.
Optical absorption spectra of SMDPPEH thin films prepared
on glass substrates (precoated with 25 nm thick PEDOT:PSS
films) are presented in Figure 5. All films show a peak at
420 nm that has been previously attributed to a π–π* transi-
tion for both the thiophene and the central diketopyrrolopyr-
[
66]
role (DPP) units.
In the 500–900 nm wavelength region,
intense absorptions are found that can be fit well with a set
of three Gaussian peaks (see Figures S23 and S24; Table S2).
The relative intensities and positions of the absorption peaks
are related to crystal packing, as peaks arise from both intra-
and intermolecular charge transfer. The apparent absorption
at long wavelengths beyond the band gap for neat films of RS-
SMDPPEH is attributed to light scattering. Given that neat
unannealed films of syn-SMDPPEH, com-SMDPPEH, and RS-
SMDPPEH show similar absorption spectra (Figure 5a) with
peaks from Gaussian fitting centered at 610, 655, and 720 nm,
they presumably share the same phase (with RS-SMDPPEH
crystallizing to a greater degree). The results suggest that neat,
unannealed films of syn-SMDPPEH and com-SMDPPEH—
stereoisomeric mixtures—may be composed of the crystalline
RS-SMDPPEH component with disordered regions containing
the RR- and SS-SMDPPEH isomers in between. The conclu-
sion is supported by the XRD data (Figure 3a). On the other
hand, neat unannealed films of RR- and SS-SMDPPEH show
slightly blue-shifted absorption peaks, with Gaussian fits cen-
tered at approximately 600, 650 and 720 nm, with the shoulder
at 600 nm becoming more prominent. As seen in the XRD
analysis above, these films crystallize into a different phase
than the RS-SMDPPEH containing films.
1
0.0 nm, respectively.
Without any annealing, films blended with PCBM are
quite flat and generally featureless, with RMS roughness of
approximately 0.6 nm for all five materials (Figure S22). After
annealing, however, two distinct morphologies are observed.
For annealed syn-SMDPPEH:PCBM, com-SMDPPEH:PCBM,
and RS-SMDPPEH:PCBM films (Figures 4f–h), aggregates
protrude from the surface with heights of 5–20 nm and diam-
eters of 30–150 nm (although there are fewer in the case of RS-
SMDPPEH:PCBM). Randomly oriented crystallites are visible
in the background. For annealed RR- and SS-SMDPPEH:PCBM
films (Figures 4i,j), long, narrow crystallites decorate an
undulating surface with peak-to-valley height of 30–40 nm.
The annealed blended films were generally flatter than the
annealed neat films, although less so for the RR- and SS- iso-
mers, with RMS roughnesses of 1.3, 1.5, 3.3, 7.4 and 1.0 nm
for annealed syn-SMDPPEH:PCBM, com-SMDPPEH:PCBM,
SS-SMDPPEH:PCBM, RR-SMDPPEH:PCBM, and RS-
SMDPPEH:PCBM films, respectively. In summary, very similar
surface morphology is observed for unannealed films of all five
After annealing, the spectral shape of the absorption from
neat syn- and com-SMDPPEH fi lms is relatively unchanged
(Figure 5b), while a ∼ 10 nm blue shift is observed for the
longest wavelength peak from 720 nm to 710 nm. The RS-
SMDPPEH absorption spectrum (Figure 5b), with peaks cen-
tered at 615, 660, and 730 nm (after annealing), no longer
matches the absorption spectra of the isomeric mixtures,
6
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Figure 5. Absorption spectra for spin-coated films of SMDPPEH on glass/PEDOT:PSS. (a,c) were not annealed while (b,d) were annealed at 100 °C
for 5 minutes. (a,b) are neat films while (c,d) are 1:1 by weight blends with PCBM.
suggesting that the meso compound adopts a different phase,
in agreement with XRD measurements. Finally, after annealing,
the SS-SMDPPEH and RR-SMDPPEH fi lms also show nearly
identical absorption spectra. Taking the neat film XRD and
absorption data together, before annealing, the RS-SMDPPEH
component of the isomer blends crystallizes the fastest, and
thus dominates the absorption and XRD spectra of unannealed
syn- and com-SMDPPEH fi lms. However, after annealing, the
isomer blends take on a crystal structure that is different from
both the RR- and SS- SMDPPEH fi lms on the one hand, and
the RS-SMDPPEH fi lm on the other. It can be concluded that
after annealing the isomer blends crystallize into a structure
that incorporates all three of the isomers.
conclusion is not supported by the AFM and XRD measure-
ments in this work, which show that the surface morphology
and degree of crystallization of annealed RS-SMDPPEH:PCBM,
syn-SMDPPEH:PCBM, and com-SMDPPEH:PCBM films are
quite similar.
2.5. Photovoltaic Device Performance
In order to probe the effect of side chain stereochemistry on
the optoelectronic properties of the SMDPPEH materials, pho-
tovoltaic devices were fabricated with the structure: indium tin
oxide (ITO)/PEDOT:PSS/SMDPPEH:PCBM/Al. Active layer
formation was carried out in an identical fashion to the films
analyzed above.
Unannealed devices showed very similar performance with
the exception of the RS-SMDPPEH:PCBM device (Table 3,
Figure 6a–c). For the other four materials, relatively low fill fac-
tors (FF) of 34–36% presumably result from fast recombination
of charge carriers in the amorphous blends due to a lack of
efficient charge extraction pathways. External quantum efficien-
cies (EQEs) in all four cases are also very similar (Figure 6c); as
the light absorption efficiency was found to be the same, this
suggests that the internal quantum efficiencies are nearly iden-
tical. The stereochemical purity of the 2-ethylhexyl side chains
is not expected to significantly affect the electrical properties
of the films when they are not allowed to crystallize. Thus, it
is inferred that the more readily crystallizing RS-SMDPPEH
material begins to phase segregate from the PCBM sans
The absorption spectra for unannealed SMDPPEH:PCBM
blended films are nearly identical (Figure 5c) as a result of the
low crystallinity of these films (for XRD, see Figure 3c). After
annealing, however, all of the films are found to crystallize,
resulting in differentiation of the absorption spectra (Figure 5d).
The most pronounced change is observed for the annealed RR-
and SS-SMDPPEH:PCBM films, for which the 600 nm peak
becomes dominant, while the peak at 700 nm is strongly sup-
pressed (see peak fitting in Figure S24). In agreement with the
AFM and XRD results, the annealed RS-SMDPPEH:PCBM
film absorption spectrum resembles that of the syn- and com-
SMDPPEH:PCBM films, albeit with a reduction in intensity
of the 700 nm peak. This peak has been previously assigned
to intermolecular charge transfer in syn-SMDPPEH due to
aggregate species,[ and thus a reduction in its intensity may
be interpreted as a reduction in crystallinity. However, such a
66]
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7

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Table 3. Characteristics for SMDPPEH photovoltaic devices.
Material
Annealed
Voc
Jsc
FF
[%]
PCE
[%]
2
[
V]
[mA/cm ]
5.4 ± 0.2
5.3 ± 0.2
5.5 ± 0.2
5.2 ± 0.4
6.5 ± 0.2
7.3 ± 0.2
7.5 ± 0.3
7.8 ± 0.3
7.6 ± 0.3
5.1 ± 0.2
syn-SMDPPEH
com-SMDPPEH
SS-SMDPPEH
RR-SMDPPEH
RS-SMDPPEH
syn-SMDPPEH
com-SMDPPEH
SS-SMDPPEH
RR-SMDPPEH
RS-SMDPPEH
no
no
0.742 ± 0.005
0.742 ± 0.006
0.756 ± 0.005
0.752 ± 0.007
0.740 ± 0.005
0.800 ± 0.005
0.766 ± 0.005
0.705 ± 0.005
0.703 ± 0.005
0.688 ± 0.005
35 ± 1
34 ± 1
36 ± 1
35 ± 2
45 ± 2
52 ± 1
50 ± 1
52 ± 1
50 ± 1
55 ± 1
1.4 ± 0.1
1.3 ± 0.1
1.5 ± 0.1
1.4 ± 0.2
2.2 ± 0.2
3.0 ± 0.1
2.9 ± 0.1
2.9 ± 0.1
2.7 ± 0.1
1.9 ± 0.1
no
no
no
yes
yes
yes
yes
yes
annealing, leading to enhanced charge extraction in the corre-
sponding device and a significantly higher FF of 45%.
results in an improved photocurrent generation efficiency. For
these annealed devices, the EQE spectral shape matches the
film absorption spectra above, with RR- and SS-SMDPPEH
showing stronger absorption from the 600 nm peak and
syn- and com-SMDPPEH showing stronger absorption from
the 700 nm peak. On balance, the spectral shift results in a
For annealed devices (Figures 6d–f and Table 3) the per-
formance is again quite similar for all of the devices except
those featuring RS-SMDPPEH. The most dramatic change
after annealing for the other four materials is the increase in
FF to 50–52%, a 45% relative increase compared to the unan-
nealed cases. Additionally, peak EQE increases from 30%
for the unannealed devices up to 43–45% for the annealed
devices, resulting in a relative increase in JSC by 35–45%.
Clearly, the crystallization that occurs during the annealing
step for the syn-SMDPPEH:PCBM, com-SMDPPEH:PCBM,
SS-SMDPPEH:PCBM, and RR-SMDPPEH:PCBM devices
2
similar short-circuit current (JSC) of 7.3–7.8 mA/cm for these
four annealed devices. Differences in device performance
for RR- and SS-SMDPPEH may be related to differences in
the degree of donor crystallization, as observed in the XRD
data. A significant reduction in open-circuit voltage (VOC) is
observed for the RR- and SS-SMDPPEH devices (0.703 V and
0.705 V, respectively) relative to the syn- and com-SMDPPEH
Figure 6. Characterization of SMDPPEH bulk heterojunction photovoltaic devices. (a,d) show current density under 1 sun AM1.5G illumination, (b,e)
show dark current density, and (c,f) are EQE spectra. (a–c) were not annealed, while (d–f) were annealed at 100 °C for 5 minutes.
8
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devices (0.800 V and 0.766 V, respectively). If these differences
3
. Conclusions
in VOC resulted from a change in band gap (E ), we would
g
expect to see a red shift of 30–50 nm in the absorption onset
of the RR- and SS-SMDPPEH devices relative to the syn- and
com-SMDPPEH devices. A red shift is indeed observed in the
onset of EQE, but only by approximately 5–10 nm. However,
the dark current in Figure 6e can fully account for the differ-
ence in VOC between the devices, specifically due to the dif-
ference in diode reverse-bias saturation current density, which
The goal of the current work has been to evaluate the conse-
quences of 2-ethylhexyl solubilizing group chirality, and the
isomeric complexity it creates, on small π-conjugated molecule
morphology and bulk heterojunction photovoltaic device per-
formance. To this end, SMDPPEH, a commercially-available
organic semiconductor, was prepared in isomerically defined
form from enantiomerically pure reagents. The crystallization
behavior and optoelectronic properties—neat and as blends
with PCBM—of the three pure isomers (RR- , SS- , and RS-
SMDPPEH) were systematically compared to typical ∼ 1:1:2
isomer mixtures purchased commercially (com-SMDPPEH) or
prepared in the laboratory (syn-SMDPPEH).
was found to be 1.8±×± 10− mA/cm for the RR-, SS-, and RS-
7
2
SMDPPEH devices, and 4.4±×± 10− and 1.9±×± 10 mA/cm2
for the syn- and com-SMDPPEH devices, respectively (see fit-
ting results in Figure S25). The discrepancy may arise from
a difference in the relative shift of the optical and transport
gaps.[
8
−8
83]
Expectedly and gratifyingly, the enantiomers (RR- and SS-
SMDPPEH) showed overall similar thin film morphology
and absorption (neat and as blends; annealed or unannealed),
and consequently bulk heterojunction device performance
throughout the studies. Although different in these respects
from the enantiomers, and despite small differences in
isomer composition, syn- and com-SMDPPEH showed sim-
ilar morphologies and optoelectronic characteristics. All four
SMDPPEH compositions were amorphous as 1:1 blends with
PCBM without post-deposition thermal annealing, a situation
where the crystallinity, and therefore stereochemical informa-
tion, was lost. RS-SMDPPEH was found to crystallize most
readily of the pure isomers, in the presence and absence of
PCBM, and consequently dominated the absorption and XRD
profiles of the neat isomeric mixtures. Indeed, RS-SMDPPEH
maintained some crystallinity in the presence of PCBM prior
to thermal annealing resulting in a 50–60% improvement
in photovoltaic device performance relative to the other four
compositions.
After thermal annealing, RR/SS-SMDPPEH, syn/com-
SMDPPEH, and RS-SMDPPEH revealed different crystal
structures and morphologies suggesting that the isomer mix-
tures (syn- and com-SMDPPEH) adopted a phase that incorpo-
rated all three components. Blending with PCBM had a strong
effect on the crystal packing (by XRD), where again the more
strongly crystallizing RS-SMDPPEH dominated the profiles of
the isomer mixtures. For syn-, com-, SS- , and RR-SMDPPEH,
a substantial increase in photovoltaic device performance
was observed after thermal annealing, while RS-SMDPPEH
showed a decrease in device performance. Overall, the stereo-
center affected the morphology of the active layer and the thin
film absorption spectra, but had a relatively weak effect on the
overall photovoltaic performance.
Unlike the other four devices, annealing of the RS-
SMDPPEH:PCBM device results in a reduction in EQE, and
therefore a reduction in JSC. Differences in overall absorp-
tion cannot explain the reduction in EQE observed here, as
peak EQE for the RS-SMDPPEH:PCBM device is only 26%
(Figure 6f). Instead, some internal element of photocur-
rent generation efficiency must be negatively affected by the
annealed RS-SMDPPEH:PCBM film’s morphology.
Hole-only
devices
with
the
structure
ITO/
PEDOT:PSS/SMDPPEH:PCBM/MoO /Au were fabricated in
x
order to investigate charge transport in these materials (Figures
S26 and S27). A transition from Ohmic to space-charge-limited
current (SCLC) was observed at approximately 0.5 V for all of
the SMDPPEH compositions. The SCLC region of each cur-
rent-voltage curve was fit according to Child’s law to determine
[
84]
−5
−6
−5
the hole mobility. Values of 1.1±×± 10 , 6.9±×± 10 , 2.0±×± 10 ,
−6
−5
2
9
.6±×± 10 , and 1.7±×± 10 cm /V·s were found for unannealed
films of syn-SMDPPEH:PCBM, com-SMDPPEH:PCBM,
SS-SMDPPEH:PCBM, RR-SMDPPEH:PCBM, and RS-
SMDPPEH:PCBM, respectively. Upon thermal annealing,
the hole mobility of all five of the films increased by approxi-
mately half to one full order of magnitude, with values of
−
5
−5
−5
−5
−5
4
.8±×± 10 , 6.8±×± 10 , 9.1±×± 10 , 8.3±×± 10 , and 6.9±×± 10
2
cm /V·s for syn-SMDPPEH:PCBM, com-SMDPPEH:PCBM,
SS-SMDPPEH:PCBM, RR-SMDPPEH:PCBM, and RS-
SMDPPEH:PCBM, respectively.
Overall, we find that the photovoltaic device performance is
only weakly affected by the differences in morphology between
the pure stereoisomers SS-/RR-SMDPPEH and the stereoi-
someric mixtures syn-/com-SMDPPEH. Shifted absorption
spectra balance out to give approximately the same coverage of
the solar spectrum. Moreover, although very different surface
morphologies are observed with AFM, coincidently the ability
of each of these materials to transport charges is only weakly
affected. However, the RS-SMDPPEH devices perform much
differently from those made with the other four materials.
This isomer crystallizes more rapidly, resulting in differences
in optimum film processing. Most notably, while the PCE of
the other devices improved upon mild thermal annealing at
The extent to which alkyl side chain stereochemistry should
be considered an important tunable parameter in materials
design is most certainly device/application dependent. To wit,
Nguyen and coworkers demonstrated that while the isolated
stereoisomers of DPP(TBFu)2 showed similar morphologies,
RS-DPP(TBFu) displayed much higher FET mobility (before
2
and after annealing) as a consequence of its crystal structure
with tighter π–π stacking, as well as a more homogenous
domain shape/size and smaller film roughness (versus RR- and
1
00 °C, the performance of the RS-SMDPPEH:PCBM device
deteriorated with the same treatment. It is expected that a more
thorough optimization of processing conditions for this mate-
rial could bring the overall efficiency closer to that of the other
four materials.
SS-DPP(TBFu) ). The study conveys the importance of isomeric
2
composition/purity on a small molecule’s carrier mobility,
and the observation appears generally extendable to other
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π-conjugated systems.[40] Our work shows that the side chain
chirality, while strongly impacting the thin film morphology
and optical properties of SMDPPEH, has modest effects on the
photovoltaic performance of the material as blends with PCBM.
While this does not mean that side chain stereochemistry does
not influence blend structure, it does suggest that the struc-
tural consequences lead to fortuitously compensatory optoelec-
tronic effects in this context. Studies among additional classes
of semiconductors will expose whether side chain chirality can
be employed to rationally improve OPV efficiency or whether it
can be safely “ignored”.
law. Molar extinction coefficients (ε) were determined from the linear
plot for each compound (where A =±εbc).
Thin Film Characterization: UV-vis absorption (thin film)
measurements were carried out with a calibrated Newport 818-UV
Si photodiode illuminated by a Newport Oriel Apex illuminator and
monochromator system chopped at 400 Hz. The signal was detected
using
a Stanford Research Systems SR830-DSP lock-in amplifier.
Atomic force microscopy was carried out using a Veeco Innova AFM in
tapping mode with a silicon tip (radius ∼ 8 nm) at 325 kHz with a force
constant of approximately 40 N/m. X-ray diffraction measurements were
performed using a PANalytical X’Pert Powder diffractometer operated in
the θ-2θ mode using Cu Kα radiation (λ = 1.54 Å).
Device Fabrication and Characterization: Organic photovoltaic devices
were fabricated on commercial indium-tin oxide (ITO) coated glass
substrates with a sheet resistance of ∼15 Ω/square, while films for XRD
and AFM analysis were fabricated on Si(100) substrates. The substrates
were sequentially sonicated for 15 minutes in detergent, water,
acetone and isopropanol before UV-ozone treatment for an additional
4
. Experimental Section
General Synthetic Methods and Molecular Characterization:
1
5 minutes. PEDOT:PSS (Clevios AI4083) was spin-coated in air at
Reagents and solvents were purchased from commercial sources and
used without further purification unless otherwise specified. THF,
Et O, CH Cl , and DMF were degassed in 20 L drums and passed
8
000 rpm to form a ∼25 nm thick layer, which was annealed at 150 °C
for 30 minutes in air before passing into a nitrogen glovebox (H O ∼
1
2
2
2
2
ppm). Photovoltaic active layers were spin-coated from a 5:5 mg/mL
through two sequential purification columns (activated alumina;
molecular sieves for DMF) under a positive argon atmosphere. 2-(5′-
Hexyl-[2,2′-bithiophen]-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and
solution of SMDPPEH:PC BM in CHCl at 500 rpm to form a ∼110 nm
61
3
thick layer. Films were then passed into a vacuum chamber pumped
−6
down to 10 Torr, and a 100 nm Al cathode was evaporated through a
shadow mask with thicknesses monitored by a quartz crystal monitor.
[
1,1′-bis(diphenylphosphino)ferrocene]dichloro palladium(II) (complex
with dichloromethane, [Pd(dppf)Cl ·CH Cl ]) were purchased from
2
2
2
2
The cross-bar geometry was used to define an active area of 4 mm for
Sigma-Aldrich and used as received. Thin layer chromatography (TLC)
was performed on SiO 60 F aluminum plates with visualization by
the organic photovoltaic cells. After aluminum deposition, some devices
were annealed at 100 °C for 5 minutes in a nitrogen glovebox, and then
encapsulated with a UV-curable epoxy layer to prevent degradation from
exposure to ambient oxygen and water before characterization in air.
Devices were characterized under illumination from a 150W Xe-arc
lamp solar simulator with a KG1 filter, calibrated to 1 sun intensity using
the AM1.5G spectrum. External quantum efficiency measurements were
carried out with a calibrated Newport 818-UV Si photodiode illuminated
by a Newport Oriel Apex illuminator and monochromator system
chopped at 400 Hz. The photocurrent signal was detected using a
Stanford Research Systems SR830-DSP lock-in amplifier.
2
–
254
UV light or staining. Flash column chromatography was performed
using Silica gel technical grade, pore size 60 Å, 230−400 mesh
particle size, 40−63 µm particle size from Sigma-Aldrich. Specific
Optical rotations were obtained on a JASCO P-2000 Series Polarimeter
(
5
wavelength = 589 nm) and then corresponded to the literature.
1
13
00 (125) MHz H ( C) NMR were recorded on an INOVA 500
spectrometer. Chemical shifts (δ) are given in parts per million (ppm)
relative to TMS and referenced to residual protonated solvent purchased
from Cambridge Isotope Laboratories, Inc. (CDCl : δ 7.26 ppm, δC
3
H
7
7.16 ppm; DMSO-d : δ 2.50 ppm, δ 39.52 ppm). Abbreviations used
6 H C
are s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), hp
heptet), b (broad), and m (multiplet). ESI-TOF-, APCI-TOF-, and DART-
(
TOF-MS spectra were recorded on an Agilent 6210 TOF spectrometer
with MassHunter software. EI-MS (70 eV) spectra were recorded on
a Thermo Scientific DSQ MS after sample introduction via GC with
data processing on Xcalibur software (accurate masses are calculated
with the CernoBioscience MassWorks software). MALDI-TOF-MS was
performed on a AB Sciex TOF/TOF 5800 in reflectron mode while the
data is processed with Data Explorer. Samples were prepared by mixing
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
the molecule of interest in dithranol (DTL) and then applied onto the Acknowledgements
MALDI plate. Isomeric purity was determined by chiral HPLC analysis
R. K. C. acknowledges financial support from the National Science
Foundation (CHE-1057411). J.X. and R.K.C. also acknowledge
partial financial support from the Research Corporation for Science
Advancement (Scialog Award 20316) and the University of Florida Office
of Research.
(
Shimadzu) using a Chiralpack IA column (8% i-PrOH in hexanes,
0
.8 mL/min, 350 nm).
Thermal Analysis: Thermal gravimetric analysis (TGA) was performed
using a TA Instruments TGA Q5000–0121 V3.8 Build 256 at a heating
rate of 10 °C/min using 1–3 mg of sample in a 100 µL platinum pan
(under nitrogen). The data was analyzed on Universal Analysis 2000
Received: March 31, 2014
Revised: May 19, 2014
Published online:
4
.4A software. Differential Scanning Calorimetry (DSC) was performed
using a TA Instruments DSC Q1000–0620 V9.9 at a heating/cooling rate
of 10 °C/min using 1–3 mg of sample in a sealed aluminum pan, with
respect to an empty aluminum reference pan. Three cycles of heating
and subsequent cooling were performed. The data was analyzed on
Universal Analysis 2000 4.4A software.
Solution Absorption Measurements: Absorption spectra were measured
for at least six different concentrations (2.5–30 µM) of SMDPPEH using
a Cary 100 Bio spectrophotometer and 1 cm quartz cuvettes. All solvents
were spectrophotometric grade and purchased from Sigma-Aldrich. The
absorption intensity at λmax was then plotted against the concentration
in all cases to confirm, by linearity, that the compounds followed Beer’s
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[
2
[
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