Structural, electrochemical and optical properties of di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah)

Article abstract of DOI:10.1016/S0022-2860(03)00048-6

Crystals of di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah), obtained from a dmso (dimethylsulfoxide) solution of dpkfah, are in the monoclinic space group, P2₁/n. Structural analysis reveals planar hydrazone moiety, non-coplanar pyridine rings and infinite chains of anti-parallel dpkfah dimers interlocked via a web of hydrogen bonds. Electrochemical measurements on dpkfah in non-aqueous solvents show solvent dependence, single and multi-electronic transfers and electrochemical transformation(s) following the first oxidative or reductive electronic transfer. Optical measurements on dpkfah in non-aqueous solvents show strong solvent dependence. In non-polar solvent such as CH₂Cl₂ a single electronic absorption band with extinction coefficient of 18,600 ± 2000 M^-1 cm^-1 appeared at 325 nm and in polar solvents a low-energy absorption band at ～396 nm and a high-energy absorption band at ～320-335 nm appeared that are concentration, temperature and salt dependent. In the presence and absence of NaBF₄ and NaBH₄, extinction coefficients of 21,000 ± 2000 and 22,500 M^-1 cm^-1, and 17,200 ± 2,000 and 23,000 M^-1 cm^-1 were calculated for the low and high energy electronic states of dpkfah in dmf and dmso, respectively at 295 K. Thermo-optical measurements on dpkfah in dmso and dmf confirmed the reversible interconversion between the high and low energy electronic states of dpkfah and allowed calculations of their thermodynamic activation parameters and gave changes in enthalpy (ΔH^?) of +47.5 ± 1.2 and -16.3 ± 0.4 kJ mol^-1, entropy (ΔS^?) of +147.7 ± 3.8 and -64.4 ± 1.64 J mol^-1 K^-1 and free energy (ΔG^?) of +3.49 ± 0.2 and +2.85 ± 0.2 kJ mol^-1 and hence equilibrium constant (K) of +0.25 ± 0.05 and +0.32 ± 0.05 in dmso and dmf, respectively. The reversible BH₄^-/BF₄^- interconversion of the electronic states of dpkfah points to weak non-covalent interactions between these species and dpkfah and possible use of dpkfah as a spectrophotometric sensor for a variety of physical and chemical stimuli. Chemical stimuli in concentrations <1.00 × 10^-6 M can be detected and determined using dpkfah in non-aqueous media.

Full text of DOI:10.1016/S0022-2860(03)00048-6

Journal of Molecular Structure 649 (2003) 133–142
www.elsevier.com/locate/molstruc
Structural, electrochemical and optical properties of di-2-pyridyl
ketone 2-furoic acid hydrazone (dpkfah)
*
Mohammed Bakir , Colin Gyles
Department of Chemistry, The University of the West Indies, Mona Campus, Kingston 7, Jamaica, WI
Received 2 October 2002; revised 6 January 2003; accepted 6 January 2003
Abstract
Crystals of di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah), obtained from a dmso (dimethylsulfoxide) solution of
dpkfah, are in the monoclinic space group, P21=n: Structural analysis reveals planar hydrazone moiety, non-coplanar pyridine
rings and infinite chains of anti-parallel dpkfah dimers interlocked via a web of hydrogen bonds. Electrochemical measurements
on dpkfah in non-aqueous solvents show solvent dependence, single and multi-electronic transfers and electrochemical
transformation(s) following the first oxidative or reductive electronic transfer. Optical measurements on dpkfah in non-aqueous
solvents show strong solvent dependence. In non-polar solvent such as CH2Cl2 a single electronic absorption band with
extinction coefficient of 18,600 ^ 2000 M 21 cm21 appeared at 325 nm and in polar solvents a low-energy absorption band at
,396 nm and a high-energy absorption band at ,320–335 nm appeared that are concentration, temperature and salt
dependent. In the presence and absence of NaBF4 and NaBH4, extinction coefficients of 21,000 ^ 2000 and 22,500 M21 cm21
,
and 17,200 ^ 2,000 and 23,000 M21 cm21 were calculated for the low and high energy electronic states of dpkfah in dmf and
dmso, respectively at 295 K. Thermo-optical measurements on dpkfah in dmso and dmf confirmed the reversible
interconversion between the high and low energy electronic states of dpkfah and allowed calculations of their thermodynamic
activation parameters and gave changes in enthalpy (DHB) of þ47.5 ^ 1.2 and 216.3 ^ 0.4 kJ mol21, entropy (DSB) of
þ147.7 ^ 3.8 and 264.4 ^ 1.64 J mol21 K21 and free energy (DGB) of þ3.49 ^ 0.2 and þ2.85 ^ 0.2 kJ mol21 and hence
equilibrium constant ðKÞ of þ0.25 ^ 0.05 and þ0.32 ^ 0.05 in dmso and dmf, respectively. The reversible BH24 /BF42
interconversion of the electronic states of dpkfah points to weak non-covalent interactions between these species and dpkfah
and possible use of dpkfah as a spectrophotometric sensor for a variety of physical and chemical stimuli. Chemical stimuli in
concentrations ,1.00 £ 1026 M can be detected and determined using dpkfah in non-aqueous media.
q 2003 Elsevier Science B.V. All rights reserved.
Keywords: Sensors; Hydrazones; di-2-pyridyl ketone; X-ray
1. Introduction
Bistability, i.e. the ability of the system (substrate plus
surrounding molecules) to exist in two states (elec-
tronic or conformational), is essential for molecular
recognition as the interconversion between states
allows to explore their structural relaxation and
interactions with their environment [9]. We have
been interested in the chemistry of di-2-pyridyl ketone
and its oxime and hydrazone derivatives and described
The development of molecular sensors is of interest
for their use to monitor their surroundings [1–8].
*
Corresponding author. Tel.: þ1-876-935-8164; fax: þ1-876-
977-1835.
E-mail address: mbakir@uwimona.edu.jm (M. Bakir).
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-2860(03)00048-6

134
M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
the synthesis, optical, electrochemical and structures
of a series of di-2-pyridyl ketone derivatives [10–22].
Optical and thermodynamic measurements on di-2-
pyridyl ketone hydrazones in polar non-aqueous
solvents revealed reversible interconversion between
two interlocked electronic states due to intermolecular
interaction between these species and their surround-
ings [11,12,14,15]. The high values for their extinction
coefficients and low values for their activation
parameters allowed for the use of these systems (di-
2-pyridyl ketone hydrazones and surrounding mol-
ecules) as molecular sensors for a variety of chemical
and physical stimuli that include metal ions and
biomolecules [11–15]. X-ray structural analysis on a
series of di-2-pyridyl ketone hydrazones revealed the
presence of a web of non-covalent interactions that
include solvent-solute, solute-solute, and p-stacking
which may account for their interlocked charge-
transfer bands and molecular sensitivity [13–18].
Although the ligand di-2-pyridyl ketone benzoyl
hydrazone (dpkbz) has been widely used as a sensitive
optical analytical reagent for the determination of trace
amounts of metal ions in aqueous and non-aqueous
media, the use of di-2-pyridyl ketone 2-furoic acid
hydrazone as an optical sensor is scarce [13,23,24].
Two reports appeared on the optical determination of
iron and scandium using di-2-pyridyl ketone 2-furoic
acid hydrazone in aqueous media [23,24]. In this
report, we describe the structure, thermodynamics and
optosensing behavior of dpkfah in non-aqueous
solvents.
2. Experimental
2.1. Reagents
Solvents were reagent grade and thoroughly
deoxygenated prior to use. All other reagents were
obtained from commercial sources and used without
further purification. The compound dpkfah was
prepared by refluxing a mixture of di-2-pyridyl ketone
and 2-furoic acid hydrazide, ethanol and a few drops
of HCl in a procedure similar to that reported for
the synthesis of dpkbz and other related compounds
[10,15].
2.2. X-ray crystallography
Single crystals of dpkfah were obtained from a
DMSO solution of dpkfah when allowed to stand at
room temperature for several days. A single crystal
was selected and mounted on a glass fiber with epoxy
cement. A Bruker AXS with a Mo Ka radiation and a
graphite monochromator was used for data collection
and the SHELXTL software package version 5.1 was
used for structure solution [25,26]. Cell parameters
and other crystallographic information are given in
Table 1 along with additional details concerning data
collection, and Table 2 gives a list of atomic
coordinates. All non-hydrogen atoms were refined
with anisotropic thermal parameters.

M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
135
Table 1
Table 2
Atomic coordinates ( £ 104) and equivalent isotropic displacement
Crystal data and structure refinement for dpkfah
2
3
˚
parameters (A £ 10 ) for dpkfah. U(eq) is defined as one third of
Empirical formula
Formula weight
Temperature
C16H12N4O2
the trace of the orthogonalized Uij tensor
292.30
298(2) K
˚
0.71073 A
x
y
z
U(eq)
Wavelength
Crystal system,
space group
Monoclinic, P21=n
O(2)
2758(2)
2327(2)
1022(2)
833(2)
2831(2)
4856(2)
3323(2)
2355(2)
5869(3)
3643(2)
4719(2)
2178(3)
5559(3)
6764(3)
6591(3)
1879(2)
3106(3)
431(2)
9512(1)
57(1)
52(1)
46(1)
48(1)
57(1)
41(1)
39(1)
43(1)
51(1)
55(1)
59(1)
51(1)
42(1)
57(1)
55(1)
44(1)
66(1)
62(1)
61(1)
49(1)
64(1)
65(1)
N(1)
12280(2)
10556(2)
9826(2)
12957(2)
10887(2)
11679(2)
9334(2)
11775(2)
13077(2)
12472(2)
10056(2)
10508(2)
8394(1)
7927(2)
8556(2)
9694(2)
9664(2)
10134(2)
10535(2)
7653(2)
7352(2)
˚
Unit cell dimensions
a ¼ 10:706ð2Þ A, a ¼ 908
N(3)
˚
b ¼ 9:9008ð17Þ A, b ¼ 108:76ð2Þ8
N(4)
˚
c ¼ 13:8795ð15Þ A g ¼ 908
C(11)
C(1)
235(3)
58(2)
3
˚
Volume
1393.1(4) A
Z; calculated density
Absorption coefficient 0.096 mm21
4, 1.394 Mg/m3
C(15)
C(2)
420(3)
1729(3)
1490(3)
975(3)
Fð000Þ
608
C(14)
C(12)
C(13)
N(2)
Crystal size
0.6 £ 0.9 £ 0.7 mm
2.11–17.508
Theta range for
data collection
Limiting indices
Reflections
1752(2)
21469(2)
21323(3)
251(2)
21 # h # 9; 21 # k # 8; 211 # l # 11
1282/879 [R(int) ¼ 0.0316]
C(25)
O(1)
collected/unique
Completeness to
theta ¼ 17.50
C(32)
C(31)
C(22)
C(21)
C(23)
C(24)
C(34)
C(33)
2029(2)
684(3)
100.0%
1395(2)
1146(3)
2043(3)
1380(3)
3300(3)
3844(2)
23788(3)
22684(4)
23643(3)
22403(3)
212(3)
Absorption correction None
Refinement method
Data/restraints/
parameters
Full-matrix least-squares on F2
879/0/200
2483(3)
2366(3)
Goodness-of-fit on F2 1.010
1264(3)
Final R indices
[I . 2sigma(I)]
R indices (all data)
R1 ¼ 0:0275; wR2 ¼ 0:0987
R1 ¼ 0:0289; wR2 ¼ 0:1017
UV/VIS/NIR spectrometer l-19: Baseline corrections
on blank solvents were recorded prior to measure-
ments. A Lauda-Brinkmann RM6 circular bath was
used for temperature control. Electrochemical
measurements were performed with the use of a
Princeton Applied Research (PAR) Model 173
potentiostat/galvanostat and Model 276 interface in
conjunction with a 286 PC. Data were acquired with
the EG&G PARC Headstart program and manipulated
using the Microsoft Excel Program. Measurements
were performed in solutions that were 0.1 M in [N(n-
Bu)4](PF6). The Ep,a, Ep,c and Ep ¼ (Ep,a þ Ep,c)/2
values were referenced to the potassium chloride
saturated calomel electrode, SCE, at room tempera-
ture and are uncorrected for junction potentials. The
number of electrons on the redox waves
was determined using the oxidative peak
current of the reversible one-electron couple of
FeCp2/FeCpþ2 couple as an internal standard. Ep(-
(FeCp2/FeCpþ2 ) ¼ þ0.47 ^ 0.02, þ0.45 ^ 0.02
and þ0.45 ^ 0.02 V 200.6 mV s21 versus
SCE in CH2Cl2, CH3CN and DMF, respectively.
Extinction coefficient 0.050(6)
23
˚
0.101 and 20.084 e A
Largest diff. peak
and hole
2.3. Optical sensing studies
A stock solution of dpkfah in polar non-aqueous
solvent and stock solutions of stimulus (ZnCl2, CdCl2,
HgCl2, NaBH4 or NaBF4) in polar non-aqueous solvent
were prepared separately. Optical changes (electronic
absorption spectral changes) were measured on a
solution prepared by mixing appropriate volumes of
dpkfah and stimulus solutions to prepare the desired
solution (see figure captions for details). With the
exception of the thermodynamic studies all measure-
ments were made at room temperature.
2.4. Physical measurements
Electronic absorption spectra were recorded on a
HP-8452A spectrophotometer or a Perkin–Elmer

136
M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
Electrochemical cells were of conventional design
based on scintillation vials or H-cells. A glassy-
carbon disc was used as the working electrode and Pt-
wire as a counter electrode.
3. Results and discussion
3.1. Structural studies
The molecular structure of dpkfah is shown in
Fig. 1 and selected bond distances and angles are
given in Table 3. The furan acetic hydrazone moiety
and the pyridine ring of N1 are planar and the pyridine
rings are not co-planar. The non-coplanarity of the
pyridine rings are normal and similar to those
observed in dpkbz and other related compounds.
Fig. 1. The molecular structure of dpkfah. The thermal ellipsoids are
drawn at the 30% probability level and H atoms are shown as small
spheres of arbitrary radii.
Table 3
˚
3 Selected bond distances (A), angles (8) and hydrogen bonds for dpkfah
N(1)–C(15)
N(1)–C(11)
C(11)–C(12)
C(15)–C(1)
N(2)–C(21)
N(2)–C(25)
C(24)–C(25)
C(25)–C(1)
C(1)–N(3)
1.334(3)
1.341(3)
1.366(3)
1.490(3)
1.333(3)
1.352(3)
1.379(3)
1.499(3)
1.296(3)
1.362(2)
1.356(3)
1.232(3)
1.446(3)
1.347(3)
1.369(2)
1.327(3)
1.360(3)
C(15)–N(1)–C(11)
117.5(2)
124.6(2)
121.4(2)
117.9(3)
118.3(2)
121.1(2)
115.8(2)
122.9(3)
112.9(2)
126.6(2)
120.3(2)
119.4(2)
120.3(2)
124.4(2)
120.7(2)
115.0(3)
132.0(3)
109.2(2)
118.8(3)
110.9(2)
106.0(2)
N(1)–C(11)–C(12)
N(1)–C(15)–C(14)
N(1)–C(15)–C(1)
C(21)–N(2)–C(25)
N(2)–C(25)–C(24)
N(2)–C(25)–C(1)
C(24)–C(25)–C(1)
N(3)–C(1)–C(15)
N(3)–C(1)–C(25)
C(15)–C(1)–C(25)
C(1)–N(3)–N(4)
C(2)–N(4)–N(3)
O(2)–C(2)–N(4)
O(2)–C(2)–C(31)
N(4)–C(2)–C(31)
C(32)–C(31)–C(2)
C(32)–C(31)–O(1)
O(1)–C(31)–C(2)
C(33)–C(34)–O(1)
C(34)–O(1)–C(31)
N(3)–N(4)
N(4)–C(2)
C(2)–O(2)
C(2)–C(31)
C(31)–C(32)
C(31)–O(1)
C(33)–C(34)
C(34)–O(1)
Donor-H· · ·Acceptor
N(4)–H(4)· · ·O(1)
d(D–H)
0.86
d(H· · ·A)
2.30
d(D· · ·A)
2.677(3)
2.627(3)
3.352(3)
2.889(3)
3.312(3)
3.277(4)
,(D–H· · ·A)
106.6
N(4)–H(4)· · ·N(2)
0.86
1.96
133.7
C(12)–H(12)· · ·O(2)1
C(24)–h(24)· · ·n(1)
C(24)–H(24)· · ·O(2)2
C(32)–H(32)· · ·N(1)3
0.93
2.59
139.0
0.93
2.41
111.9
0.93
2.51
145.2
0.93
2.53
137.8
Symmetry transformations used to generate equivalent atoms: 1. 2x þ 1=2; y þ 1=2; 2z þ 5=2; 2., 2x; 2y þ 1; 2z þ 2 and 3.
x þ 1=2; 2y þ 1=2; z 2 1=2x:.

M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
137
Fig. 2. A view of the packing of dpkfah.
˚
The bond distances and angles are of the same order as
those reported for dpkbz, di-2-pyridylketone-2-ami-
nobenzoylhydrazone (Hdpa), di-2-pyridyl ketone 2-
pyridylhydrazone (dpkph) and other related species
[13–18,27]. For example in dpkbz, the hydrazone
CyN, N–N and N–C bond distances of 1.30(2),
1.36(2), 1.36(3) and bond angle of C–N–N and N–
N–C 118.5(2) and 120.6(2) of were observed [13].
The molecules pack reveals infinite chains of anti-
parallel dpkfah (see Fig. 2) interlocked via a network
of hydrogen bonds. Table 3 gives a list of hydrogen
bonds and Fig. 3 shows views of intermolecular
hydrogen bonds. The bond distances and angles of
these bonds are of the same order as those reported for
a variety of compounds containing such bonds. [13–
18,27–30]. For example, d(H· · ·A) and ,(D–H· · ·A)
of 2.46 A and 1368 were observed for the C–H· · ·O
hydrogen-bond in Naþ·C5H3N2O23 ·3H2O27 and
˚
hydrogen bond parameters of 2.61 A and 1298 were
reported for the (N–H…N) in dpkph [28].
3.2. Electrochemical properties
The electrochemical properties of dpkfah in non-
aqueous solvents were investigated using voltam-
metric techniques. Cyclic voltammograms of dpkfah
in DMF, CH3CN and CH2Cl2 are shown in Fig. 4. On
a reductively initiated scan in dmf (Fig. 4(a)) an
irreversible one electron reduction appeared at
Ep,c ¼ 21.74 V, irreversible two electrons oxidation
appeared at Ep,a ¼ þ1.86 V and a reductively gener-
ated product wave appeared at Ep,a ¼ þ0.62 V. On an

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M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
Fig. 4. Cyclic voltammograms of dpkfah in DMF (a and b), CH3CN
(c and d) and CH2Cl2 (e and f) solutions 0.1 M in [N(n-Bu)4](PF6) at
a glassy carbon working electrode at a scan rate of 400 mVs21
versus SCE.
Fig. 3. Views of the intermolecular hydrogen bonds in dpkfah.
Symmetry transformations used to generate equivalent atoms: 1.
2x þ 1=2; y þ 1=2; 2z þ 5=2; 2. 2x; 2y þ 1; 2z þ 2 and 3. x þ
1=2; 2y þ 1=2; z 2 1=2x:
appeared at Ep,a ¼ þ2.14 V. These voltammetric
results reveal sensitivity of the redox potentials of
dpkfah to solvent variations and point to solvent-
solute interactions. No signs of reversible redox
behavior were observed when the scan rates were
varied from 100 to 10000 mV s21. The irreversibility
of the reduction waves suggests electrochemical
reactions following the electronic transfer(s). The
appearance of electrochemically generated product
waves following the first electronic transfer suggest
that the electrochemically generated radical anion is
unstable and undergoes decomposition as shown in
Scheme I. The appearance of multi-electronic
reduction (three electrons) following the first elec-
tronic reduction in DMF and CH3CN may hint to
electron(s)/proton(s) coupled transfer(s) leading to the
reduction of the imine radical to ammine (see Scheme
I). The proposed scheme is similar to that we reported
for the reduction of di-2-pyridyl ketone oxime to di-2-
pyridylmethyl amine in non-aqueous media [22].
oxidatively initiated scan (Fig. 4(b)) the electroche-
mically generated product wave at Ep,a ¼ þ0.62 V
disappeared and new oxidatively generated product
waves appeared at Ep,c ¼ 20.72 and 21.37 V. On a
21.0 to 23.0 reductive scan (Fig. 5(a)) a three
electrons reduction wave appeared at Ep,c ¼ 2
2.53 V and on an oxidatively generated scan from
þ1.0 to þ3.0 V (Fig. 5(b)) two irreversible reduction
waves two electrons each appeared at Ep,a ¼ þ1.80
and þ2.05 V. In CH3CN, electrochemical behavior
similar to that observed in dmf was noted except on
oxidative scan from þ1.0 to þ3.0 V sequential
electronic transfers followed the first multi-electronic
oxidation. In CH2Cl2 under similar scanning con-
ditions as in dmf and CH3CN a single one electron
irreversible reduction appeared at Ep,c ¼ 22.19 V
and a single irreversible one electron oxidation

M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
139
3.3. Optical behavior
The electronic absorption spectra of dpkfah in non-
aqueous solvents are shown in Fig. 6. In non-polar
solvent, CH2Cl2, a single absorption band with
extinction coefficient of 18,600 M21 cm21 appeared
at 325 nm and polar solvents (see Fig. 6) two
absorption bands appeared at 396 and 335–320 nm.
These spectra hint to strong solvent-solute inter-
actions as apparent from the dependence of the
absorption bands on solvent variations. In DMF, the
low- and high-energy absorption bands at 396, and
325 are concentration and temperature dependent. As
the concentration of dpkfah decreases the ratio of
A396/A325 increases and a plot of A325 versus
concentration is non-linear point to direct solvent-
solute interactions and deviation from Beer’s Law. In
mixed binary polar/non-polar co-solvent systems, the
intensity of the low energy absorption band of dpkfah
increases as the volume % of polar solvent increases.
In DMSO, as the temperature increases the ratio of
A396/A325 increases and the reverse was observed
when the temperature was decreased. These results
point to reversible interconversion between the high-
and low-energy electronic states of dpkfah and
Fig. 5. The same as above on 21.0 to 23.0 V reductive scans and
þ1.0 to 3.0 V oxidative scans at 200 mV s21 versus SCE.
Scheme 1.

140
M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
Fig. 8) the low-energy electronic state becomes
favorable and when stoichiometric amounts of
NaBF4 were allowed to interact with the resulting
solution, the reverse was observed. In dmf in the
presence of excess NaBH4 the equilibrium shifts all
the way to the low-energy conformation and that
gave
an
extinction
coefficient
of
21,000 ^ 2000 M21 cm21 for the low-energy elec-
tronic state of dpkfah and hence extinction
coefficient of 17,200 ^ 2000 M21 cm21 for the
high energy absorption band at 295 K. In the case
of DMSO, extinction coefficients of 22,500 ^ 2000
and 23,000 ^ 2000 M21 cm21 were calculated at
295 K for the low- and high-energy absorption
bands from temperature studies. These results
demonstrate the reversible interconversion between
the high and low electronic states of dpkfah and
point to weak non-covalent intermolecular inter-
actions which may be due to donor/acceptor
hydrogen bonding between dpkfah and stimulus
as demonstrated in this study using the electron
rich/deficient NaBH4/NaBF4, solvent and tempera-
ture variations. This is consistent with the web of
Fig. 6. The electronic absorption spectra of dpkfah in (1) DMF,
2.0 £ 1025 M; (2) DMSO 2.20 £ 1025 M; (3) CH3CN
2.00 £ 1025 M and (4) CH2Cl2 2.00 £ 1025 M.
allowed calculations of its thermodynamic activation
parameters in a procedure similar to that we reported
for fac-Re(CO)3(dpknph)Cl and dpkbh [11,13]. A plot
of ln(A396/A396 versus 1=T £ 103 K21 (Fig. 7) gave
straight lines with a slope of 2570 ^ 70 and
þ197 ^ 23 with intercepts of þ17.70 ^ 2.0 and –
7.75 ^ 0.5 that gave changes in enthalpy (DHB) of
þ47.5 ^ 1.2 and 216.3 ^ 0.4 kJ mol21, entropy
(DSB)
of
þ147.7 ^ 3.8
and
264.4 ^ 1.64 J mol21 K21 and free energy (DGB)
of þ3.49 ^ 0.2 and þ2.85 ^ 0.2 kJ mol21 and hence
equilibrium constant (K) of þ0.25 ^ 0.05 and
þ0.32 ^ 0.05 in DMSO and DMF, respectively.
The reversible interconversion between the high-
and low-energy electronic states of dpkfah, their low
activation parameters and high value for the extinc-
tion coefficient of the high-energy electronic states
allows for the use of these systems (dpkfah and
surrounding molecules) as molecular sensors for
variety of stimuli. These results are similar to those
reported for the optical behavior of fac-Re(CO)3(-
dpknph)Cl, dpkbh and other related species [11,13–
16].
When NaBH4 in dmf were allowed to
interact with a DMF solution of dpkfah (see
Fig. 7. A plot of ln(A396/A325) versus 1=T £ 103 K21 of DMSO and
DMF solutions of dpkfah 1.0 £ 1025 M.

M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
141
hydrogen bonds observed in the solid state and low
values of the activation parameters for the inter-
conversion between the high and low energy
conformations of dpkfah. The disturbance of the
equilibrium distribution of the high and low
electronic states of dpkfah upon the addition of
stimulus improves the optosensing properties of
dpkfah as relaxation to the equilibrium position
allows for the exploration of the structural
relaxation and the use of these systems (dpkfah
and surrounding molecules) as molecular sensors
for a variety of species. When DMF solutions of
Group 12 metal ions were added to DMF solutions
of dpkfah the intensity of the low-energy absorp-
tion band increases and the intensity of the high-
energy absorption band decreases. This is in
contrast to the optosensing behavior of fac-
Re(CO)3(dpknph)Cl where the intensity of the low
energy absorption band decreases and the intensity
of the high energy absorption band increases upon
the addition of Group 12 metal ions in dmf [11]. A
plot of the absorbance of dpkfah versus concen-
tration of Group 12 metal ions in DMF is shown in
Fig. 9. These results reveal that chemical stimuli in
concentration , 1 £ 1026 M can be detected and
determined using dpkfah in non-aqueous media. In
previous studies on the optical determination of
trace amounts of iron and scandium using dpkfah
in aqueous media competing species such as
NH4Cl, NH4OH, NaOH and others, all of which
interact with dpkfah and influence its optical
properties, were used to adjust the pH [23,24].
Owing its convenient synthesis and rich physico-
chemical properties, together with our continued
interest in molecular sensors, the coordination chem-
istry and optosensing behavior of dpkfah towards a
variety of stimuli, including biomolecules, remain
under investigation in our laboratories.
Fig. 8. Electronic absorption spectra of dpkfah 2.00 £ 1025 M in
DMF (1) in the presence of 5.00 £ 1025 M (2), 1.00 £ 1024 M (3),
2.00 £ 1024 M (4) and 5.00 £ 1024 M (5) NaBH4.
4. Conclusion
Structural and optical studies of dpkfah have
revealed the presence of hydrogen bonds in the solid
state, reversible interconversion between its high- and
low-energy electronic states and use of dpkfah as an
optical sensor for a variety of chemical stimuli.
Fig. 9. A plot of the absorbance of dpkfah 2.00 £ 105 M in DMF at
396 nm (1, 2 and 3) and 325 nm (4,5 and 6) versus concentration of
ZnCl2 (series 1 and 4), CdCl2 (series 2 and 5) and HgCl2 (series 3
and 6).

142
M. Bakir, C. Gyles / Journal of Molecular Structure 649 (2003) 133–142
[12] M. Bakir, C. Gyles, Talanta 56 (2002) 1117.
Acknowledgements
[13] M. Bakir, O. Brown, J. Mol. Struct. 609 (2002) 129.
[14] M. Bakir, O. Brown, J. Mol. Struct. 641 (2002) 183.
[15] M. Bakir, Eur. J. Inorg. Chem. (2002) 481.
[16] M. Bakir, Inorg. Chim. Acta 332 (2002) 1.
[17] M. Bakir, O. Green, Acta Cryst. C58 (2002) 0263.
[18] M. Bakir, Acta Cryst. C57 (2001) 1371.
[19] M. Bakir, Acta Cryst. C57 (2001) 1154.
[20] M. Bakir, J.A.M. McKenzie, Electroanal. Chem. 425 (1997)
61.
We acknowledge funding from The University of
the West Indies and Inter-America Development
Bank.
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