Syntheses, structures, and fluorescent properties of 2-(1H-imidazol-2-yl) phenols and their neutral Zn(II) complexes

Article abstract of DOI:10.1021/ic9008703

A series of 2-(imidazole-2-yl)phenol ligands L1-L6 with the general composition 4-R⁴-5-R³-6-R²-2-(4,5-R ¹,R¹-1H-imldazole-2-yl)phenol (L1 : R¹ = C ₂H₅, R² =

Full text of DOI:10.1021/ic9008703

Inorg. Chem. 2009, 48, 9133–9146 9133
DOI: 10.1021/ic9008703
Syntheses, Structures, and Fluorescent Properties of 2-(1H-Imidazol-2-yl)phenols
and Their Neutral Zn(II) Complexes
Abiodun Omokehinde Eseola,†,§ Wen Li,† Rong Gao,‡ Min Zhang,‡ Xiang Hao,‡ Tongling Liang,‡
Nelson Okpako Obi-Egbedi,§ and Wen-Hua Sun*,‡
†Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and
Chemistry, The Chinese Academy of Sciences, Beijing 100190, China, ‡Key Laboratory of Engineering Plastics
and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences,
Beijing, 100190, China, and §Chemical Sciences Department, Redeemer’s University, Redemption City, Ogun,
Nigeria
Received May 5, 2009
A series of 2-(imidazole-2-yl)phenol ligands L1-L6 with the general composition 4-R4-5-R3-6-R2-2-(4,5-R1,R1-1H-
imidazole-2-yl)phenol (L1: R1 = C2H5, R2 = R3 = R4 = H; L2: R1 = C6H5, R2 = R3 = R4 = H; L3: R1 = C6H5, R3 = OCH3,
R2 = R4 = H; L4: R1 = C6H5, R4 = OCH3, R2 = R3 = H; L5: R1 = C6H5, R3 = H, R2 = R4 = CH3; L6: R1 = C6H5, R3 = H, R2 =
R4 = t-Bu) and L7 (2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol) and their neutral Zn(II) complexes
(Z1-Z7) were synthesized and characterized by spectroscopic and elemental analyses. Molecular structures of L1,
L5, Z1, and Z2 were confirmed by single-crystal X-ray diffraction. L1 crystallized in the monoclinic Cc space group,
while L5, Z1, and Z2 all crystallized in the triclinic P1 space group. One-dimensional arrays based on continuous π-π
stacking interactions and hydrogen bonding were observed for L1 and Z1, while L5 existed as discrete dimeric stack
units. Z2 formed hydrogen-bonded 1D network structures but was completely devoid of π-π stacking interactions.
Emission processes were found to be more dependent on the substituents on phenol as well as condensed media. In
contrast to general conclusions on closely related systems in the literature, significant photorelaxation from the excited
enol state was observed in the cases of L1 in methanol and L4 in both THF and methanol. Therefore, there exists a
certain unusual hindering factor to keto-enol phototautomerism in the ligand-solvent systems. The sensing property
of zinc(II) complexes was explored regarding the effects of substituents in their ligands. It was observed that
coordination to the zinc(II) ion led to emission quenching for L1 and L2 while causing an enhancement of fluorescent
intensity for L3, L4, L5, and L6. A linear relationship was observed between the emission intensity and the
concentration of the zinc ion at the 10-8 M level. Compared to other zinc compounds in this work, fluorescence
enhancement in Z3 and Z4 showed that the methoxyl substituent is favorable for fluorescent enhancement.
Introduction
emission of tris-(8-hydroxyquinolinato)aluminum(III) (Alq3).5
Luminescent metal complexes have advantages such as the
combination of emitting and electron transferring roles, higher
environmental stability, and a better extent of diversity that is
achievable through tuning of electronic properties by virtue of
structural and metal-center variability.6 Due to the advantages
of vacuum-film-forming or thermal deposition properties, zinc
complexes chelated by N-O ligands have attracted attention
and have proved to be suitable metal-organic precursors of
both academic and industrial relevance.7-9 Through different
Research efforts on organic or metal-coordinated organic
materials with luminescent properties have been of great interest
for decades because of their potential applications as electro-
luminescent components.1,2 Luminescent materials for light-
emitting diodes can be classified into three groups: organic
dyes,3 polymers,4 or metal complexes.5 Luminescent materials
based on N/\O coordination compounds have been extensively
investigated since the initial discovery of a bright and stable
*To whom correspondence should be addressed. Tel.: þ86 10 62557955.
Fax: þ86 10 62618239. E-mail: whsun@iccas.ac.cn.
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2009 American Chemical Society
Published on Web 09/03/2009
pubs.acs.org/IC

9134 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Scheme 1. A Summary of the Usual Photocycle of Events Following Excitation of Phenols Bearing Proximal Base (B) Donor Fragments to the
Hydroxyl Group
coordination modes, complexes with the same ligands and
metal centers can form monomeric or multinuclear structures
by virtue of the oxygen acting as a terminal or bridging donor
atom, respectively.6,10 The 2-(imidazol-2-yl)phenol ligand fra-
mework is considered a model for the study of Tyrz-His190
cofactor in photosystem II, and its luminescent properties are
widely utilized in research studies of biological interest.11-13
The introduction of multiple phenyl substituents of chro-
mophores usually improves thermal and amorphous proper-
ties in the light-emitting materials as well as affects optical
performance by preventing π-π stacking,14-16 which is
believed to interfere with emission processes.17 Extension of
conjugation by introducing a methoxy group in order to
exploit its electron-rich nature has been known to produce an
anticipated red shift in electronic spectral bands of organic
chromophores.18
solutions of related ligands are normally assigned to relaxation
from the respective excited state keto tautomers rather than
from the primary excited enol states.6,9-12,20 Though Zn(II)
chelate complexes and others have been extensively explored in
search of optimal performance for electroluminescent applicat-
ions,6-10 systematic investigations of the optical properties of
the ligands and zinc complexes are relatively scarce.21,22 There-
fore, it would be helpful to design new promising fluorophores
through a comparative study of optical properties of N/\O
ligated zinc complexes and the corresponding ligands with
an understanding of substituent effects and the influence of
condensed media intermolecular interactions. Herein, we pre-
sent the syntheses, characterization, and photoluminescent
properties of 2-(1H-imidazol-2-yl)phenols and their neutral
Zn(II) complexes.
Experimental Section
According to Kasha’s rule, fluorescence usually results from
the first excited singlet state (S1) tothegroundstate(S0), andthe
emission spectral shape is usually similar to the corresponding
absorption spectra except for the Stokes shift.19 On photoexci-
tation, phenols bearing a proximal proton acceptor usually
undergo a photoinduced cycle either described as excited state
intramolecular proton-electron transfer (ESIPT)20 or con-
certed proton-electron transfer,11,12 which is followed by an
enol-keto tautomerism (Scheme 1). ESIPT is reported to be
extremely rapid in phenols bearing ortho-substituent frag-
ments.11 Therefore, phototautomerization in both a solid and
All manipulations of synthesizing organic and complex
compounds were performed under a nitrogen atmosphere
using standard Schlenk techniques. THF was refluxed and
distilled oversodium and benzophenone, while methanol was
dried and distilled over CaH2. All starting materials were
obtained commercially as analytical-grade and used without
further purification. 2-Hydroxy-3,5-dimethylbenzaldehyde
was prepared according to the literature method.23 All
2-(imidazole-2-yl)phenol derivatives were prepared using a
modified literature procedure24 except for 2,4-di-tert-butyl-
6-(4,5-diphenyl-1H-imidazol-2-yl)phenol (L6), which was
prepared according to its reported procedure.25 The synthe-
sized organic compounds were purified on a silica gel column
to exclude impurities. Elemental analyses were performed on
a Flash EA 1112 microanalyzer. In the cases where there were
difficulties of getting satisfactory data for elemental analysis
(due to incorporation of the solvent in solid samples), the
compounds were dissolved in THF followed by the addition
of 5 mL of heptane, evaporation, and vacuum drying for 4 h.
1H and 13C NMR spectra were recorded on a Bruker ARX-
400 MHz instrument using TMS as an internal standard. IR
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(11) Markle, T. F.; Rhile, I. J.; DiPasquale, A. G.; Mayer, J. M. Proc.
Natl. Acad. Sci. U. S. A. 2008, 105, 8185–8190.
(12) Rhile, I. J.; Markle, T. F.; Nagao, H.; DiPasquale, A. G.; Lam, O. P.;
Lockwood, M. A.; Rotter, K.; Mayer, J. M. J. Am. Chem. Soc. 2006, 128,
6075–6088.
(13) Lipscomb, W. N.; Strter, N. Chem. Rev. 1996, 96, 2375–2434.
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Tsai, Z.-W.; Chen, C.-T. Org. Lett. 2003, 5, 1261–1264.
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N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147–5157.
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Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9135
spectra were recorded on a Nicolet 6700 FT-IR spectrometer
as KBr discs in the range of 4000-600 cm-1. The steady-state
fluorescent spectra were measured on an F4500-FL fluores-
cence spectrophotometer; meanwhile, fluorescence lifetimes
were obtained using the time-correlated single-photon count
technique (Edinburgh Analytical Instruments F900 fluores-
cence spectrofluorimeter). Thin films of the samples were
prepared on quartz slides (1 cm) through spin-coating. Fluor-
escence quantum yields (ΦF) were calculated using the com-
parative method26,27 using anthracene inethanol(ΦF = 0.27)
as a standard.
7.6 Hz, 1H); 6.95 (dd, J = 8.4 Hz, 1H). 13C NMR (100 MHz,
TMS, CDCl3): 131.35, 130.43, 129.68, 129.01, 128.81, 128.65,
128.03, 127.96, 127.83, 119.69. Anal. Calcd for C21H16N2O: C,
80.75; H, 5.16; N, 8.97. Found: C, 80.66; H, 5.13; N, 8.78.
5-Methoxy-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (L3). In
a similar manner as described for L2, benzil (1.00 g, 4.76 mmol),
2-hydroxy-4-methoxybenzaldehyde (0.72 g, 4.76 mmol), and
ammonium acetate (7.40 g) were treated to obtain white-gray
microcrystals of L3 (1.36 g, 91.4%). Mp. 220-221 °C. Selected
IR peaks (KBr, cm-1): ν 3217s, 3060m, 2999w, 1627s, 1602s,
1169m, 1143m, 1076m, 699s. 1H NMR (400 MHz, TMS,
CDCl3): δ 12.5-13.5 (s, br, 1H); 9.0-9.2 (s, br, 1H); 7.56
(s, br, 4H); 7.36 (m, br, 7H); 6.63 (d, J = 2.5 Hz, 1H); 6.50
(dd, J = 2.5, 8.6 Hz, 1H); 3.84 (s, 3H). 13C NMR (100 MHz,
TMS, CDCl3): 161.74, 159.28, 146.12, 128.72, 127.75, 124.09,
106.59, 105.79, 101.95, 55.37. Anal. Calcd for C22H18N2O2: C,
77.17; H, 5.30; N, 8.18. Found: C, 77.30; H, 5.36; N, 8.08.
4-Methoxy-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (L4). In
a similar reaction procedure as described for L2, benzil (0.67 g,
3.21 mmol), 2-hydroxy-5-methoxybenzaldehyde (0.49 g, 3.21
mmol), and ammonium acetate (4.95 g) reacted, and the crude
product was purified on a silica gel column with ethylacetate/
petroleum ether (1:4). L4 was obtained as white micro-needles
(0.70 g, 64%). Mp. 158-159 °C. Selected IR peaks (KBr, cm-1):
ν 3204s, 3059w, 3012w, 2829m, 1602m, 1584m, 1500vs, 762vs.
1H NMR (400 MHz, TMS, CDCl3): δ 7.55 (d, J = 6.9 Hz, 4H);
7.35 (m, 6H); 7.01 (m, 2H); 6.87 (dd, J = 2.9 Hz, 9.0 Hz, 1H);
3.80 (s, 3H). 13C NMR (100 MHz, TMS, CDCl3): 152.28,
151.58, 145.46, 128.73, 128.23, 127.93, 127.83, 118.42, 116.73,
112.34, 108.45, 56.12. Anal. Calcd for C22H18N2O2: C, 77.17; H,
5.30; N, 8.18. Found: C, 77.01; H, 5.57; N, 8.02.
!
R
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2
1 -10-A
nx
std
IF xðνÞ dν
,
R
ΦF ¼ ΦF
,
x
,std
1 -10-A
x
IF stdðνÞ dν
,
nstd
IF,x(ν) and IF,std(ν) are fluorescence intensities at wavelength
ν for the sample and the standard, respectively. Ax and Astd
are the absorbance at the excitation wavelength for sample x
andthestandardrespectively.nx andnstd arerefractiveindices
of the solvents employed for the sample and standard, re-
spectively.
Preparation of Ligands. 2-(4,5-Diethyl-1H-imidazol-2-yl)-
phenol (L1). When the modified synthetic procedure of imida-
zole derivatives was employed,24 CH2Cl2 (20 mL), EtOH
(10 mL), catalytic glacial acetic acid (0.5 mL), 3,4-hexanedione
(2.2 mL, 17.52 mmol), and salicylaldehyde (1.93 mL, 17.52 mmol)
were added via syringe to ammonium acetate (20 g, 0.26 mol)
under nitrogen. The reaction mixture was refluxed for 2 h. After
cooling and extraction with dichloromethane (100 mL), the
combined organic extracts were dried and purified on a silica
gel column using dichloromethane/petroleum ether/ethyl acetate
(2:6:1) as the eluent to obtain the target compound in two
fractions as a free molecule (L1, 0.42 g, 11.1%, eluting first)
2,4-Dimethyl-6-(4,5-diphenyl-1H-imidazol-2-yl)phenol (L5).
In a similar reaction procedure as described for L2, benzil
(0.35 g, 1.65 mmol), 2-hydroxy-3,5-dimethylbenzaldehyde
(0.25 g, 1.65 mmol), and ammonium acetate (2.54 g) were
refluxed for 2 h. The products were separated by column
chromatography on silica gel (ethyl acetate/petroleum ether,
1:10) to obtain colorless crystals (0.50 g, 90%). Mp. 171-
172 °C. Selected IR peaks (KBr, cm-1): ν 3325vs, 3022m,
2952m, 2917s, 1603s, 1587s, 1485vs, 1391s, 1232vs, 1033s,
773vs, 695vs. 1H NMR (400 MHz, TMS, CDCl3): δ 9.25
(s, br, 1H); 7.92 (d, J = 7.8 Hz, 2H); 7.68 (s, br, 2H); 7.47 (dd,
J = 7.6 Hz, 4H); 7.39 (M, 4H); 1.57 (s, 6H). C23H20N2O: C,
81.15; H, 5.92; N, 8.23. Found: C, 81.10; H, 5.80; N, 7.88.
2,4-Ditert-butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)phe-
nol (L7). In a similar reaction procedure as described for L2,
phenanthrenequinone (2.00 g, 9.61 mmol), ammonium acetate
(15.00 g), and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (2.25 g,
9.61 mmol) were refluxed for 2 h and purified on a silica gel
column with dichloromethane/petroleum ether (1:20) to obtain
L7 (0.60 g, 14.8%). Mp. (dec.) >320 °C. Selected IR peaks
(KBr, cm-1): ν 3481s, 3053m, 2949vs, 2906s, 2863s, 1616s,
and as an acetic acid adduct (L1 CH3COOH, 0.68 g, 14.0%,
3
eluting second). Analytical data are as follows. L1: Mp. 154-155
°C. Selected IR peaks (KBr, cm-1): ν 3256vs, 2964vs, 2930s,
2871m, 1610s, 1588vs, 1490vs, 1388vs, 1261vs, 1134m, 692m. IH
NMR (400 MHz, TMS, CDCl3): δ 7.36 (dd, J = 1.3, 7.8 Hz, 1H);
7.20 (dd, J = 7.1, 8.4 Hz, 1H); 7.20 (d, J = 8.3 Hz, 1H,); 7.03
(d, J = 7.7Hz, 1H); 6.83(dd, J = 7.3Hz, 1H); 2.60(q, J = 7.6 Hz,
4H); 1.25 (t, J = 7.6 Hz, 6H). 13C NMR (100 MHz, TMS,
CDCl3): 156.55, 141.73, 131.01, 128.91, 125.97, 118.52, 117.37,
109.76, 17.41, 13.97. Anal. Calcd for C13H16N2O: C, 72.19;
H, 7.46; N, 12.95. Found: C, 72.45; H, 7.36; N, 12.90.
L1 CH3COOH: Mp. 152-153 °C. Selected IR peaks (KBr,
3
cm-1): ν 3253vs, 2964vs, 2932s, 2873m, 1710vs, 1651s, 1611vs,
1588vs, 1390vs, 1261vs, 1133m, 692m. IH NMR (400 MHz, TMS,
CDCl3): δ 7.42 (d, J = 1.3, 7.8 Hz, 1H); 7.16 (dd, J = 1.5, 7.7 Hz,
1H); 7.00 (d, J = 7.7 Hz, 1H,); 6.79 (dd, J = 7.7 Hz, 1H); 2.59
(q, J = 7.6 Hz, 4H); 2.18 (s, 3H); 1.25 (t, J = 7.6 Hz, 6H). 13
C
NMR (100 MHz CDCl3, TMS): 177.51, 156.55, 141.73, 131.01,
128.91, 125.97, 118.52, 117.37, 109.76, 31.97, 22.60, 17.41.
2-(4,5-Diphenyl-1H-imidazol-2-yl)phenol (L2). To a solution
of benzil (2.00 g, 9.51 mmol) and ammonium acetate (14.67 g, 20
equivalent) in refluxing glacial acetic acid (20 mL) was added
salicylaldehyde (1.00 mL, 9.51 mmol), and it was refluxed for a
further 2 h. The reaction mixture was allowed to cool, trans-
ferred to 40 mL of water, carefully neutralized with concen-
trated aqueous ammonia, and the crude product filtered. After
washing with water, the dried solid was recrystallized from
ethanol to yield L2 (2.10 g, 71%). Mp. 200-201 °C. Selected
IR peaks (KBr, cm-1): ν 3211s, 3057m, 1601s, 1539m, 1137m,
1071m, 692s. 1H NMR (400 MHz, TMS, CDCl3); δ12.83 (br, s,
1H); 9.37 (br, s, 1H); 7.70-7.50 (br, 4H); 7.49 (d, J = 7.6 Hz,
1H); 7.50-7.30 (br, 6H); 7.32 (dd, J=8.4 Hz, 1H); 7.12 (d, J =
1
1543m, 1510m, 1469vs, 1442vs, 1243s, 747vs. H NMR (400
MHz, TMS, CDCl3): δ 13.48 (s, 1H); 9.92 (s, 1H); 8.77 (d, J =
8.1 Hz, 1H); 8.69 (dd, J = 8.2 Hz, 2H); 8.13 (d, J = 7.7 Hz, 1H);
7.72 (m, 2H); 7.66 (m, 2H); 7.51 (s, 1H); 7.47 (s, 1H). Anal. Calcd
for C29H30N2O: C, 82.43; H, 7.16; N, 6.63. Found: C, 82.32; H,
7.10; N, 6.53.
Preparation of Complexes. Bis(2-(4,5-diethyl-1H-imidazol-
2-yl)phenoxy)zinc (Z1). A solution of 2-(4,5-diethyl-1H-imida-
zol-2-yl)phenol (L1; 0.11 g, 0.51 mmol) was made in dichlor-
omethane (2 mL), and zinc acetate dihydrate (56.00 mg, 0.25
mmol) in ethanol (2 mL) was layered over the dichloromethane
solution. The reaction system was covered and kept standing for
two weeks; complex Z1 was obtained as colorless crystals (80.00
mg, 29%). Mp./dec 330-332 °C. Selected IR peaks (KBr,
cm-1): ν 3170m, 3116, 3028m, 2967s, 1622s, 1557s, 1481vs,
1305vs, 1254vs, 1140vs, 760. Anal. Calcd for C26H30N4O2Zn:
C, 62.97; H, 6.10; N, 11.30. Found: C, 62.82; H, 6.02; N, 11.17.
ꢀ
ꢁ
(26) Hrdlovic, P.; Kollar, J.; Chmela, S. J. Photochem. Photobiol. A:
Chem. 2004, 163, 289–296.
(27) Dawson, W. R.; Windsor, M. W. J. Phys. Chem. 1968, 72, 3251–3260.

9136 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Bis(2-(4,5-diphenyl-1H-imidazol-2-yl)phenoxy)zinc (Z2). 2-(4,5-
Diphenyl-1H-imidazol-2-yl)phenol (L2; 0.26 g, 0.83 mmol) and
zinc acetate dihydrate (0.09 g, 0.42 mmol) were dissolved in 20
mL of ethanol and refluxed at 70 °C for 3 h. The resulting
precipitate was filtered, washed with a few milliliters of ethanol,
and dried to obtain Z2 as white microcrystals (0.26 g, 88%).
Mp./dec 336-338 °C. Selected IR peaks (KBr, cm-1): ν 3621s,
3053m, 1602s, 1554m 1532m, 1305vs, 1246s, 696vs. Anal. Calcd
for C42H30N4O2Zn: C, 73.31; H, 4.39; N, 8.14. Found: C, 73.13;
H, 4.41; N, 7.98.
-100 °C. Cell parameters were obtained by global refinement of
the positions of all collected reflections. Intensities were cor-
rected for Lorentz and polarization effects and empirical (Z2,
L1, and L5) or numerical (Z1) absorptions. The structures were
solved by direct methods and refined by full-matrix least squares
on F2. All non-hydrogen atoms were refined anisotropically,
and all hydrogen atoms were placed in calculated positions.
Structure solution and refinement were performed using the
SHELX-97 package.28 Crystallographic data of compounds L1,
L5, Z1, and Z2 are summarized in Table 1.
Bis(5-methoxy-2-(4,5-diphenyl-1H-imidazol-2-yl)phenoxy)zinc
(Z3). 5-Methoxy-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (L3;
0.30 g, 0.88 mmol) and zinc acetate dehydrate (0.10 g, 0.44
mmol) were dissolved in 20 mL of ethanol and stirred at room
temperature to produce Z3 as a white powder (0.24 g, 73%).
Mp./dec 356-358 °C. Selected IR peaks (KBr, cm-1): ν 1606vs,
1533m, 1538m, 1490s, 1442s, 1335m, 1209vs, 1144s, 697vs Anal.
Calcd for C44H34N4O4Zn: C, 70.64; H, 4.58; N, 7.49. Found: C,
70.57; H, 4.71; N, 7.56.
Results and Discussion
Synthesis of Ligands. 2-Hydroxy-3,5-dimethylbenzalde-
hyde23 and the imidazolyl ligands24,25 were prepared by
modified procedures of reported methods. A condensation
reaction of R-dicarbonyls and aldehydes was conducted in
the presence of excess ammonium acetate as the source of the
imidazole nitrogen atoms. In order to alter the electronic
environments of the five-membered imidazole and six-mem-
bered phenol rings of the 2-(1H-imidazol-2-yl)phenol nu-
cleus, a rational reaction of appropriate salicylaldehyde
derivatives with selected R-dicarbonyls was considered for
the preparation of L1-L7. A general synthetic sketch of the
ligands is presented in Scheme 2.
During condensation of 3,4-hexandione and salicylal-
dehyde in attempts to prepare 2-(4,5-diethyl-1H-imida-
zol-2-yl)phenol (L1), employing glacial acetic acid as a
catalytic reagent as well as a reaction medium was found
to be unfavorable. This is probably due to the fate
suffered by the dicarbonyl in the presence of such a large
amount of acetic acid. Employing ethanol/dichloro-
methane (1:1) as a reaction medium in the presence of a
catalytic amount of glacial acetic acid (0.5 mL) afforded
L1 in acceptable yields (total 25%) as a free ligand (L1)
Bis(4-methoxy-2-(4,5-diphenyl-1H-imidazol-2-yl)phenoxy)zinc
(Z4). In the same manner as for Z2, 4-methoxy-2-(4,5-diphenyl-
1H-imidazol-2-yl)phenol (0.36 g, 1.06 mmol) was reacted with
zinc acetate dihydrate (0.12 g, 0.53 mmol) to obtain Z4 as a
white powder (0.32 g, 70.7%). Mp./dec 310-312 °C. Selected IR
peaks (KBr, cm-1): ν 3627m, 3053m, 2951m, 2835m, 1591s,
1568s, 1493vs, 1229vs, 1136s, 1042s, 697s. Anal. Calcd for
C44H34N4O4Zn: C, 70.64; H, 4.58; N, 7.49. Found: C, 70.73;
H, 4.85; N, 7.34.
Bis(2,4-dimethyl-6-(4,5-diphenyl-1H-imidazol-2-yl)phenoxy)zinc
(Z5). In a similar manner as for Z1, 2,4-dimethyl-6-(4,5-diphen-
yl-1H-imidazol-2-yl)phenol (L5; 0.13 g, 0.39 mmol) was reacted
with zinc acetate dihydrate (42.00 mg, 0.19 mmol) to obtain
microcrystalline solids of Z5 (0.50 g, 90%). Mp./dec 302-304
°C. Selected IR peaks (KBr, cm-1): ν 3223br,s, 3054m, 2965m,
2916m, 2859w, 1613s, 1590s, 1476vvs, 1247vs, 1049s, 734vs,
696vs. Anal. Calcd for C46H38N4O2Zn: C, 74.24; H, 5.15; N,
7.53. Found: C, 74.20; H, 5.42; N, 7.42.
and an acetic acid adduct (L1 CH3COOH). There were
3
Bis(2,4-di-tert-butyl-6-(4,5-diphenyl-1H-imidazol-2-yl)phenox-
y)zinc (Z6). 2,4-Di-tert-butyl-6-(4,5-diphenyl-1H-imidazol-2-
yl)phenol (L6; 0.21 g, 0.50 mmol) in THF (2 mL) and zinc
acetate dihydrate (54.91 mg, 0.25 mmol) in ethanol (5 mL) were
mixed and refluxed for 1 h in the presence of triethylamine (1.4
mL). On cooling, the precipitate was filtered and washed with a
little ethanol and then dried to afford Z6 as yellowish micro-
crystals (64.70 mg, 27%). Mp. 319-320 °C. Selected IR peaks
(KBr, cm-1): ν 3660m, 3476m, 3200br,s, 3052m, 2949vs, 2902s,
2865s, 1607s, 1590m, 1527vs, 1326s, 1259vs, 1145s, 773vs, 694vs.
Anal. Calcd for C58H62N4O6Zn: C, 76.34; H, 6.85; N, 6.14.
Found: C, 76.20; H, 6.97; N, 6.58.
signals at δ 2.18(s, 3H) ppm, 177.5 ppm, and 1710 cm-1
1
respectively in the H NMR, 13C NMR, and IR spectra
assigned to the acetic acid fragment of L1 CH3COOH. A
3
similar observation was previously reported.29 In the
preparation of 2,4-di-tert-butyl-6-(1H-phenanthro[9,10-
d]imidazol-2-yl)phenol (L7), a relatively low yield (15%)
was recorded for the reaction of phenanthrenequinone
with 3,5-di-tert-butyl-2-hydroxybenzaldehyde, and this is
due to the competing formation of the oxazole analogue,
which was typically isolated in greater yields than the
target imidazole ligand. All organic compounds were
characterized by NMR, IR, and elemental analysis. Melt-
ing point analysis revealed a rather high melting point/
decomposition temperature value for L7 (>320) and
probably suggests the existence of a zwitterionic form in
the solid state.
Bis(2,4-di-tert-butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)-
phenoxy)zinc (Z7). In a similar manner as for Z6, 2,4-di-tert-
butyl-6-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol (L7; 0.20
g, 0.47 mmol) was reacted with zinc acetate dihydrate (52.00 mg,
0.24 mmol) to form Z7 as yellowish microcrystals (50.30 mg,
21%). Dec. 408-410 °C. Selected IR peaks (KBr, cm-1): ν
3054m, 2955vs, 2907s, 2868s, 1574vvs, 1471vvs, 1399vvs, 1330s,
1260s, 1023s, 755s. Anal. Calcd for C58H58N4O2Zn: C, 76.68; H,
6.43; N, 6.17. Found: C, 76.91; H, 6.54; N, 6.02.
Synthesis of Complexes. Neutral zinc complexes con-
taining two ligands are generally recognized as potentially
important candidates for the preparation of excellent elec-
troluminescent materials.1,2,6,7,21 The challenge associated
with obtaining suitable single crystals for bidentate zinc
complexes was recently discussed21,30 and has necessitated
X-Ray Measurements. Single crystals of L1 and L5 were
obtained by slow evaporation of their ethanol and dichloro-
methane solutions, respectively. Single crystals of complex Z1
were obtained during its synthesis by layering an ethanol solu-
tion of zinc acetate over the dichloromethane solution of L1.
Slow evaporation of the solvent from a methanol solution of
Z2 yielded suitable single crystals. Suitable crystals of com-
pounds L1, L5, and Z2 were mounted on a Rigaku R-AXIS
Rapid diffractometer, while compound Z1 was mounted on a
Rigaku Saturn 724 diffractometer. Both diffractometers employ
graphite-monochromated Mo KR radiation and operated at
::
::
(28) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen,
Germany, 1997.
(29) Wen, H.-L.; He, M.; Liu, C.-B. Acta Crystallogr. 2008, E64, o1949–
o1949.
(30) Tong, Y.-P.; Zheng, S.-L.; Chen, X.-M. Inorg. Chem. 2005, 44, 4270–
4275.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9137
Table 1. Crystal Data and Processing Parameters for L1, L5, Z1, and Z2
L1
L5 CH3CH2OH
3
Z1 CH3CH2OH
3
Z2 3CH3OH
3
formula
fw
temp (K)
C
216.28
173(2)
0.71073
monoclinic
Cc
13.748(3)
20.316(4)
13.030(3)
90.00(0)
99.34(3)
90.00(0)
3591.4(12)
12
1.200
13H16N2O
C23H20N2O C2H6O
386.48
173(2)
0.71073
triclinic
P1
8.2745(2)
11.232(2)
12.311(3)
73.11(3)
73.80(3)
88.26(3)
1049.7(4)
2
1.223
C26H30N4O2Zn C2H6O
495.91
173(2)
0.710747
triclinic
P1
11.207(2)
11.295(2)
12.353(3)
101.06(3)
110.62(3)
102.46(3)
1366.2(5)
2
C42H30N4O2Zn C3H12O3
784.22
173(2)
0.71073
triclinic
P1
12.401(3)
12.590(3)
14.729(3)
67.34(3)
67.89(3)
75.34(3)
1950.3(8)
2
1.335
3
3
3
˚
wavelength (A)
cryst syst
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
volume (A )
Z
calcd density (g m-3
)
1.206
0.685
784
3
μ (mm-1
F(000)
)
0.077
1392
0.078
412
0.681
820
cryst size (mm)
θ range (deg)
limiting indices
0.90 ꢀ 0.35 ꢀ 0.15
1.81-27.48
-17 e h e þ17
-23 e k e þ26
-16 e l e þ16
7181
7181
0.0422
99.8, θ = 27.48
447
1.059
R1 = 0.0720
wR2 = 0.1373
R1 = 0.1165
wR2 = 0.1517
0.483, -0.628
0.70 ꢀ 0.40 ꢀ 0.26
1.80-27.48
-10 e h e þ10
-14 e k e þ14
-15 e l e þ15
8674
4798
0.0313
99.6, θ = 27.48
277
1.281
R1 = 0.0734
wR2 = 0.1697
R1 = 0.0983
wR2 = 0.1876
0.281, -0.518
0.35 ꢀ 0.14 ꢀ 0.13
1.93-27.49
-14 e h e þ14
-14 e k e þ14
-16 e l e þ16
16816
6209
0.0367
98.8, θ = 27.49
298
1.122
R1 = 0.0566
wR2 = 0.1665
R1 = 0.0636
wR2 = 0.1739
0.514, -0.823
0.58 ꢀ 0.57 ꢀ 0.39
1.95-25.00
-12 e h e þ12
-11 e k e þ11
-26 e l e þ26
12723
6853
0.0286
99.69, θ = 25.00
511
1.106
R1 = 0.0431
wR2 = 0.0917
R1 = 0.0550
wR2 = 0.0957
0.302, -0.378
no. of rflns collected
no. of unique rflns
R(int)
completeness (%) to θ (deg
no. of params
)
goodness-of-fit on F2
final R indices (I > 2σ(I))
R indices (all data)
-3
˚
largest diff. peak and hole (e A
)
Scheme 2. Synthesis of Ligands L1-L7
Scheme 3. Synthesis of the Zinc Complexes
attempts through high-vacuum sublimation of the powder
complexes,6 hydrothermal methods of complex prepara-
tion, solvent diffusion or evaporation, and so forth. How-
ever, it may not pose such a difficulty to obtain the zinc
complex crystals. After obtaining the above imidazolyl--
phenol derivatives (L1-L7), their zinc complexes were
easily formed through the direct reactions of such imida-
zolyl-phenol derivatives with zinc salt in solutions
(Scheme 3).
Due to the different substituents, however, the ligands
exhibited various reactivities toward the zinc salt. The
ligands with an unsubstituted phenol ring (L1 and L2) are
capable of forming the respective complexes (Z1 and Z2)
by mixing in ethanol at room temperature. The ligands
(L3 and L4) bearing an electron-donating methoxyl sub-
stituent exhibited faster precipitation of the complexes
(Z3 and Z4), in which the formation of Z4 was faster than
that of Z3 because of the stronger influence of the para-
substituent in L4 than that of the meta-substituent in L3.
With a bulky substituent of the tert-butyl group, ligands
L6 and L7 were most reluctant in the chelation and
required the addition of base to aid the reaction.

9138 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Figure 2. 1D-linkage hydrogen-bonding interactions in L1.
Figure 1. Molecular structures of L1 showing three crystallographically
independent units with thermal ellipsoids drawn at the 50% probability
level. Some hydrogen atoms and labels have been omitted for clarity, and
C37 is disordered.
the imidazole ring. The steric bulk of the two phenyl rings
in L5 would be effective in contributing to the reduced
coplanarity between the imidazolyl and phenol rings. The
bond lengths, angles, and hydrogen-bond parameters are
provided in the Supporting Information.
All complexes were characterized by elemental analysis
and IR spectroscopy. The vibrations of active phenolic
protons of the ligands were absent in their complexes, and
IR peaks around the 1600 cm-1 region were shifted to
lower wavenumbers in the zinc complexes (e.g., 1622
cm-1 and 1600 cm-1 for L1 compared to 1610 cm-1
and 1588 cm-1 for Z1; 1590 cm-1 for L5 compared 1568
cm-1 for Z5; and 1615 cm-1 for L7 compared to 1587
cm-1 for Z7). These are evidence indicating the coordina-
tion of ligand with the zinc center. These zinc complexes
likely decomposed before melting, but Z6 exhibited melt-
ing into light yellow oil without apparent signs of decom-
position. Moreover, the molecular structures of
representative ligands (L1 and L5) and complexes (Z1
and Z2) were confirmed by single-crystal X-ray diffrac-
tion.
Structures. The molecular structures of L1 and L5 in
the solid state were determined by X-ray diffraction, and
both compounds clearly showed interactions between
molecules. Meanwhile, the compound L5 crystallized
along with an ethanol solvate. L1 crystallized in the
monoclinic Cc space group and revealed three indepen-
dent molecules per unit cell (Figure 1), in which C37 is
disordered. The phenol and imidazolyl rings are almost
coplanar with each other, with dihedral angles in the
range of 3.36-4.72°. The bond lengths, angles, and
hydrogen-bond parameters are provided in the Support-
ing Information.
In the solid of L5, there exist discrete dimeric π-π
stackings locked on either side by hydrogen bonds invol-
ving solvent molecules (Figure 4, Table 3). As observed
for L1, the phenol ring of one molecule interacts in an
“offset face-to-face” fashion with the imidazolyl ring of
the partner molecule in the dimeric unit of L5 with an
˚
interplanar separation of 3.484 A. In addition, the in-
tramolecular and intermolecular hydrogen bonds be-
tween molecules of L5 and ethanol solvate involved in
pairing interaction are within expected values (Table 3).
The molecular structure of complex Z1 incorporated
with one ethanol molecule is shown in Figure 5, and its
selected bond lengths and angles are reported in Table 4.
Though the geometry at each zinc center can be best
described as a distorted tetrahedron in which two chelat-
ing ligands are placed in a similar disposition about the
metal center, slight differences still occur: a longer Zn-
˚
(1)-O(1) bond length of 1.946(2) A and smaller dihedral
angle (C(1)-C(6)-C(7)-N(1)) of 1.00° were observed
for one ligand, while the respective values for the second
˚
ligand are 1.918 A for Zn(1)-O(2) and 7.49° for C-
(14)-C(19)-C(20)-N(3). These dihedral angles repre-
sent a relative twist in the phenolate-imidazole ring
planes about the C-C bond joining the rings.
In the packing pattern observed for complex Z1
(Figure 6), the π-π stacking interaction (alternating
˚
˚
face-to-face distances of 3.396 A and 3.501 A) assisted
by solvent-mediated hydrogen bonding (Table 5) held
each complex unit to the left and right neighboring units
along a 1D array. The ethanol lattice solvent in Z1 is
disordered and was squeezed31,32 before writing the final
structure refinement files. The average location of the
solvent is x = 0.500, y = 0.000, and z = 1.000, and its
In the solid of L1, the phenol ring of one molecule
interacts in an “offset face-to-face” fashion with another
imidazolyl ring of a contiguous moiety (Figure 2), and the
˚
interplanar distance is 3.306 A as a π-π interaction. The
intramolecular and intermolecular hydrogen bonds
(Table 2) furnished a 1D array of the ligand molecules,
which is similar to the observation of arrangement in 2-(2-
hydroxyphenyl)benzimidazole and 5-amino-2-(1H-ben-
zoimidazol-2-yl)phenol.21
volume is 224 A-3 with 96 electrons contained.
˚
As encountered for Z1, complex Z2 also bears a
distorted tetrahedral geometry about the zinc center
(Figure 7). The basal plane is composed of O1, N1, and
N3, and the zinc atom deviates from the basal plane by
Unlike for L1, L5 crystallized with incorporating etha-
nol molecules in the triclinic P1 space group (Figure 3), in
which C25 is disordered. The dihedral angle of the phenol
ring with imidazolyl ring is 5.86°, which is notably larger
than that of L1 (3.36-4.72°). Structural differences are
caused by steric effects of ethyl and phenyl substituents on
˚
0.496 A. The dihedral angles between the coordination
(31) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(32) Vandersluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194–201.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9139
Table 2. Hydrogen Bond Parameters for L1a
˚
˚
A (A)
˚
A (A)
D-H (A)
D-H A (deg)
D
H
3 3 3
3 3 3
3 3 3
O(1)-H(1) N(1)
0.840
0.840
0.840
0.880
0.880
0.880
149.09
149.69
149.43
171.34
172.54
172.38
2.581
2.589
2.589
2.894
2.899
2.906
1.824
1.835
1.830
2.020
2.024
2.032
3 3 3
O(2)-H(2) N(3)
3 3 3
O(3)-H(3) N(5)
3 3 3
N(2)-H(2A) O(1)#1
3 3 3
N(4)-H(4A) O(2)#1
3 3 3
N(6)-H(6A) O(3)#2
3 3 3
a Symmetry transformations used to generate equivalent atoms: (#1) x, -y, z þ 1/2; (#2) x, -y, z - 1/2.
Figure 5. Ortep plot of Z1 C2H5OH with thermal ellipsoids drawn at
3
Figure 3. Ortep plot of L5 CH3CH2OH with thermal ellipsoids drawn
3
the 50% probability level. Hydrogen atoms linked on carbon atoms have
been omitted for clarity.
at the 50% probability level. Some hydrogen atoms have been omitted for
clarity, and C(25) of the ethanol molecule is disordered.
˚
Table 4. Selected Bond Lengths (A) and Angles (Degree) for Z1 CH3CH2OH
3
and Z2 3CH3OH
3
Z1 CH3CH2OH
3
Z2 3CH3OH
3
Bond Lengths
Zn(1)-O(2)
Zn(1)-O(1)
Zn(1)-N(1)
Zn(1)-N(3)
1.918(2)
1.946(2)
1.953(3)
1.959(2)
1.9383(19)
1.9392(17)
1.978(2)
1.965(2)
Bond Angles
Figure 4. Discrete π-π stacking interactions observed in the crystal
pack of L5 CH3CH2OH.
3
O(2)-Zn(1)-O(1)
O(2)-Zn(1)-N(1)
O(1)-Zn(1)-N(1)
O(2)-Zn(1)-N(3)
O(1)-Zn(1)-N(3)
N(1)-Zn(1)-N(3)
116.33(10)
115.12(10)
96.13(10)
96.61(10)
109.94(10)
124.00(11)
109.69(8)
110.42(8)
95.05(8)
96.10(8)
121.09(8)
124.51(8)
Table 3. Hydrogen Bond Parameters for L5 CH3CH2OH
3
D-H
A
3 3 3
(deg)
˚
˚
˚
D-H (A)
D
A (A) H A (A)
3 3 3
3 3 3
and angles for Z1 and Z2 are in the range of values
reported for other zinc N-O systems.6,10
In the packing pattern observed for Z2 (Figure 8), each
complex unit interacts with another through hydrogen
bonding via incorporated solvent molecules and forms a
1D array (Table 6).
Absorption Properties. The absorption spectra of all
organic ligands and complexes were recorded under the
same instrument settings and at room temperature (about
23 °C). All ligands exhibit the wavelength absorption
peaks around 320 nm in their solutions (5 ꢀ 10-5 M) of
tetrahydrofuran (THF), methanol, or dimethylforma-
mide (DMF), which can be considered to be absorptions
belonging to π-π* transitions on the basis of extinction
O(1)-H(1O) N(2)
0.903
0.961
1.000
146.39
161.14
177.04
2.584
2.878
2.868
1.783
1.952
1.869
3 3 3
O(2)-H(2O) O(1)
3 3 3
N(1)-H(2N) O(2)
3 3 3
ring atoms on the phenol and imidazole rings of the two
chelated ligands are significantly larger (15.82 and 21.52°)
than those observed for Z1 (1.00 and 7.49°). Moreover,
one coordination plane (N(1)-O(1)-Zn(1)) in Z2 is
approximately perpendicular to the other (N(3)-O(2)-
Zn(1)) with a dihedral angle of 89.38°, which is bigger
than the corresponding value for Z1 (84.00°). The sig-
nificant differences observed are considered to result
from the steric effects between the small ethyl and the
bulky phenyl substituents. Metal-ligand bond lengths

9140 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Figure 6. Continuous 1D-array in Z1 C2H5OH achieved by π-π
3
stacking and ethanol mediated H-bonding.
Table 5. Hydrogen Bond Parameters for Z1 CH3CH2OH
3
D-H
(A)
D-H
A
D
A
H
(A)
A
3 3 3
(deg)
3 3 3
3 3 3
˚
˚
˚
(A)
Figure 7. Molecular structure of Z2 3CH3OH with thermal ellipsoids
3
drawn at the 50% probability level. Some hydrogen atoms and three
methanol solvent molecules have been omitted for clarity.
N(4)-H(4A) O(1)
3 3 3
0.880
0.950
0.950
140.08
101.68
101.82
2.788
2.883
2.886
2.056
2.537
2.538
C(5)-H(5) N(2)
3 3 3
C(18)-H(18) N(4)
3 3 3
Table 6. Hydrogen Bond Parameters for Z2 3CH3OH
3
coefficients (ελmax, Table 7). The solvents are selected on
the basis of their varying dielectric constants (THF, 7.5;
MeOH, 33; DMF, 38) and protic/aprotic natures. In
general, absorption peaks were slightly blue-shifted in
methanol compared to those in THF. When the absorp-
tion positions of ligands in THF and DMF were com-
pared, the peaks hardly showed a response to solvent
polarities (i.e., 0 - 2 nm shifts), except for L1, with a shift
of 13 nm. Polarities of the solvents might not be impor-
tant in affecting electronic properties of the compounds,
and the preliminary observation is in agreement with the
prominence of a π-π* nature of transition to the excited
states30,33 rather than n-π*. Considering the 2-(imidazol-
2-yl)phenol nucleus as the fundamental skeleton of the
chromophores, absorption data shows that conjugation
of the π system was extended by a methoxy substituent
para to the hydroxyl group or by the fused aromatic
phenanthrene group on the 4,5 positions of the imidazole
ring, leading to bathochromic shifts (≈24 nm) and hy-
perchromism (L1 versus L4 and L7; Table 7). All ligands
generally presented a red shift on coordination to zinc,
except in Z7, which was mainly unaffected. Zinc com-
plexes did not seem to follow a definite trend on the basis
of solvent or substituent effects. In the solid thin films, the
ligands generally maintained similar absorption spectral
positions to those of the solution except for the t-Bu
substituted ligands (L6 and L7), which were significantly
red-shifted. Apart from highlighting the role of the
phenol ring in the transitions, this observation suggests
that extensive interactions exist in solution similar
to observed intermolecular interactions in the analyzed
solid-state structures and also reveal the vibronic
nature of t-Bu substituted L6 and L7, since lesser vibra-
tion in the solid state probably enabled stronger hyper-
conjugation.
D-H
(A)
D-H
A
D
A
H
(A)
A
3 3 3
(deg)
3 3 3
3 3 3
˚
˚
˚
(A)
O(5)-H(O5) O(3) 0.782
155.32
168.49
177.88
177.40
171.58
99.65
2.711
2.679
2.855
2.701
2.870
2.916
1.981
1.933
1.976
1.923
1.997
2.602
3 3 3
O(4)-H(O4) O(1) 0.757
3 3 3
N(2)-H(2A) O(5) 0.879
3 3 3
O(3)-H(O3) O(2) 0.778
3 3 3
N(4)-H(4A) O(4) 0.880
3 3 3
C(5)-H(5A) N(2) 0.951
3 3 3
presented in Table 8 and Figure 9a-f, respectively. In
Figure 9, spectral traces of ligands and their correspond-
ing zinc complexes in the same solvent have been plotted
on identical coordinates to show changes in spectral
intensities (I) of ligands in relation to the chelated analo-
gues. In THF and methanol, an additional near-infrared
emission band was observed around 800 nm for L1 and
L2, which was enhanced and blue-shifted on coordination
to zinc in Z1 and Z2. Luminescence lifetimes at all
emission bands indicate a singlet excited state nature
rather than a triplet one.
In general, the fluorescence lifetimes of the ligands L1-L7
are longer coupled with higher-emission intensities than the
respective zinc chelates. For L1, L2, Z1, and Z2, which
possess two emission bands, the fluorescence lifetime mon-
itored for both emission bands revealed similar lifetime
values. L1, which is considered to best represent the parent
2-(imidazole-2-yl)phenol fluorophores nucleus of the series
except for the 4,5-diethyl substitution, exhibited the strongest
emission intensity with a quantum yield as high as 0.57 in
DMF. The narrow blue emission of L1 is also notable, with a
width at half-maximum (fwhm) of 2818-2870 cm-1 in all
solvents. The presence of substituents on the basic
2-(imidazole-2-yl)phenol framework only leads to a decrease
of fluorescence quantum yields, with substituents on the
phenol ring being more important in deciding the extent of
emission intensity reduction. Generally, L1, L2, L3, and L7
exhibited higher quantum yields compared to L4, L5, and
L6. Relative to L1, the spectra of ligands in THF (Figure 9a;
Table 8) show a gradual decrease in photoluminescence
properties coupled with an increasing peak width for
L2 (4,5-diphenyl-substituted; fwhm = 3058 cm-1) and L7
(phenanthro-substituted with two t-Bu added to the phenol
ring; fwhm = 2978 cm-1). Similar trends were observed in
methanol and DMF solutions, except that the L7 emission
Ligand Fluorescent Properties. The solutions of L1-L7
and Z1-Z7 were prepared at the same concentration,
and excitation and emission experiments were all con-
ducted under the same spectrophotometer settings at
room temperature to enable a direct comparison of
spectral results. Fluorescence data and comparative over-
lays of emission spectra of all synthesized compounds are
(33) Park, S.; Seo, J.; Kim, S. H.; Park, S. Y. Adv. Funct. Mater. 2008, 18,
726–731.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9141
Figure 8. 1D network formed by Z2 3CH3OH via H-bonding.
3
Table 7. Absorption Properties of L1-L7 and Z1-Z7
media
ligands
λabs-max (nm)
ε
(λmax) (M-1.cm-1
)
complexes
λabs-max (nm)
ε
(λmax) (M-1.cm-1
)
THF
L1
313
311
326
331
320
316
321
322
318
316
320
323
337
335
337
337
325
323
326
327
322
322
322
343
337
335
336
381
15120
18320
15360
Z1
342
329
341
329
350
336
346
23080
22460
25740
MeOH
DMF
solid
THF
L2
L3
L4
L5
L6
L7
29420
29680
18580
Z2
Z3
Z4
Z5
Z6
Z7
28000
37840
31940
MeOH
DMF
solid
THF
43400
26720
22580
343
331
341
338
338
359
372
44120
46840
42200
MeOH
DMF
solid
THF
22240
21160
19080
37450
27720
25160
MeOH
DMF
solid
THF
27680
25340
23000
326
343
345
332
323
336
339
341
338
335
336
371
31080
26100
25280
MeOH
DMF
solid
THF
30840
23020
23280
39260
29220
26040
MeOH
DMF
solid
THF
28200
28600
27180
35620
37520
23000
MeOH
DMF
solid
intensity became slightly higher than that for L2 (Figure 9c
and e). Rate constants for radiative (kr) and nonradiative
(knr) processes were calculated and are presented in Table 8.
It is interesting to observe that the pendant diphenyl substit-
uents on the 4- and 5-imidazolyl positions of L2-L6 ob-
viously caused larger nonradiative decay rate constants at
the expense of the radiative decay rates (Table 8; kr and knr;
L2-L6 versus L1, L7 and Z2-Z6 versus Z1 and Z7). This is
attributable to vibrational effects. Substitution of the meth-
oxyl group at the mposition to the phenolic OH group for L3
caused a reduction of nonradiative decay rates relative to L4
with the methoxy group in the p position. Generally, the
complexes showed relatively larger knr values than the free
ligands, which probably reflect a stronger vibronic nature for
the excited state of the complexes. Solvent environments did
not seem to produce a definite trend in affecting kr and knr
values.
by the various substituents could be concluded. There-
fore, the effect of substituents on reducing emission
capacity could be attributed to vibrational quenching
or a loss of conjugation between imidazole and phenol
rings through a loss of coplanarity, especially in the cases
bearing the bulky phenyl or t-Bu substituents (Figure 9a,
c, and e). This is also supported by the relatively larger
ellipsoids observed in the solid state for the distal phenyl
ring carbons of coordinated L2 (C18, C19; Figure 7). A
more extensive conjugated system is formed by virtue of
the phenanthrene group in L7, which is evidenced by a
consistently large spectral red shift. L7 is thus electro-
nically robust and different in the series. Consequently,
spectral parameters for L7 are largely unperturbed
in different solvents, unlike for L1-L6 (Table 8 and
Figure 9:a, c, and e).
On the basis of the L2 molecular framework, L3-L6
molecular structures are only different by substituents R2,
R3, and R4 (Scheme 2). These substituents all caused the
quantum yield to decrease greatly (Table 8), especially
with R2 and R4 having greater influence. The alkyl or
-OMe groups can lead to some extent of loss of copla-
narity between the five-membered imidazole ring and
Emission intensities decreased greatly for L4, L5, and
L6 in the three solvents and were mainly red-shifted
relative to L1. However, L3 has a moderate emission at
almost the same position as L1 in THF and MeOH, or
even blue-shifted in DMF. Regarding the general red shift
with respect to L1, some form of extension of conjugation

9142 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Table 8. Solution and Solid Emission Data for L1-L7 and Their Zinc Complexes (Z1-Z7)
Ligands
Zinc complexes
λmaxEm
(nm)
λmaxEx
(nm)
Δλa
(nm)
τ
kr ꢀ 108 knr ꢀ 108
λmaxEm λmaxEx
Δλa
kr ꢀ 108 knr ꢀ 108
Media
(ns) ΦF
(S-1
)
(S-1
)
(nm)
(nm)
(nm) τ(ns) ΦF
(S-1
)
(S-1
)
THF
L1 434b
325
324
320
327
344
341
340
330
348
340
356
337
355
339
371
337
348
346
365
343
349
343
361
342
361
359
369
377
109
4.0 0.45
3.3 0.57
4.2 0.56
1.125
1.727
1.333
1.375
1.303
1.048
Z1 387b
364
328
363
353
375
362
374
368
374
360
369
366
384
388
397
372
373
371
382
375
367
373
377
410
378
372
372
386
23
50
23
62
31
32
28
26
32
34
38
49
57
40
43
49
48
47
39
42
52
50
44
33
3.9 0.19
2.4 0.19
1.0 0.26
0.487
0.792
2.600
2.077
3.375
6.634
MeOH
DMF
solid
353, 416b
427
418
29, 92
107
91
105
89
103
114
95
73
52
378b
386
415
THF
L2 449b
2.4 0.26
2.4 0.39
2.4 0.28
1.083
1.625
1.167
3.083
2.542
3.000
Z2 406b
394b
402
0.9 0.24
1.0 0.22
1.0 0.20
2.667
2.200
2.000
8.444
7.800
8.000
MeOH
DMF
solid
430b
443
444
395
THF
L3 443
413
408
2.9 0.13
2.2 0.23
1.3 0.11
0.448
1.045
0.846
3.000
3.500
6.846
Z3 406
394
407
1.1 0.23
1.2 0.25
1.4 0.24
2.091
2.083
1.714
7.000
6.250
5.429
MeOH
DMF
solid
422
85
24, 145 1.3 0.06
34, 128 1.0 0.13
415
THF
L4 379, 500
373, 467
443
0.462
1.300
0.231
7.231
8.700
7.462
Z4 441
428
440
1.1 0.08
1.1 0.12
1.3 0.17
0.727
1.091
1.308
8.364
8.000
6.385
MeOH
DMF
solid
72
1.3 0.03
486
149
123
106
55
122
120
113
61
132
122
112
115
104
421
THF
L5 471
452
420
2.3 0.08
1.2 0.05
0.8 0.01
0.348
0.417
0.125
4.000
7.917
12.375
Z5 421
418
421
2.0 0.15
0.8 0.09
0.9 0.12
0.750
1.125
1.333
4.250
11.375
9.777
MeOH
DMF
solid
465
417
THF
L6 469
456
422
2.5 0.07
1.3 0.07
1.1 0.01
0.280
0.538
0.090
3.720
7.154
9.000
Z6 419
423
421
2.3 0.09
0.9 0.15
0.9 0.15
0.391
1.667
1.667
3.957
9.444
9.444
MeOH
DMF
solid
474
443
THF
L7 483
471
484
2.9 0.34
1.9 0.25
3.1 0.35
1.172
1.316
1.129
2.276
3.947
2.097
Z7 432, 466
468
482
54, 88 2.9 0.15
96
110
62
0.517
1.421
0.935
2.931
3.842
2.290
MeOH
DMF
solid
1.9 0.27
3.1 0.29
481
448
a Δλ = Stokes shift b Additional emission was observed around 800 nm.
phenol ring, which results in a weaker fluorescence. For
instance, the X-ray structure of L1 showed an imidazo-
le-phenol plane tilt of 3.36° (between plane of C35-C34-
C33-N5-N6 and plane C27-C32-C28-C29-C31-
C30); meanwhile, 5.86° (between N1-N2-C7-C8-C9
and C1-C2-C3-C4-C5-C6) was observed for L5 with
a dimethyl substituent. The drastic and progressive lower-
ing of the emissive property in going from L2 (R2 =
R3 = R4 = H) to L5 (R2 = R4 = Me; R3 = H) and L6
(R2 = R4 = t-Bu) (Figure 9a, c, and e) could be attributed to
the role of vibrational deactivation and the importance of the
phenol ring in the electronic transitions.
Although both L3 and L4 bear the methoxyl group on
the phenol ring, the substituent effects differ due to the
substituent position. Particularly, it is noteworthy that a
methoxyl substituent at the para position from a hydroxyl
group in L4 resulted in a significant decrease in radiative
relaxation in all solvents compared to that in L2 and in
contrast to the case for L3 with the same substituent at the
meta position.
The spectra of L1 in methanol and those of L4 in
methanol and THF (Figure 10a, b) show emission bands
that significantly overlapped with the respective longer-
wavelength excitation bands (Stokes shifts within 27-34
nm; Table 8). Therefore, photorelaxation for L1 and L4 in
the mentioned media gave emissions originating from
both the primary enol excited state and the excited state
tautomer. It could be concluded that phototautomeriza-
tion is, to some extent, hindered in the affected systems.
The rapid and total conversion of the excited state enol
form into the excited state keto tautomer, which has
hitherto been concluded by kinetic,11 theoretical,21,30,34
and previous experimental findings,12,20-30,33-35 was not
strictly followed in the current work. Emission bands for
L4 with a Stokes shift showed a value of 72 nm in DMF,
145 nm in MeOH, and 128 nm in THF belonging to
ESIPT emissions. The lower Stokes shift bands appeared
alongside the ESIPT bands in THF and MeOH, which
corresponds to a significant primary photorelaxation for
L4 in THF and MeOH (Table 8). Therefore, one obvious
factor responsible for this unusual phenomenon is a
reduction in overlap between energies of excited enol
configuration and the excited keto tautomer configura-
tion (Scheme 4). The effect of solvent in affecting the
transformation of excited state configurations in the
concerned solutions is such that conversion to the keto
tautomer is less favored. A potential barrier to intramo-
lecular proton transfer of the ligands is due to intermo-
lecular hydrogen bonding or proton transfer between the
ligand and solvent (e.g., between the imidazole nitrogen
base and methanol proton or between THF and the
phenolic proton). However, the exact role of the solvent
is not yet clear. Measurements of fluorescence lifetime
gave undistinguishable values at both excited enol and
excited keto tautomer emissions, which suggests that the
relaxation process depends on the population of enol
tautomer and supports a conclusion about a delayed
ESIPT process. Solid-state emission positions for the
(34) Zheng, S.-L.; Zhang, J.-P.; Chen, X.-M.; Huang, Z.-L.; Lin, Z.-Y.;
(35) Park, S.; Kwon, O.-H.; Kim, S.; Park, S.; Choi, M.-G.; Cha, M.;
Wong, W.-T. Chem.;Eur. J. 2003, 9, 3888–3896.
Park, S. Y.; Jang, D.-J. J. Am. Chem. Soc. 2005, 127, 10070–10074.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9143
Figure 9. Relative emission intensities of ligands L1-L7 and their corresponding complexes Z1-Z7 in THF (a and b), MeOH (c and d), and DMF (e and
f) solutions, respectively.
ligands are generally within the values observed in their
solutions. This suggests a dominance of the extensive
hydrogen bonding interactions over stacking.
Fluorescent Properties of Zinc Complexes. The spectral
overlay of the neutral zinc complexes in THF, methanol,
and DMF are shown in Figure 9b, d, and f, respectively.
All emissions occur with significant blue shifts relative to
corresponding ligands. In order to rationalize the factors
responsible for the blue-shifted fluorescence spectra of
complexes, electronic perturbation of the chelated ligands

9144 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Figure 10. Normalized excitation and emission spectra showing overlap of primary emission bands with excitation spectra for (a) L1 and (b) L4.
Scheme 4. Schematic Representation of the Unusual Photocycle Observed for L1 and L4
can be mentioned. On ligand coordination with Zn2þ, net
valence transfer is from the ligand to the zinc ion. Due to
the transfer of frontier orbital electrons toward the Zn2þ
ion orbital, all local frontier π-orbital energies are lower,
and both the absorption and the emission spectrum bands
should undergo a blue shift. The overlap of excitation and
emission spectral bands coupled with a corresponding
lower Stokes shift both support a loss of phototautomer-
ization possibilities due to the absence of a phenolic
proton (Figure 11, Table 8).36
ligands. Such an observation is contrary to some zinc
complexes reported to consistently intensify,37-39
quench,40 or maintain no influence21,36 on the fluores-
cence of ligands after chelation. Generally, the emission
intensity of Z3 was observed to be higher than that of
other complexes. It is probable that the methoxy group
has the capacity to compensate for the valence electron
transfer of the ligand π orbital to the zinc ion when
located on the meta position relative to the oxo atom. It
is observable that each case of quenching after coordina-
tion was accompanied by a significant blue shift of the
emission band (e.g., L1, L2, and L7; Figure 12a), while the
cases involving enhancement show emission peaks at
roughly the same positions as the free ligands (e.g., L4,
L5, and L6; Figure 12b).
In the series, fluorescence intensities of chelated ligands
can be classified as enhanced (Z3, Z4, Z5, and Z6) or
quenched (Z1, Z2, and Z7) with respect to those of free
::
::
(36) Gorner, H.; Khanra, S.; Weyhermuller, T.; Chaudhuri, P. J. Phys.
Chem. A. 2006, 110, 2587–2594.
(37) Huston, M. E.; Haider, K. W.; Czarnik, A. W. J. Am. Chem. Soc.
1988, 110, 4460–4462.
(39) Ruf, M.; Durfee, W. S.; Pierpont, C. G. Chem Commun. 2004, 2004,
1022–1023.
(38) Koike, T.; Abe, T.; Takahashi, M; Ohtani, K.; Kimura, E.; Shirob,
M. J. Chem. Soc., Dalton Trans. 2002, 2002, 1764–1768.
(40) Sohna, J.-E. S.; Jaumier, P.; Fages, F. J. Chem. Res. (S). 1999, 1999,
134–135.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9145
Considering solvent effects, methanol has almost
no significant influence relative to THF solvent except
for a shortening of lifetimes to some extent for L1-
L6. No regular difference could be observed between
solution and solid emission properties of the zinc complex
series.
Fluorescent Response of L2 and L4 under Zn2þ Titration.
In an effort to explore the potential of this ligand system
as zinc sensors, emission scans were performed on 1 ꢀ
10-4 M solutions of L2 (in MeOH) and L4 (in DMF) in
the presence of 0-5 ꢀ 10-8 M zinc(II) ions. The repre-
sentative ligands studied were selected on the basis of
quenching (L2) and enhancing (L4) behaviors resulting
from coordination to Zn2þ. It was observed that the
emission intensities displayed a significant and linear
response to the presence of Zn2þ even at Zn2þ/ligand
molar ratios as low as 1:10 000 to 5:10 000. These findings
suggest the applicability of the ligand systems for the
quantitative estimation of free Zn2þ in water miscible
solvents such as methanol and DMF.
Figure 11. Excitation and emission spectra of L4 and Z4 in DMF.
Figure 12. (a) Quenching of L1, L2, and L7; (b) enhancement of L4, L5, and L6.
Figure 13. Fluorescence response on Zn2þ titration (0-5 ꢀ 10-8 M) for L2 (a) and L4 (b).

9146 Inorganic Chemistry, Vol. 48, No. 19, 2009
Eseola et al.
Conclusions
processes of the family of ligands. The coordination of zinc
with ligands (L1-L7) caused enhancement or quenching
effects of intensities of fluorescence relative to free ligands.
Electron-rich substituents such as the methoxyl group were
found to be suitable for ensuring better emission intensity in
the zinc complexes. The linear quenching or enhancement of
A series of 2-(imidazole-2-yl)phenol ligands, L1-L7, and
their bis-chelate Zn(II) complexes (Z1-Z7) were prepared
and characterized. Structures of L1, L5, Z1, and Z2 were
confirmed by single-crystal X-ray diffraction experiments.
One-dimensional arrays based on continuous π-π stacking
interactions and hydrogen bonding were observed in the solid
states of L1, L5, and Z1. Z2 also adopted hydrogen-bonding
1D networks without π-π stacking interactions. Photolumi-
nescence properties of the ligands and complexes were
studied and comparatively discussed on the basis of experi-
mentally observed substituent and condensed media effects.
The presence of substituents on the 2-(imidazole-2-yl)phenol
skeleton only leads to a reduction in radiative relaxation
properties, and substituents on the phenol ring are more
relevant for controlling emission properties of the basic
skeleton. L1 should be a potential candidate for organic
emitting applications based on high fluorescence intensitiesin
solid film and solutions. In contrast to the general conclu-
sions on the ESIPT photoprocess for phenols bearing a base
as the ortho substituent, significant photorelaxation from
excited enol states was observed for L1 in methanol and for
L4 in both THF and methanol. Therefore, we can conclude
that there exists a certain unusual hindering factor to en-
ol-keto phototautomerism and that substituents as well as
condensed media play important roles in luminescence
fluorescence observed in the presence of Zn2þ at 10-8
M
orders of concentration and Zn2þ/ligand molar ratios of
1:10000 indicates applicability of these ligand systems for
the quantitative analytical estimation of free Zn2þ samples in
water miscible solvents.
Acknowledgment. The authors thank the National Nat-
ural Science Foundation of China for support of this
research (Grant No. 20773149). A.O.E. is grateful to the
Chinese Academy of Science (CAS) and the Academy of
Science for The Developing World (TWAS) for the
Postgraduate Fellowships and Redeeme’s University for
study leave. Prof. Dr. Joshua A. Obaleye (Department of
Chemistry, University of Ilorin, Ilorin Kwara state,
Nigeria) is thanked for his kind proofreading of the
manuscript.
Supporting Information Available: Crystallographic details
for all structures in the form of CIF files and solid-state
photoluminescence spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.