X-ray and DFT studies of the structure and spectral property of 2-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide

Article abstract of DOI:10.1016/j.molstruc.2007.09.003

A hemicyanine dye: 2-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide (1) was synthesized and characterized by X-ray diffraction, ¹H NMR, MS, IR, UV-Vis spectra and elemental analysis. The crystals are monoclinic, space group P2₁/c, with a = 7.1231(8), b = 20.672(2), c = 10.9642(12) ?, V = 1605.7(3) ?³, and Z = 4 (at 298(2) K). Crystal stacking scheme indicates the hydrogen bonds and intermolecular π?π interactions; the molecular planes arrange in a head-to-tail fashion along the ab plane due to the strong 'push-pull' electron system. Molecular geometries, frequencies and absorption properties were calculated at DFT/TD-DFT level using two hybrid exchange-correlation functionals, B3LYP and PBE1PBE. These calculations on (1) provide deep insight into its electronic structure and property.

Full text of DOI:10.1016/j.molstruc.2007.09.003

Available online at www.sciencedirect.com
Journal of Molecular Structure 881 (2008) 117–122
www.elsevier.com/locate/molstruc
X-ray and DFT studies of the structure and spectral property
of 2-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide
a
a,
a
b
a
X.H. Zhang , L.Y. Wang *, G.H. Zhai , Z.Y. Wen , Z.X. Zhang
a
Department of Chemistry, Northwest University, Xi’an 710069, China
Institute of Modern Physics, Northwest University, Xi’an 710069, China
b
Received 19 June 2007; received in revised form 29 August 2007; accepted 1 September 2007
Available online 12 September 2007
Abstract
A hemicyanine dye: 2-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-pyridinium iodide (1) was synthesized and characterized by
1
X-ray diffraction, H NMR, MS, IR, UV–Vis spectra and elemental analysis. The crystals are monoclinic, space group P21/c, with
3
˚
˚
a = 7.1231(8), b = 20.672(2), c = 10.9642(12) A, V = 1605.7(3) A , and Z = 4 (at 298(2) K). Crystal stacking scheme indicates the hydro-
gen bonds and intermolecular pÁ Á Áp interactions; the molecular planes arrange in a head-to-tail fashion along the ab plane due to the
strong ‘push-pull’ electron system. Molecular geometries, frequencies and absorption properties were calculated at DFT/TD-DFT level
using two hybrid exchange-correlation functionals, B3LYP and PBE1PBE. These calculations on (1) provide deep insight into its elec-
tronic structure and property.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Hemicyanine dyes with pyridine nucleus; X-ray diffraction; H-bond; pÁ Á Áp Interaction; TD-DFT
1. Introduction
mance of the dyes. The tool of theoretical chemistry can be
valuable, which can help to rationalize the design of dyes
and to refine their structure in order to optimize their
absorption color, solubility, and/or solid-state structure.
Among a variety of quantum chemistry techniques, DFT
calculations of large organic molecules [14] and the chro-
mophoric properties of organic dyes [15,16] have been
made accessible for practical applications.
In this paper the new synthesis method, crystal structure
and UV–Vis spectral characteristic of (1) are reported. The
data can serve as a basis for the contribution to the under-
standing of the nature of the hemicyanine dyes with pyri-
dine nucleus. Detailed results and discussion are
elaborated in the following sections.
Hemicyanine dyes with pyridine nucleus, which have
strong electron donors and acceptors, are among the most
important organic functional dyes. They are widely used as
sensitizers and other additives in the photographic industry
and solar cell [1,2], as optical recording medium in laser
disks [3], and as fluorescent probes in biochemistry area
[4–6]. Because of their large first hyperpolarizability b
and other interesting features, hemicyanine dyes have
gained much attention in nonlinear optical research and
molecular electronics [7–13].
Considering the multifunction of the hemicyanine dyes
with pyridine nucleus, it is important to study the single
crystal X-ray structure of the dyes, which reveals molecular
conformation, intra- and intermolecular interactions in
solid state and the structural information gained from the
analysis assists towards understanding the technical perfor-
2. Experimental
2.1. Synthesis
*
A mixture of 0.24 g (1 mmol) 2-methylpyridinium
Corresponding author. Tel.: +86 29 88303403; fax: +86 29 88303798.
E-mail address: wanglany@nwu.edu.cn (L.Y. Wang).
methiodide, 0.15 g (1 mmol) 4-N,N-dimethylamino benzal-
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.09.003

118
X.H. Zhang et al. / Journal of Molecular Structure 881 (2008) 117–122
Table 2
dehyde and a few drops of piperidine was subjected to
microwave irradiation at 163 W and 2.5 min under sol-
vent-free conditions in a Galanz microwave oven. After
cooling, the reaction mixture was recrystallized from
EtOH–H2O to afford pure 2-[2-(4-dimethylaminophe-
nyl)ethenyl]-1-methyl pyridinium iodide (1). Single crystals
of (1) for X-ray diffraction experiments were grown from
methanol–water mixture (4:1).
Atomic coordinates and equivalent isotropic displacement parameters
2
(A ·103) for (1)
˚
Atom
x
y
z
U(eq)
I(1)
N(1)
N(2)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
C(15)
C(16)
H(1A)
H(1B)
H(1C)
H(2A)
H(2B)
H(2C)
H(4)
0.0622(1)
0.6646(7)
0.7690(7)
0.6933(11)
0.6090(10)
0.6865(7)
0.7372(8)
0.7600(8)
0.7373(8)
0.6854(8)
0.6601(8)
0.7666(7)
0.7580(8)
0.7881(8)
0.8334(8)
0.8550(9)
0.8325(9)
0.7909(8)
0.7270(10)
0.8180
0.3452(1)
0.3515(2)
0.6347(2)
0.2827(3)
0.3855(3)
0.3815(3)
0.3487(3)
0.3804(3)
0.4464(3)
0.4798(3)
0.4487(3)
0.4771(3)
0.5403(3)
0.5704(3)
0.5349(3)
0.5656(4)
0.6308(3)
0.6647(3)
0.6765(3)
0.2719
0.7547(1)
0.0633(4)
0.6767(4)
0.546(6)
-0.0481(6)
0.1754(5)
0.2843(5)
0.3947(5)
0.4055(5)
0.2957(5)
0.1849(5)
0.5236(5)
0.5460(5)
0.6654(5)
0.7743(5)
0.8843(6)
0.8926(6)
0.7892(6)
0.5691(5)
0.0906
82(1)
74(2)
61(1)
105(3)
96(2)
55(1)
65(2)
65(2)
55(2)
65(2)
68(2)
63(2)
57(2)
54(1)
70(2)
79(2)
73(2)
70(2)
86(2)
157
2.2. Measurements
A
red crystal with approximate dimensions of
0.23 · 0.14 · 0.09 mm3 was selected for data collection.
The X-ray diffraction data were collected on a Bruker
SMART APEX II CCD diffractometer equipped with a
graphite monochromated Mo Ka radiation (k =
˚
0.71073 A) by using x–2h scan technique at room tempera-
ture. The structure was solved by direct methods with
SHELXS-97 [17], and refined using the full-matrix least-
squares method on F2 with anisotropic thermal parameters
for all non-hydrogen atoms using SHELXL-97 [18]. Hydro-
gen atoms were generated geometrically. The crystal data,
details concerning data collection and structure refinement
are given in Table 1. Atomic coordinates and equivalent dis-
placement parameters are listed in Table 2. Molecular illus-
trations were prepared using the XP package [19].
Parameters in CIF format are available as Electronic Sup-
0.6790
0.6019
0.4956
0.5864
0.7076
0.7561
0.7921
0.6677
0.2699
0.2604
0.4094
0.3551
0.4147
0.3042
0.3564
0.5244
À0.0300
157
157
144
144
144
78
78
78
0.0976
À0.0397
À0.1142
À0.0652
0.2824
0.4655
0.2983
H(5)
H(7)
H(8)
0.6247
0.4726
0.1143
82
H(9)
0.7943
0.4502
0.5910
75
H(10)
H(12)
H(13)
H(14)
H(15)
H(16A)
H(16B)
H(16C)
0.7302
0.8486
0.8858
0.8457
0.7767
0.8250
0.7200
0.6083
0.5674
0.4903
0.5416
0.6514
0.7094
0.6721
0.7207
0.6640
0.4789
0.7707
0.9554
0.9684
0.7936
0.5159
0.5952
0.5259
68
85
95
88
84
128
128
128
Table 1
Crystal data and structure refinement for (1)
Empirical formula
Formula weight
Temperature (K)
Wavelength (A)
Crystal system
Space group
C16H19IN2
366.23
298(2)K
0.71073
Monoclinic
P21/c
˚
U(eq) is defined as one third of the trace of the orthogonalized Uij tensor.
˚
Unit cell dimensions (A)
a
b
c
7.1231(8)
20.672(2)
10.9643(12)
1605.7(3)
plementary Publication from Cambridge Crystallographic
Data Centre (CCDC 646676).
3
˚
Volume (A )
Z
Calculated density (mg/m3)
Absorption coefficient (mmÀ1
F(000)
Crystal size (mm3)
h Range for data collection
Limiting indices h, k, l
Refinement method
Reflections collected/unique/Rint
Completeness to h = 25.05ꢁ
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R1
4
Melting points were taken on a XT-4 micromelting
apparatus and uncorrected. Elemental analyses were per-
formed with Vario EL-III instrument. IR spectra in cmÀ1
were recorded on a Bruker Equiox-55 spectrometer. 1H
NMR spectra were recorded at 400 MHz on a Varian Ino-
va-400 spectrometer and chemical shifts were reported
relative to internal Me4Si. MS were taken on a HP5989B
instrument. The absorption spectra were recorded on a
Shimadzu UV-1700 UV–vis spectrometer.
1.515
1.984
728
)
0.23 · 0.14 · 0.09
1.97–25.05
À8/8, À14/24, À13/11
Full- matrix least-square on F2
6637/2821/0.0364
99.0%
2821/0/175
1.021
2-[2-(4-Dimethylaminophenyl)ethenyl]-1-methyl-pyridi-
nium iodide (1): red crystals; m.p. 273–274 ꢁC; UV–vis
0.0437
0.0786
1
(MeOH) kmax: 462.0 nm; H NMR (DMSO-d6, 400 MHz)
wR2
d: 3.04 [s, 6H, N(CH3)2], 4.29 (s, 3H, N+CH3), 6.82 (d,
J = 9.2 Hz, 2H, ArH), 7.24 (d, J = 16.0 Hz, 1H, CH@CH),
7.65–7.74 (m, 3H, one Py-H and two ArH), 7.92 (d,
J = 16.0 Hz, 1H, CH@CH), 8.34 (t, J = 8.0 Hz, 1H, Py-
R indices (all data)
R1
0.0897
0.0941
0.346 and À0.346
wR2
À3
˚
Largest diff. peak and hole (e A
)

X.H. Zhang et al. / Journal of Molecular Structure 881 (2008) 117–122
119
Table 3
H), 8.45 (d, J = 8.8 Hz, 1H, Py-H), 8.75 (d, J = 6.0 Hz, 1H,
˚
Some selected bond lengths (A), bond angles and selected torsion angles
(ꢁ) for (1) determined by X-ray diffraction and DFT calculations
Py-H); IR (KBr) m: 3030, 2993, 2903, 1630, 1593, 1558,
1528, 1456, 1373, 968, 814 cmÀ1; MS (70 eV) m/z (%):
223 (100), 207 (26), 180 (6), 142 (15), 127 (6), 77 (40). Anal.
Calcd. for C16H19IN2: C, 52.47; H, 5.23; N, 7.65; found: C,
52.16; H, 5.12; N, 7.47.
X-ray (1) B3LYP
PBE1PBE
6-31G* 6-311G** 6-31G* 6-311G**
Bond lengths
N(1)–C(1)
N(1)–C(2)
N(1)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–C(8)
1.441(7) 1.462 1.463
1.451 1.451
1.451 1.451
1.353 1.349
1.421 1.419
1.373 1.37
1.414 1.412
1.415 1.413
1.371 1.368
1.425 1.423
1.423 1.42
1.372 1.369
1.427 1.424
1.415 1.413
1.372 1.369
1.405 1.403
1.366 1.363
1.378 1.376
1.429(7) 1.462 1.463
1.370(6) 1.359 1.356
1.388(7) 1.425 1.424
1.372(7) 1.376 1.373
1.380(7) 1.419 1.417
1.403(7) 1.420 1.418
1.370(7) 1.375 1.371
1.408(7) 1.429 1.428
1.438(7) 1.427 1.425
1.332(7) 1.376 1.373
1.445(7) 1.430 1.428
1.410(7) 1.420 1.418
1.358(7) 1.375 1.372
1.362(9) 1.410 1.408
1.339(8) 1.369 1.365
1.344(6) 1.387 1.384
1.375(7) 1.367 1.366
1.467(7) 1.475 1.475
2.3. Computational
In this work, the ground-state geometries were fully
optimized at the DFT level using two hybrid exchange-cor-
relation functionals, B3LYP [20,21] and PBE1PBE [22],
including 20% and 25% of Hartree–Fock (HF) exchange,
respectively. The geometries were fully optimized at the
DFT level using 6-31G* and 6-311G** basis sets. The verti-
cal excitation energies and oscillator strengths in low-lying
singlet excited states were calculated at the TD-DFT
method. In our calculation, the anion was neglected as
the experiment proved that the anion would not affect
the spectra of the cationic dyes. Analytic frequency calcula-
tions were done to confirm the optimized structures to be
an energy minimum. All calculations reported in this work
were carried out with the GAUSSIAN 03 program [23].
C(3)–C(8)
C(6)–C(9)
C(9)–C(10)
C(10)–C(11)
C(11)–C(12)
C(12)–C(13)
C(13)–C(14)
C(14)–C(15)
N(2)–C(11)
N(2)–C(15)
N(2)–C(16)
1.36
1.359
1.464 1.463
Bond angles
C(3)–N(1)–C(1)
C(4)–C(3)–C(8)
C(5)–C(4)–C(3)
C(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(8)–C(7)–C(6)
C(10)–C(9)–C(6)
C(9)–C(10)–C(11)
N(2)–C(11)–C(12)
C(13)–C(12)–C(11)
C(11)–N(2)–C(15)
C(12)–C(13)–C(14)
C(15)–C(14)–C(13)
C(14)–C(15)–N(2)
120.1(5) 120.6 120.8
116.3(5) 117.3 117.2
121.4(6) 120.7 120.8
123.0(6) 122.4 122.4
115.8(5) 116.6 116.6
121.9(5) 121.9 121.9
126.4(5) 127.8 127.9
125.7(5) 124.6 124.6
117.0(5) 116.1 116.2
120.3(6) 122.0 122.0
121.7(5) 121.8 121.8
121.2(6) 119.8 119.8
118.6(6) 118.1 118.1
121.2(6) 122.2 122.2
120.5 120.6
117.4 117.4
120.6 120.6
122.3 122.3
116.8 116.8
121.8 121.8
127.7 127.8
3. Results and discussion
3.1. Synthesis and structural confirmation
The
2-[2-(4-dimethylaminophenyl)ethenyl]-1-methyl-
124
124.2
pyridinium iodide (1) was conventionally prepared by reflux-
ing the mixture of 2-methylpyridinium methiodide, 4-N,
N-dimethylamino benzaldehyde and piperidine for 5 h in
methanol [24]. However, in this paper, 2-[2-(4-dimethylam-
inophenyl)ethenyl]-1-methyl-pyridinium iodide was success-
fully synthesized with high yields 98% within 2.5 min. The
reaction proceeded well even when both the starting reac-
tants were solids and the reaction temperature was main-
tained below the melting points of both components. The
product could be easily purified by recrystallization from
EtOH–H2O. Besides, the waste disposal of solvents and
excess chemicals in classical synthesis of the dye was avoided
or minimized. The approach provided an efficient and envi-
ronmentally friendly pathway to synthetic dyes.
116.3 116.3
121.8 121.9
122
119.7 119.8
118 118
121.9
122.1 122.2
Torsion angles
C(1)–N(1)–C(3)–C(8)
C(2)–N(1)–C(3)–C(4)
C(5)–C(6)–C(9)–C(10)
C(6)–C(9)–C(10) –C(11) 179.9(5) 180
C(9)–C(10)–C(11)–C(12) 0.8(9)
C(16)–N(2)–C(11)–C(12) 178.3(5) 180
UV–Vis absorption
maxima (nm)
179.6(6) 180
178.8(6) 180
176.7(6) 180
180
180
180
180
0
179.4 179.7
179.6 179.6
177.3 179.3
178.4 179.5
0
10.8
3.2
180
179.2 179.8
Exp.a
462.0
6-31G* 6-311G** 6-31G* 6-311G**
459.3 460.4 447.0 445.9
a
The solvent is methanol.
The structure of (1) was confirmed by elemental analy-
sis, UV–vis absorption spectra, IR, 1H NMR and MS data.
IR spectra of (1) shows typical aromatic absorption
(t@C–H, 3033–2903 cmÀ1, tC@C, 1630–1456 cmÀ1, d@CH
,
are shown in Fig. 1, and the crystal packing of (1) along
the a axis is presented in Fig. 2. Table 3 lists some selected
bond lengths, bond angles and selected torsion angles for
the dye molecule.
968–814 cmÀ1) and resonance conjugated unsaturated
stretching modes in the chromophore (C@C, C@N,
1
1456–1373 cmÀ1). The H NMR chemical shift data, MS
The investigated compound crystallizes in the P21/c
space group with four molecules per unit cell. It can be
found that the carbon-carbon bond lengths on the dye
molecular skeleton are basically intermediate between typ-
m/z values and UV–vis absorption spectra (Table 3) are
in accordance with the structure of (1).
3.2. Crystal structure
˚
˚
ical C–C single (1.54 A) and C@C double (1.34 A) bonds,
and carbon–nitrogen bond lengths are also intermediate
between C–N typical single (1.47 A) and C@N double
The molecular structure and atom numbering of 2-[2-(4-
dimethylaminophenyl)ethenyl]-1-methyl pyridinium iodide
˚

120
X.H. Zhang et al. / Journal of Molecular Structure 881 (2008) 117–122
due to the strong ‘push-pull’ electron system. Specifically,
the pyridinium moiety from one molecule interacts with
the benzene ring containing the dimethyl amino group from
another molecule. The iodide anion interacts via Coulombic
attractions with the adjacent nitrogen atoms. The short con-
˚
tacts are: N(2)Á Á ÁI(1) 4.556(2) A (1 À x, 0.5 + y, 1.5 À z),
˚
˚
5.018(6) A (1 À x, 1 À y, 1 À z), and N(1)Á Á ÁI(1) 5.182(6) A
(1 À x, 1 À y, 1 À z) (Table 4). We also found that C(14)–
H(14)Á Á ÁI(1), C(15)–H(15)Á Á ÁI(1) and C(16)–H(16A)Á Á ÁI(1)
hydrogen bonds exist between molecules (Table 4). The rea-
son is that because of the electron-attracting inducement of
C@N+, the r-bonds’ electron cloud of C(14)–H(14),
C(15)–H(15) and C(16)–H(16A) displace to the carbon
atoms, leading to form polar C(dÀ)–H(d+) groups. Hence,
there is attraction between H(d+) and the IÀ. The domination
hydrogen-bonding interaction in the crystal structure is
between the pyridinium donor and the IÀ acceptor. There-
fore, supramolecular interactions, including pÁ Á Áp stacking,
C–HÁ Á ÁI hydrogen bonds and Coulombic attractions, gener-
ate a unique supramolecular architecture in the solid state of
the dye.
Fig. 1. ORTEP diagram of (1) with ellipsoids drawn at the 30%
probability level.
3.3. DFT calculations
The ground-state geometries were optimized by the
B3LYP and PBE1PBE level of theory, using two basis sets,
6-31G* and 6-311G**, respectively.
The selected optimized bond lengths, bond angles and
torsion angles are given in Table 3. The computed bonds
Table 4
Geometry of the C–HÁ Á ÁI hydrogen bonds and N+Á Á ÁIÀ in the crystals of
˚
(1) (A and ꢁ)
Fig. 2. Crystal packing of (1), showing the C–HÁ Á ÁI hydrogen bonds
(dashed lines) along the a axis with ellipsoids drawn at the 30% probability
level.
D–HÁ Á ÁI
d(D–H)
d(HÁ Á ÁI)
d(DÁ Á ÁI)
\(DHI)
C(14)–H(14)Á Á ÁI(1)
C(15)–H(15)Á Á ÁI(1)
C(16)–H(16A)Á Á ÁI(1)
N(2)Á Á ÁI(1)
0.93(7)
0.93(6)
0.96(7)
3.04(5)
3.10(5)
3.17(5)
3.894(7)
3.918(6)
4.021(6)
6.239(4)
4.556(2)a
5.018(6)b
6.421(7)
5.182(6)c
153(4)
148(4)
148(4)
˚
(1.27 A) bonds. All C–C–C, C–N–C and C–C–N bond
angles are close to 120ꢁ. It indicates that the p electrons
in the dye molecule are delocalized. The dimethyl amino
phenyl group is coplanar with C(9)=C(10), as indicated
by C(5)–C(6)–C(9)–C(10) torsion angle of 176.7(6)ꢁ. The
torsion angles of C(1)–N(1)–C(3)–C(8) and C(2)–N(1)–
C(3)–C(4) [179.6(6)ꢁ and 178.8(6)ꢁ, respectively] show that
the amino group is not significantly distorted both from
planarity and from coplanarity with the phenyl ring. The
torsion angles of the dye molecular skeleton are closely
to 180ꢁ, which indicates that the molecular framework of
the dye is planar. In addition, the molecule has trans-con-
figuration as indicated by the torsion angle C(6)–C(9)–
C(10)–C(11) (179.9(5)ꢁ).
N(1)Á Á ÁI(1)
Symmetry code: a1 À x, 0.5 + y, 1.5 À z; b1 À x, 1 À y, 1 À z; c1 À x,
1 À y, 1 À z.
Table 5
Main calculated optical transitions for (1)
Band Cal.
(nm)
f a
Transition characterb
Transition
feature
Exp.
(nm)
1
2
3
460.4
400.4
322.4
0.9299 HOMO fi LUMO
(67.17%)
0.3278 HOMO fi LUMO+1
(84.06%)
0.0083 HOMO-2 fi LUMO
(91.60%)
p fi p
462.0
*
In the crystal packing along the a axis, as shown in Fig. 2,
adjacent molecules in (1) are stacked through strong offset
pÁ Á Áp aromatic stacking interactions, with a face-to-face dis-
˚
a
tance of 3.734 A. Interestingly, the conjugated molecular
Oscillator.
b
The proportion of the main transition are given in parenthesis.
planes arrange in a head-to-tail fashion along the ab plane

X.H. Zhang et al. / Journal of Molecular Structure 881 (2008) 117–122
121
Fig. 3. Sketch of the main molecular orbitals involved in the dominant dipole-allowed transition for (1), obtained at the B3LYP/6-311G** level of
approximation.
are in agreement with experimental results. We also
checked the effect of basis sets on the calculations. The cal-
culated results show that the larger basis set 6-311G** is
more suitable than 6-31G*. The B3LYP/6-31G* results
2 to LUMO but from HOMO to LUMO, and the assigned
p fi p* transition which is localized on the main molecular
skeleton is also confirmed. The calculated outcomes are in
good agreement with experimental data, and accurately
explain the experimental results. Overall, these initial
DFT and TD-DFT calculations on the molecule provide
deep insight into their electronic structures and properties.
˚
deviate in the range from 0.004 to 0.048 A for bond
lengths, from 0ꢁ to 1.7ꢁ for bond angles and from 0.1ꢁ to
3.3ꢁ for torsion angles, and B3LYP/6-311G** results devi-
˚
ate in the range from 0.001 to 0.046 A for bond lengths,
from 0ꢁ to 1.7ꢁ for bond angles and from 0.1ꢁ to 3.3ꢁ for
torsion angles with respect to the experimental values.
Table 3 also lists some experimental and theoretical kmax
of (1). It can be seen that the kmax obtained by PBE1PBE
method significantly deviates from the experimental data.
B3LYP/6-31G* underestimates kmax 2.7 nm and B3LYP/
6-311G** underestimates kmax 1.6 nm relative to experi-
mental data. Therefore, B3LYP/6-311G** method cor-
rectly reproduces the signs of the bond lengths, bond
angles, torsion angles and absorption maxima, which is
useful for investigating the characteristic of some structur-
ally related hemicyanine dyes.
4. Conclusion
We describe a rapid and highly efficient method for the
synthesis
of
2-[2-(4-dimethylaminophenyl)ethenyl]-1-
methyl-pyridinium iodide under solvent-free microwave
irradiation by the condensation of 1-methyl-2-picolinium
iodide with 4-N,N-dimethylamino benzaldehyde in the pres-
ence of piperidine. Single-crystal X-ray diffraction study
indicates that the p electrons in the dye molecule are delocal-
ized. Intermolecular hydrogen bonding and p-stacking inter-
actions are displayed in the molecular system. In particular,
DFT and TD-DFT calculations have allowed a detailed
understanding of the electronic structure and absorption
spectra of the dye. B3LYP/6-311G** method shows that
LUMO and HOMO are highly delocalized on whole molec-
ular skeleton and reproduces the p – p* type absorption
bands of the dye.
3.4. Assignment of the calculated transition
The optical and electronic properties of the dye were
investigated by means of TD-DFT calculations at the
molecular level. Table 5 lists the main orbital compositions
of the computed lower-lying singlet excited states and tran-
sition feature of the dye molecule obtained at the B3LYP/
6-311G** level. From Table 5, we can see that the lowest
energy absorption found at 462.0 nm belongs to the
dipole-allowed p fi p* transition from HOMO to LUMO,
corresponding to 460.4 nm with the largest oscillator
strength calculated by B3LYP/6-311G** method. This
result is also suggested by the analysis of the orbital popu-
lation. Fig. 3 shows that HOMO is mainly localized on the
whole molecular skeleton, the same as LUMO, which is
obviously different from LUMO+1/HOMOÀ2. That is
why the longest absorption band of (1) is a charge transfer
transition not from HOMO to LUMO+1 or from HOMO-
Acknowledgements
We appreciate the financial support for this research by
a grant from the Natural Science Foundation of Shaanxi
Province (No. 2004B23) and the Science Research Startup
Foundation of Northwest University.
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