Syntheses, structures, photoluminescence and theoretical studies of two dimeric Zn(II) compounds with aromatic N,O-chelate phenolic ligands

Article abstract of DOI:10.1016/j.molstruc.2006.04.037

Two neutral and dimeric Zn(II) complexes [Zn₂(epbm)₄] (1) and [Zn₂(bpbm)₄] (2) (Hepbm = 3-ethyl-2-(2-hydroxyphenyl)-3H-benzimidazole, Hbpbm = 3-n-butyl-2-(2-hydroxyphenyl)-3H-benzimidazole) have been prepared an

Full text of DOI:10.1016/j.molstruc.2006.04.037

Journal of Molecular Structure 826 (2007) 104–112
www.elsevier.com/locate/molstruc
Syntheses, structures, photoluminescence and theoretical studies of two
dimeric Zn(II) compounds with aromatic N,O-chelate phenolic ligands
Yi-Ping Tong a,b, Shao-Liang Zheng a,¤, Xiao-Ming Chen a,¤
a State Key Laboratory of Optoelectronic Materials and Technologies/BISC Lab, School of Chemistry and Chemical Engineering,
Sun Yat-Sen University, Guangzhou 510275, China
b Department of Chemistry, Hanshan Normal College, Chaozhou 521041, China
Received 20 February 2006; accepted 20 April 2006
Available online 27 June 2006
Abstract
Two neutral and dimeric Zn(II) complexes [Zn2(epbm)4] (1) and [Zn2(bpbm)4] (2) (Hepbm D 3-ethyl-2-(2-hydroxyphenyl)-3H-benz-
imidazole, HbpbmD 3-n-butyl-2-(2-hydroxyphenyl)-3H-benzimidazole) have been prepared and characterized by X-ray crystallography
and photoluminescent studies. Time-dependent density functional theory (TDDFT) energy level calculation and molecular orbital calcu-
lation show their absorption and luminescence properties are ligand-centered. The spectral shifts caused by the eVects of deprotonation/
complextion, and substitution/dimerization have been also discussed. Both 1 and 2 show blue emission nature.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Zn(II) complexes; Photoluminescence; Alkylation; Time-dependent density functional theory (TDDFT); Oligomerization; Benzimidazole
derivatives
1. Introduction
monomers to oligomers or polymers (Scheme 1) [5–8]. With
the O-donor atoms of deprotonated phenolate rings func-
tioning as either terminal or bridging atoms, such ligands
show a variety of coordination modes in ligating metal
ions. Zn(II) ions may have diVerent coordination numbers
(commonly 4–6) in ligation to such phenolic N,O-donor
derivatives in a variety of coordination geometries, which
can be expected to have high luminescence in the blue or
blue-green region [5–8]. An increase of the dihedral angle
between the imidazole and phenolic rings, concomitant
with an increase of the N- and O-donor separation, will
usually enhance the probability of a bridging coordination
mode, leading to polymerization of the resulting metal
complexes.
We recently reported the synthesis, crystal structures,
photoluminescence and theoretical calculations of a series
of neutral and blue-luminescent mononuclear Be(II) and
Zn(II) complexes, namely [Be(pbm)2], [Be(pbx)2], [Be(pbt)2]
(HpbxD 2-(2-hydroxylphenyl)benzoxazole, and Hpbt D
2-(2-hydroxyl-phenyl)benzothiazole) [9], [Zn(pbm)2] and
[Zn(apbm)2] [5]. We anticipated that an alkylation
As a basic component of full-color display in organic light-
emission diodes (OLEDs), neutral and blue emission materials
are required to be thermally stable [1]. Among a number of
metal complexes found with excellent light emitting property
in recent years [2,3], Zn(II) complexes with phenolic
N,O-donor ligands showed application potential as blue or
blue-green emitters in electroluminescence [4], as exempliWed
by monomeric [Zn(pbm)2] and [Zn(apbm)2] (HpbmD2-(2-
hydroxyphenyl)-benzimidazole; and HapbmD 5-amino-2-
(1H-benzoimidazol-2-yl)phenol) [5], dimeric [Zn2(btz)4]
(Hbtz D 2-(2-hydroxyphenyl)-benzothiazole) [6], trimeric
[Zn3(ppo)6] (Hppo D 2-(2-hydroxyphenyl)-5-phenyl-1,3-
oxazole) [7], and tetrameric [Zn4(q)8] (HqD8-hydroxyquino-
line) [8].
Phenolic N,O-donor derivatives can be used to construct
neutral Zn(II) complexes with the structures ranging from
*
Corresponding authors.
E-mail address: cxm@mail.sysu.edu.cn (X.-M. Chen).
0022-2860/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.04.037

Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
2.1. Preparation of Hepbm·HCl
Monomer
105
O
N
O
Zn
Hpbm was synthesized using similar methods based on
the literature [5,10]. Hepbm·HCl was synthesized from
Hpbm based on a modiWed literature method [11]. Hpbm
(0.210g, 1 mmol) in acetone (100mL) was added with KOH
(0.169g, 3 mmol) and then heated to reXux temperature.
The mixture further reacted with excessive bromoethane
(0.327g, 3mmol) by slow addition in small portions in ca.
1 h, and the reaction was maintained at the reXux condition
for ca. 3h before it was allowed to cool to room tempera-
ture, Wltered oV and washed with acetone. The combined
Wltrate was evaporated under reduced pressure to give a
brown viscous oil. The crude material from the above reac-
tion was then dissolved into pyridine (1mL), and concen-
trated hydrochloric acid (1.2mL) was added carefully with
cooling in ice water. The water was distilled out of the
mixture over a period of 1h, and the reXux temperature of
pyridinium hydrochloride (ca. 493 K) was then attained.
The reaction was maintained for 1h at this temperature
before allowed to cool to ca. 373K and then poured into
cold water (ca. 10mL) with stirring. The crude product was
decolored with charcoal and Wltered oV and evaporated to
dryness. After recrystallization, colorless crystals of
Hepbm·HCl were isolated. The yield was ca. 70% calculated
based on Hpbm. Calcd. for C15H15Cl1N2O: C, 65.57; H,
5.50; N, 10.20. Found: C, 65.39; H, 5.69; N, 10.05%;
1H NMR (500 MHz,DMSO-d6): ꢀ 8.11 (m, 1H), 7.88
(m,1H), 7.65 (m,4H), 7.31 (d,J D 8.10 Hz, 1H), 7.10
(t,J D7.80 Hz, 1H), 4.38 (q, J D 7.50 Hz, 2H), 1.36
(t, J D 7.50 Hz, 3H) ppm; FT-IR data (cm¡1): 3050s, 2794s,
2725s, 1608s, 1497m, 1461s, 1480m, 1352m, 1281m,
1262m, 1220m, 1160m, 1205w, 1117w, 1087w, 1057w,
831m, 756s, 686m, 588w, 538w, 421w.
N
N
O
N
O
O
N
Dimer
Zn
N
O
O
N
O
N
Trimer
N
O
Zn
O
N
Zn
N
N
O
N
O
O
O
N
O
N
Tet ramer
Polymer
N
O
O
N
Zn
Zn
Zn
N
O
N
O
O
N
O
N
O
N
N
O
O
N
Zn
Zn
N
O
O
N
n
Scheme 1. Coordination modes of Zn(II) complexes with N,O-donor phe-
nolic ligands.
modiWcation of Hpbm at the imidazole-N position with ste-
ric hindrance can result in a markedly larger separation
between N- and O-donor atoms and hence lead to the for-
mation of new neutral and polymeric Zn(II) complexes.
Indeed, we have synthesized two alkylated derivatives,
namely 3-ethyl-2-(2-hydroxyphenyl)-3H-benzimidazole
hydrochloride (Hepbm·HCl) and 3-n-butyl-2-(2-hydroxy-
phenyl)-3H-benzimidazole hydrochloride (Hbpbm·HCl·H2
O), which led to the generations of two new dimeric Zn(II)
complexes [Zn2(epbm)4] (1) and [Zn2(bpbm)4] (2). Herein
we report the synthesis, crystal structures, photolumines-
cence and time-dependent density functional theory
(TDDFT) studies of these compounds.
2.2. Preparation of Hbpbm·HCl·H2O
The synthetic approach was similar to that of
Hepbm·HCl using 1-bromo-butane in place of bromoe-
thane. The yield was ca. 70%. Calcd. For C17H21Cl1N2O2: C,
63.65; H, 6.60; N, 8.73. Found: C, 63.71; H, 6.48; N, 8.79%;
1H NMR (500 MHz,DMSO-d6): ꢀ 11.22 (s,1H), 8.09
(m,1H), 7.87 (m,1H), 7.63 (m,4H), 7.23 (d,J D7.80Hz,1H),
7.10 (t,J D7.80 Hz,1H), 4.37 (t,J D7.20 Hz,2H), 1.70
(m,J D7.20 Hz,2H), 1.14 (m,J D7.20 Hz,2H), 0.71 (t,J D
6.60 Hz,3H) ppm; FT-IR data (cm¡1): 3384m, 2961s, 1610s,
1594m, 1556m, 1499s, 1477m, 1463s, 1434m, 1357m, 1298m,
1325m, 1298m, 1212m, 1175m, 1159m, 1111m, 753s, 683w,
591w, 541w, 525m.
2. Experimental
All the reagents and solvents employed were commer-
cially available and used as received without further
puriWcation. The C, H and N micro-analyses were carried
out with an Elementar Vario El elemental analyzer. The
1H NMR data were recorded with a Varian INOVA500NB
spectrometer. The FTIR spectra were recorded using a Bru-
ker Vector22 spectrometer in the KBr pellets in the range
4000–400cm¡1. The UV–vis spectra were recorded by a
Varian Cary-100 spectrometer. The ESI-MS was carried
out on a high-resolution Finnigan MAT LCQ mass spec-
trometer. The steady-state Xuorescence lifetimes were deter-
mined with an Edinburgh Instrument FLS920 Xuorescence
spectrophotometer using laser light resource.
2.3. Preparation of [Zn2(epbm)4] (1)
To a solution of zinc acetate dihydrate (0.110g,
0.5mmol) and Hepbm·HCl (0.275 g, 1.0mmol) in ethanol
(50mL) was added a small amount of triethylamine and
acetic acid to control the solution pH to ca. 6.0, the resultant
solution was allowed to stand for very slow evaporation for

106
Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
over 1 year at ambient temperature, colorless block crystals
of 1 were obtained (yield ca. 0.108g, 40%). ESI-MS
(CH2Cl2): m/z D 1080.8 [Zn2L4 + H]+. Calcd. for C60H52
N8O4Zn2: C, 66.73; H, 4.85; N, 10.38. Found: C, 66.63; H,
Bruker Smart Apex CCD diVractometer (MoKꢁ,
ꢂ D0.71073 Å´). Absorption corrections were applied by
using SADABS [12]. The structures were solved with direct
methods and reWned with full-matrix least-squares tech-
nique using SHELXTL program package [13]. The organic
hydrogen atoms were generated in ideal positions and the
aqua hydrogen atoms were located from diVerence maps.
Anisotropic thermal parameters were applied to all non-
hydrogen atoms. Experimental details of the X-ray analyses
are provided in Table 1. Selected bond lengths and angles
are listed in Table 2.
1
5.08; N, 10.25%; H NMR (500 MHz,DMSO-d6): ꢀ 7.67
(d,J D 5.40 Hz,8H), 7.44 (d,J D 6.90 Hz,4H), 7.29 (t,J D7.20
Hz,12H), 6.94 (s,8H), 4.27 (q,J D 7.60Hz,8H), 1.33 (t,J D
7.60Hz,12H) ppm; FT-IR data (cm¡1): 1596s, 1543m,
1504m, 1475s, 1428s, 1403m, 1309m, 1319m, 1261m, 1153m,
1133m, 1033m, 1011w, 865m, 748s, 736s, 704w, 574w, 547w,
480w.
2.4. Preparation of [Zn2(bpbm)4] (2)
2.6. Calculation details
To an ethanol solution (50 mL) of zinc acetate dihydrate
(0.110 g, 0.5mmol) and Hbpbm·HCl·H2O (0.320 g,
1.0 mmol) was added a small amount of triethylamine and
acetic acid to control the solution PH to ca. 6.0, the resul-
tant solution was allowed to stand for very slow evapora-
tion for over 14 months at ambient temperature, colorless
block crystals of 2 were obtained (yield ca. 0.122g, 41%).
ESI-MS (CH2Cl2): m/z D1193.1 [Zn2L4 + H]+. Calcd. for
C68H68N8O4Zn2: C, 68.51; H, 5.75; N, 9.40. Found: C, 68.29;
Based on the optimized geometries (with no constraints
during the optimizations), time-dependent density func-
tional (TDDFT) calculations and molecular orbital calcu-
lations were performed at the B3LYP level with 6-31G¤¤
basis set for C, H, N and O atoms, and eVective core poten-
tials basis set LanL2DZ for Zn atoms, employing the
Gaussian98 suite of programs [14]. The electron density
diagrams of molecular orbitals were obtained with the
Molden 3.5 graphics program [15].
1
H, 5.55; N, 9.39%; H NMR (500 MHz,DMSO-d6): ꢀ 7.67
(d, J D 5.40Hz, 8H), 7.46 (d, J D 6.90Hz, 4H), 7.28 (m, 12H),
6.91 (m, 8H), 4.32 (t, J D7.30 Hz, 8H), 1.72 (m, 8H), 1.17 (m,
8H), 0.77 (t, J D6.70 Hz, 12H) ppm; FT-IR (cm¡1): 1601m,
1560w, 1496s, 1501m, 1428m, 1406m, 1340m, 1309w,
1265m, 1219w, 1151w, 1125w, 1032w, 1011w, 861w, 751m,
743m, 575w.
3. Results and discussion
3.1. Synthesis and stability
Ethyl and n-butyl groups were selected as substituent
groups to modify Hpbm, which led to successful controlling
the formations of new dimeric Zn(II) complexes diVerent
from the neutral, monomeric complex [Zn(pbm)2]. The syn-
thetic routines for Hpbm and its N-ethyl or N-n-butyl
substituted derivatives (Hepbm·HCl and Hbpbm·HCl·
H2O) are shown in Scheme 2.
2.5. X-ray crystallography
DiVraction intensities for 1, 2, Hepbm·HCl and
Hbpbm·HCl·H2O were collected at 293K or 123 K on a
Table 1
Crystal data and structure reWnement for 1, 2, Hepbm·HCl and Hbpbm·HCl·H2O
Compound
Hepbm·HCl
Hbpbm·HCl·H2O
1
2
Formula
Formula mass
Crystal system
Space group
a/Å
b/Å
c/Å
ꢁ/°
ꢃ/°
C15H15ClN2O
274.74
C17H21ClN2O2
320.81
C60H52N8O4Zn2
1079.84
Monoclinic
P21/c (No. 14)
10.528(2)
21.150(4)
14.398(2)
90
130.257(9)
90
2446.6(7)
2
C68H68N8O4Zn2
1192.04
Triclinic
P-1 (No. 2)
10.912(1)
11.0509(11)
13.5192(13)
75.749(2)
86.102(2)
64.946(2)
1430.2(2)
1
Triclinic
P-1 (No. 2)
8.3349(8)
9.2688(8)
9.7149(9)
94.259(2)
113.091(1)
91.377(2)
687.33(11)
2
Monoclinic
P21/c (No. 14)
9.2883(8)
19.8996(18)
9.5452(9)
90
103.858(2)
90
1712.9(3)
4
ꢄ/°
V/Å3
Z
Dc/g cm¡3
ꢀ/mm¡1
Unique reX.
S on F2
1.328
0.271
1973
1.048
1.244
0.231
3362
1.078
1.466
1.041
4788
0.981
1.384
0.897
5842
1.084
R1 [I > 2ꢅ(I)]
wR2 [I > 2 ꢅ(I)]
R1 (all data)
wR2 (all data)
0.0355
0.0912
0.0396
0.0945
0.0679
0.1734
0.0848
0.1858
0.0669
0.1250
0.1379
0.1517
0.0476
0.1196
0.0602
0.1311

Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
107
Table 2
tals of X-ray quality, including conventional evaporation,
diVusion and solvo(hydro)thermal methods, the single crys-
tals were only successfully isolated by conventional evapo-
ration after 1 year, hence we can have a systematic
investigation of their emission properties using TDDFT
calculations and/or molecular orbital calculations, avoiding
dubious conclusions of the luminescent property arguments
without the X-ray structures that are much more compli-
cated than their empirical formulae.
Thermal stabilities of 1 and 2 were characterized by
thermo-gravimetric analysis (TGA) under Xowing N2 gas at
a heating rate of 10°C/min. 2 had a much higher thermally
stable temperature of 454°C, while 1 was thermally unsta-
ble above 267°C, where 1 likely experienced a de-ethyl pro-
cess, as indicated by a weight loss of ca. 10.70%.
Selected bond length (Å) and angles (°) for 1, 2, Hepbm·HCl and
Hbpbm·HCl·H2O
1
Zn(1)–O(2a)
Zn(1)–N(1)
Zn(1)–O(1)
1.973(3)
2.032(4)
2.035(3)
Zn(1)–N(3)
Zn(1)–O(2)
2.042(4)
2.210(3)
O(2a)–Zn(1)–N(1)
O(2a)–Zn(1)–O(1)
N(1)–Zn(1)–O(1)
O(2a)–Zn(1)–N(3)
N(1)–Zn(1)–N(3)
Zn(1a)–O(2)–Zn(1)
117.7(2)
104.1(2)
87.2(2)
119.4(2)
119.6(2)
106.3(1)
O(1)–Zn(1)–N(3)
O(2a)–Zn(1)–O(2)
N(1)–Zn(1)–O(2)
O(1)–Zn(1)–O(2)
N(3)–Zn(1)–O(2)
96.9(2)
73.7(1)
96.8(2)
175.9(1)
81.5(2)
2
Zn(1)–O(2a)
Zn(1)–O(1)
Zn(1)–N(3)
O(2a)–Zn(1)–O(1)
O(2a)–Zn(1)–N(3)
O(1)–Zn(1)–N(3)
O(2a)–Zn(1)–N(1)
O(1)–Zn(1)–N(1)
Zn(1a)–O(2)–Zn(1)
1.980(2)
Zn(1)–N(1)
Zn(1)–O(2)
2.046(2)
2.186(2)
2.020(2)
2.043(2)
97.76(8)
115.66(8)
99.32(8)
122.38(8)
88.37(8)
105.73(8)
N(3)–Zn(1)–N(1)
O(2a)–Zn(1)–O(2)
O(1)–Zn(1)–O(2)
N(3)–Zn(1)–O(2)
N(1)–Zn(1)–O(2)
119.67(9)
74.27(8)
171.98(7)
83.69(8)
96.63(8)
3.2. Crystal structures of the ligands
Single-crystal X-ray structure analysis indicated that
Hpbm is a neutral and approximately planar molecule with
the dihedral angle between benzimidazole and phenolic
rings at ca. 2.2° [5], being similar to other planar phenolic
N,O-donor ligands Hpbx and Hpbt. The distance between
the N- and O-donor atoms is ca. 2.564 Å for Hpbm.
The organic species for crystalline Hepbm·HCl and
Hbpbm·HCl·H2O were characterized to be protonated in
the solid (Fig. 1). Compared to the un-alkylated Hpbm, the
N-ethyl or N-butyl group in [H2epbm]+ or [H2bpbm]+
Hepbm·HCl
O(1)–C(1)
N(1)–C(7)
N(1)–C(13)
N(2)–C(7)
1.351(3)
1.336(2)
1.383(3)
1.337(2)
N(2)–C(8)
N(2)–C(14)
C(6)–C(7)
1.400(2)
1.471(3)
1.462(3)
C(7)–N(1)–C(13)
C(7)–N(2)–C(8)
C(7)–N(2)–C(14)
C(8)–N(2)–C(14)
109.3(2)
108.2(2)
126.7(2)
124.9(2)
N(1)–C(7)–N(2)
N(1)–C(7)–C(6)
N(2)–C(7)–C(6)
C(8)–C(13)–N(1)
109.2(2)
122.2(2)
128.5(2)
106.5(2)
Hbpbm·HCl·H2O
N(2)–C(7)
N(2)–C(8)
N(2)–C(14)
O(1)–C(1)
a
1.343(4)
1.397(4)
1.462(4)
1.356(3)
N(1)–C(7)
N(1)–C(13)
C(7)–C(6)
1.332(4)
1.379(4)
1.467(4)
C(7)–N(2)–C(8)
C(7)–N(2)–C(14)
C(8)–N(2)–C(14)
C(7)–N(1)–C(13)
108.0(2)
126.9(2)
125.0(2)
109.7(3)
N(1)–C(7)–N(2)
N(1)–C(7)–C(6)
N(2)–C(7)–C(6)
C(13)–C(8)–N(2)
109.0(2)
123.1(3)
127.6(3)
107.0(2)
Symmetric codes: 1: a D ¡x + 1, ¡y, ¡z + 1; 2: a D ¡x, ¡y + 1, ¡z + 2.
HO
OH
NH2
NH2
PPA
N
+
COOH
520 K
N
H
b
(Hpbm)
RBr
RO
KOH
H
HO
N+
493 K
N
Cl-
N
R
N
R
C5H5N.HCl
R = ethyl or butyl
Scheme 2. The synthetic routes for Hepbm·HCl (R D ethyl) and
Hbpbm·HCl·H2O (R D butyl).
Similar to other related neutral complexes [5,9], the sin-
gle crystals of 1 and 2 were very hard to grow. Although
diVerent techniques were employed to grow the single crys-
Fig. 1. Perspective views of Hepbm·HCl (a) and Hbpbm·HCl·H2O
(b). Thermal ellipsoids are plotted at the 50% probability level.

108
Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
generates signiWcantly steric hindrance, and hence larger
dihedral angles of ca. 58.2° and 56.6° between the imidazole
and phenolic rings, as well as much larger separations
between the N- and O-donor atoms (ca. 3.852 Å for
[H2epbm]+ and 3.892 Å for [H2bpbm]+).
a
In the crystal packing of Hepbm·HCl, each chloride ion
accepts two hydrogen bonds from adjacent phenolic and
imidazole groups [O(1A),Cl(1) 3.078(2)Å; N(1),Cl(1)
3.059(2)Å] of two [H2epbm]+ ions (Fig. 2(a)), while strong
ꢆ–ꢆ stacking interactions (interplanar distance ca. 3.47Å)
between the phenolic rings of adjacent [H2epbm]+ cations
are also observed. While in the molecular packing of
Hbpbm·HCl·H2O, each chloride ion accepts three hydro-
gen bonds from the phenolic group [O(1),Cl(1A)
3.057(2)Å] and two lattice water molecules [O(1WA)
,Cl(1A) 3.180(3) and O(1W),Cl(1A) 3.164(3)Å, (see
Fig. 2(b))]; a strong hydrogen bond between the imidazole
group and a lattice water molecule [N,O 2.693(4)Å] is
also found.
b
3.3. Crystal Structures of 1 and 2
Both 1 and 2 have similar centrosymmetrical Zn2O2
dimer structures (Fig. 3), which are planar and similar to
those Zn2O2 cores documented previously [6]. Each Zn2O2
core is surrounded by four deprotonated epbm¡ or bpbm¡
ligands. A pair of these ligands act in the bridging mode
and the other pair of ligands act in the chelate mode as
Fig. 3. Perspective view of 1 (a) and 2 (b) with the thermal ellipsoids at
50% probability level. Symmetry code for 1: A: 1 ¡ x, ¡y, 1 ¡ z; 2: A: ¡x,
1 ¡ y, 2 ¡ z.
terminal ligands, furnishing in a trigonal-bipyramidal
arrangement for each Zn(II) ion with N(1), N(3) and O(2A)
atoms at the equatorial positions, O(1) and O(2) atoms at
the axial positions. The dihedral angles between benzimid-
azole and phenolate rings are 39.7° and 41.5° for 1, and
40.9° and 43.9° for 2, indicating non-coplanar nature.
The hydrogen bonding and ꢆ–ꢆ stacking interactions are
obviously weaker in both 1 and 2, compared with the
un-alkylated compound [Zn(pbm)2] [5]. This fact can be
readily attributed to the presence of bulky alkyl groups that
can reduce intermolecular ꢆ–ꢆ stacking interactions, while
alkylation of the imidazole group can also reduce the
formation of strong hydrogen bonds between benzimid-
azole donor and other acceptor atoms.
3.4. UV–vis and luminescent spectra
Fig. 2. Perspective views of hydrogen-bonding and ꢆ–ꢆ interactions for
Hepbm·HCl (a) and Hbpbm·HCl·H2O (b). Symmetry code for
Hepbm·HCl: A: 1 ¡ x, 1 ¡ y,1 ¡ z; Hbpbm·HCl·H2O: A: x,1/2 ¡ y,¡1/2 + z.
The lowest-energy absorption bands of Hepbm·HCl and
Hbpbm·HCl·H2O occur at 281 and 282nm for the

Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
109
Table 3
Calculated and experimental absorption wavelengths (nm) and transition nature for 1, 2, Hepbm·HCl and Hbpbm·HCl·H2O with oscillator strengths in
CH2CI2 and solid
Compound
Transition Nature
Wavelength
Oscillator strengths
b
Exp. in CH2Cl2
Exp. in solid
Calcd.
Hepbm·HCl
ꢆ ! ꢆ¤
ꢆ ! ꢆ¤
ꢆ ! ꢆ¤
ꢆ ! ꢆ¤
ꢆ ! ꢆ¤
ꢆ ! ꢆ¤
280
281
323
322
330
331
281
282
342
343
345
377
301
302
313
351
353
364
0.15
0.16
0.44
0.14
0.15
0.24
Hbpbm·HCl·H2O
Hpbma (non-alkylated product)
1
2
[Zn(pbm)2]a (non-alkylated product)
a
The calculated and experimental results were reported in Ref. [5].
b
The solution concentration is ca. 10¡6 M.
powdered solid samples, and 280 and 281 nm in dichloro-
methane (ca. 10¡6 M), respectively, while those of 1 and 2 lie
in the visible region (343 and 345nm for solid samples, and
322 and 330 nm for dichloromethane solutions (ca. 10¡6 M),
respectively) (Table 3, Fig 4). In general, the lowest-energy
absorption bands may be attributed to the transitions from
the ground-states to the higher-energy excited-states in UV
spectra [16]. According to our TDDFT calculations, the
lowest energy absorption maxima for [H2epbm]+ and
[H2bpbm]+ cations (301 and 302nm, respectively), which
are consistent with those of experimental results, mainly
arise from the transitions from the ground state to the Wrst
singlet excited state (S0–S1, with corresponding oscillator
strengths 0.15 and 0.16, respectively) (Fig. 5(a)). The close
approach of the calculated lowest energy absorption bands
between them implies that the substitutions of ethyl and
n-butyl group have an approximately identical eVect on the
absorption spectra, in accordance with the experimental
results. However, the lowest energy absorption maxima for
1 and 2 are markedly red-shifted in the spectra, when com-
pared to their ligands. Based on our TDDFT calculations,
the corresponding lowest energy absorptions (351 and
353 nm, respectively) arise mainly from the transition from
S0–S6 with oscillator strengths of 0.14 and 0.15 for 1 and 2
(Fig. 5(b)), respectively, while the transitions from S0–S1
(further into visible region of 371 and 372 nm, respectively)
are relatively weaker (oscillator strength being 0.03). The
calculated lowest energy absorption maxima (351nm for 1
and 353 nm for 2) are in good agreement with the
experimental observations. The excited-state–ground-state
(ES–GS) separations derived from TDDFT calculations
are in agreement with those observed experimentally.
For the [H2epbm]+ and [H2bpbm]+ cations, the S0–S1
transitions are mainly associated with their HOMOs and
LUMOs with conWguration interaction coeYcient up to
0.67; the HOMOs are ꢆ bonding orbitals and LUMOs are
ꢆ¤ anti-bonding orbitals (Fig. 6), thus the transitions should
be ascribed to ꢆ ! ꢆ¤ transition in nature. However, the
S0–S1 transitions are mainly associated with transitions
a
S
6
5
4
7
6
S
S
S
5
4
S
S
3
2
S
1
S
0
0
[H2epbm]+
[H2bpbm]+
0.4
1
2
b
Hepbm.HCl
3.9
0.3
S
S
Hbpbm.HCl.H2O
10
9
S
8
S
7
0.2
3.6
S
6
5
S
S
4
S
S
0.1
0.0
3
2
1
S
3.3
0
S
250
300
350
400
0
1
Wavelength / nm
2
Fig. 4. The UV–vis spectra for Hepbm·HCl, Hbpbm·HCl·H2O, 1 and 2 in
solid state.
Fig. 5. The singlet excited state diagrams of ligands ([H2epbm]+ and
[H2bpbm]+) (a) and complexes (1 and 2) (b).

110
Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
a
a
b
b
LUMO+3
LUMO
LUMO+2
HOMO
Fig. 6. Contour plots of HOMOs and LUMOs of the cations of
[H2epbm]+ (a) and [H2bpbm]+ (b).
LUMO+1
LUMO
from HOMO¡ 1 to LUMO with conWguration interaction
coeYcients up to 0.52 and 0.48 for 1 and 2, respectively, and
from HOMO to LUMO+ 1 with the conWguration interac-
tion coeYcients up to 0.47 and 0.51, respectively. The
HOMO and HOMO¡ 1 orbitals are ꢆ bonding orbitals of
the deprotonated ligands, and LUMO and LUMO+ 1
orbitals are ꢆ¤ anti-bonding orbitals of deprotonated
ligands (Fig. 7), therefore the transitions should be also
ascribed to ꢆ ! ꢆ¤ in nature and should be of ligand-cen-
tered charge transfer (LCCT).
For the S0–S6 transitions of 1 and 2, the transition com-
positions are mainly HOMO¡1 ! LUMO + 2 (coeYcient
up to 0.43) and HOMO! LUMO+ 3 (coeYcient up to
0.41) for 1 and HOMO¡1 ! LUMO + 3 (coeYcient up to
0.43) and HOMO! LUMO+ 2 (coeYcient up to 0.47) for
2, although such transitions are also assigned to be ꢆ ! ꢆ¤
transition in nature in terms of our TDDFT results (Fig. 7).
Similarly, they are ligand-centered charge transfer (LCCT).
It is notable that n-butyl is a slightly stronger electron-
donor than ethyl group, thus 2 should be slightly smaller
in energy gap than 1, similarly, Hbpbm·HCl·H2O is
slightly smaller in energy gap with respect to Hepbm·HCl.
These are in good agreement with the ES–GS separation
of our TDDFT calculations and experimental UV spectra
both in solid and in dichloromethane. However, com-
pared with un-alkylated Hpbm and [Zn(pbm)2] (Table 3)
[5], the alkylation twists the ligand into a non-planar
structure, as indicated by the dihedral angle between
benzimidazole and phenolic rings, resulting in the reduc-
tion of electron delocalization energy, thus an increase of
energy gap can be assumed. Meanwhile, the reduction of
energy gap for the alkylated products caused by the elec-
tron-donating n-butyl or ethyl group is much smaller, thus
a signiWcant overall increase of energy separation result-
ing from the alkylation and/or oligomerization can be
expected, in good agreement with the fact that the S0–S1
energy separation of our TDDFT calculations and experi-
mental absorption maxima both in solid and dichloro-
methane solution for the alkylated products are all larger
in energy than those of their un-alkylated species. More-
over, the variations of supramolecular interactions caused
by the alkylation and/or oligomerization (mentioned
HOMO
HOMO-1
Fig. 7. Contour plots of relevant molecular orbitals (from HOMO ¡ 1 to
LUMO + 3) of 1 (a) and 2 (b).
above) are also favoured for the blue-shifts of UV spectra
in solid with respect to their un-alkylated parents [17,18].
Deprotonation and complexation of ligands with
Zn(II) ions will usually reduce signiWcantly the energy
gaps between LUMOs and HOMOs [3,5,19] and therefore
according to the energy-gap law for radiationless deacti-
vation [20,21], leading to the reduction of emission energy.
The
emission
maxima
for
Hepbm·HCl
and
Hbpbm·HCl·H2O are 382 nm (ꢂex D 325 nm) and 388 nm
(ꢂex D 330 nm), respectively (Fig. 8), while those for 1 and
2 are 422 nm (ꢂex D 367 nm) and 424 nm (ꢂex D 372 nm) in
solid, respectively. According to our TDDFT calcula-
tions, the S0–S1 transitions for 1 and 2 are red-shifted
when compared to their parent ligands, and this fact can
also be readily related to the observations that 1 and 2
have smaller emission energy with respect to their parents.
However, when compared with their un-alkylated species
Hpbm and [Zn(pbm)2] [5], the emission energy is

Y.-P. Tong et al. / Journal of Molecular Structure 826 (2007) 104–112
111
Acknowledgements
Hepbm·HCl
Hbpbm·HCl·H O
1
2
Emission
1.00
0.75
0.50
0.25
0.00
Excitation
2
This work was supported by the National Natural
Science Foundation of China (Grant No. 20531070) and a
ScientiWc and Technological Project of Guangdong
Province (No. 04205405).
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