Experimental and theoretical study on the structure and optical properties of 2-acyl-1,3-indandiones - Conformational effects

Article abstract of DOI:10.1016/j.molstruc.2010.07.008

A series of six 2-acyl-1,3-indandione derivatives is studied by means of experimental (single crystal X-ray diffraction, NMR, electron absorption and emission spectroscopy) and theoretical methods (HF, DFT and TD-DFT). All possible tautomers and rotamers

Full text of DOI:10.1016/j.molstruc.2010.07.008

Journal of Molecular Structure 981 (2010) 10–20
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical study on the structure and optical properties
of 2-acyl-1,3-indandiones – Conformational effects
b
c
d
e
Anife Ahmedova a, , Gordana Pavlovic , Diana Zhiryakova , Dubravka Šišak , Neyko Stoyanov ,
*
´
Michael Springborg f, Mariana Mitewa a
a Faculty of Chemistry, University of Sofia, 1, J. Bourchier av., 1164 Sofia, Bulgaria
b Laboratory of General Chemistry, Department of Applied Chemistry, Faculty of Textile Technology, University of Zagreb, Prilaz baruna Filipovi´ca 28a, HR-10000 Zagreb, Croatia
c Institute of Organic Chemistry and Phytochemistry, Bulgarian Academy of Sciences, Acad. Bonchev str. Bl.9, Sofia 1113, Bulgaria
d Laboratory of Crystallography, Pliva R&D, Prilaz baruna Filipovic´a 29, HR-10000 Zagreb, Croatia
e Rousse University, Technological Branch – Razgrad, 3, Aprilsko Vastanie Str., Razgrad-7200, Bulgaria
f Physical and Theoretical Chemistry, University of Saarland, Saarbruecken 66123, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of six 2-acyl-1,3-indandione derivatives is studied by means of experimental (single crystal X-ray
diffraction, NMR, electron absorption and emission spectroscopy) and theoretical methods (HF, DFT and
TD-DFT). All possible tautomers and rotamers are described quantum-chemically on the basis of their
structure, energy and optical properties. Although the heterocyclic substituent may easily rotate about
a C–C bond, according to the calculated energies, the conformation in the most stable gas-phase opti-
mized structures is the same as in the crystalline form. These results suggest that the intermolecular
interactions are very weak compared to the intramolecular ones, like strong intramolecular hydrogen
Received 29 May 2010
Received in revised form 6 July 2010
Accepted 6 July 2010
Available online 29 July 2010
Keywords:
1,3-Indandione
Optical properties
TD-DFT
bonding and
p-electron delocalization. Therefore, a good account of the experimental results was
obtained theoretically by considering only the conformational modes of the isolated molecules in ground
and excited states. The experimentally observed spectra of the studied series of compounds are charac-
terized by very strong absorption in the visible region and weak fluorescence showing moderate to strong
Stokes shift.
Crystal structure
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
have recently undertaken a detailed study of the structural, optical
and complexation properties of 2AID derivatives in which the
The 2-substituted 1,3-indandiones have attracted an increasing
interest during the last few decades due to their photophysical
properties [1–5]. Very strong fluorescence was reported for 2-(pyr-
idyl)- and 2-(quinolyl)-1,3-indandiones [6,7]. On the other hand, 2-
acyl derivatives of 1,3-indandione posses intramolecular hydrogen
bond and have been the subject of experimental and theoretical
studies on the possible tautomeric equilibria [8–10]. The existence
of Excited State Intramolecular Proton Transfer (ESIPT) was
experimentally observed for 2-acetyl-1,3-indandione (2AID) and
theoretically supported through quantum chemical (AM1) calcula-
tions in ground and excited singlet state [11]. The suggested fast
and reversible tautomerization of 2AID gave the first clue that it
should be photostable upon irradiation with UV-light. Later this
has been experimentally proven, showing that 2AID and its metal
complexes with Zn(II) and Cu(II) are even more photostable than
some commercially available sunscreens, and therefore were pro-
posed as promising sunscreen agents [12]. Of these reasons we
methyl group is substituted by extended p-conjugated system,
i.e., 2-cinnamoyl-1,3-indandione (compound 1 in Fig. 1) [13,14].
The 1,3-indandione fragment is a very strong electron acceptor
and when coupled with electron donating substituents of different
strength the photophysical properties of the so-formed dipolar
‘‘push–pull” molecules can be modified [1,15]. Several studies have
been devoted to excited state twisting observed in N,N-dimethyl-
aminobenzylidene-1,3-indandione [16,17], its solid-state optical
properties [18,19], and potential application of its derivatives as
non-linear optical materials [20]. Recently, Sigalov et al. examined
the role of inter- and intramolecular hydrogen bonding in stabil-
ization of different conformers in solution of 1,3-indandione deriv-
atives directly conjugated with a heterocyclic ring, such as 2-
pyrrolyl-1,3-indandione [15]. The photophysical properties in the
solid phase of 1,3-indandiones, conjugated with butadiene, simi-
larly to the structures in Fig. 1, were recently studied in relation
to the strong influence of the crystal packing [21].
In the current study, the structural and optical properties of
a series of 2-acyl-1,3-indandiones, substituted with a heterocy-
clic ring and depicted in Fig. 1, were investigated by means of
* Corresponding author. Fax: +359 2 962 5438.
E-mail address: ahmedova@chem.uni-sofia.bg (A. Ahmedova).
0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.07.008

A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
11
and
p–p stacking interactions for structures 2, 3 and 4 are given
1
2
in Tables 2 and 3.
The IR spectra were recorded on a Specord 75-IR (Carl-Zeiss,
Jena, Germany) in Nujol mulls. The UV–Vis absorption spectra
were recorded on a JASCO V-570-UV/Vis/NIR. Fluorescence spectra
were obtained in three different solvents using Perkin–Elmer LS-5
and Varian Eclipse fluorometers. All solvents were spectroscopic
grade and were used without further purification. Acetonitrile
was dried over P2O5 and freshly distilled before use. The NMR spec-
tra were recorded in CDCl3 using a Bruker DRX-250 spectrometer
operating at 250.13 MHz for 1H- and 62.90 MHz – for the 13C-nu-
clei. The chemical shifts are related to TMS used as reference.
NMR spectra of compounds 5 and 6 were not recorded due to very
low solubility in most of the available solvents (CDCl3, DMSO-d6 or
CD3CO).
S
O
3
1
4
R=
S
OH
10
6
5
2
3
7
8
4
5
9
O
12
11
13
R
O
N
N
6
2.2. Synthesis and characterization
Compounds 1–6 were obtained by refluxing 4.7 g (0.025 mol) 2-
acetyl-1,3-indandione, 0.05 mol of the corresponding aldehyde (1
– benzaldehyde, 2 – thiophene-2-aldehyde, 3 – thiophene-3-alde-
hyde, 4 – furan-2-aldehyde, 5 – pyridine-2-aldehyde, 6 – pyridine-
3-aldehyde) and 0.012 mol piperidine for 1 h. To the formed poly-
crystalline precipitate 50 mL of ethanol was added and the reaction
mixture was boiled for a half hour more. After cooling down the
formed crystals were filtered off and repeatedly washed with eth-
anol. The filtrate was further diluted with water, acidified and the
additionally formed amount of the product was collected. The total
amount of the products were recrystalized from ethanol.
Fig. 1. Structure of the substituted 2-acyl-1,3-indandiones; compounds 1–6.
experimental and theoretical methods. As can be seen, these struc-
tures allow for the formation of different keto–enol tautomers
involving the 2-acyl-1,3-indandione fragment, as well as different
cis–trans isomers in the substituted allylidene fragment. The struc-
tures were determined by X-ray diffraction and/or quantum chem-
ical (ab initio and DFT) methods, whereas the optical properties
were studied by UV–Vis absorption and emission spectroscopy in
solution. The main question was to identify the most stable con-
formers in solid state and gas phase as well as the origin of the
forces (intra- or intermolecular) determining their stabilization.
Thus, the focus was put on the effect of the remote heterocyclic
substituents on the interplay between inter- and intramolecular
interactions in defining the most stable isomers and the corre-
sponding photophysical properties.
Compound 1; 2-(1-hydroxy-3-phenyl-allylidene)-2H-inden-1,3-
dione, Yield 70%, M.p. 186–187 °C. Anal. Found, %: C, 78.09; H,
4.50. C18H12O3; Calcd, %: C, 78.25; H, 4.38. IR (Nujol)
>3200 ( O–H), 1700 ( C@O), 1650 (mC@O C@C), 1610 (mC@O
1585 ( C@C, (Ph)), 1550 ( C@C, (Ph)), 1290 (dOH), 1135 (dCH), 980 (c@C–
H), 750 (
Ar–H). 1H NMR (CDCl3), d, ppm: 13.15 (s, 1H), 7.85–8.00
m
, cmꢀ1: broad
C@C, (Ph)),
m
m
,
m
, m
m
m
c
(m, 2H), 7.64–7.73 (m, 4H), 7.4–7.42 (m, 5H). 13C NMR (CDCl3), d,
ppm: 197.4 (C7), 188.5 (C9), 173.1 (C10), 144.8 (C11), 141.0 (C6),
138.7 (C5), 134.9 (C2), 134.7 (C3), 134.0 (C13), 131.1 (C12), 129.0
(C17, C15), 129.0 (C18, C14), 122.6 (C1), 122.3 (C4), 117.9 (C16),
107.8 (C8).
2. Experimental
2.1. X-ray diffraction and spectroscopic experiments
Selected crystallographic and refinement data for structures 2, 3
and 4 obtained by the single crystal X-ray diffraction method are
reported in Table 1. The data collection for all three structures
was carried out on an Oxford Xcalibur Nova diffractometer with
Compound 2; 2-(3-thiophen-2-yl-1-hydroxy-allylidene)-2H-in-
den-1,3-dione, Yield 45%; M.p. 172–173 °C. Anal. Found, %: C,
68.47; H, 3.89; S, 12.48. C16H10O3S. Calcd, %: C, 68.07, H, 3.57, S,
11.36. IR (Nujol)
C@C), 1605 (mC@O
(Ph)), 1500 ( C@C, (Ph)), 1400 (
@C–H), 750 (
Ar–H). 1H NMR (CDCl3), d, ppm: 11.9 (s, 1H), 7.91–
m
, cmꢀ1: 3400 broad (
m
O–H), 1700 (
mC@C, (Ph)), 1590 (mC@C, (Ph)), 1555 (mC@C,
C@C, (Ph)), 1295 (dOH), 1135 (dCH), 980
mC@O), 1650
CCD detector and k(Mo Ka) = 0.71703 Å for compound 2 and
(mC@O
,
m
,
k(Cu K ) = 1.54184 Å for compounds 3 and 4. Data collection for
a
m
m
all structures has been performed by applying the CrysAlis Soft-
ware system, Version 1.171.32.29 [22] The Lorentz-polarization
effect was corrected and the intensity data reduced by the CrysAlis
RED application of the CrysAlis Software system, Version
1.171.32.29 [22].
(c
c
7.58 (H, arom), 7.57–7.31 (m, 3H), 7.20–6.97 (m, 2H). 13C NMR
(CDCl3), d, ppm: 197.2 (C7), 188.3 (C9), 172.6 (C10), 140.8 (C6),
140.5 (C5), 138.6 (C13), 137.1 (C11), 134.7 (C2), 133.9 (C3), 132.4
(C12), 130.8 (C14), 128.5 (C16), 122.4 (C1), 122.1 (C4), 116.4
(C15), 107.4 (C8).
The diffraction data were scaled for absorption effects by the
multi-scanning method. Both structures were solved by direct
methods and refined on F2 by weighted full-matrix least squares.
Programs SHELXS97 [23] and SHELXL97 [23] integrated in the
WinGX software system [24] were used to solve and refine the
structures. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed in geometrically idealized posi-
tions [Csp2–H 0.93 Å with Uiso (H) = 1.2 Ueq (C)] and were con-
strained to ride on their parent atoms by using the appropriate
SHELXL97 HFIX instructions. The hydrogen atom from the hydro-
xyl group was also found in difference Fourier maps at distances
0.86(5) and 0.97(2) Å in 3 and 4, respectively and was refined
freely. The molecular geometry calculations and drawings were
performed by ORTEP-3, [25] PLATON [26] and Mercury [27]. The
main geometrical features along with hydrogen bond geometry
Compound 3; 2-(3-thiophen-3-yl-1-hydroxy-allylidene)-2H-in-
den-1,3-dione, Yield 65%; M.p. 193–194 °C. Anal. Found, %: C,
68.29; H, 3.93; S, 13.40. C16H10O3S. Calcd, %: C, 68.07; H, 3.57; S,
11.36. IR (Nujol)
C@C) 1605 (mC@O
mC@C, (Ph)), 1550 (mC@O mC@C, (Ph)), 1320 (
(dCH), 985 ( @C–H), 820 ( Ar–H), 780 (
Ar–H). 1H NMR (CDCl3), d,
m
, cmꢀ1: 3400 broad (
C@C) 1580 (mC@O
m
O–H), 1700 (
mC@C, (Ph)), 1570 (mC@O
mN–Ar), 1285 (dOH), 1150
mC@O), 1625
(mC@O
,
m
,
m
,
,
,
c
c
c
ppm: 12.15 (s, 1H), 7.76–7.61 (m, 4H), 7.56–7.45 (m, 2H), 7.44–
7.33 (m, 3H). 13C NMR (CDCl3), d, ppm: 197.4 (C7), 188.6 (C9),
173.5 (C10), 140.9 (C6), 138.7 (C5), 138.3 (C13), 138.2 (C11),
134.8 (C2), 134.0 (C3), 130.3 (C12), 127.3 (C14), 125.7 (C15),
122.6 (C1), 122.2 (C4), 117.6 (C16), 107.5 (C8).
Compound 4; 2-(3-furan-2-yl-1-hydroxy-allylidene)-2H-inden-
1,3-dione, Yield 86%; M.p. 217–218 °C. Anal. Found, %: C, 72.5; H,

12
A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
Table 1
Summary of data collection and structure refinement details for compounds 2, 3 and 4.
Compound
2
3
4
Formula
C16H10O3S
C16H10O3S
C16H10O4
Mr
282.30
282.300
266.24
Crystal system, colour and habit
Orthorhombic, orange plates
P n a 21
Orthorhombic, red–orange plates
P n a 21
Monoclinic, red–orange plates
P 21/c
Space groupa
Crystal dimensions (mm)
0.60 ꢁ 0.30 ꢁ 0.20
0.47 ꢁ 0.29 ꢁ 0.24
0.22 ꢁ 0.14 ꢁ 0.08
Unit cell parameters
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
23.260(3)
4.7307(6)
12.0110(14)
–
1321.7(3)
4
1.419
296
27.2298(5)
3.8827(1)
12.3329(2)
–
1303.90(5)
4
1.438
296
11.8206(2)
3.8600 (1)
27.3872(4)
101.25(1)
1225.58(4)
4
1.443
296
Dc (g cmꢀ3
T (K)
)
l
(mmꢀ1
)
0.248
2.248
0.868
F (0 0 0)
584
584
552
h Range for data collection (°)
h, k, l range
Scan type
4–28
5–73
3–73
ꢀ10 to 30, ꢀ6 to 5, ꢀ14 to 15
ꢀ33 to 32, ꢀ3 to 4, ꢀ15 to 12
ꢀ14 to 14, ꢀ4 to 4, ꢀ33 to 33
x
x
x
No. measured reflections
No. independent reflections (Rint
No. observed reflections, I P 2r(I)
4438
2523 (0.0420)
1127
3453
1873 (0.0285)
1815
10,736
2424 (0.0325)
2158
)
No. refined parameters/restraints
194/1
185/1
185/0
g1, g2 in w
R, wR [I P 2r(I)]
0.0321, 0.000
0.049, 0.0719
0.0918,0.1513
0.898
0.0972, 0.0414
0.0432, 0.1168
0.0443, 0.1185
1.056
0.0913, 0.0965
0.0452, 0.1246
0.0497, 0.1291
1.032
R, wR [all data]
Goodness of fit on F2, S
Max., min. electron density (e Åꢀ3
)
0.231, ꢀ0.135
0.205, ꢀ0.163
0.207, ꢀ0.231
Maximum
D/r
<0.001
<0.001
0.001
a
Flack parameter for 3: 0.06(2).
Table 2
Selected interatomic distances (Å) and valence angles (°) for the compounds 2, 3 and 4 obtained from X-ray data and B3LYP/6-31G** optimizations.
2
3
4
X-ray data
B3LYP/6-31G**
X-ray data
B3LYP/6-31G**
X-ray data
B3LYP/6-31G**
Bond distances
O1–C9
O2–C7
O3–C10
C7–C8
C8–C9
C8–C10
C10–C11
C11–C12
C12–C13
1.229(4)
1.244(5)
1.338(4)
1.456(6)
1.438(6)
1.367(6)
1.428(6)
1.337(6)
1.441(6)
1.225
1.244
1.332
1.454
1.472
1.390
1.442
1.357
1.437
1.219(3)
1.251(3)
1.335(3)
1.446(3)
1.469(3)
1.378(3)
1.435(3)
1.342(3)
1.439(3)
1.226
1.243
1.331
1.454
1.472
1.390
1.444
1.354
1.449
1.2119(17)
1.2403(17)
1.3369(17)
1.4443(18)
1.4726(18)
1.3829(19)
1.4274(19)
1.348(2)
1.225
1.244
1.333
1.454
1.473
1.390
1.442
1.357
1.429
1.422(2)
Bond angles
C2–C1–C6
C1–C2–C3
C2–C3–C4
C3–C4–C5
C4–C5–C6
C1–C6–C5
117.0(6)
120.9(5)
121.9(6)
117.5(5)
122.1(4)
120.7(5)
117.8
120.9
121.0
117.9
121.1
121.3
117.6(3)
121.3(2)
120.1(3)
118.4(3)
121.2(2)
121.3(3)
117.8
120.9
121.0
117.9
121.0
121.3
118.04(13)
121.07(13)
120.84(13)
118.08(12)
121.11(11)
120.85(12)
117.8
120.9
121.0
117.9
121.0
121.3
3.7. C16H10O4. Calcd, %: C, 72.18; H, 3.79. IR (Nujol)
3500 broad ( O–H), 1700 ( C@O), 1645 (mC@O C@C), 1615 (mC@O
C@C), 1590 (mC@O mC@C, (Ph)), 1530 (mC@O mC@C, (Ph)), 1280 (dOH),
@C–H), 850 (
Ar–H). 1H NMR (CDCl3 + DMSO-d6),
d, ppm: 11.15 (s, 1H), 7.74–7.5 (m, 4H), 6.76–6.75 (m, 3H), 6.49–
6.5 (m, 2H). 13C NMR (CDCl3 + DMSO-d6), d, ppm: 196.5 (C7),
187.6 (C9), 172.0 (C10), 150.9 (C13), 145.8 (C16), 140.3 (C6),
138.0 (C5), 134.3 (C3), 133.5 (C2), 129.8 (C12), 121.9 (C1), 121.5
(C4), 117.0 (C11), 114.6 (C14), 112.6 (C15), 106.9 (C8).
m
, cmꢀ1: 3400–
(
mC@O
,
mC@C, (Ph)), 1575 (mC@O, mC@C, (Ph)), 1280 (dOH), 1150 (dCH),
@C–H).
m
m
,
m
,
975 (
c
m
,
c
,
Compound 6; 2-(3-pyridin-3-yl-1-hydroxy-allylidene)-2H-inden-
1,3-dione, Yield 85%; M.p. 234–235 °C. Anal. Found, %: C, 73.60; H,
4.48; N, 5.16. C17H11O3N. Calcd, %: C, 73.64, H, 4.00; N, 5.05. IR (Nu-
jol)
1630 (mC@O
1545 (mC@O
1150 (dCH), 985 (
c
m
, cmꢀ1: 3500 broad (
C@C), 1595 (mC@O
C@C, (Ph)), 990 ( @C–H), 835 (c
m
O–H), 1700 (
mC@C, (Ph)), 1580 (mC@O
Ar–H).
m
C@O), 1650 (mC@O
, mC@C),
,
m
,
,
mC@C, (Ph)),
,
m
c
Compound 5; 2-(3-pyridin-2-yl-1-hydroxy-allylidene)-2H-inden-
1,3-dione, Yield 79%; M.p. 221–222 °C. Anal. Found, %: C, 74.05; H,
4.39; N, 5.18. C17H11O3N. Calcd, %: C, 73.64; H, 4.00; N, 5.05. IR (Nu-
2.3. Quantum-chemical calculations
The quantum-chemical calculations were performed with full
geometry optimization without any symmetry restrictions. The po-
tential energy surface for all studied compounds was searched for
jol)
m
, cmꢀ1: 3450 broad (
C@C), 1650 (mC@O
mO–H), 2500–2600 (
m
C@N), 1690 (
mC@O),
1675 (mC@O
,
m
,
m
C@C), 1630 (mC@O,
mC@C, (Ph)), 1585

A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
13
Table 3
Hydrogen bond geometry and geometrical parameters of
p–p interactions (Å, °) in the crystal structures of compounds 2, 3 and 4.
D–Hꢂ ꢂ ꢂA
D–H
Hꢂ ꢂ ꢂA
Dꢂ ꢂ ꢂA
\Hꢂ ꢂ ꢂA
Symmetry code
2
O3–H3Oꢂ ꢂ ꢂO2
C3–H3ꢂ ꢂ ꢂO2
C14–H14ꢂꢂꢂO1
0.82
0.93
0.93
1.89
2.618
2.689
2.629(5)
3.393(7)
3.324(6)
149
141(1)
126(1)
–
ꢀx + 1/2 + 1,+y + 1/2,+z ꢀ 1/2
ꢀx + 1,ꢀy + 1,+z + 1/2
3
O3–H3Oꢂ ꢂ ꢂO2
C3–H3ꢂ ꢂ ꢂO2
C14–H14ꢂꢂꢂO1
C15–H15ꢂ ꢂ ꢂO2
0.86(5)
0.93
0.93
1.83(5)
2.676
2.550
2.746
2.650(3)
3.417(4)
3.246(4)
3.520(4)
162(5)
137(1)
132
–
ꢀx + 1/2, +y ꢀ 1/2, +z ꢀ 1/2
1 ꢀ x, 2 ꢀ y, 1/2 + z
ꢀx + 1, ꢀy + 1, +z ꢀ 1/2
0.93
141(1)
4
O3–H3Oꢂ ꢂ ꢂO2
C16–H16ꢂ ꢂ ꢂO1
C14–H14ꢂꢂꢂO2
C2–H2ꢂꢂꢂO2
0.97(2)
0.93
0.93
1.73(2)
2.510
2.859
2.789
2.629(2)
3.216(2)
3.770(2)
3.475(2)
153(2)
133
166(1)
131(1)
–
1 ꢀ x, ꢀy, 1 ꢀ z
ꢀx, ꢀy + 1, ꢀz + 1
ꢀx, +y ꢀ 1/2, ꢀz + 1/2 + 1
0.93
Interactiona
Cg–Cg distance
Cgꢂ ꢂ ꢂP1b
Cgꢂ ꢂ ꢂP2c
bd
Slippage
3
Cg(2)ꢂ ꢂ ꢂCg(3)i
4
3.792(2)
3.638(1)
3.479(1)
3.500(1)
3.495(1)
3.524(1)
22.85
14.36
1.473
0.902
Cg(2)ꢂ ꢂ ꢂCg(3)i
Symmetry code: i = x, 1 + y, z. Cg(2): centroid of cyclopentane plane in 3 and 4. Cg(3): centroid of benzene plane in 3 and 4.
a
The dihedral angle between planes which define the centroids Cg(2) and Cg(3) is 0.72(14)° in 3 and 1.55(7)° in 4.
b,c
Cgꢂ ꢂ ꢂP1 (or P2) is the perpendicular distance of corresponding centroid to a plane. Planes P1 or P2 are defined by the atoms, which define the corresponding centroids.
d
b is the angle between Cgꢂ ꢂ ꢂCg vector and vertical line on corresponding plane.
stable conformers. Geometries of six to eight possible rotamers and
31G** optimizations are depicted in Fig. 2. It can be seen that both
methods predict that the exocyclic enolic tautomers a, of the s-
trans isomers, are energetically the most favorable ones. Energy
diagrams of all optimized conformers for each compound are pre-
sented in Supplementary material. The calculated relative stabili-
ties of the possible isomers of compounds 1–6 are estimated on
the basis of ZPVE-corrected electronic energies. (Inclusion of the
zero-point vibrational energy (ZPVE) corrections do not change
the energy order of the studied isomers, as could be seen from
Table SM1 in the Suppl. Mater.) In summary, the calculations show
that the energy differences between the a and b tautomers are ca.
4.0 kcal/mol (varying from 4.04 for 4 and 5 to 3.94 kcal/mol for 6)
according to DFT and ca. 5.0 kcal/mol according to HF data. A rota-
tion about the C10–C11 single bond leads to s-cis isomer that,
according to the DFT calculations, have energies that are 2.3–
4.6 kcal/mol higher than those of the most stable structures, and
even 4.1–6.2 kcal/mol according to the HF calculations. Also inter-
esting is the orientation of the heterocyclic ring, i.e. the C12–C13
bond rotation. As can be seen from Fig. 2, the thiophene and furan
substituents in compounds 2, 3 and 4 are oriented so that the het-
erocyclic double bond position leads to a s-trans configuration of
the C12–C13 bond, too. In this way all s-trans configuration,
regarding the configuration of the single C10–C11 and C12–C13
bonds, is realized. A similar trend can be identified for the struc-
tures 5 and 6, although here deviations occur due to strong delocal-
ization in the pyridine ring. Nevertheless, the hetero-atom in
compound 5 is directly bonded to C13 and is positioned closer to
the 1,3-indandione fragment, similarly to structures 2 and 4. Com-
pounds 3 and 6 are positional isomers of 2 and 5, respectively, and
this affects the orientation of the heterocyclic substituents as can
be seen in Fig. 2. The orientation of the heterocyclic rings raises
the question about the role of intra- and/or intermolecular forces
for stabilization of different rotamers.
tautomers were optimized by the semiempirical AM1 and ab initio
Hartree–Fock methods. Vibrational frequencies and intensities
were computed at the same level of theory as used for the struc-
ture optimization in order to confirm that local energy minima
were attained. Selected structures were further optimized using
the density functional theory (DFT) and the hybrid B3LYP func-
tional [28,29]. For all calculation the 6-31G** basis set was used.
At the equilibrium geometries of the stable conformers vertical
excitation energies were calculated employing ZINDO and time-
dependent DFT (TD-DFT) with the above mentioned functional
and basis set. TD-DFT was also applied in combination with IEF-
PCM to account for the solvent effect. All calculations were carried
out using the Gaussian 03 program package [30]. Supplementary
material contains the energy diagrams of the calculated isomers/
tautomers of compounds 1–6 and a complete list of the obtained
total energies (in a.u. and kcal/mol) and zero-point vibrational
energies.
3. Results and discussion
3.1. Theoretical calculations
From a structural point of view, the studied series of com-
pounds allow two different kinds of intramolecular rearrange-
ments: intramolecular proton transfer, i.e. tautomerization, and
cis–trans-isomerizations. Of simplicity, we will call the exo- and
endo-cyclic enolized forms tautomers a and b, respectively. As
far as the C–C bond rotations are concerned, we shall study isomer-
izations via the single C10–C11 and C12–C13 bond rotations. A
rotation about C10–C11 bond leads to the s-cis and s-trans isomers,
and rotation about C12–C13 bond affects the orientation of the
heterocyclic substituent (see Figs. 1 and 2). We found that in all
optimized structures the C11–C12 double bond has trans-orienta-
tion of the H-atoms. (The structures and energies of cis-isomers
were computed for the more stable tautomer and showed much
higher energies and therefore were not considered further.) The
most stable structures obtained from HF/6-31G** and B3LYP/6-
It is interesting to note that the energy of the isomeric forms
that result from rotating the heterocyclic substituents about the
C12–C13 bond is less than 1 kcal/mol higher than the most stable
structures in Fig. 2. The largest energy difference between these
rotamers is obtained for compound 3 (0.97 and 1.26 kcal/mol
according to DFT and HF calculations, respectively) and the small-

14
A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
O
O
1
4
1
4
OH
10
OH
6
5
6
5
2
2
7
7
8
8
10
9
9
3
3
12
12
11
11
O
O
O
1a
4a
O
O
1
1
4
OH
10
OH
10
6
6
5
2
2
7
7
8
8
9
9
3
3
12
12
5
11
11
4
O
O
S
S
2a
5a
3a
O
O
1
1
OH
10
OH
10
6
6
5
2
2
7
7
8
8
9
9
3
3
12
12
5
11
11
4
4
O
O
N
N
6a
Fig. 2. Structures of the most stable conformers of compounds 1–6, according to the HF/6-31G** and B3LYP/6-31G** calculations.
est one is for compound 5 (0.36 and 0.12 kcal/mol according to DFT
and HF calculations, respectively). This implies that rotation of the
heterocyclic substituent about C12–C13 bond easily can result
from external factors like temperature, light, solvents, etc. In an at-
tempt to identify a structural parameter that could possibly reflect
the relative energies of the discussed rotamers it was noted that
the DFT-optimized C12–C13 bond length is shorter by 0.003 Å in
the most stable rotamers in compounds 2, 3 and 4 and only by
0.001 Å in compound 5. This might influence the degree of electron
delocalization in the molecule leading to stabilization of the corre-
sponding rotamer. Another interesting fact is that the crystal struc-
tures of compounds 2, 3 and 4, described further below, agree very
well with the results of the isolated molecule gas-phase optimiza-
tions (HF/6-31G** and B3LYP/6-31G**) predicting the same confor-
mation as those obtained in the crystals. The same holds for the
structure of compound 6, which was solved from powder diffrac-
tion [31]. Selected optimized structural parameters are compared
with the experimental ones in Table 2.
Planarity of 1,3-indandione moiety in the molecules is con-
firmed by the dihedral angle values of 1.1(2)°, 0.72(14)° and
1.55(7)° between the planes of the benzene and cyclopentane rings
in 2, 3 and 4, respectively. The keto oxygen atoms O1 and O2 are
0.006(3) and ꢀ0.039(3) Å in 2, ꢀ0.002(2) and ꢀ0.002(3) Å in 3
and 0.013(1) and ꢀ0.010(1) Å in 4 apart of the cyclopentane ring
plane. The geometry of the fused benzene ring with saturated
five-membered ring in indane fragment of the molecule is affected
so that the bonds at C1 and C4 are shortened (C1–C6 1.397(8) Å,
C4–C5 1.357(7) Å in 2, (C1–C6 1.380(3) Å, C4–C5 1.373(4) Å in 3
and C1–C6 1.386(2) Å, C4–C5 1.384(2) Å in 4). The corresponding
bond angles decrease and are less than 120° (C2–C1–C6 117.0(5)°
and C3–C4–C5 117.4(5)° in 2, C2–C1–C6 117.6(3)° and C3–C4–C5
118.4(3)° in
3 and C2–C1–C6 118.04(13)° and C3–C4–C5
118.08(12)° in 4). The shortening of those bonds is accompanied
with the elongation of the others within the benzene ring.
The b-diketone fragment is characterized by the existence of a
resonance-assisted intramolecular hydrogen bond (RAHB) [32,33]
O3–H3Oꢂ ꢂ ꢂO2 whose geometry (Oꢂ ꢂ ꢂO distance about 2.6 Å and
O–HꢂꢂꢂO angle in the range 150–160°; Table 3) is as expected for
RAHB due to the presence of the enol form. The RAHB forms well
recognizable supramolecular synthon described by the graph-set
analysis as the S(6) type. The geometry of the RAHB is not influ-
enced by the type of the substituent i.e. thiophene or furan ring
in 2, 3 and 4, respectively.
3.2. Crystal structure descriptions of compounds 2, 3 and 4
The molecular structures of compounds 2, 3 and 4 are depicted
in Figs. 3a, 4a and 5a, respectively. All compounds are planar in the
crystalline state and are in a trans configuration regarding the spa-
tial orientation of the substitutents at C11@C12 double bond. The
planarity of the molecules is indicated by the dihedral angle value
of 6.35(17)° and 5.35(14)° between 1,3-indandione fragment and
thiophene plane in 2 and 3, respectively, and 3.12(9)° between
1,3-indandione fragment and furan plane in 4.
The participation of the keto O2–C7 group in the RAHB forma-
tion influences the O2–C7 bond length (the bond is elongated) in
comparison with the keto O1–C9 bond length (O1–C9 1.229(6) Å
and O2–C7 1.244(7) Å in 2, O1–C9 1.219(3) Å and O2–C7

A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
15
Fig. 3. ORTEP drawing of compound 2 including the atom numbering scheme. The thermal ellipsoids are drawn at the 50% probability level at 296 K (3a); the packing of
molecules in ac plane and herringbone mode of packing of molecules in 2 viewed in ab plane (3b).
1.251(3) Å in 3 and O1–C9 1.212(2) Å and O2–C7 1.240(2) Å in 4).
On the contrary, the O3–C10 bond in 2, 3 and 4 (1.338(5),
1.335(3) Å and 1.337(2) Å, respectively; Table 2), has a character
derivatives like the 2-pivaloyl-1,3-indandione structure [35], 2-
acetyl-1,3-indandione [36], 2-(hydroxy(amino)methylidene)-1,3-
indandione [9] and 2-acylindan-1-one [37].
of
r
Car–OH bond as those found in phenols [34]. Such a molecular
The geometry of allylidene moiety is characterized by p-elec-
geometry of the b-diketone fragment in 2, 3 and 4 is in agreement
tron delocalization through C–C single and double bonds (C8–
with what is found in similar 1,3-indandione and indan-1-one
C10 and C11–C12 bond lengths are 1.366(7) Å and 1.337(7) Å in

16
A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
Fig. 4. ORTEP drawing of compound 3 including the atom numbering scheme. The thermal ellipsoids are drawn at the 50% probability level at 296 K (4a); the packing of
molecules in ac plane and herringbone mode of packing of molecules 3 viewed in ab plane (4b).
2, 1.378(3) Å and 1.342(3) Å in 3 and 1.383(2) Å and 1.348(2) Å in
4, while C10–C11 and C12–C13 bond lengths are 1.428(6) Å and
1.442(6) Å in 2, 1.435(3) Å and 1.439(3) Å in 3 and 1.427(2) Å
and 1.422(2) Å in 4) (Table 2).
The compounds 2 and 3 are isostructural on the basis of similar-
ities of molecular geometry, morphological accordance, resem-
blance of unit cell parameters (Table 1) and intermolecular
interactions in their crystal structures (Table 3, Figs. 3b and 4b).
The keto O1 atom in 2 and 3 participate in intermolecular hydro-
gen bond formation with the thiophene C–H group (C14–H14) (Ta-
ble 3) thus forming infinite zig-zag chains along the b axis. The
molecules are arranged in a herringbone fashion in ab plane which
enables antiparallel molecular stacking (Figs. 3b and 4b). An excep-
tion is compound 4. Here, the discrete centrosymmetric dimers are
formed via C16–H16ꢂ ꢂ ꢂO1 intermolecular hydrogen bond (Table 3).
The dimers form 20-membered rings with two donor and two
acceptor groups, which according to a graph-set analysis can be de-
scribed as R22ð20Þ (Fig. 5b).
Moreover,
p–p
stacking geometry in 3 and 4 (Table 3) reveal
-overlap exists between the 1,3-indandione
that considerable
p
fragments of two adjacent molecules shifted along the shortest b
axis (Table 3, symmetry code: x, 1 + y, z). Almost ideal slippage is
found in 3 (Table 3). Interestingly, b axis in 2 is also the shortest
one, but longer to some extent (Table 1) than in 3 and 4 and
aromatic stacking with reasonable geometry is not found in 2
(the shortest slippage is about 2 Å). The criteria for aromatic
p–p
p–p

A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
17
Fig. 5. ORTEP drawing of compound 4 including the atom numbering scheme. The thermal ellipsoids are drawn at the 50% probability level at 296 K (5a); centrosymmetric
discrete dimers as a packing mode of molecules 4 viewed down b axis (5b).
stacking geometry is based on the following values: perpendicular
distance between the corresponding centroids of two planes no
longer than 3.8 Å (3.3–3.8 Å; CgP1 and CgP2) and dihedral angles
(between P1 and P2) no larger than 20° [38–40]. The calculated
slippage on the basis of Cgꢂ ꢂ ꢂCg distances as well as the value of
the angle between Cg...Cg vector and vertical line on corresponding
plane has been taken into account as one of the geometrical crite-
i.e., predicting higher excitation energy: the wavelength is re-
duced by ca. 30 nm compared to the experimental data. On the
other hand, when including the solvent effects, the calculations
give good agreement with the observed absorption maxima (Ta-
ble 4, ethanol); the largest deviation (ca. 10 nm) is obtained for
compounds 2, 3 and 4. Furthermore, TD-DFT calculations were
performed for the optimized structures of the other possible rota-
mers and tautomers in gas phase. The results show that cis–trans
isomerisations lead to small shifts in the vertical excitation ener-
gies (less than 10 nm) whereas keto–enol tautomerization results
in drastic changes in the calculated excitation energies. Moreover,
according to the gas-phase calculations tautomerization results in
bathochromic shift of the long-wave-length band, by ca. 30 nm,
and a similar shift but in the opposite direction of the other band.
The former has only small oscillator strength, whereas the latter
appears at about 330 nm and has much larger oscillator strength;
ten times larger in compounds 2, 3 and 4. Such drastic changes in
the experimental spectra, in six different solvents, were not ob-
served except the strong hypsochromic shift seen in aqueous
solution of compounds 2, 3 and 4. Poor solubility of all com-
pounds in water, however, made a more detailed analysis not
possible. The calculated energies of the a and b tautomers (Suppl.
Mater.) do not show evidence for possible tautomerization. Fur-
ther quantum-chemical calculations, explicitly taking account
for the solvent effect, may reveal a possible tautomerization as-
sisted by solvent molecules.
ria, too (approx. 1.5 Å for p–p stacking interactions).
3.3. Optical properties
Absorption spectra of all compounds in different solvents were
recorded at room temperature. The data are summarized in Table 4
and compared with the calculated values for the vertical excitation
energies for the most stable conformers of compounds 1–6 (shown
in Fig. 2) obtained from TD-DFT B3LYP/6-31G** calculations in gas
phase and in ethanol. The absorption spectra of the studied com-
pounds are characterized by a high molar absorptivity (40,000–
45,000 Mꢀ1 cmꢀ1) and composite structure. The latter is more pro-
nounced in a non-polar solvent like cyclohexane (not shown). Polar
solvents cause slight bathochromic shift. Interestingly, however,
the spectra in water are blue shifted by ca. 40 nm for 2, 3 and 4.
The other compounds do not show this shift. In comparison with
the absorption properties of 2-acetyl-1,3-indandione (2AID) the
expected strong bathochromic shift for compounds 1–6 (due to
Another optical property that may give information on tautom-
erism is the emission spectrum. Steady-state fluorescence spectra
were, therefore, recorded in three different solvents and it was
found that they are characterized by a broad and structureless
emission band. The estimated Stokes shift is about 100 nm (see
Table 4), and the observed large Stokes shift (e.g. 125 nm for 1 in
acetonitrile) suggests the occurrence of some intramolecular rear-
rangements in excited state. Similarly to the 2AID fluorescence,
characterized by Stokes shift of 200 nm, the possibility of intramo-
lecular proton transfer in the excited state of compound 1 was
studied through theoretical calculations. We applied optimization
procedure in which a single structural parameter was changed in
small step in order to calculate the potential energy surface along
the tautomerization or cis–trans isomerization process. The tauto-
merization was studied by changing the O3ꢂ ꢂ ꢂH distance (see atom
the presence of elongated
p-conjugation) is observed. Another
structural parameter defining the position of the absorption max-
ima is the electron donating strength of the conjugated substituent
as well as the planarity of the whole molecule. According to the
DFT data, all studied compounds in their most stable conformation
are very close to being completely planar; the largest dihedral an-
gle between the plane of the 1,3-indandione fragment and that of
the heterocyclic substituent is 0.07° for compound 2. Comparing
the absorption properties of compounds 1–6, shown in Table 4, it
can be concluded that molecular planarity, leading to a strong elec-
tron delocalization, is the main factor governing their optical
properties.
The calculated vertical excitation energies of the most stable
conformers of compounds 1–6 (TD-DFT B3LYP/6-31G**) in vac-
uum show a qualitative agreement with the experimental data,

18
A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
numbering in Figs. 3a, 4a and 5a) from 0.900 to 1.700 Å, whereas
for the cis–trans isomerization we varied the C8–C10–C11–C12
dihedral angle (see Fig. 6). These calculations were performed in
the energetically lowest singlet and triplet states. To consider the
energetically lowest triplet state instead of the first excited singlet,
was used as an approximation that has successfully been applied
for Stokes-shift calculation and motivated elsewhere [41]. For fur-
ther confirmation of the applicability of this approximation, we
performed analogous calculation for the tautomerization process
in ground and excited triplet state for 2AID (show in Fig. 6), which
has previously been studied by experimental and theoretical meth-
ods at the semiempirical level [11]. Our DFT calculation, with this
approximation, are in agreement with the previous results for
2AID, showing that the relative stability of the a and b tautomers
is different in the ground and the excited state, and thus explains
the experimentally observed indication for ESIPT and its high
photostability [12]. As can be seen from Fig. 6, the results for com-
pound 1 show that in the ground and the excited state the relative
stability of the studied tautomers is not reversed, i.e. in both states
tautomer a is the most stable one. Similar results were obtained
from calculations for the reaction coordinate for the cis–trans isom-
erization. These findings suggest that most probably the large
Stokes shift is caused instead by strong charge redistribution tak-
ing place during the electronic transitions.
4. Conclusion
The structures of six 2-acyl-1,3-indandione derivates that pos-
ses intramolecular hydrogen bonding and extended
p-electron
conjugation, were determined in crystalline state and in gas phase
using X-ray diffraction and quantum-chemical calculations,
respectively. The results show that the molecular conformation
in gas phase is the same like in solid state although C–C bond rota-
tions are energetically allowed and despite the intermolecular
interactions present in the crystal packing. This implies that the
main component of stabilization energy of this series of com-
pounds is the degree of
p-electron delocalization between the
1,3-indandione fragment and the heterocyclic ring through the
butadiene bridge. The optical (absorption) properties of the com-
pounds were studied using both experimental and by theoretical
methods, too. It was shown that the calculated vertical excitation
energies are not sensitive to cis–trans isomerizations but differ
considerably in the keto–enol tautomers. The calculated energy
differences between both possible tautomers, however, are too
large to allow tautomerization. Nevertheless, the spectra of com-
pounds 2, 3 and 4 in water show strong hypsochromic shift, ex-
pected for the
b tautomer. To justify theoretically that
tautomerization might take place in water would require a more
accurate description of the process including an explicit treatment
of the solvent molecules so that a proton transfer can be described.
Our quantum-chemical calculations of the energetics along two
possible reaction coordinates of compound 1 (tautomerization
and rotamerization) in the ground and the energetically lowest
triplet state show that the relative stability of the studied tautom-
ers and rotamers is the same in both states. Thus, the experimen-
tally observed strong Stokes shift of this compound could not be
explained through intramolecular rearrangements in the excited
state, in contrast to the case for 2AID, but is more likely caused
by significant charge redistribution during the electronic transi-
tions. Although the gas-phase calculations do not predict intramo-
lecular rearrangements (tautomerization or rotamerizations) in the
ground or excited state, the experimental data would require a
more thorough theoretical description with an explicit description
of the solvent molecules. This is, however, beyond the scope of the
present work.

A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
19
-649,58
-649,60
-649,62
-649,64
-649,66
-649,68
-649,70
2AID; B3LYP/6-31G**
Exc. Triplet a->b
Ground Singlet a->b
1,0
1,2
1,4
1,6
O ... H distance [A]
-918,74
-918,75
-918,76
-918,77
-918,78
-918,79
-918,80
-918,81
-918,82
-918,83
-918,84
-918,85
compound 1
Exc. Triplet ; 1: a -> b
Ground Singlet ; 1: a -> b
1,0
1,2
1,4
1,6
O ... H distance [A]
-918,74
-918,76
-918,78
-918,80
-918,82
-918,84
compound 1
Exc. Triplet ; 1:
s-cis -> s-trans
Ground singlet ; 1:
s-cis -> s-trans
0
20
40
60
80
100
120
140
160
180
dihedral angle [º]
Fig. 6. Energy diagram for tautomerization (a ? b) or cis–trans isomerization (s-cis ? s-trans) processes for 2-acetyl-1,3-indandione (2AID) and compound 1 followed by
B3LYP/6-31G** calculations in ground and excited triplet states.

20
A. Ahmedova et al. / Journal of Molecular Structure 981 (2010) 10–20
[14] A. Ahmedova, V. Atanasov, P. Marinova, N. Stoyanov, M. Mitewa, Cent. Eur. J.
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CCDC numbers 775554–775556 for compounds 2, 3 and 4,
respectively, contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cam-
bridge Crystallographic Data Centre (CCDC), 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 (0) 1223 336033; email: depos-
it@ccdc.cam.ac.uk]. Structure factors table is available from the
authors. Supplementary Material contains energy diagrams of all
optimized conformers with B3LYP/6-31G** and HF/6-31G** of
compounds 1–6 as well as a table listing the calculated total ener-
gies, ZPVE and energy differences. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.molstruc.2010.07.008.
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