Structure and Phase Transition of 4,7-Bis-(4′-cyano-biphenyl-4-yl)-[1, 10]phenanthroline

Article abstract of DOI:10.1007/s10870-015-0614-y

4,7-Bis-(4′-cyano-biphenyl-4-yl)-[1, 10]phenanthroline (cnbiphphen) is synthesized from a palladium mediated coupling between 4-cyanobenzene boronic acid and 4,7-bis(4-bromophenyl)-[1,10]phenanthroline. The single crystal X-ray structure was measured at b

Full text of DOI:10.1007/s10870-015-0614-y

J Chem Crystallogr (2015) 45:453–460
DOI 10.1007/s10870-015-0614-y
ORIGINAL PAPER
Structure and Phase Transition of 4,7-Bis-(40-cyano-biphenyl-4-
yl)-[1, 10]phenanthroline
Brenden Carlson1 Werner Kaminsky2 Linyue Tong3 Gregory D. Phelan4
•
•
•
Received: 9 September 2015 / Accepted: 20 October 2015 / Published online: 30 October 2015
Ó Springer Science+Business Media New York 2015
Abstract 4,7-Bis-(40-cyano-biphenyl-4-yl)-[1, 10]phenan-
throline (cnbiphphen) is synthesized from a palladium
mediated coupling between 4-cyanobenzene boronic acid and
4,7-bis(4-bromophenyl)-[1,10]phenanthroline. The single
crystal X-ray structure was measured at both 298(2) K and
90(2)K.Theasymmetricunitofthetitlecompoundinthelow-
temperature phase, C38H22N.4 HCCl3, is comprised of one
cnbiphphen molecule and one chloroform solvent molecule.
Additional symmetry relates one-half of the title compound to
the other at room temperature. During cooling the structure
experiences a reversible phase transition from A 2/a (different
setting of C 2/c, with a and c exchanged to match lattice
parametersofbothphases)toP21/c. Thecrystalpackingofthe
title compound consists primarily of electronic charge inter-
actions between the cyano moieties. The chloroform solvent
mol-ecule is situated in a cavity between two phenanthroline
systems and is associated to two nitrogen atoms of cnbiph-
phen molecules through Cl3C–H_Nphen van der Waals
interaction.
Graphical Abstract The synthesis, structure, analytical
characterization, optical properties and phase transition of
4,7-bis-(4’-cyano-biphenyl-4-yl)-[1,10]phenanthroline is
presented.
& Brenden Carlson
wcarlson@seattlepolymer.com
1
Seattle Polymer Company, 454 North 34th Street, STE 100,
Seattle, WA 98103, USA
2
X-ray Laboratory, Chemistry Department, University of
Washington, Box 351700, Seattle, WA 98195, USA
3
Department of Chemistry, Binghamton University, State
University of New York, Binghamton, NY 13902, USA
4
Chemistry Department, State University of New York
College at Cortland, Cortland, NY 13045, USA
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J Chem Crystallogr (2015) 45:453–460
Keywords Cyano Á Nitrile Á Biphenyl Á Phenanthroline Á
Scheme 2) of this report has extended cyano functional
biphenyl moieties in the 4,7 positions on the phenanthro-
line ring system.
Phase transition
The p-system of the phenanthroline structure is exten-
ded by electron withdrawing cyano groups which can
promote other chemical reactions in cnbiphphen. We
studied previously the effect of fluorination on the packing
of phenanthroline compounds and found that inter-molec-
ular F_F interactions played a significant role [17]. We
find similar inter-molecular interactions for cnbiphphen,
but the molecule packing differs from that of the fluori-
nated phenanthrolines causing larger C:N interactions.
Introduction
[1,10]Phenanthroline (Scheme 1) is a polyaromatic com-
pound consisting of three fused rings with nitrogen atoms in
the 1 and 10 positions on the ring system, and as such, these
compounds are classified as pyridyls. The structure and
orientation of the nitrogen atoms of [1,10]phenanthroline
makes it particularly useful in binding to Lewis acid centers
such as those of metal cations [1]. Phenanthroline derivatives
have found applications in enzyme inhibition [2–4], creation
of luminescent compounds with a wide variety of metal
centers [5–7], for the photometric determination of the fer-
rous ion and determination of organometallic reagents [8–
10], and in various types of organic electronics [11, 12].
[1,10]Phenanthroline with numbered positions is shown
in Scheme 1. Substitutions at all positions of the phenan-
throline structure have been reported and some are known
industrial chemicals [13–15]. Bathophenanthroline is a
common derivative with phenyl groups substituted in the 4
and 7 positions. Bathocouprine is a derivative with phenyl
groups in the 4 and 7 positions and methyl groups in the 2
and 9 positions and neocouprine is a derivative with methyl
substitution in the 2 and 9 positions. Substituting the
phenanthroline unit modifies the chemical and electronic
properties of the molecule and of larger structures that
incorporate the molecule. Applying cross coupling tech-
niques such as Suzuki coupling allows for the p-systems of
phenanthroline to be extended and a variety of functional
groups to be substituted on phenanthroline systems [16].
The novel [1,10]phenanthroline derivative 4,7-bis-(40-
cyano-biphenyl-4-yl)-[1,10]phenanthroline (cnbiphphen,
Scheme 1 Chemical structure
6 5
and numbering positions of
[1,10]phenanthroline
7
4
8
3
9 N
N 2
10
1
N
N
N
N
Scheme 2 The chemical structure of 4,7-Bis-(40-cyano-biphenyl-4-
yl)-[1,10]phenanthroline (cnbiphphen). HCCl3 in this report
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J Chem Crystallogr (2015) 45:453–460
455
Two structures of cnbiphphen at 298 K and 90 K are here
presented, derived by single crystal X-ray diffraction,
which are related by a reversible structural order–disorder
phase transition.
temperature and added to 75 mL of a 70 % ethanol—water
mixture. The solids were filtered and washed with 70 %
ethanol—water (3 9 50 mL) and dried. The solids were
dissolved into 99.5 HCCl3:0.5 EtOH and purified by column
chromatography using silica. The title compound was the
first band to elute. Large, clear, colorless crystals were grown
from the chloroform solution to yield 4,7-bis-(40-cyano-
biphenyl-4-yl)-[1,10]phenanthroline (1.045 g, 1.955 mmol)
in 75 % yield. 1H NMR (300 MHz, DCCl3): d 9.3196 (d, 2H,
J = 5 Hz), 7.9455 (s, 2H), 7.8040(d, 9H, J = 3 Hz), 7.7844
(t, 3H, J = 2 Hz), 7.7053 (t, 3H, J = 2 Hz), 7.6836 (m, 2H,
J = 2H), 7.6727 (s, 1H). Mass spectrometry (ESI) calculated
for C38H22N4: 554.18; found 535.19 for M ? H, 557.17 for
M ? Na. A fragment appears at 434.16 for cleavage of a
cyanophenyl moiety. Anal. calcd for C38H22N4: C, 85.37; H,
4.15; N, 10.48; found C, 84.89; H, 4.21; N, 10.38.
Experimental
4-Cyanophenyl boronic acid was purchased from Aldrich
and used without further purification. 4,7-bis(4-bro-
mophenyl)-[1,10]phenanthroline was synthesized accord-
ing to literature preparation [18]. UV–Visible and emission
data were measured on a SpectraMax M5 spectropho-
tometer by Molecular Devices. Differential scanning
calorimetry experiments were run on a DSC Q200 V23.10
Build 79 instrument at a temperature change rate of
10.0 K min-1 and under a nitrogen atmosphere.
X-ray Crystallograph
Preparation of 4,7-bis-(40-cyano-biphenyl-4-yl)-
[1,10]phenanthroline (Scheme 3)
Room-temperature (RT) single crystal X-ray data was
collected at 298(2) K on a Nonius Kappa CCD FR590 and
the low-temperature (LT) collected on a Bruker APEX II
diffractometer at 90(2) K, both with Mo-radiation. Color-
less plates, measuring 0.50 9 0.24 9 0.10 mm3 (0.9 9
0.27 9 0.05 mm3; LT) were mounted on a loop with oil.
Crystallographic parameters for both the RT and LT
structures of cnbiphphen are summarized in Table 1.
Crystal-to-detector distance was 30 mm, RT (40 mm
LT), exposure time 60 s per degree for all sets, and the scan
width was 2° (0.5°; LT). Data collection was 96.9 % (99.7;
LT) complete to 25o in 0. Rint = 0.0543 (0.0429; LT)
indicated that the data were better than average quality
(0.07) for both the RT and LT structures. Indexing and unit
cell refinement indicated an A-centered monoclinic lattice
at room temperature, matching the metric at LT. The space
groups were found to be A 2/a at RT and P 21/c at LT.
The room temperature data was integrated, scaled, and
corrected for absorption effects utilizing multi-scan meth-
ods using hkl-SCALEPACK [19]. This program applies a
multiplicative correction factor (S) to the observed inten-
sities (I) and has the following form:
4,7-Bis(4-bromophenyl)-[1,10]phenanthroline
(1.278 g,
2.607 mmol) was added to a 25 mL three neck round bottom
flask along with 4-cyanophenylboronic acid (0.804 g,
5.474 mmol) and K2CO3 (1.250 g, 9.044 mmol). DMF,
10 mL, and saturated aqueous K2CO3, 5 mL were added to
the round bottom flask. The system was vigorously purged
with argon for an hour at which tetrakis(triphenylphosphine)
palladium(0) (0.030 g, 0.026 mmol) was added. The system
was heated to 100 °C in the dark and under an argon atmo-
sphere for 18 h. The system was then cooled to room
Br
NH2
O
N+
Cl
O-
SnCl2
Br
H3AsO4
H3PO4
EtOH, reflux
O
N
NO2
100-150 o
C
Br
Br
Cl
Br
Br
O
H3AsO4
H3PO4
2
2
S = ðeÀ2Bðsin hÞ=k Þ=scale
N
N
100-150 oC
NC
N
NH2
CN
S is calculated from the scale and the B factor deter-
mined for each frame and is then applied to I to give the
corrected intensity (Icorr).
Br
Br
The LT data was processed (integration, scaling, and
absorption correction employing multi-scan methods) with
SAINT and SADABS within the Bruker APEXII software
package [20]. Solution by direct methods (SHELXS, SIR97
[21, 22]) produced complete heavy atom phasing models
consistent with the proposed structures. The structures
were completed by difference Fourier synthesis with
NC
B(OH)2
Pd(TPP)4; DMF
N
N
N
N
K2CO3; H2O, 100 o
C
cnbiphphen
Scheme 3 Schematic of the synthesis method for producing
cnbiphphen
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J Chem Crystallogr (2015) 45:453–460
Table 1 Crystallographic data
for the structures of
cnbiphphen provided
Temperature
298 (2) K
90 (2) K
Data collection (Mo-ka)
Crystal system
Nonius Kappa CCD
Monoclinic
A 2/a
Bruker APEX II
Monoclinic
P 21/c
Space group
Unit cell dimensions
˚
a (A)
12.9603(9)
10.9899(11)
23.486(3)
97.508(7)
12.8978(8)
10.9590(6)
22.7671(14)
98.347(3)
˚
b (A)
˚
c (A)
c (°)
Volume
3
˚
3
˚
3316.4 (6) A
3184.0 (3) A
Z
4
4
Density (calculated)
Absorption coefficient
F(000)
1.310 Mg/m3
0.311 mm-1
1344
1.364 Mg/m3
0.324 mm-1
1344
Crystal size
0.50 9 0.24 9 0.10 mm3
0.90 9 0.27 9 0.05 mm3
Theta range for data collection
Index ranges
2.66–26.37°.
-16 Bl B 16, -12 B k B 13,
-29 B h B 29
2.45–28.40°.
-17 B h B 17, -14 B k B 14,
-30 B l B 30
Reflections collected
32188
169327
Independent reflections
Completeness to theta = 25.00°
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
3358 [R(int) = 0.0577]
98.9 %
7969 [R(int) = 0.0429]
99.7 %
0.9696 and 0.8602
2949/6/222
0.9840 and 0.7595
7969/0/443
1.000
1.024
Final R indices [I [ 2sigma(I)]
R indices (all data)
R1 = 0.0592, wR2 = 0.1386
R1 = 0.1419, wR2 = 0.1736
-3
R1 = 0.0433, wR2 = 0.1068
R1 = 0.0608, wR2 = 0.1188
-3
˚
˚
Largest diff. peak and hole
0.188 and -0.149 e.A
0.433 and -0.598 e.A
Fig. 1 ORTEP plot of the title compound measured at 298(2) K
(a) and 90(2) K (b). Displacement ellipsoids are drawn at the 50 %
level. The crystallographic two-fold axis, as indicated, relates one half
of the molecule to the other at 298(2) K. The N–H van der Waals
interaction distance between the chloroform and the phenanthroline
˚
are 2.299(2) and 2.443(2) A for the structure measured at 90(2) K;
˚
2.37–2.53 A at 298(2) K
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J Chem Crystallogr (2015) 45:453–460
457
DMF. 1H NMR, mass spectrometry, elemental analysis and
single crystal crystallography are all consistent in con-
firming the molecule. The title compound was purified on a
silica column using chloroform:ethanol (99.5:0.5) as the
mobile phase. The 1H NMR of the title compound in
DCCl3 is shown in the supplementary materials. The pro-
tons in the 2,9 positions are observable at d 9.32, the 5,6
protons at d 7.95, and the 3,8 protons at d 7.81 with other
signals being due to the biphenyl structure. The absorbance
initiates at 377 nm, reaches a maximum at 287 nm, and
features a shoulder at 315 nm (Fig. 2). The experimental
extinction coefficient at 287 nm is 65,000 L mol-1
cm-1 10 %. The compound luminesces when excited at
290 nm in chloroform solution (Fig. 2) at a concentration
of 10-8 M. Cnbiphphen features a structured Gaussian
emission profile that was identical for the two concentra-
tions. The emission maximum is at 393 nm while a
shoulder appears at 413 nm.
Fig. 2 Absorbance (blue line) and emission (red line) of cnbiphphen
in chloroform solutions. Absorbance maximum occurs of 287 nm
with an extinction coefficient of 65,000 L mol-1 cm-1. Emission
maximum measured at 10-8 mol L-1 occurs at 393 nm with a
shoulder appearing at 413 nm (Color figure online)
Structures of cnbiphphen at RT and at LT are shown in
a side by side comparison in Fig. 1. Cnbiphphen under-
goes a structural order–disorder phase transition upon
cooling from space group A 2/a to P21/n. Single bonds
between the phenanthroline, the biphenyl and within the
˚
biphenyl at a distance of 1.482–1.495 A allow for rotation
SHELXL97 [23, 24] Scattering factors are from Waasmair
and Kirfel [25]. Hydrogen atoms were placed in geomet-
rically idealized positions and constrained to ride on their
of the phenyl moieties. The cyanophenyl group is rota-
tionally disordered with site occupancies between the dis-
ordered moieties of 0.65 and 0.35 at RT. A two-fold
rotation of the space group approximately through the C-H
group of the disordered chloroform relates one half of the
Cnbiphphen to the other as indicated in Fig. 1.
˚
parent atoms with C–H distances in the range 0.95–1.00 A.
Isotropic thermal parameters Ueq were fixed such that they
were 1.2Ueq of their parent atom Ueq for CH’s and 1.5Ueq
of their parent atom Ueq in case of methyl groups. All non-
hydrogen atoms were refined anisotropically by full-matrix
least-squares.
We note that phenyl C7–C12 (mean deviation of phenyl
atoms from the planar average, Rms = 0.0116) is rotated
towards ring N1, C1–C5 (Rms = 0.0087) of the phenan-
throline by 70.14(9)o at room temperature. One of the
disordered phenyls, C13–C18, is rotated to phenyl C7–C12
by 12.44(45)o (Rms = 0.0044) and the second disordered
phenyl by 15.5(1.2)o (Rms = 0.0096), with 27.9 o between
them. The phenanthroline system itself is nearly planar,
torsion N1–C12–C11–N2 = 4.36°, torsion C4–C5–C6–
C7 = 0.04°, the Rms calculated for all atoms of the
phenanthroline is 0.0384. At LT we have two different
angular sequences of phenanthroline-ring to phenyl to
phenyl. The ring-phenyl and phenyl–phenyl angles are
53.74(4)°, 24.61(6)o and 76.43(4)°, 28.62(5)°, respectively.
The Rms readings for the 6 rings are all below 0.02. While
the Rms of 0.0571 for the phenanthroline indicates no
dramatic change of geometry between RT and LT, the
phenyl rotations have changed significantly as a result of
the phase transition. Specifically, below the phase transi-
tion, rotations of the 40-cyanobiphenyl groups with respect
to the phenanthroline ring structure are quite different.
The unit cells contain interacting molecules of cnbiph-
phen as shown in Fig. 3 with inter-molecular distance
Phenyl C13–C18, with CN group C19–N2 is disordered
at RT via a rotation about axis C10–C16. The chloroform is
heavily disordered with four orientations, two of which are
generated via the two-fold rotation of the space group
which relates one half of the molecule to the other as
indicated in Fig. 1. The chloroform molecule is disordered
to a lesser amount, the phenyl disorder of cnbiphphen is
resolved, and the rotational symmetry has vanished at LT.
Restraints where required in the refinement of the RT
structure due to the disorder, but none in absence of phenyl
disorder at LT.
Table 1 summarizes the data collection details. Figure 1
shows an ORTEP [26] of the asymmetric unit at both LT
and RT.
Results and Discussion
Cnbiphphen was obtained from the palladium mediated
coupling reaction between 4,7-bis(4-bromophenyl)-
[1,10]phenanthroline and 4-cyanobenzene boronic acid in
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J Chem Crystallogr (2015) 45:453–460
Fig. 3 ORTEP of molecular packing at 90(2) K of the unit cell as
viewed perpendicular to the a–c plane with chloroform and hydrogen
atoms omitted for clarity. Two coplanar molecules of the title
compound associate with each other ‘wedge like’ at a distance of
˚
10.959(2) A along the b-axis. The ‘wedge’ direction alternates along
the c-axis which enables C:N interaction. A close up view of a pair
of molecules shows the wedge like fitting. The inter-molecular
distance between phenanthroline nitrogens and closest carbon of the
adjacent phenanthroline is 7.348(2), and it is this cavity between two
phenanthroline systems where the chloroform molecule is situated.
Ellipsoid probability is set at 50 %. The geometry is the same at room
temperature with small angular deviations and distances, caused by
the cyanophenyl disorder
Fig. 4 Plot of the opposing
arrangement of molecules of the
title compound at 90(2) K. The
arrangement allows for
interaction of the C:N
moieties at an inter-molecular
˚
distance of 3.563(2) A and
˚
3.595(2) A (298(2) K:
˚
˚
3.66(2) A and 3.49(4) A). The
arrangement also illustrates the
‘zigzag’ packing of the
opposing structures that the
C:N moieties impart.
Ellipsoids drawn at the 50 %
probability level. Hydrogen
atoms and chloroform are
omitted for clarity
˚
between adjacent molecules along the b-axis of 10.959(2) A
at LT. Pairs of coplanar molecules fit wedge like into each
other. The N–C inter-molecular cavity between two
for electrostatic interactions of nitrile units as shown in
Fig. 4. The inter-molecular distances of the interacting
˚
˚
˚
C:N groups are 3.563(2) A and 3.595(2) A at LT (3.66(2) A
˚
phenanthroline systems in the wedge structure is 7.348(2) A
˚
and 3.49(4) A, RT), which is substantially shorter than the
leaving space for one chloroform. The electron withdrawing
nature of the chlorines gives a partial positive charge to the
carbon and hydrogen atoms of the chloroform molecule
which the electron rich nitrogen atoms of the phenanthroline
‘‘chelate’’. The N–C (phenanthroline-chloroform) inter-
inter-molecular distances between aromatic rings. The
energy of interaction between two C:N groups is large,
reported to be 8–12 kJ/mol for crystalline aromatic nitriles
[27], and as such the molecules arrange themselves to
maximize the interaction between C:N groups. The ‘zig-
zag’ pattern of molecules is similar to what has been
observed for trifluoromethylbiphenyl substituted phenan-
throline [8]. The difference being that there are two ‘out of
phase’ ‘zigzag’ chains for cnbiphphen.
˚
molecular distances are 3.171(2) A and 3.196(2) A (LT).
˚
˚
The phenanthroline systems are at a distance of 12.898(2) A,
seemingly without further interactions. Individual pairs of
the title compound face in opposing directions which allows
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J Chem Crystallogr (2015) 45:453–460
459
The material experiences a structural order–disorder
phase transition upon cooling which resolves the disorder
found in the cyanophenyl of the RT structure. Both struc-
tures have similar lattice parameters (Table 1) and the
transition temperature of 243 K seems to be independent of
the direction by which the temperature is driven. The
crystal stays intact throughout the completely reversible
phase transition and shows no domain structure in the
primitive monoclinic phase. The lattice parameters show
hardly any variation in the course of the phase transition,
however, the number of peaks for indexing rises sharply
when the A-center extinction rule (peaks for k ? l = 2n
only exist, n integer) is lost and replaced by fewer
restrictions in the primitive lattice (Fig. 5).
Differential scanning calorimetry (DSC) results from the
heating and cooling of a cnbiphphen crystal, weighing
8.200 mg, are shown in Fig. 6. The DSC measurement
confirms the observations from the X-ray analysis of the
phase transition. The difference in transition temperatures
between warming (241.5 K) and cooling (236.6 K) direc-
tion is within the limits of accuracy. The enthalpy change,
DH, for the order–disorder phase transition is calculated
from the DSC data to be *443 J mol-1 and the entropy
change, DS, to be *1.9 J mol-1 K-1, indicating a mostly
thermodynamic second-order type transition mechanism.
Many order–disorder phase transitions have been
observed previously for polymeric and inorganic systems
[28–31]. Cyano substituted biphenyls in liquid crystal
systems have been reported to exhibit order–disorder phase
transitions [32]. In the reports on cyano substituted
biphenyls it was discussed that the length of alkyl chain
substitution impacted the nature and presence of the dis-
order of the cyano substituted biphenyls, However, cn-
biphphen is different in that the cyanobiphenyl moiety is
bonded to a phenanthroline system rather than alkyl
substitution.
Fig. 5 Peak-search results during crystal indexing under fixed
conditions while lowering the temperature. One crystal, mounted
inside the diffractometer, was cooled to a desired temperature,
indexed via 36 frames of three different runs at different kappa angles
of 12 frames each separated by phi-rotations of 0.5°. Peak search
parameters were kept the same for the next repetitions of the same
procedure at different temperatures and the number of peaks found
was recorded. Tc was determined from the intersection of the two
slopes least-squares-fitted to the data in the two different symmetries.
Below Tc = 243 K the structure indexed primitive. The structure was
found to be monoclinic A-centered for temperatures above Tc. The
number of indices found is not absolute as it depends on many
parameters including sample size and X-ray exposure. This number is
expected to increase smoothly towards lower temperatures due to less
diffuse scattering, which is responsible for the rising number of peaks
especially below 120 K
Conclusions
4,7-Bis-(40-cyano-biphenyl-4-yl)-[1,10]phenanthroline,
abbreviated cnbiphphen in this report, was synthesized
using a palladium coupling reaction. Spectral analytic
results were confirmed by single crystal X-ray studies. The
C : N substituted phenyl of the biphenyl moiety on the
4,7-positions of the phenanthroline exhibits rotational dis-
order at 298 K. Cooling the crystal to 90 K eliminated the
disorder with a transition temperature measured at 243 K
which was confirmed using DSC with DH *443 J mol-1
and DS *1.9 J mol-1 K-1. The cnbiphphen molecules
pack in an alternating pattern which maximizes C : N
interactions with lesser or none contributions from other
molecular interactions.
Fig. 6 DSC measurement for cnbiphphen. The order–disorder phase
transition is observed at 236.6 K in the cooling direction (black) and
241.5 K in the warming direction (red). Tc was identified from the
first derivative of the data to find peak maxima (Color figure online)
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J Chem Crystallogr (2015) 45:453–460
14. Case FH (1951) J Org Chem 16:1541
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(eds) Methods in enzymology. Academic Press, New York,
pp 276–307
Supplementary material CCDC 1417150 and CCDC
1417151 contains the crystallographic data for the LT and
RT structures discussed in this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: ?44 1223 336033; email:
deposit@ccdc.cam.ac.uk).
20. Bruker (2007) APEX2 (Version 2.1-4), SAINT (version 7.34A),
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Guagliardi A, Moliterni AGG, Polidori G, Spagna RJ (1999)
Appl Cryst 32:115
Acknowledgments The authors wish to thank Andranik Aprikyan
and Kilian Dill of Stemgenics for the use of their spectrophotometer.
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