Merocyanines based on 1,3-indanedione: Electronic structure and solvatochromism

Article abstract of DOI:10.1002/poc.1821

A series of merocyanines derived from 1,3-indanedione and heterocycles of various electron-donating properties was studied in detail. Their solvatochromic properties were explored in a wide range of solvent polarities to reveal the dependences of their chromacity and electronic structure on the key structural parameters - the properties of a donor heterocycle and the polymethine chain length. Also the dyes were studied by NMR spectroscopy and by quantum chemical calculations, both with the semiempirical AM1 and the non-empirical density functional theory/B3LYP method. The solvatochromic properties of the explored dyes are rather close to those of merocyanines derived from malononitrile as acceptor group. Appreciable distinctions were observed only in protic ethanol; obviously, they are connected with the formation of solvent-solute H-bonds in the case of 1,3-indanedione derivatives. The electron-acceptor properties of 1,3-indanedione were found to be somewhat stronger in comparison with those of malononitrile even in aprotic solvents, contrary to the known literature data. Analysis of the merocyanines' molecular orbitals and simulation of their electronic spectra were carried out both in vacuum and in the solvent matrix, and the absorption electronic transitions were analyzed. Static nonlinear optical properties were calculated for both the new merocyanines and the corresponding cationic and anionic cyanine dyes. Copyright

Full text of DOI:10.1002/poc.1821

Research Article
Received: 10 June 2010,
Revised: 27 September 2010,
Accepted: 15 October 2010,
Published online in Wiley Online Library: 13 December 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1821
Merocyanines based on 1,3-indanedione:
electronic structure and solvatochromism
a
a
a
*
Andrii V. Kulinich , Nadia A. Derevyanko , Elena K. Mikitenko
and Alexander A. Ishchenkoa
A series of merocyanines derived from 1,3-indanedione and heterocycles of various electron-donating properties was
studied in detail. Their solvatochromic properties were explored in a wide range of solvent polarities to reveal the
dependences of their chromacity and electronic structure on the key structural parameters – the properties of a donor
heterocycle and the polymethine chain length. Also the dyes were studied by NMR spectroscopy and by quantum
chemical calculations, both with the semiempirical AM1 and the non-empirical density functional theory/B3LYP
method. The solvatochromic properties of the explored dyes are rather close to those of merocyanines derived from
malononitrile as acceptor group. Appreciable distinctions were observed only in protic ethanol; obviously, they are
connected with the formation of solvent–solute H-bonds in the case of 1,3-indanedione derivatives. The electron-
acceptor properties of 1,3-indanedione were found to be somewhat stronger in comparison with those of mal-
ononitrile even in aprotic solvents, contrary to the known literature data. Analysis of the merocyanines’ molecular
orbitals and simulation of their electronic spectra were carried out both in vacuum and in the solvent matrix, and the
absorption electronic transitions were analyzed. Static nonlinear optical properties were calculated for both the new
merocyanines and the corresponding cationic and anionic cyanine dyes. Copyright ß 2010 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: 1,3-indanedione; merocyanines; solvatochromism; quantum-chemical calculations
limit (A2) the molecular dipole moment remains practically
INTRODUCTION
constant at excitation. Thus, in merocyanines with a ground-state
electronic structure diverted from A2 to this or that side, an
excitation causes a light-induced CT. It was also shown that in the
A2 state an excitation results in a unique p-charge reversal in
the chromophore. This phenomenon was termed as charge
resonance according to Mulliken’s nomenclature.[1]
Since the long-wavelength electronic transition in the ideal
polymethine state is accompanied by the least change of bond
orders, the vibronic interactions (VI) in A2 are weaker than in
A1 and A3. This results in narrowing of the spectral bands and
increasing of their peak intensity (extinction) and favors an
increase of the fluorescence quantum yield.[1] Also, the energy of
excitation to the first excited singlet state in A1 and A3 is
relatively high and goes through a minimum in A2.
Due to the remarkable variability of their electronic
structure, connected both with structural modifications of the
dye molecule and their sensitivity to changes of the solvent
polarity, merocyanines find an increasing application in modern
technologies. For example, the positively solvatochromic merocy-
anines with ground state structures between A1 and A2 display
Merocyanine dyes are polymethines for which a significant
intramolecular charge transfer (CT) from donor (D) to acceptor
(A) terminal groups takes place (this term means the fictitious
charge shift within the p-system of a real molecule in relation to
the hypothetical resonance structures of the theory of resonance
or mesomerism). The merocyanine electronic structure can be
visually described by means of superposition of three ideal
structures A1–A3 (Scheme 1). They correspond to three ideal
states – neutral polyene (A1), ideal polymethine (A2), and dipolar
polyene (A3).[1–3]
Depending on the terminal groups’ structure, polymethine
chain length, and solvent nature, merocyanines can reach any of
the three limiting states as well as states intermediate between
A1 and A2, A2, and A3. Changing of the merocyanine electronic
structure in the direction from A1 to A2 and from A2 to A3 is
accompanied by a growth of the dipole moments of their
molecules. The p-electrons are appreciably localized in A1 and
A3, but maximal delocalized in A2, which has been experimen-
tally proved by determination of the solvent dependence of the
specific polarizability of cyanines and merocyanines.[1] Also, A2 is
characterized by a maximal p-charge alternation along chromo-
phore. For that reason A2 having a symmetrical p-electron
distribution through the polymethine chain has been designated
as ideal polymethine state or cyanine limit.
*
Correspondence to: A. V. Kulinich, Institute of Organic Chemistry, National
Academy of Sciences of Ukraine, Murmanskaya Street 5, 02094 Kiev, Ukraine.
E-mail: andrii.kulinich@gmail.com
For merocyanines the electronic p ! p*-transition (usually
S0 ! SF1C) is accompanied by a significant growth of the
dipole moment for structures within the interval A1–A2 and
by its reduction for structures within A2–A3, while in the cyanine
a
A. V. Kulinich, N. A. Derevyanko, E. K. Mikitenko, A. A. Ishchenko
Institute of Organic Chemistry, National Academy of Sciences of Ukraine,
Murmanskaya Street 5, 02094 Kiev, Ukraine
J. Phys. Org. Chem. 2011, 24 732–742
Copyright ß 2010 John Wiley & Sons, Ltd.

MEROCYANINES BASED ON 1,3-INDANEDIONE
Scheme 1.
distinct nonlinear optical (NLO) and electro-optical properties[3–6]
;
it is characterized by high symmetry, has only one type of
heteroatoms, and its carbonyl groups are spatially accessible
to solvation. It was also found that merocyanines based on
malononitrile can possess very large hyperpolarizabilities (b0),[17]
which are promising for nonlinear optics. The deeper coloring of
1,3-indanedione dyes (thanks to the more extended intrinsic
p-system of this nucleus), as well as their position at the
scale A1–A2–A3 close to that in malononitrile dyes (due to
close electron-acceptor properties of the 1,3-indanedione
and malononitrile residues) should cause a further growth of
their b0 values.
materials based on them are competitive to traditional NLOs –
inorganic crystals. Since the position and shape of electronic
spectral bands of merocyanines, as well as their fluorescence
quantum yields, strongly depend on the solvent nature, they are
used (e.g. MC-540) as polarity indicators, biological probes, and
labels.[7,8] Donor–acceptor dyes have also significant potential in
other fields related to light energy transformations.[9,10] There-
fore, the finding and generalization of regularities connecting
electronic structure and properties of merocyanines with their
chemical structure are of great interest.
Earlier in our work numerous merocyanines with the electronic
structure varied over the whole scale A1–A2–A3 due to changes
in their terminal donor and acceptor groups, polymethine
chain lengths, and solvent polarity[11–14] were investigated. As an
acceptor group the residues of malononitrile, barbituric, and
thiobarbituric acid were used.
Thereby, the purpose was to investigate the electronic
structure, chromaticity, and solvatochromism of merocyanines
1–9 in comparison with the corresponding characteristics of the
malononitrile derivatives 10–18 studied before (Scheme 3).[11,13]
This work deals with the investigation of merocyanines 1–9
(Scheme 2), based on 1,3-indanedione as an acceptor group and
heterocycles of moderate (1–3), strong (4–6), and feeble (7–9)
electron-donating abilities. Compound 1 was earlier described in
Ref.[15]; dyes 2–9 are newly synthesized.
According to,[16] the electron-acceptor properties of the
1,3-indanedione residue in polymethine dyes are close to that
of the malononitrile residue. In addition, its carbonyl groups,
unlike nitriles, are inclined to form hydrogen bonds with protic
solvents, and the lone electron pairs on the carbonyl oxygen
atoms can play an important role in the spectral-luminescent and
solvatochromic properties of merocyanines. The 1,3-indanedione
residue is a convenient object for analysis of these effects since
MATERIALS AND METHODS
The UV/Vis spectra were recorded with a spectrophotometer
Shimadzu UV-3100 at room temperature. Solvents were purified
according to methods given in Ref.[18] Chromatography was
carried out on silica gel 60 and aluminium oxide 80 (Merck). The
purity of the dyes was checked by TLC-control (Silufol UV-254,
CHCl3, or CH2Cl2 as the eluent). Melting (decomposition) points
were measured in an open capillary and were not corrected.
1H NMR spectra were measured with
Varian VXR-300 (299.943 MHz for H-atoms) in CDCl3 with TMS
as an internal standard. The 13C, COSY-HH, COSY-CH, ROESY,
and TOCSY spectra were registered for dye 3 at a Bruker-600
a spectrometer
Scheme 2.
Scheme 3.
J. Phys. Org. Chem. 2011, 24 732–742
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

A. V. KULINICH ET AL.
by the method AM1 with a standard set of parameters.[24]
Geometry optimizations with the use of a limited Hartree–
Fock methodꢁa1nd Polak–Ribiere algorithm with an accuracy of
0.001 kcal ꢀ A ꢀ molꢁ1 have been preliminary carried out. The
˚
maxima of the electronic absorption bands were determined
with allowance for singly excited configurations, arising from all
possible electronic transitions from five HOMOs to five LUMOs.
All non-empirical calculations were carried out using the PC
Gamess/Firefly program.[25] Besides dyes 1–30, the anionic dyes
31–33, parent dyes for merocyanines 10–18, were calculated.
The ground-state geometries of all molecules have been optimized
using the density functional theory (DFT) at the B3LYP/6-31G(d,p)
level. For calculation of the vertical excitation energies, the TDDFT
methodology was used. Solvent effects on the excitation energies
of merocyanines 3 and 12 have been estimated within the
polarizable continuum model (PCM). The static polarizabilities (a),
first hyperpolarizabilities (b), and second hyperpolarizabilities (g)
have been calculated using TDHF approach.
Scheme 4.
For an analysis of the bond-order alternation in the
polymethine chain of dyes the parameters BLA and BOA[3] were
used. For the calculation of the charges alternation in chromophore
(600.334 MHz for H-atoms). The atom numbering used in NMR
spectra description is shown in Scheme 4 (H-atoms possess
the same numbers as the corresponding C-atoms, i.e. 1-H is the
H-atom at C-1).
P
in the S0 state we used the formula: Dq ¼ ð1=mÞ jqiꢁqiþ1j,
i
where qi and qi þ 1 are the Lowden charges on the neighboring
atoms of the chromophore, and m is the total number of items
(pairs of atoms taken into account). The parameter Dq was
calculated for the chromophore N-1–C-2–. . .–C-9 for merocyanines
(Scheme 4), as well as for the polymethine chain of cationic
polymethines (from N- to N-atom) and anionic dyes (only for the
open part of chromophore).
It is well known that polymethine dyes, both charged
and neutral ones, readily aggregate in solvents of low polarity.
The absorption spectra of merocyanines 1–9 were shown to obey
Beer’s law in a concentration interval up to 5 ꢀ 10ꢁ5 mol Lꢁ1 (the
corresponding data were not obtained for dyes 7–9 in n-hexane
owing to their extremely weak solubility in this solvent).
Mathematical processing of the long-wavelength absorption
bands of the dyes by means of the method of moments[19] allows,
besides the bands maxima (lmax), to characterize quantitatively
their centers (Mꢁ1), oscillator strength ( f), and shape (s, g1, g2,
and F). The parameter Mꢁ1 is averaged on all vibronic transitions,
and allows to compare objectively, unlike lmax, curves of a various
shapes. The integrated parameter f possesses similar advantage
in comparison with the extinction e. The s value characterizes
a diversion of points of a single band from its gravity center
ðv ¼ 107=Mꢁ1Þ, it is an analog of the traditionally used band
half-width. The coefficients g1, g2, and F characterize symmetry,
steepness (peakedness), and structure of bands accordingly.
To calculate the deviations Dl and DM (based on lmax and Mꢁ1
accordingly), which are quantitative measures of the electronic asy-
mmetry ofdyes,[15] the spectral characteristics of the corresponding
anionic (20 and 21) parent polymethine dyes[20,21] were explored
(Supplementary Information). The corresponding data for the
parent cationic dyes 22–30 were taken from Refs.[19,22] (Scheme 5).
The lowest vinylog of the anionic parent dyes, 19 (n ¼ 0),
cannot be used for the correct calculation of deviations as steric
hindrances in its molecule cause a significant bathochromic
shift of the long-wavelength absorption band.[23] In low polar
n-hexane and toluene, the ionic dyes 19–30 are either non-
soluble or form close ion pairs,[21,22] which also excludes a reliable
evaluation of deviations. The anionic dyes 20 and 21 are poorly
soluble and inclined to aggregate even in dichloromethane,
therefore only their absorption maxima (registered for solutions
with an approximate concentration of c ꢂ 10ꢁ6 mol Lꢁ1) were
used to calculate deviations of merocyanines’ spectral bands in
this solvent.
EXPERIMENTAL PART
Preparation of dyes 1–9
2-[(2E)-2-(1,3,3-Trimethyl-1,3-dihydro-2H-indol-2-ylidene)ethylidene]-
1H-indene-1,3(2H)-dione (1): (2E)-(1,3,3-trimethyl-1,3- dihydro- 2H-
indol-2-ylidene)acetaldehyde (34) (100 mg, 0.5 mmol) and 1H-
indene-1,3(2H)-dione (35) (78 mg, 0.5 mmol) were boiled for
3 min in acetic anhydride (3 mL). The precipitate was filtered off,
washed with cold ethanol, dried, two times chromatographed on
SiO2 (CHCl3 as the eluent), and crystallized from ethanol to yield
orange crystals of 1 (70 mg, 42%, m.p. 209–210 8C (the only dye
1
from the series that melts without decomposition)). H NMR, d
(CDCl3, 300 MHz): 1.75 (s, 6H; (CH3)2), 3.53 (s, 3H; NCH3), 7.00 (d,
J ¼ 7.8 Hz, 1H, 3-H), 7.18 (t, J ¼ 7.4 Hz, 1H, 5-H), 7.30–7.39 (m, 2H,
4-H þ 6-H), 7.44 (d, J ¼ 14.0 Hz, 1H, 10-H), 7.58–7.69 (m, 2H),
7.75–7.87 (m, 2H), 8.13 (d, J ¼ 14.0 Hz, 1H, 11-H). Anal. calcd for
C22H19NO2 (329.4): C 80.22, H 5.81, N 4.25. Found: C 80.15, H 5.77,
N 4.30.
2-[(2E,4E)-4-(1,3,3-Trimethyl-1,3-dihydro-2H-indol-2-ylidene)but-
2-enylidene]-1H-indene-1,3(2H)-dione (2): 160 mg (0.36mmol) of
2-{(1E,3E)-4-[acetyl(phenyl)amino]buta-1,3-dienyl}-1,3,3-trimethyl-
3H-indolium perchlorate (36) and 60 mg (0.41 mmol) of 35 was
boiled for 5 min in 3 mL of absolute ethanol. The isolation and
purification of 2 were similar to those for dye 1. Dark-blue needles
1
of 2 were isolated (41 mg, 32% yield, m.p. 250–251 8C). H NMR,
d (CDCl3, 300 MHz): 1.65 (s, 6H; (CH3)2), 3.35 (s, 3H; NCH3), 5.88
(d, J ¼ 12.2 Hz, 1H; 10-H), 6.88 (d, J ¼ 8.0 Hz, 1H, 3-H), 7.07
(t, J ¼ 7.3 Hz, 1H, 5-H), 7.21–7.31 (m, 2H, 4-H þ 6-H), 7.55–7.90
(m, 7H). Anal. calcd for C24H21NO2 (355.4): C 81.10, H 5.96, N 3.94.
Found: C 81.14, H 5.92, N 4.01.
Semiempirical quantum-chemical calculations of the electronic
structure of all dye molecules were performed for the gas phase
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 732–742

MEROCYANINES BASED ON 1,3-INDANEDIONE
2-[(2E,4E,6E)-6-(1,3,3-Trimethyl- 1,3-dihydro-2H-indol-2-ylidene)
hexa-2,4-dienylidene]-1H-indene-1,3(2H)-dione (3): The synthesis
is analogous to that of dye 2 (the only difference is that 2-
{(1E,3E,5E)-6-[acetyl(phenyl)amino]hexa-1,3,5-trienyl}-1,3,3-tri-
methyl-3H-indolium perchlorate (37) was used instead of
hemicyanine 36). The product was isolated as olive needles
(20% yield, m.p. 196–197 8C). 1H NMR, d (CDCl3, 600 MHz): 1.62 (s,
6H; (CH3)2), 3.29 (s, 3H; NCH3), 5.60 (d, J ¼ 12.8 Hz, 1H; 10-H), 6.41
(t, J ¼ 12.8 Hz, 1H; 12-H), 6.78 (d, J ¼ 7.9 Hz, 1H, 3-H), 6.99 (t,
J ¼ 7.4 Hz, 1H, 5-H), 7.16–7.26 (m, 3H, 4-H þ 6-H þ 13-H), 7.36 (t,
J ¼ 13.0 Hz, 1H, 11-H), 7.55 (d, J ¼ 12.6 Hz, 1H, 15-H), 7.64–7.70 (m,
2H), 7.73 (t, J ¼ 13.0 Hz, 1H, 14-H), 7.83–7.90 (m, 2H). 13C NMR, d
(CDCl3, 151 MHz): 28.4 (C(CH3)2), 29.7 (NCH3), 46.9 (C-8), 98.5
(C-10), 107.5 (C-3), 121.7 (C-5), 121.8 (C-6), [122.1, 122.2 (C-19,
C-22)], 122.6 (C-16), 123.1 (C-14), 125.0 (C-12), 128.1 (C-4), [133.9,
134.0 (C-20, C-21)], 139.4 (C-7), [140.8, 142.1 (C-18, C-23)], 142.8
(C-11), 144.1 (C-2), 145.7 (C-15), 155.7 (C-13), 164.1 (C-9), [191.2,
191.4 (C-17, C-24)]. Anal. calcd for C26H23NO2 (381.5): C 81.86, H
6.08, N 3.67. Found: C 81.90, H 6.04, N 3.65.
refluxed for 30 min. The precipitate was filtered off and washed
thoroughly with hot ethanol to yield 21 mg (25%) of a dark-violet
powder (m.p. >260 8C). H NMR, d (CDCl3, 300 MHz): 5.32 (s, 2H;
1
CH2), 7.01 (d, J ¼ 7.0 Hz, 1H), 7.27–7.46 (m, 5H), 7.47–7.57 (m, 2H),
7.62–7.73 (m, 2H), 7.75–7.90 (m, 4H), 7.99 (d, J ¼ 7.8 Hz, 1H), 8.50
(d, J ¼ 7.5 Hz, 1H), 8.59 (d, J ¼ 13.4 Hz, 1H; 11-H). Anal. calcd for
C29H19NO2 (413.5): C 84.24, H 4.63, N 3.39. Found: C 84.31, H 4.64,
N 3.44.
2-[(2E,4E)-4-(1-benzylbenzo[cd]indol-2(1H)-ylidene)but-2-enyl-
idene]-1H-indene-1,3(2H)-dione (8): A solution of 1-benzyl-2-
methyl-benzo[cd]indolium perchlorate 43 (140 mg, 0.4 mmol),
hemicyanine 40 (130 mg, 0.4 mmol), and Et3N (ꢃ0.2 mL) in
absolute ethanol (10 mL) was refluxed for 40 min. The precipitate
was filtered off and washed thoroughly with hot ethanol to yield
20 mg (11%) of a greyish green powder (m.p. 248–250 8C). 1H
NMR, d (CDCl3, 300 MHz): 5.18 (s, 2H; CH2), 6.26 (d, J ¼ 12.4 Hz, 1H;
10-H), 6.77–6.85 (m, 1H), 7.21–7.45 (m, 6H), 7.61–7.92 (m, 7H), 8.09
(t, J ¼ 12.4 Hz, 1H; 11-H), 8.13 (d, J ¼ 7.2 Hz, 1H). Anal. calcd for
C31H21NO2 (439.5): C 84.72, H 4.82, N 3.19. Found: C 84.65, H 4.78,
N 3.20.
2-[2-(1,3-Diphenyl-1,3-dihydro-2H-benzimidazol-2-ylidene)
ethylidene]-1H-indene-1,3(2H)-dione (4): A solution of 2-methyl-
1,3-diphenyl-3H-benzimidazolium chloride (38) (160 mg,
0.5 mmol) and 3-[(dimethylamino)methylene]-1H-indene-1,3
(2H)-dione 39 (160 mg, 0.55 mmol) in pyridine (2 mL) was
refluxed for 20 min in the presence of Et3N (ꢃ0.2 mL). The dye
was filtered off, washed with ethanol, dried, chromatographed on
SiO2 (CHCl3 as the eluent), and crystallized from ethanol to yield
orange crystals of 4 (80 mg, 36%, m.p. >260 8C). Merocyanines 5
and 6 were synthesized and isolated by the same procedure from
salt 38 and the correspondent hemicyanines 40 and 41 (vinylogs
of 39). 1H NMR, d (CDCl3, 300 MHz): 6.91 (d, J ¼ 15.0 Hz, 1H; 10-H),
6.97–7.07 (m, 3H, 11-H þ half of AA0BB0-system of the benzimi-
dazole residue), 7.17–7.26 (m, 2H, half of AA0BB0-system of the
benzimidazole residue), 7.35–7.42 (m, 2H), 7.43–7.51 (m, 2H),
7.56–7.64 (m, 4H), 7.71–7.83 (m, 6H). Anal. calcd for C30H20N2O2
(440.5): C 81.80, H 4.58, N 6.36. Found: C 81.74, H 4.59, N 6.27.
2-[(2E)-4-(1,3-Diphenyl-1,3-dihydro-2H-benzimidazol-2-ylidene)
but-2-enylidene]-1H-indene-1,3(2H)-dione (5): Greyish green
2-[(2E,4E,6E)-6-(1-benzylbenzo[cd]indol-2(1H)-ylidene) hexa-
2,4-dienylidene]-1H-indene-1,3(2H)-dione (9): A solution of 43
(140 mg, 0.4 mmol), hemicyanine 41 (140 mg, 0.4 mmol), and Et3N
(ꢃ0.2 mL) in absolute ethanol (10 mL) was refluxed for 2 h. The
precipitate was filtered off and washed with hot ethanol, then
dissolved in 10 mL of CHCl3, filtered off, and chromatographed at
SiO2 (CHCl3 as an eluent) to yield 7 mg (4%) of a dark-blue powder
1
(m.p. 226–228 8C). H NMR, d (CDCl3, 300 MHz): 5.10 (s, 2H; CH2),
6.02 (d, J ¼ 12.4 Hz, 1H; 10-H), 6.54 (t, J ¼ 12.4 Hz, 1H; 12-H), 6.66
(d, J ¼ 6.8 Hz, 1H), 7.14–7.38 (m, 8H), 7.50 (d, J ¼ 12.0 Hz, 1H; 15-H),
7.60–7.71 (m, 4H), 7.72–7.89 (m, 4H), 7.98 (d, J ¼ 7.0 Hz, 1H). Anal.
calcd for C33H23NO2 (465.6): C 85.14, H 4.98, N 3.01. Found: C
85.12, H 4.91, N 3.08.
The initial chemicals were purchased from Merck (34 and 35)
or obtained by methods described in Refs.[10,15,18,26] (36–43).
1
RESULTS AND DISCUSSION
crystals (26% yield, m.p. >260 8C). H NMR, d (CDCl3, 300 MHz):
5.51 (d, J ¼ 14.2 Hz, 1H; 10-H), 6.56 (t, J ¼ 13.6 Hz, 1H; 12-H), 6.85
(d, J ¼ 14.2 Hz, 1H; 13-H), 6.96–7.05 (m, 2H, half of AA0BB0-system
of the benzimidazole residue), 7.19 (t, J ¼ 13.6 Hz, 1H; 11-H),
7.22–7.29 (m, 2H, half of AA0BB0-system of the benzimidazole
residue), 7.40–7.46 (m, 2H), 7.50–7.63 (m, 6H), 7.70–7.81 (m, 6H).
Anal. calcd for C32H22N2O2 (466.6): C 82.38, H 4.75, N 6.00. Found:
C 82.45, H 4.69, N 5.95.
2-[(2E,4E)-6-(1,3-Diphenyl-1,3-dihydro-2H-benzimidazol-2-
ylidene)hexa-2,4-dienylidene]-1H-indene-1,3(2H)-dione (6): Dark-
blue needles (14% yield, m.p. 223–224 8C). 1H NMR, d (CDCl3,
300 MHz): 5.26 (d, J ¼ 14.0 Hz, 1H; 10-H), 5.88 (t, J ¼ 13.2 Hz, 1H;
12-H), 6.24 (t, J ¼ 13.2 Hz, 1H; 11-H), 6.437 (d, J ¼ 13.2 Hz, 1H;
13-H), 6.89–7.00 (m, 2H; half of AA0BB0-system of the benzimi-
dazole residue), 7.09–7.33 (m, 4H; 14-H þ 15-H þ half of
AA0BB0-system of the benzimidazole residue), 7.38–7.47 (m,
2H), 7.51–7.63 (m, 6H), 7.68–7.83 (m, 6H). Anal. calcd for
C34H24N2O2 (492.6): C 82.91, H 4.91, N 5.69. Found: C 82.97, H
4.93, N 5.59.
Electronic structure, chromaticity, and solvatochromism of
the merocyanines 1–9
All spectral parameters of merocyanines 1–9 are collected in
Table 1.
It is obvious from the data of Table 1 (arrays lmax and Mꢁ1) and
Fig. 1, that dyes 1–3 possess a distinct positive solvatochromism.
The range of solvatochromism increases with lengthening of
the polymethine chain, similarly to other vinylogous series of
merocyanines,[9] which is related to the larger polarizability of the
higher vinylogs.
A change from low polar n-hexane to toluene, and further
to moderate and high polar solvents is accompanied by a
bathochromic shift, narrowing (reduction of s values), growth
of peak intensity (e), integrated intensity ( f), asymmetry (g1),
steepness (g2), and structuredness (F) of their long-wavelength
absorption bands (Table 1). In n-hexane as a solvent the shapes of
the spectral bands of compounds 1–3 (Fig. 1) approaches
to those for typical polyenes – carotenoids (i.e. they are
wide, symmetrical, with distinct vibrational maxima distanced
on 1150–1250 cmꢁ1 from each other). In polar DMF and ethanol
they are, vice versa, narrow, intense, and unsymmetrical,
i.e. similar to those for symmetrical polymethine dyes.
2-[(2E)-2-(1-Benzylbenzo[cd]indol-2(1H)-ylidene)ethylidene]-1H-
indene-1,3(2H)-dione (7):
A solution of 1-benzyl-2-[(E)-2-
(dimethylamino)ethenyl]-benzo[cd]indol-1-ium tetrafluoroborate
42 (80 mg, 0.2 mmol), 1H-indene-1,3(2H)-dione 35 (35 mg,
0.24 mmol), and Et3N (ꢃ0.1 mL) in absolute ethanol (3 mL) was
J. Phys. Org. Chem. 2011, 24 732–742
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A. V. KULINICH ET AL.
Table 1. Parameters of the long-wavelength Vis-absorption bands of the merocyanines 1–9, measured in solvents of different
polarity at room temperature
Dye
Solvent
lmax nm
Dl nm
e ꢀ 10ꢁ4
Mꢁ1 nm
DM nm
f
s cmꢁ1
g1
g2
F ꢀ 10ꢁ2
1
n-Hexane
Toluene
CH2Cl2
DMF
474
489
494
495
—
—
—
—
—
—
—
23.5
11
5.5
—
—
44
19
9.5
—
—
—
—
—
4
6
10.5
—
10.88
11.47
11.33
11.24
11.09
8.47, 6.04
9.81
12.61
13.43
15.61
7.28,8.25
7.38,7.38
10.37
12.63
15.24
12.83
10.27
9.62
458.9
473.3
478.0
479.1
478.4
518.8
551.2
564.2
570.7
573.5
553.1
590.6
626.6
645.5
653.2
480.8
470.8
467.3
457.4
581.7
572.9
560.1
540.0
672.8
672.4
640.7
603.1
546.8
562.7
566.6
569.8
572.3
577.7
604.8
622.7
631.1
640.3
600.0
623.4
650.3
665.8
676.9
—
—
—
—
—
—
—
—
13.1
7.1
—
—
—
32
18.2
—
0.836
0.891
0.929
0.958
0.978
0.868
0.996
1.013
1.026
1.085
1.142
1.150
1.344
1.354
1.352
0.893
0.924
0.912
0.929
1.016
1.079
1.060
1.121
1.178
1.225
1.169
1.343
—
1190
1090
1080
1090
1110
1270
1200
1080
1040
980
1520
1520
1420
1320
1250
980
1.48
1.51
1.49
1.47
1.43
1.03
1.14
1.47
1.59
1.67
1.14
1.12
1.16
1.28
1.46
1.55
1.61
1.46
1.15
1.83
1.81
1.74
1.59
1.87
2.14
1.82
1.41
1.23
1.16
1.18
1.19
1.35
0.95
0.99
1.03
1.03
1.11
0.99
0.99
0.91
0.88
0.95
3.7
3.8
3.7
3.7
3.5
2.0
2.3
3.6
4.2
4.6
2.4
2.3
2.3
2.6
3.1
4.2
4.1
3.7
2.3
5.6
5.4
4.9
4.0
5.9
7.2
5.2
3.2
2.5
2.1
2.2
2.3
3.1
1.7
1.9
1.9
1.9
2.0
2.3
2.2
1.9
1.7
1.8
7.6
8.3
7.7
7.4
7.3
4.3
4.5
7.0
8.0
8.6
5.1
4.5
5.2
6.7
8.3
7.6
7.4
7.1
4.5
10.4
10.0
10.2
9.7
10.3
13.2
11.8
8.2
6.7
6.2
6.4
6.5
8.0
3.5
3.7
5.1
5.0
5.2
4.4
4.5
3.7
3.5
3.8
EtOH
496
2
3
n-Hexane
Toluene
CH2Cl2
DMF
542, 512
571
584
589
592
599, 564
630,598
667
685
690
493
488
487
478
593
590
583
575
697
693
685
670
585, 545
599, 558
603, 560
606, 564
609, 565
630, 593
617
EtOH
n-Hexane
Toluene
CH2Cl2
DMF
EtOH
4
5
6
7
Toluene
CH2Cl2
DMF
—
—
—
—
1210
1240
1400
800
EtOH
7.45
Toluene
CH2Cl2
DMF
18.77
17.46
13.90
9.97
18.86
21.51
12.36
8.10
—
980
12.3
29.5
—
1190
1560
1030
1010
1530
1970
1320
1260
1240
1250
1230
1400
1400
1390
1390
1340
1600
1620
1660
1700
1700
EtOH
Toluene
CH2Cl2
DMF
4
—
3.5
18.5
—
—
—
—
—
—
—
87.5
33
16
—
28.1
61.6
—
—
—
—
—
—
—
EtOH
n-Hexane
Toluene
CH2Cl2
DMF
—
6.89, 5.40
7.52, 5.63
7.28, 5.46
9.08, 6.07
—
7.14
7.39
6.53, 7.39
8.23, 7.84
—
0.709
0.754
0.747
0.806
—
0.947
1.022
1.042
1.099
—
EtOH
8
9
n-Hexane
Toluene
CH2Cl2
DMF
631
—
683, 639
693, 644
628
649
671
55.7
41.4
—
—
—
EtOH
n-Hexanea
Toluene
CH2Cl2
DMF
—
6.01
6.07
5.78
5.71
0.957
1.014
1.013
0.995
156.5
139.5
118.5
686
698
115.2
99.5
EtOH
a n-Hexane contains 5% of CH2Cl2.
Hence, the electronic structure of merocyanines 1–3 falls in the
range from A1 to A2. In weakly solvating media it comes nearer
to A1, and the contribution of the ideal polymethine A2 increases
with increasing solvent polarity. But is the state A2 achieved for
them at least in ethanol?
A lengthening of the polymethine chain in going from di- (1) to
tetramethinemerocyanine (2) causes a slight narrowing of the
absorption bands even in high polar DMF and ethanol (Table 1).
The spectral bands of vinylog 3 are broader than those of 2 in all
solvents used, while the bands of the corresponding parent dyes
narrow in going from 20 to 21 and from 23 to 24 (Supplementary
Information and Ref.[22]). A growth of the s values in going from
2 to 3 can be caused both by increasing of VI due to greater
bond-order alternations in the polymethine chromophore,
and by the amplification of the intermolecular solute–solvent
interactions (ISSI) due to increasing the number of solvation
centers and polarizability of the chromophore.
The vinylene shifts (VS) of merocyanines 1–3 both on
maxima (VSl) and on band centers (VSM) are <100 nm (the
typical value for symmetrical polymethine dyes[22]), and decrease
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MEROCYANINES BASED ON 1,3-INDANEDIONE
higher vinylog 6, a slight long-wavelength band shift caused by a
solvent change from CH2Cl2 to toluene can be explained by
the greater refractive index (nD) of toluene (which influences
the band position accordingly to the Bayliss function:
r ¼ ðnD2 ꢁ1Þ=ð2nD2 þ 1Þ[27]), and not by an approach of the
electronic structure of dye 6 to the A2 state in toluene.
The VSl in the series 4–6 are close to those of the symmetrical
polymethines. However, the VSM values in DMF and ethanol (in
the pair 5–6 even in toluene) are less than the classical value of
Dl ¼ 100 nm. For example, in going from dye 4 to 5 and than to 6,
they amount to Dl ¼ 82.6 and 63.1 nm in ethanol. The bandwidth
decreases in going from 4 to 5 and than increases for the highest
vinylog 6 (Table 1). In protic ethanol, in which the s values of dyes
4–6 are maximal, only a broadening of the spectral bands at a
lengthening of the polymethine chain is observed. The deviations
(Dl and DM) in the series 4–6 increase with increasing solvent
polarity and polymethine chain lengthening.
Figure 1. Vis absorption spectra of dye 3, measured in n-hexane (1),
CH2Cl2 (2), DMF (3), and ethanol (4) ar room temperature
Therefore, the electronic structure of dyes 4 and 5 changes in
the range from A2 to A3, as much as possible approaching the
A2 state in low polar toluene. The electronic structure of
merocyanine 6 in toluene slightly deviates from A2 toward the
non-polar polyene A1, achieves A2 in dichloromethane, and is
shifted toward A3 in polar DMF and ethanol.
The shape of merocyanines 7–9 absorption bands depends
extremely both on solvent polarity and on the polymethine chain
length (Fig. 3). Therefore, band centers (Mꢁ1) have a definite
advantage over band maxima for analysis of their electronic
spectra.
with lengthening the polymethine chain or reducing the solvent
polarity (Table 1). For example, in DMF the VSl and VSM values
amount to 94 and 74.8 nm accordingly in going from 2 to 3.
The deviations Dl and DM increase with lengthening of the
polymethine chain and are at maximum in low-polar solvents
(Table 1). They are essential even in the high-polar solvents DMF
and ethanol. Therefore, for dyes 1–3 the ideal polymethine state
A2 is not achieved in any of the solvents chosen.
Merocyanines 4–6, derivatives of the strong electron-donating
benzimidazole, have an extremely low solubility in n-hexane;
hence even qualitative spectra were not be available in this
solvent. In going from toluene to dichloromethane and further to
DMF and ethanol as solvent, the absorption bands of dyes 4–6
undergo hypsochromic shifts both on lmax and on Mꢁ1 (Table 1);
the solvatochromic shifts grow, as usual, with lengthening of the
polymethine chain. On the contrary, they decrease for the higher
vinylogs in the pair of solvents toluene – CH2Cl2, that is especially
well traced on the band centers (DMꢁ1 ¼ 10, 8.8, and 0.4 nm for
merocyanines 4, 5, and 6, respectively). For the vinylogous dyes 4
and 5 the narrowest, most intensive (on e), unsymmetrical and
structured bands are observed in toluene, but for compound 6 in
more polar dichloromethane (Table 1, Fig. 2). It seems, that for the
Dyes 7–9 possess positive solvatochromism (Fig. 3 and
Table 1); their solvatochromic shifts (DMꢁ1) are comparable with
those of 1–3. The long-wavelength absorption bands of 7–9 are
less intensive, wider, and more symmetrical than those of 1–3
(cf. the corresponding parameters (e, f, s, g1, and g2) in Table 1).
For example, the absorption band of merocyanine 8 in ethanol
has the s value of 1340 cmꢁ1, that is 360 cmꢁ1 larger than that
for its analog 2 with the same polymethine chain length, and
the extinction coefficient (e) of 8 is almost half as great. The
dependence of the band shape on solvent polarity for the
benzo[cd]indole derivatives 7–9 is less expressed, compared to
the correspondent derivatives of the more electron-donating
1,3,3-trimethyl-3H-indole (cf. s, g1, g2, and F in Table 1). For
merocyanines 7 and 8 an increase in solvent polarity results only
Figure 2. Vis absorption spectra of dyes 4–6, measured in toluene (solid
line), CH2Cl2 (dashed line), and ethanol (dotted line) at room temperature
Figure 3. Vis absorption spectra of dyes 7–9, measured in toluene (solid
line) and ethanol (dashed line) at room temperature
J. Phys. Org. Chem. 2011, 24 732–742
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A. V. KULINICH ET AL.
in a slight narrowing of their absorption bands, and for the
higher vinylog 9, despite its positive solvatochromism, even in a
broadening. Probably, it is connected with an amplification of ISSI
in the case of 7–9, since benzo[cd]indole has a larger (than indole
or benzimidazole), and therefore more polarizable p-system.
A lengthening of the polymethine chain causes in all solvents a
broadening of the absorption bands of 7–9. The VSM in the series
7–9 are much less than 100 nm and the deviations (DM) are very
large even in highly polar solvents (Table 1). All these data
testify the strong alternation of the p-electron distribution in the
polymethine chain of merocyanines 7–9. Thus, their electronic
structure changes within the range A1–A2, and the contribution
of A1 is greater than for compounds 1–3.
Comparison of the spectral characteristics of merocyanines
1–9 and their malononitrile-based analogs 10–18[11,13] has
allowed us to reveal the following conclusions.
First, as one would expect, the absorption bands of the
1,3-indanedione dyes 1–9 lie in more long-wavelength spectral
ranges (Dl ꢂ 50–60 nm). Second, the spectral properties of dyes
1–9, unlike 10–18, are significantly influenced by the formation
of hydrogen bonds with molecules of protic solvents (ethanol;
Fig. 4).
In ethanol merocyanines 1–3 are characterized by the
narrowest, most intensive and asymmetric long-wavelength
absorption bands (Table 1), while for malononitrile derivatives
10–12 a change from dichloromethane to ethanol as solvent
is accompanied by only slight changes of shape and position
of their absorption bands.[11] Hence, as well as in the case of
merocyanines derived from barbituric or thiobarbituric acids,[12]
the electronic structure of dyes 1–9 in ethanol is characterized by
a maximal shift toward dipolar structures A2 and A3.
Third, the absorption bands of positively solvatochromic
merocyanines 1–3, and 7, 8 in all solvents used are narrower,
more intensive and more unsymmetrical, than those of their
analogs 10–12 and 16, 17 (cf. the data of Table 1 and Refs.[11,13]),
that testifies a greater contribution of A2 in the S0 ground state
of the 1,3-indanedione derivatives. For benzimidazole dyes
replacement of malononitrile by 1,3-indanedione causes even
greater effects. While merocyanines 4 and 5 possess negative
solvatochromism at changing from toluene to dichloromethane
as solvent (Table 1), dyes 13 and 14 demonstrate positive
solvatochromism[13] in this pair of solvents, in spite of the lack of
specific solvation of the 1,3-indanedione carbonyl groups in
the solvents given. Hence, the residue of 1,3-indanedione as
terminal group of merocyanines is a stronger acceptor than the
malononitrile one. This conclusion contradicts the data of[16] but
agrees with their pKa values (to the best of our knowledge, 7.2
for 1,3-indanedione[28] and 11.2 for malononitrile[29]). In work[16]
the deviations of the styryl-like compounds 44 and 45 (Scheme 6)
were used as a measure of the electron-acceptor abilities of
terminal groups.
In the molecule of the parent anionic dye 19 (whose
absorption maximum was used for the calculation of deviation
of styryl dye 44) a chromophore planarity is distorted much more
strongly, than in the corresponding malononitrile derivative 31. A
chromophore planarity violation results in this case in an
additional bathochromic shift of the long-wavelength absorption
band of 19[23] and an overestimation of the deviation value of 44.
On the contrary, possible planarity violation in the chromophore
of 44, should result in a short-wavelength shift of its spectral
band, which also causes an increase of its deviation, since in this
case a rotational displacement should occur round the bond with
a bond order close to 1.[23] Hence, the approach used in[16] has
some limitations. In the recent paper, Dominguez and Rezende[30]
has made an attempt to unify the three types of solvatochromism
of phenolate betaine dyes. The model, based on the calculation
of the chemical hardness of donor and acceptor fragments was
suggested, and the investigated compounds were successfully
classified according to the sum of the hardnesses of their non-
interacting fragments. Therefore, chemical hardness can be used
as another parameter for evaluation of relative donor–acceptor
abilities of the terminal groups.
NMR spectroscopy and quantum chemical calculations of
merocyanines 1–9
The electronic structure of polymethine dyes can be investigated
also by NMR spectroscopy,[1,2] since chemical shifts correlate with
the electronic density on the corresponding atoms, and spin–spin
coupling constants between vicinal H-atoms of the polymethine
chain correlate with the corresponding carbon–carbon bond
orders.[1] In Refs.[12,31] the NMR spectra of merocyanines based on
malononitrile and N,N-diethylthiobarbituric acid were analyzed. It
was shown, that chemical shifts of the polymethine chain atoms
are the most sensitive to a change of electronic density in the
chromophore (atoms C-9–C-16 and 10-H–15-H according to the
numbering given at Scheme 4).
1
Analysis of the H NMR spectra of merocyanines 3 and 12[31]
confirms the conclusion about a somewhat larger electron-
acceptor ability of the 1,3-indanedione residue as compared to
malononitrile. Above all, it is proved by weak-field shifts of the
signals of 10-, 11-, 12-, and 13-H in 3 (the chemical shifts of 14-
and 15-H are strongly influenced by the strongly anisotropic
carbonyl groups of 1,3-indanedione and should not be taken into
account). It is confirmed also by the reduction of the spin–spin
coupling constants alternation in the polymethine chain of dye 3
in comparison with 12. In 13C NMR spectra the changes are very
small and no regularities were found: the signals of C-10, C-12,
and C-13 of dye 3 are shifted to somewhat weaker field, but
that of C-9 and C-11 to a higher field, than the corresponding
signals of 12.
The results of AM1 quantum-chemical calculations of dyes 1–9
(Table 2) and 10–18 (Ref.[13]) do not fit with the conclusions
made from the experimental data. Thus, the BLA (BOA) values of
1,3-indanedione dyes 1–9 are in all cases slightly greater than
Figure 4. Vis absorption spectra of dyes 3 and 12, measured in CH2Cl2
(solid line) and ethanol (dashed line) at room temperature
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MEROCYANINES BASED ON 1,3-INDANEDIONE
those of 10–18, testifying the greater contribution of A1 to
the electronic structure of 1–9. Moreover, these parameters are
almost non-sensitive to a change of the electron-donating
abilities of the terminal heterocycles. Even for negative
solvatochromic benzimidazole derivatives 4–6, the calculated
lengths and orders of the chromophore bonds (see BLA in Table
2) correspond to strongly alternating polyene A1. An inadequacy
between calculated and experimental data appears also at a
comparison of the calculated dipole moments in the ground (S0)
and excited (S1) states of the merocyanines with the data
obtained by other methods.[17] The AM1 calculated absorption
maxima and VS are far from the experimental results (cf. the data
in Tables 1 and 2).
O
O
19 (n = 0)
n
20
21
(n = 1)
(n = 2)
Me4N
ClO4
O
O
Me
Me
Me Me
22 (n = 1)
23
24
(n = 2)
(n = 3)
N
Me
N
Me
n
Ph Ph
25
(n = 1)
(n = 2)
N
N
ClO4 26
Also a lack of correlation between the AM1 calculated charges
on the carbon atoms of dye molecules and the chemical shifts of
the H-atoms connected to them was found. For example, for
H-atoms of the polymethine chain of dye 3, the correlation
factor (r) amounts to 0.392, while for the malononitrile dye 12
r ¼ 0.980.[31] The obvious explanation of this fact is connected
with the strong anisotropic effect of the carbonyl groups in 1–9,
which cause additional weak-field shifts of signals of the adjacent
H-atoms (14-, 15-, 19-, and 22-H). The correlations of the 13C NMR
data of dye 3 with the corresponding calculated charges are
also unsatisfactory. Even for C-atoms of the polymethine chain of
3 only (C-10–C-15), the correlation with the calculated charges is
practically missed (r ¼ 0.717), whereas for dye 12 r ¼ 0.998).[31]
For a more rigorous analysis of the electronic structure of
merocyanines, as well as for the corresponding cationic and
anionic parent dyes, DFT/B3LYP quantum chemical calculations,
which are now widely used for the investigation of polymethine
dyes,[32–34] were made. In the work,[32] the basis set 6-31G(d,p)
was shown to be an optimal concerning quality/price ratio, so it
was chosen for the current study. In the case of the rather small
molecule of 10, calculations with more advanced basis sets were
executed, and it was shown, that the results are very close to that
obtained at 6-31G(d,p)-level. Marked differences were observed
only between the calculated bond orders and atomic charges,
but it is well known that these parameters depend essentially on
the chosen basis set, especially if diffuse functions are applied.[35]
The results of the DFT/B3LYP calculations (Table 3; data for the
parent ionic dyes 19–30 are given in Supplementary Information)
agree somewhat better with the experimental data, than that of
the AM1 calculations. Particularly, they adequately reflect both a
growth of the contribution of A1 at the merocyanine poly-
methine chain lengthening, and a significant equalization of
bond lengths in the polymethine chain (i.e. an approach to A2) in
going from dyes 1–3 to strong electron-donating benzimidazole
derivatives 4–6 (Table 3). The DFT-calculated dipole moments of
merocyanines and their changes on electronic excitation agree
also better with the data gained by other methods, for example
from the Lippert–Mataga relationship.[17] However, the calculated
BLA (BOA) values are larger, and Dq values are smaller for dyes
1–9, than for 10–18 (Table 3). Thus, contrary to the conclusions
made on the electronic spectra, DFT-calculation, as well as AM1,
predict greater contribution of the structure A1 for 1–9 in
comparison with malononitrile derivatives 10–18.
27 (n = 3)
N
Ph
N
Ph
n
28 (n = 1)
29 (n = 2)
ClO4
30
(n = 3)
N
N
n
Ph
Ph
NC
CN
31
32
(n = 0)
(n = 1)
n
Me4N
CN
CN
33 (n = 2)
Scheme 5.
Scheme 6.
Table 2. Results of the quantum-chemical calculation of
merocyanines 1–9 by the AM1 method
˚
BLA, A
Dye
mg, D
me, D
DES0–S1, nm
1
2
3
4
5
6
7
8
9
0.068
0.083
0.088
0.064
0.079
0.086
0.049
0.072
0.081
2.223
2.218
2.262
2.349
2.605
2.743
7.259
6.866
6.737
5.305
6.996
8.299
4.936
7.074
8.774
8.51
395
413
428
378
396
402
421
426
422
The DFT (B3LYP) calculated atomic charges correlate better
with the NMR data, than gained by AM1, but nevertheless the
correlations are unsatisfactory for dyes 1–9 (e.g. for merocyanine
3, taking into account only the polymethine chain atoms,
r ¼ 0.604 and 0.862, respectively for 1H and 13C spectral data;
cf. with abovementioned figures for AM1).
10.54
12.18
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A. V. KULINICH ET AL.
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MEROCYANINES BASED ON 1,3-INDANEDIONE
More adequate calculations of the electronic structure and
spectral properties of merocyanine dyes demands considering
the solvent effect.[32,33] Therefore, for dyes 3 and 12 the solvent
influence was simulated by the method of PCM. Methanol has
been chosen as a model solvent, because it is the most polar one
(with exception of water) whose PCM-parameters are included
into the software used, and hence the expected effect was largest
for it. For both dyes a geometry optimization within the frames of
PCM resulted in an essential alignment of the polymethine chain
bonds lengths and bonds orders (i.e. in approaching of the dyes’
electronic structure to the ideal polymethine state A2), and also
in long-wavelength shifts of their absorption bands (Table 3).
However, smaller absolute values of BLA and BOA were received
for dye 12 and not for 3.
Analysis of the MOs of merocyanines 1–9 shows, that
both HOMOs and LUMOs of these dyes are localized mainly
on the polymethine chain atoms with much smaller coefficients
at atoms of the terminal groups (Fig. 5).
According to Fig. 5, an alignment of the electronic density
in the chromophore of donor–acceptor dyes takes place at
electronic excitation, i.e. they approach structure A2.
(LUMOþ1)s of dyes 1–9 are localized on the acceptor fragment of
the molecules (1,3-indanedione), and theirs energies are close
enough to the energies of the LUMOs. Therefore, they give an
appreciable contribution to the low-lying excited states of these
dyes (to S3 for compounds 1–3, 7–9, and even to S1 and S2 states
for benzimidazole derivatives 4–6). On the contrary, in 10–18
(LUMOþ1)s are localized mainly on the donor terminal group and
on the polymethine chain (in 13–15 (LUMOþ1)s lie on the
NPh-groups of the 1,3-diphenylbenzimidazole residue). It was
found that the (LUMOþ1)s of dyes 1–9 resemble the corres-
ponding MOs of anionic polymethines 19–21, and the
(LUMOþ1)s of malononitrile merocyanines 10–18 resemble
the (LUMOþ1)s of cationic dyes 22–30. The same result was
obtained when the semiempirical method AM1 was used.
TDDFT calculations predict that the most long-wavelength
absorption transition of dye 1 (from HOMOꢁ1 to LUMO),
*
unlike malononitrile derivative 10, has n ! p -character. But this
transition cannot be found in the experimental electronic
spectra as it is characterized by negligible oscillator strength
( f). For the higher vinylogs 2 and 3 the energies of the cyanine
*
*
p ! p -transitions are less than the energies of the n ! p -ones.
The TDDFT calculated VS in the series 1–3 amount to Dl ¼ 38 and
44 nm (Table 3), which corresponds better to the experimental
data, than the results of AM1 calculations (Table 2).
Besides two boundary orbitals, it is necessary to single out
the (LUMOþ1)s of the explored merocyanines. It was found, that
Solvation is known to reduce greater the energy of more
dipolar state.[27] For merocyanines a p ! p -excited state is
*
*
usually more dipolar than a n ! p one. Indeed, for dye 3 in
*
vacuum the n ! p -transition (here from HOMOꢁ1 to LUMO) has
the energy of 2.77 eV and determines its S2 state. In methanol
*
(PCM) the energy of the same n ! p -transition (S0 ! S3 in this
case, from HOMOꢁ2 to LUMO) increases to 3.08 eV, whereas the
*
energy of a cyanine-type p ! p -transition S0 ! S1 (from HOMO
to LUMO) decreases from 2.53 to 2.42 eV. In going from vacuum to
methanol the energy of HOMO-to-LUMOþ1 transition decreases
to a much greater extent (from 2.97 to 2.61 eV), and it becomes
S0 ! S2.
The main peculiarity of the calculated electronic spectrum of
dye 4 is the presence of two closely located and intensive
*
long-wavelength p ! p -transitions. The first one (i.e. S0 ! S1)
is determined mainly by transitions from HOMO to LUMO
and LUMOþ1 with comparable contributions of both, the S0 ! S2
is a transition from HOMO to LUMO, and the S0 ! S3 has
*
n ! p -nature.
Due to a greater VS of the cyanine-type HOMO ! LUMO
transition, it becomes S0 ! S1 for the vinylogs 5 and 6. Also a
significant decrease of the oscillator strengths of the S0 ! S2
transitions (from HOMO to LUMO and LUMOþ1) is observed for
them. Probably, the high intensity of such a transition for dye 4 is
related mainly to a strong coupling of the closely located states S1
and S2.
The calculated TDDFT spectra of dyes 7–9 are similar to those
of 1–3, with the difference, that due to larger p-system of the
*
benzo[cd]indole nucleus, the energy of the p ! p -transition is
*
smaller than of n ! p -one even for dimethinemerocyanine 7.
The first hyperpolarizability of dye 12, calculated using TDHF
(DFT/B3LYP) method, is very close (bv ¼ 3.65 ꢀ 10ꢁ28 esu) to the
experimentally determined[17] value bexp ¼ (3.9 ꢄ 0.2) ꢀ 10ꢁ28 esu;
therefore, the results obtained are reliable enough. As it was
assumed, the hyperpolarizabilities (bv) of indanedione derivatives
1–3 are appreciably larger than those of their analogs with a
malononitrile residue 10–12 (Table 3). Also one can observe that
dyes 1–9 possess very large average second hyperpolarizabilities
Figure 5. The MOs of dye 6 (contour value 0.03 is chosen for plotting)
J. Phys. Org. Chem. 2011, 24 732–742
Copyright ß 2010 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

A. V. KULINICH ET AL.
(gav), i.e. they are promising for application in organic photo-
refractive materials.
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man, K. M. Hahn, J. Phys. Chem. A 2007, 111, 10849–10860.
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[10] A. Mishra, R. K. Behera, P. K. Behera, B. K. Mishra, G. B. Behera, Chem.
Rev. 2000, 100, 1973–2011.
CONCLUSION
[11] A. V. Kulinich, N. A. Derevyanko, A. A. Ishchenko, Russ. Chem. Bull.
2005, 54, 2820–2830.
[12] A. V. Kulinich, N. A. Derevyanko, A. A. Ishchenko, J. Photochem.
Photobiol. A 2007, 188, 207–217.
[13] A. A. Ishchenko, A. V. Kulinich, S. L. Bondarev, V. N. Knyukshto, Opt.
Spectrosc. 2008, 104, 57–68.
[14] A. V. Kulinich, N. A. Derevyanko, A. A. Ishchenko, S. L. Bondarev, V. N.
Knyukshto, J. Photochem. Photobiol. A 2008, 197, 40–49.
[15] L. G. S. Brooker, G. H. Keyes, R. H. Sprague, E. VanLare, G. VanZandt, F.
L. White, H. W. J. Cressman, J. S. G. Dent, J. Am. Chem. Soc. 1951, 73,
5332–5350.
The differences in the solvatochromic behavior of dyes 1–9
and 10–18 are mainly related to the formation of hydrogen
bonds with protic solvent (ethanol) in the case of 1,3-indanedione
derivatives. The residue of 1,3-indanedione, contrary to the
literature data, is a stronger acceptor than the malononitrile
residue. This even causes a reversal of the solvatochromic sign in
the pair of solvents toluene – dichloromethane for based on the
strong electron-donating benzimidazole nucleus merocyanines 4
and 5 (negative solvatochromism) in comparison with their
analogs 13 and 14 (positive solvatochromism).
[16] T. H. James, The Theory of the Photographic Process, 4th edn, Mac-
millan, New York, 1977.
[17] S. L. Bondarev, S. A. Tikhomirov, V. N. Knyukshto, A. A. Turban, A. A.
Ishchenko, A. V. Kulinich, I. Ledoux, J. Lumin. 2007, 124, 178–186.
The TDDFT calculations demonstrate that the electronic
transitions in the series of 1,3-indanedione derivatives 1–9 differ
significantly from those of malononitrile merocyanines 10–18. It
is shown that the S2 or S3 states (and even S1 in the case of dye 1)
[18] A. J. Gordon, R. A. Ford, The Chemist’s Companion.
A
Handbook of Practical Data Techniques and References, Wiley,
New York, 1972.
[19] A. A. Ishchenko, V. A. Svidro, N. A. Derevyanko, Dyes Pigments 1989,
10, 85–96.
[20] Zh. A. Krasnaya, T. S. Stytsenko, D. G. Gusev, E. P. Prokof’ev, Izv. Akad.
Nauk SSSR, Ser. Khim. 1986, 1596–1602.
[21] A. S. Tatikolov, Zh. A. Krasnaya, L. A. Shvedova, V. A. Kuzmin, Int. J.
Photon 2000, 2, 23–30.
[22] A. A. Ishchenko, Structure, Spectral and Luminescent Properties of
Polymethine Dyes, Naukova Dumka, Kyiv, 1994 (in Russian).
[23] A. M. Kolesnikov, F. A. Mikhailenko, Russ. Chem. Rev. 1987, 56,
275–287.
[24] J. J. P. Stewart, J. Comput. Aid. Mol. Des. 1990, 4, 1–105.
[25] A. A. Granovsky, Firefly Version 7.1.H, http://www.classic.chem.-
msu.su/gran/firefly/index.html.
[26] F. M. Hamer, The Cyanine Dyes and Related Compounds,
Wiley-Interscience Publication, New York/London, 1964. This book
can be found at http://onlinelibrary.wiley.com/book/10.1002/
9780470186794
*
of compounds 1–9 result from their n ! p -transitions (close in
*
*
energy to cyanine-type p! p -transitions). The n ! p -transitions
of dyes 10–18 have sufficiently high energies and are related to
their S5 or S6 states.
Interestingly, it follows from the analysis of the MOs’ shapes
of merocyanines, that the (LUMOþ1)s of the malononitrile
derivatives 10–18 resemble the same MOs of the parent cationic
dyes. On the contrary (LUMOþ1)s of 1,3-indanedione dyes 1–9
are localized mainly on the acceptor terminal group and
somewhat resemble the corresponding MOs of anionic parent
dyes 19–21. This fact can be interpreted either in favor of
a greater acceptor strength of 1,3-indanedione group or it is
connected with its more extended p-system.
[27] C. Reichardt, Solvents and Solvent Effects in Organic Chemistry, 3rd
edn, WILEY-VCH, Weinheim, 2003.
[28] M. W. Whitehouse, J. E. Leader, Biochem. Pharmacol. 1967, 16,
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2011, 24 732–742