Merocyanines based on 1,2-diphenyl-3,5-pyrazolidinedione

Article abstract of DOI:10.1039/c9nj03275d

Vinylogous series of merocyanines have been synthesized with indole, benzimidazole, and benzo[cd]indole as the donor termini and 1,2-diphenyl-3,5-pyrazolidinedione as the acceptor terminal group. Their absorption and fluorescence spectra have been studied in different polarity solvents. From the analysis of their solvatochromism, vinylene shifts, Brooker's deviations, and spectral band shapes, the effects of structural modifications, viz. the donor group strength and the polymethine chain length, on their electronic structures have been revealed. Based on the obtained data, the acceptor strength of the 1,2-diphenyl-3,5-pyrazolidinedione residue has been re-evaluated, showing-contrary to the literature data-that it is not a record-strong acceptor. The (TD)-DFT/B3LYP calculations of the studied dyes have been performed in three different solvents using the PCM solvation model. From the analysis of TD-B3LYP electronic transitions, the low fluorescence of the studied dyes has been explained by the fast competitive unimolecular non-radiative decay associated with the low-lying non-fluorescent ππ?-excited state involving the molecular orbital localized on the acceptor terminal group.

Full text of DOI:10.1039/c9nj03275d

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DOI: 10.1039/C9NJ03275D
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Merocyanines based on 1,2-diphenyl-3,5-pyrazolidinedione
Heorhii V. Humeniuk, Nadezhda A. Derevyanko, Alexander A. Ishchenko and Andrii V. Kulinich*
Received 00th January 20xx,
Accepted 00th January 20xx
Vinylogous series of merocyanines have been synthesized with indole, benzimidazole, and benzo[cd]indole as the donor
termini and 1,2-diphenyl-3,5-pyrazolidinedione as the acceptor terminal group. Their absorption and fluorescence spectra
have been studied in different polarity solvents. From the analysis of their solvatochromism, vinylene shifts, Brooker's
deviations, and spectral band shape, the effects of structural modifications, viz. the donor group strength and the
polymethine chain length, on their electronic structure have been revealed. Based on the obtained data, the acceptor
strength of the 1,2-diphenyl-3,5-pyrazolidinedione residue has been re-evaluated, showing – contrary to the literature data
– it not being a record-strong acceptor. The (TD)-DFT/B3LYP calculations of the studied dyes have been performed in three
different solvents using the PCM solvation model. From the analysis of TD-B3LYP electronic transitions, the low fluorescence
of the studied dyes has been explained by the fast competitive unimolecular non-radiative decay associated with the low-
lying non-fluorescent ππ*-excited state involving the molecular orbital localized on the acceptor terminal group.
DOI: 10.1039/x0xx00000x
heterocyclic), nitrogen-, oxygen-, and sulphur-containing
heterocycles, even organometallic compounds, which give rise
to the hundreds of structures with varying electronic
1. Introduction
Merocyanines – donor-acceptor polymethine dyes – are one of
the most versatile functional compounds. They are widely used
as colorimetric sensors and fluorescent probes sensitive to
environmental polarity and viscosity in physicochemical and
biomedical studies,1–4 as well as in various technologies related
to light–energy conversion such as solar cells,5–7
electroluminescent materials,8 nonlinear optics,9,10 and
information recording.11,12 The reason for such versatility is that
their photophysical properties can be tuned in the very wide
range depending on the polymethine chain length and the
chromophore electronic structure, the latter varying from the
nonpolar polyene A1 to the ideal polymethine A2 and further to
the dipolar polyene A3 (Scheme 1).13
properties.2,9,14,15 On the contrary, the number of acceptor
(electron withdrawing) terminal groups used in the synthesis of
merocyanines is rather limited (see fundamental review 16 by
D. M. Sturmer and D. W. Heseltine from Eastman Kodak Co.)
and has increased just slightly over the past four decades. Only
a few of them are characterized by extremely strong electron-
withdrawing properties, required for creation of donor-
acceptor dyes, whose electronic structure in polar solvents
approaches the dipolar polyene structure A3.9,17–19 Among the
strong acceptor groups, the derivatives of barbituric and
thiobarbituric acids have been studied most extensively; for
example, the fluorescent probe Merocyanine-540. Other strong
acceptors have found lesser attention from researchers.
Besides barbiturates, the list of the most acidic (acceptor)
terminal groups in ref. 16 includes some imidazolones and
pyrazolidinediones, the residue of 1,2-diphenyl-3,5-
pyrazolidinedione being estimated as the strongest non-
charged acceptor. To the best of our knowledge, merocyanines
comprising this acceptor can be found in patents mainly, only in
some of them their spectral properties being briefly
reported.20,21 In a few journal articles, several styryl dyes,22,23 an
indole-based dimethinemerocyanine,24 five tetramethine-
merocyanines with different donor groups,25 and one dye of the
D–π–A–π–D structure,26 which can be tentatively classified as
bis-merocyanine, are described. Their solvatochromism,
electronic structure, and fluorescent properties have not been
studied in detail, although it is noted in ref. 25 that such dyes
are characterized by low fluorescence brightness along with
comparatively high photochemical stability. Hence, the main
goal of this work was to examine spectral properties and
electronic structure of merocyanines based on 1,2-diphenyl-
3,5-pyrazolidinedione in a set of solvents of different polarity.
δ+
D
δ−
A
D
A
D
n
A
n
n
A1
A2
A3
Scheme 1 Resonance structures of merocyanines.
Merocyanines are well-known for their pronounced
solvatochromism, which is attributable to the fact that solvent
polarity is one of the factors determining the alteration of their
electronic structure within the A1–A2–A3 range.2 However, the
main factor, which predetermines the solvatochromism as well,
is the donor–acceptor properties of the terminal groups D and
A. There is a huge range of available donor (electron donating)
groups in the polymethine dye chemistry like alkyl and
arylamines, alkylamino and arylaminoarenes (both carbo- and
a. Institute of Organic Chemistry, National Academy of Sciences of Ukraine;
5 Murmanska St., 02094 Kyiv, Ukraine; e-mail: andrii.kulinich@gmail.com
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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(0.5 g, 2 mmol) was dissolved in a mixture of aceVtiiecwaAnrthiclyedOrnildinee
(3 mL) and DMF (2 mL), and the solutionDwOaI:s10h.1e0a3t9e/dC9aNtJ10332575°DC
for 2 h. After cooling, the reaction mixture was diluted with
brine. The crude product was filtered out, washed with water,
dried, and chromatographed on silica gel (CH2Cl2) to afford 12
as a white solid; yield: 0.32 g, 52%. 1H NMR: δH (300 MHz; CDCl3)
7.58 (1H, s), 7.45-7.33 (4H, m), 7.33-7.23 (4H, m), 7.15-7.05 (2H,
m), 3.83 (3H, s), 3.35 (3H, s).
2. Experimental section
The UV-Vis absorption spectra were recorded using a Shimadzu
UV-3100 spectrophotometer (Japan). The fluorescence spectra
were measured using a Solar CM2203 spectrofluorimeter
(Belarus) and corrected by the instrument sensitivity. The
fluorescence quantum yields (FQY; Φf) were determined
relative to those of Rhodamine 6G in ethanol (Φf = 95%27), Nile
blue in methanol (Φf = 27%28), and indotricarbocyanine dye (C7)
in ethanol (Φf = 28%29) and corrected taking into account
solvents' refractive indices. The long-wavelength absorption
and fluorescence bands of the studied dyes in the “mirror”
coordinates ε(ν)/ν – ν and F(ν)/ν4 – ν, suggested by
B. I. Stepanov,30 were analysed by the method of moments29 to
calculate their gravity centres (M−1 = 107/ν, where ν is the band
mean position in wavenumber) and the deviations from the
band gravity centre (σ) – the characteristic of bandwidth, which
in the case of the “ideal” Gauss, Poisson or Gaussian curves is
equal to a half of their full width at half-maximum. The
parameter M−1 can be used to characterize relative positions of
the spectral bands of different shapes, which is often the case
of merocyanines. The value σ allows comparing bandwidth of
spectral bands of different shapes. 1H NMR spectra were
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4-(3-Acetanilidoallylidene)-1,2-diphenyl-3,5-
pyrazolidinedione (13)
:
1,2-diphenyl-3,5-pyrazolidinedione
(0.5 g, mmol) and N-(3-phenyliminopropenyl)aniline
2
hydrochloride (0.51 g, 2 mmol) were heated under reflux in
acetic anhydride (6 mL) for 5 min. After cooling, the solution
was diluted with diethyl ether (15-20 mL) and left in the fridge
overnight. The precipitate was filtered out, washed with diethyl
ether, and dried to afford 13 as a dark-yellow solid; yield: 0.62
1
g, 73%. H NMR: δH (300 MHz; CDCl3) 8.49 (1H, d, J = 13.7 Hz),
7.83 (1H, d, J = 12.4 Hz), 7.61-7.52 (3H, m), 7.37-7.19 (10H, m),
7.15-7.06 (2H, m), 6.64 (1H, dd, J = 13.7, 12.4 Hz), 2.00 (3H, s).
4-(5-Acetanilido-2,4-pentadienylidene)-1,2-diphenyl-3,5-
20
pyrazolidinedione
(14)
:
from
1,2-diphenyl-3,5-
pyrazolidinedione (0.5 g, 2 mmol) and N-(5-phenylimino-1,3-
pentadienyl)aniline hydrochloride (0.57 g, 2 mmol). Crude dye
measured with
a Varian VXR-300 or Varian VXR-400
1
14 was obtained as a red solid; yield: 0.58 g, 63%. H NMR: δH
instruments; the residual solvent peaks of [D6]DMSO (δH
2.50 ppm) and CDCl3 (δH = 7.26 ppm) were used as an internal
standard.
=
(300 MHz; CDCl3) 8.15 (1H, d, J = 13.8 Hz), 7.69 (1H, d, J = 12.4
Hz), 7.61-7.46 (4H, m), 7.40-7.24 (9H, m), 7.22-7.09 (4H, m),
5.34 (1H, dd, J = 13.8, 11.4 Hz), 1.97 (3H, s).
DFT/TD-DFT calculations were performed with the Gaussian 09
program suite using the hybrid B3LYP functional and the
double-ζ basis set 6-31G(d,p).31 The solvent effect was
accounted for by the integral equation formalism polarizable
continuum model (IEFPCM).32 The ground-state geometry
optimizations (the convergence criterion on the residual forces
was set to 1×10−5 Hartree·Bohr−1 or Hartree·Rad−1) were
followed by frequency calculations to verify that the found
stationary points were true minima. In the case of fast
absorption processes (Franck–Condon principle), the nuclear
relaxation was avoided by employing state-specific solvation
calculations in terms of fast electronic cloud reorganizations
and slow solvent and solute nuclear motions.33 The NBO atomic
charges were obtained using the embedded in Gaussian-09
algorithm.34
1,2-Diphenyl-4-[(2E)-2-(1,3,3-trimethyl-2-
indolinylidene)ethylidene]-3,5-pyrazolidinedione (1) 24
:
orange
1
crystals; yield: 66 mg, 76%; mp 258-259 °C (from ethanol). H
NMR: δH (300 MHz; CDCl3) 8.25 (1H, d, J = 14.3 Hz), 7.50-7.40
(4H, m), 7.40-7.27 (7H, m), 7.20 (1H, t, J = 7.7 Hz), 7.16-7.05 (2H,
m), 7.02 (1H, d, J = 7.9 Hz), 3.51 (3H, s), 1.71 (6H, s).
1,2-Diphenyl-4-[(2E,4E)-4-(1,3,3-trimethyl-2-indolinylidene)-
2-butenylidene]-3,5-pyrazolidinedione (2) 25
a dark-green
:
solid; yield: 47 mg, 51%; mp 182-184 °C (from ethanol). 1H NMR:
δH (300 MHz; CDCl3) 7.78 (1H, t, J = 13.1 Hz), 7.73 (1H, d, J = 12.8
Hz), 7.57 (1H, t, J = 13.1 Hz), 7.47-7.40 (4H, m), 7.34-7.25 (6H,
m), 7.16-7.06 (3H, m), 6.92 (1H, d, J = 7.9 Hz), 5.87 (1H, d, J =
12.9 Hz), 3.36 (3H, s), 1.65 (6H, s).
1,2-Diphenyl-4-[(2E,4E,6E)-6-(1,3,3-trimethyl-2-
indolinylidene)-2,4-hexadienylidene]-3,5-pyrazolidinedione
Solvents for spectral measurements were purified by the
methods given in ref. 35. Column chromatography was
performed on Silica gel 60 (Merck). The purity of dyes 1–11 was
checked by TLC (Silufol UV-254, CH2Cl2 as an eluent). Melting
(decomposition) points were measured using the open capillary
method and are not corrected.
(3)
:
2-(6-acetanilido-1,3,5-hexatrienyl)-1,3,3-trimethyl-3H-
indolium tetrafluoroborate36 (92 mg, 0.2 mmol) and 1,2-
diphenyl-3,5-pyrazolidinedione (55 mg, 0.22 mmol) were
dissolved in CH2Cl2. Anhydrous potassium carbonate (200 mg,
1.45 mmol) was added, and the solution was stirred at room
temp. for 3 h, filtered, chromatographed on silica gel (CH2Cl2),
2.1.Synthetic procedures
and crystallized from ethanol to afford
3 as a peacock green
Merocyanines 1, 2 and anionic dyes 10, 11 were described
solid; yield: 27 mg, 28%; mp 159-160 °C (decomp.). Calcd. for
C32H29N3O2 (487.60): C 78.82, H 6.00, N 8.62; found: C 78.74, H
6.07, N 8.49. 1H NMR: δH (300 MHz; CDCl3) 7.70 (1H, d, J = 12.3
Hz), 7.63 (1H, t, J = 12.6 Hz), 7.49-7.38 (5H, m), 7.35-7.19 (7H,
m), 7.15-7.07 (2H, m), 7.03 (1H, t, J = 7.5 Hz), 6.83 (1H, d, J = 7.8
Hz), 6.37 (1H, t, J = 12.7 Hz), 5.66 (1H, d, J = 12.7 Hz), 3.30 (3H,
elsewhere before;20,24,25 they were synthesized and purified
according to the known procedures and characterized only by
melting points and 1H NMR spectra. The intermediate
hemicyanines were mostly used in the following synthetic
stages without further purification.
1
4-[(dimethylamino)methylene]-1,2-diphenyl-3,5-
s), 1.63 (6H, s). H NMR: δH (300 MHz; [D6]-DMSO) 7.87 (1H, t,
pyrazolidinedione (12)
:
1,2-diphenyl-3,5-pyrazolidinedione
J = 13.0 Hz), 7.70 (1H, t, J = 12.5 Hz), 7.51 (1H, d, J = 7.4 Hz), 7.47-
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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7.23 (12H, m), 7.16-7.07 (3H, m), 6.46 (1H, t, J = 12.6 Hz), 6.12 7.36-7.27 (9H, m), 7.16-7.08 (2H, m), 7.01 (1H, d, J = 7.3 Hz), 5.30
View Article Online
DOI: 10.1039/C9NJ03275D
1
(1H, d, J = 13.4 Hz), 3.48 (3H, s), 1.60 (6H, s).
4-[2-(1,3-Diphenyl-2-benzoimidazolylidene)ethylidene]-1,2-
(2H, s). H NMR: δH (300 MHz; [D6]-DMSO) 8.54 (1H, d, J = 14.1
Hz), 8.43 (1H, d, J = 7.1 Hz), 8.27 (1H, d, J = 8.2 Hz), 7.95 (1H, t,
diphenyl-3,5-pyrazolidinedione (4): a solution of 2-methyl-1,3- J = 7.7 Hz), 7.77 (1H, d, J = 7.8 Hz), 7.68-7.57 (3H, m), 7.41-7.27
diphenylbenzoimidazolium chloride (80 mg, 0.25 mmol) and (13H, m), 7.18-7.10 (2H, m), 5.46 (2H, s).
hemicyanine 12 (77 mg, 0.25 mmol) in pyridine (4 mL) was 4-[(2E,4E)-4-(1-benzyl-2-benzo[cd]indolylidene)-2-
heated under reflux for 2 h. The crude product was precipitated butenylidene]-1,2-diphenyl-3,5-pyrazolidinedione
with water, filtered out, and chromatographed on silica gel solution of 1-benzyl-2-methylbenzo[cd]indolium
(CH2Cl2) to afford as a bright-yellow solid; yield: 38 mg, 28%; tetrafluoroborate (69 mg, 0.2 mmol) and hemicyanine 13
(8)
:
a
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4
mp >270 °C. Calcd. for C36H26N4O2 (546.63): C 79.10, H 4.79, N (85 mg, 0.2 mmol) in pyridine (3 mL) was heated for 5 min under
10.25; found: C 78.98, H 4.89, N 10.13. H NMR: δH (300 MHz; reflux. The product was precipitated with water, filtered out,
1
CDCl3) 6.88 (1H, d, J = 15.0 Hz), 6.92-7.07 (5H, m), 7.11-7.23 (4H, dried, and chromatographed on silica gel (CH2Cl2). After solvent
1
m), 7.23-7.36 (6H, m), 7.52-7.60 (4H, m), 7.65-7.78 (6H, m). H removal, methanol (7 mL) was added to the residue and the
NMR: δH (300 MHz, [D6]-DMSO) 7.84-7.70 (10H, m), 7.44-7.37 mixture was heated under reflux for 3-5 min, cooled, and the
(2H, m), 7.24-7.06 (10H, m), 7.04-6.96 (2H, m), 6.72 (1H, d, J = dye was filtered out and dried. A green-gray solid; yield: 46 mg,
15.3 Hz), 6.69 (1H, d, J = 15.3 Hz).
4-[(2E)-4-(1,3-Diphenyl-2-benzoimidazolylidene)-2-
butenylidene]-1,2-diphenyl-3,5-pyrazolidinedione
42%; mp 246-247 °C. Calcd. for C37H27N3O2 (545.64): C 81.45, H
4.99, N 7.70; found: C 81.34, H 5.07, N 7.67. 1H NMR: δH
(300 MHz; [D6]-DMSO) 8.80 (1H, d, J = 7.4 Hz), 8.62 (1H, t, J =
(5)
:
a
solution of 2-methyl-1,3-diphenylbenzoimidazolium chloride 13.2 Hz), 8.12 (1H, d, J = 7.8 Hz), 8.11 (1H, d, J = 13.3 Hz), 7.82
(80 mg, 0.25 mmol), hemicyanine 13 (106 mg, 0.25 mmol), and (1H, t, J = 7.8 Hz), 7.67-7.51 (3H, m), 7.40-7.25 (14H, m), 7.17-
triethylamine (50 mg, 0.5 mmol) in pyridine (3 mL) was heated 7.10 (2H, m), 6.65 (1H, d, J = 13.1 Hz), 5.48 (2H, s).
for 5 min under reflux. Upon cooling, water (10-12 mL) was 4-[(2E,4E,6E)-6-(1-benzyl-2-benzo[cd]indolylidene)-2,4-
added to the mixture that was then left at room temp. hexadienylidene]-1,2-diphenyl-3,5-pyrazolidinedione (9)
overnight. The precipitate was filtered out and crystallized from solution of 1-benzyl-2-methylbenzo[cd]indolium
methanol to give as an ashy-pink solid; yield: 60 mg, 42%; tetrafluoroborate (69 mg, 0.2 mmol) and hemicyanine 14
: a
5
mp >270 °C. Calcd. for C38H28N4O2 (572.67): C 79.70, H 4.93, N, (90 mg, 0.2 mmol) in acetonitrile (4 mL) was heated for 10 min
1
9.78; found: C 79.58, H 4.81, N, 9.63. H NMR: δH (300 MHz; under reflux in the presence of sodium acetate (250 mg, 3
CDCl3) 7.78-7.68 (6H, m), 7.57-7.49 (4H, m), 7.44-7.35 (4H, m), mmol). The crude product was precipitated with water, filtered
7.33-7.16 (6H, m), 7.12-6.95 (5H, m), 6.84 (1H, d, J = 13.8 Hz), out, dried, chromatographed twice on silica gel (CH2Cl2 with 0.5-
6.51 (1H, dd, J = 14.3, 12.1 Hz), 5.48 (1H, d, J = 14.3 Hz).
4-[(2E,4E)-6-(1,3-Diphenyl-2-benzoimidazolylidene)-2,4-
to-2.0 v.v. % of MeOH), and crystallized from acetonitrile–
methanol (1:2) to afford as a dark green solid; yield: 16 mg,
9
hexadienylidene]-1,2-diphenyl-3,5-pyrazolidinedione (6): the 14%; mp 190-191 °C. Calcd. for C39H29N3O2 (571.68): C 81.94, H
synthesis is analogous to that of dye
(only with hemicyanine 5.11, N 7.35; found: C 81.88, H 5.06, N 7.30. 1H NMR: δH
14). Because of very low solubility, the proper crystallization of (400 MHz; CDCl3) 8.04 (1H, d, J = 7.3 Hz), 7.85 (1H, d, J = 8.1 Hz),
was not performed. Instead, the precipitated dye was washed 7.78 (1H, dd, J = 13.8, 12.5 Hz), 7.76-7.66 (3H, m), 7.44-7.33 (8H,
5
6
with hot ethanol (3×5 mL) and then with CH2Cl2 (2×5 mL). A m), 7.33-7.28 (6H, m), 7.25-7.21 (2H, m), 7.17-7.09 (2H, m), 6.74
greenish dark-gray solid; yield: 38 mg, 25%; mp 228-229 °C. (1H, dd, J = 6.8, 1.0 Hz), 6.52 (1H, dd, J = 13.8, 11.7 Hz), 6.08 (1H,
Calcd. for C40H30N4O2 (598.71): C 80.25, H 5.05, N 9.36; found: C d, J = 12.5 Hz), 5.15 (2H, s). 1H NMR: δH (400 MHz; [D6]-DMSO)
80.31, H 4.93, N 9.23. 1H NMR: δH (300 MHz; [D6]-DMSO) 7.87- 8.42 (1H, d, J = 7.4 Hz), 8.11 (1H, t, J = 12.9 Hz), 8.00 (1H, d, J =
7.74 (10H, m), 7.54-7.46 (2H, m), 7.31-7.16 (10H, m), 7.03-6.94 8.1 Hz), 7.83-7.73 (2H, m), 7.61 (1H, d, J = 12.8 Hz), 7.57-7.45
(2H, m), 6.81 (1H, t, J = 12.8 Hz), 6.67 (1H, d, J = 13.8 Hz, 1H), (3H, m), 7.40-7.30 (11H, m), 7.29-7.22 (3H, m), 7.19-7.12 (3H,
6.36-6.21 (2H, m), 6.00 (1H, dd, J = 14.0, 10.9 Hz), 5.78 (1H, d, m), 6.70 (1H, t, J = 12.7 Hz), 6.70 (1H, t, J = 12.7 Hz), 6.50 (1H, d,
J = 14.5 Hz).
J = 12.7 Hz), 5.35 (2H, s).
4-[(2E)-2-(1-benzyl-2-benzo[cd]indolylidene)ethylidene]-1,2-
Triethylammonium
(E)-4-(3-(3,5-dioxo-1,2-diphenyl-4-
diphenyl-3,5-pyrazolidinedione (7): a solution of 1-benzyl-2-[2- pyrazolidinylidene)-1-propenyl)-5-oxo-1,2-diphenylpyrazol-3-
(dimethylamino)vinyl]benzo[cd]indolium tetrafluoroborate37 olate (10) 20
:
a dark-orange solid; yield: 33 mg, 26%; mp 230-231
1
(80 mg, 0.2 mmol) and 1,2-diphenyl-3,5-pyrazolidinedione (50 °C (from methanol, decomp.) (lit. mp 233-235 °C). H NMR: δH
mg, 0.2 mmol) in pyridine (3 mL) was heated for 30 min under (300 MHz; CDCl3) 9.40 (1H, br.s), 8.20 (1H, t, J = 13.7 Hz), 7.75
reflux. Upon cooling, water–ethanol (1:1) (10 mL) was added (2H, d, J = 13.7 Hz), 7.44-7.35 (8H, m), 7.32-7.21 (8H, m), 7.15-
dropwise with stirring and the mixture was put for 2 h in a 7.02 (4H, m), 2.93 (6H, q, J = 7.4 Hz), 1.08 (9H, t, J = 7.4 Hz). 1H
fridge. The obtained dye was filtered out, washed with cold NMR: δH (300 MHz; [D6]-DMSO) 8.91 (1H, br.s), 8.15 (1H, t, J =
ethanol and dried to afford a green-gray solid with metallic 13.7 Hz), 7.52 (2H, d, J = 13.7 Hz), 7.39-7.24 (16H, m), 7.14-7.03
shine; yield: 76 mg, 73%; mp 253-254 °C. Calcd. for C35H25N3O2 (4H, m), 1.16 (9H, t, J = 7.2 Hz).
(519.60): C 80.90, H 4.85, N 8.09; found: C 80.72, H 4.80, N 8.20. Triethylammonium
1H NMR: δH (300 MHz; CDCl3) 8.76 (1H, d, J = 13.8 Hz), 8.48 (1H, pyrazolidinylidene)-1,3-pentadienyl)-5-oxo-1,2-
d, J = 7.3 Hz), 8.02 (1H, d, J = 8.1 Hz), 7.78 (1H, d, J = 13.8 Hz), diphenylpyrazol-3-olate (11) 20
a dark-bronze solid; yield:
7.76 (1H, t, J = 7.7 Hz), 7.56 (1H, d, J = 8.3 Hz), 7.51-7.41 (5H, m), 14 mg, 10%; mp 125-126 °C. 1H NMR: δH (300 MHz; CDCl3) 9.64
4-((1E,3E)-5-(3,5-dioxo-1,2-diphenyl-4-
:
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(1H, br.s), 7.54-7.19 (21H, m), 7.14-7.02 (4H, m), 2.73 (6H, q, J = bathochromic shifts of the absorption bands byVbiewotAhrticλleaOannlinde
−1
7.1 Hz), 1.11 (9H, t, J = 7.1 Hz).
Ma , their narrowing (a decrease in σa),DaOnId: 1g0r.1o0w39in/Cg9NoJf0m32o7l5aDr
absorption (Table 1, Fig. S1–S3 in ESI). The deviations Da, an
important characteristic of the electronic asymmetry of
merocyanines,36 decrease for them going from medium-polarity
DCM to DMF and ethanol as the solvent. The degree of
electronic asymmetry can also be estimated by a vinylene shift
(VS): it is close to 100 nm for symmetric polymethine dyes and
decreases with a deviation from the ideal polymethine structure
A2.10,13,39. For merocyanines 1–3, the VS values approach 100
nm only in DMF and ethanol and decrease going to less polar
solvents. In the latter, a decrease in VS with the polymethine
chain lengthening is also observed. For example, for the pairs of
3. Results and discussion
In order to track the regularities connecting the properties of
such merocyanines with the structural parameters such as the
donor strength of the group D and the polymethine chain
length, three vinylogous series of dyes – derived from medium
electron-donating indole (1–3), strongly electron-donating
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
benzimidazole
(4–6), and weakly electron-donating
benzo[cd]indole (7–9) (Scheme 2) – have been synthesized and
their spectral-fluorescence properties have been studied in
solvents of different polarities (Table 1, Fig. S1–S9, S13–S21 in
ESI). In order to calculate Brooker's deviations36 of
vinylogues
1 - 2 and 2 - 3 in toluene, the VS is equal to 88 nm
(81.1 nm by band centres Ma−1) and 74 nm (54.2 nm),
respectively.
merocyanines' spectral bands, parent anionic dyes 10, 11
(Scheme 3) have also been obtained (Table 1, Fig. S10, S11, S22,
S23 in ESI). The spectral data for the parent cationic dyes are
taken from ref. 28.
Judging by the vicinal spin-spin coupling constants for the
polymethine chain H-atoms, which are close to or exceed 12 Hz,
the all-trans conformation dominate for molecules 1–9 in the
solutions in both low-polarity CDCl3 and high-polarity (CD3)2SO.
The Bouguer–Lambert–Beer law holds for merocyanines 1–9 in
the concentration range from 1×10−6 to 5×10−5 mol/L in
dichloromethane (DCM), DMF, and ethanol; for compounds 1–
3
8
,
,
7
,
9
also in toluene. In n-hexane, and in toluene for dyes 4–6
,
the concentration-dependence study has not been
Fig. 1. Absorption spectra of dyes 1–3 in n-hexane (blue solid) and ethanol (red dash).
performed because of the low solubility. Still, their aggregation
in the solutions has not been observed, although there were
signs of adsorption of some compounds on the cuvette walls
O
O
(see ESI, Fig. S12). For anionic dyes 10, 11, the absorption bands
n
Ph
Ph
N
N
in DCM at a concentration of 10−5 mol/L are broadened as
compared to those in high-polarity solvents (Table 1), which can
be explained by the beginning of ion-pair formation.38
3.1.Electronic structure and solvatochromism
N
N
O
O
Ph
Ph
Et3NH
Scheme 3. Molecular structures of anionic dyes 10 (n = 1) and 11 (n = 2).
For merocyanines 1–3, with the medium electron-donating
indole residue, an increase in the solvent polarity results in
O
Me
O
Me
O
Me
Me
Me
Ph
Me
Ph
Ph
Ph
K
2CO3
N
N
N
+
N
O
+
n
n
Ac
N
∆
2O /
r.t.
CH2Cl2 /
Ac
N
N
N
N
N
Ph
O
Ph
Ph
BF4
O
O
Me
Me
Me
1–3
O
O
Ph
Ph
N
Ph
Ph
Et3N
N
N
+
N
n
Me
Ph
∆
n
/
N
N
Ac
Py
N
N
N
Cl
Ph
O
Ph
O
4–6
Ph
Ph
O
O
O
∆
/
Py
Ph
Ph
8
( )
N
Ph
N
N
+
NMe2
n
+
n
Ph
N
Me
Ph
N
N
NaOAc
CH3CN /
∆
/
Py
N
N
N
N
Ph
∆
O
Ph
O
Ac
Ph
O
7–9
BF4
BF4
9
Ph
Ph
( )
Scheme 2. Synthesis of merocyanines 1, 4, 7 (n = 0); 2, 5, 8 (n = 1); 3, 6, 9 (n = 2).
4 | J. Name., 2012, 00, 1-3
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Table 1. Characteristics[a] of the UV-Vis absorption and steady-state fluorescence bands of dyes 1–11 at 20°C.
View Article Online
DOI: 10.1039/C9NJ03275D
−1
Dye
Solvent
λa
(nm)
460
472
473
474
473
536; 506
560
568
571
570
601; 565
634; 594
662
673
671
463
464
455
450
442
563
569
557
545
532
658
680
660
641
ε×10−3
(m2/mol)
5.67
8.85
9.38
8.97
8.77
8.48; 6.55
11.1
15.0
15.7
17.2
4.92; 6.05
7.80; 7.22
12.6
Da
(nm)
—
—
—
—
—
—
—
9.0
0.5
−1.0
—
Ma
σa
λf
(nm)
475
487
488
492
493
565
590
592
595
592
627; 678
683
700
702
698
498
498
493
487
486
586
596
587
583
578
683
706
696
693
686
Φf
(%)
Df
(nm)
—
—
—
—
—
—
—
6.50
−0.50
1.00
—
Mf−1
(cm−1)
501.2
510.4
510.7
518.5
521.0
616.5
628.3
614.7
613.8
615.2
717.4
732.8
722.4
720.6
715.0
528.0
526.1
513.4
509.4
508.4
608.5
614.8
604.7
603.2
598.2
713.2
718.3
712.5
710.0
705.3
—
σf
(cm−1)
1540
1420
1210
1280
1330
1510
1120
930
910
960
1730
1360
830
ΔνS
(cm−1)
690
650
650
770
860
960
910
710
710
650
2950
1130
820
610
580
1520
1470
1690
1690
2050
700
800
920
1200
1500
560
540
780
1170
2310
—
(cm−1)
444.0
455.3
457.0
457.0
455.2
507.1
536.4
549.9
554.8
554.5
552.3
590.6
626.3
644.6
648.1
452.4
453.6
441.6
435.8
424.2
(cm−1)
1240
1170
1130
1130
1120
1280
1210
1050
980
1
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
n-Hexane
Toluene
DCM
DMF
EtOH
0.005
0.009
0.023
0.071
0.073
0.005
0.059
0.22
0.64
1.42
0.003
0.09
2.8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
3
4
5
6
7
8
9
940
1460
1410
1290
1150
1030
960
—
—
18
2.5
0
—
—
—
—
—
—
6.5
2.0
1.5
—
—
—
—
—
—
—
17.1
8.2
740
710
19.7
16.5
0.12
0.37
0.31
0.38
0.26
1.3
7.2
13.8
6.2
[b]
—
1850
1660
1460
1550
1620
990
890
910
960
1030
890
10.6
9.37
8.29
6.49
990
1120
1220
1480
[b]
[b]
[b]
—
—
—
18.1
15.1
10.8
7.85
—
558.1
541.6
523
830
6.5
15.5
25
—
—
6
19
68
—
—
—
—
—
—
—
13
3.5
−4.5
—
1010
1290
1590
7.0
7.5
11.0
—
502.6
4.1
2.8
[b]
[b]
[b]
—
—
—
20.3
16.4
8.82
661.8
635.3
596.9
557.4
539.8
554.5
555.9
558.6
558.2
582.1
610.7
623.2
632.3
641.9
603.4
636.5
662.7
681.6
697.9
491.3
492.7
489
950
1140
1600
1990
1390
1300
1260
1250
1220
1430
1350
1330
1310
1200
1550
1570
1600
1610
1520
970
49.0
58.5
44.0
25.7
—
—
810
760
790
890
4.0
6.0
9.0
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
592
5.73
[c]
579; 540
593; 551
593; 551
595; 553
593; 551
633; 596
665; 621
675; 627
684; 633
685; 634
635; 603
664
686
704
780; 714
502
501
4.88; 4.58
5.79; 4.89
6.69; 5.22
6.52; 5.08
7.87; 5.56
—
—
—
[c]
—
—
—
—
611; 655
614
0.009
0.018
0.037
—
0.03
0.03
0.11
0.17
—
661.8
663.1
647.1
—
793.5
767.4
765.6
759.2
—
1250
1230
1170
—
500
520
390
—
607; 652
[b]
[c]
—
—
5.05; 6.63
6.73; 7.11
7.57; 6.96
10.9; 7.41
7.37; 7.14
7.66
7.52
7.37
7.71; 8.10
13.0
14.7
700
707; 769
711; 774
705
1540
1210
1080
1070
—
750
670
560
410
—
[c]
n-Hexane
Toluene
DCM
DMF
EtOH
DCM
DMF
EtOH
DCM
—
[c]
—
—
—
—
—
—
—
—
[c]
110.5
93
74
—
—
—
—
—
—
—
—
—
826
818
526
519
519
630
630
624
0.160
0.091
0.11
0.14
0.10
5.3
893.5
868.7
545.4
540.8
538.5
647.4
644.6
640.2
1010
890
1060
1060
980
750
700
710
2100
600
910
690
810
740
660
640
10
11
—
—
—
—
—
—
840
900
1260
800
970
498
602
605
600
15.1
12.6
18.1
17.9
578.6
595
585.3
DMF
EtOH
5.0
4.1
[a] λa and λf, band maxima; Da and Df, Brooker's deviations; ΔνS, Stokes shift; ε, molar absorption; indexes “a” and “f” denote characteristics of absorption and fluorescence
bands, respectively. [b] Not determined because of low solubility. [c] Below the detection limit.
The polymethine chain lengthening in the series 1–3 is accom- edge of the band due to an increase in probability of higher
panied by a noticeable broadening of the long-wavelength vibronic transitions (Table 1, Fig. 1). In more polar solvents, the
absorption bands in low-polarity n-hexane and toluene, caused transition from
by an increase in the relative intensity at the short-wavelength from 2 to 3, the bandwidth increases even in ethanol.
1
to
2 results in a decrease of σa, but when going
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9
1
Based on their solvatochromism and trends in σa, Da, and VS, it polarity solvents. However, its H NMR spectra iVniewboArtthicleCODnClinle3
DOI: 10.1039/C9NJ03275D
can be concluded that the electronic structure of merocyanines and in [D6]-DMSO contain one set of signals, attributable to the
1–3 in the ground state S0 varies within the A1–A2 range, polymethine chain H-atoms. Taking into account that for less-
approaching the structure A2 in high-polarity solvents. In dipolar merocyanines timescale of thermal isomerization is in
addition, the σa, Da, and VS variations indicate that the the range of seconds,40 this indicates the great prevalence of
contribution of the nonpolar polyene structure A1 increases for the all-trans isomer of dye
9
in the solution. The deconvolution
in n-hexane and ethanol shows
that they both can be fitted by a set of Gaussian components
, a bathochromic shift by separated by 1040-1060 cm−1 from each other (see ESI, Fig. S26,
dyes 1–3 with the polymethine chain lengthening, at least in of the absorption bands of dye
9
low- and medium-polarity solvents.
For imidazole-based merocyanine
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
4
1 nm (by both λa and Ma−1) is observed going from n-hexane to Table S1). The relative height of higher “vibronic” components
toluene as the solvent (Table 1). However, since toluene has is smaller in polar ethanol, reflecting a greater contribution of
much greater refractive index (1.497) as compared to that of n- the ideal polymethine structure A2 in it. On the other hand, the
hexane (1.375), this shift must be explained by the Bayliss bandwidths of “vibronic” components increase substantially in
refractive index function,1 not by an approach of dye's the polar solvent (Fig. S26), which can be indicative of an
electronic structure to the ideal polymethine A2 in toluene. This enhancement in solute–solvent interactions. It can be assumed
assumption is justified by the smaller bandwidth (σa) of
hexane, since σ does not depend on nD.29 For higher vinylogues for merocyanine
and , the n-hexane–toluene bathochromic shifts are much from the vibronic and solute–solvent interactions.
4
in n- therefore, that irregular changes of the absorption bandwidth
9
are determined by variable contributions
5
6
greater: 6 nm (190 cm−1) and 22 nm (490 cm−1) by λa,
respectively. For all three dyes 4–6, a further increase in the
solvent polarity results in hypsochromic shifts of the absorption
bands, their broadening, growing of deviations, and reducing of
vinylene shifts (Table 1, Fig. S4–S6 in ESI). Hence, dye
negative solvatochromism, while its vinylogues and
4
have
has
5
6
reverse solvatochromism in the chosen range of solvent
polarities. Consequently, the electronic structures of molecules
5
and 6 are close to the ideal polymethine structure A2 in
toluene, vary within the A2–A3 range in more polar solvents,
and shift to the A1–A2 range in n-hexane.
Similarly to indole-based merocyanines 1–3, benzo[cd]indole
derivatives 7–9 demonstrate positive solvatochromism.
However, for the latter a noticeable structuring of the
absorption bands due to an increase in the intensity of higher
vibronic transitions is observed even for dimethinemerocyanine
Fig. 2. Absorption spectra of dye 9 in solvents of different polarities: n-hexane (black
solid), toluene (red dash), DCM (blue dash-dot), DMF (magenta short-dash), and ethanol
(olive dash-dot-dot).
Comparison of deviations (Da), VS, and bandwidths (σa) of dyes
1–9 (Table 1) with those of their analogues comprising other
strong acceptor groups, such as 1,3-dimethylbarbituric41 and
1,3-diethylthiobarbituric,42 reveals that the acceptor strength of
1,2-diphenyl-3,5-pyrazolidinedione residue is much lower than
that of the 1,3-diethylthiorbarbituric residue, contrary to the
data in ref. 16. The same conclusion can also be made upon
comparison of the shape of the absorption bands of dye-
analogues in the same solvent (Fig. 3, Fig. S27 in ESI).
7
(Fig. S7 in ESI). The absorption bands of dye
n-hexane and toluene are already similar in shape to those of
carotenoids (Fig. S8). For higher vinylogue , the band is
8 in low-polarity
9
carotenoid-like even in high-polarity DMF (Fig. 2). Furthermore,
the bandwidths (σa) and deviations (Da) of absorption bands of
dye
9 are much greater than those of its indole analogue 3,
while the vinylene shifts by Ma−1 – it is incorrect to calculate VS
by λa in this case, since the bandshape changes significantly and
the maxima can correspond to different vibronic transitions –
are equal to only 21–56 nm in the pair
8 - 9. Hence, one can
conclude that the contribution of the nonpolar-polyene
structure A1 to the electronic structure of merocyanines 7–9 is
higher than for indole-based dyes 1–3
.
Note, that for dye , unlike to vinylogues
9
7 and 8, the long-
wavelength absorption band broadens in the series n-hexane–
toluene–DCM–DMF, its σa value decreases only when going to
ethanol as the solvent (Table 1). The maxima of the long-
wavelength absorption band of merocyanine
9 in n-hexane are
distanced by 830 cm−1, while typically the vibronic sub-bands in
polymethines are distanced by 1050–1370 cm−1, which is
attributable to the symmetric carbon-carbon valence vibration
of the polymethine chain.39 A possible explanation of this could
Fig. 3. Normalized absorption spectra of dye 3 (black solid) and its analogues with the
1,3-dimethylbarbituric (blue dash) and 1,3-diethylthiobarbituric (red dash-dot) acceptor
groups in n-hexane.
be the presence of several isomeric forms of dye
9 in low-
6 | J. Name., 2012, 00, 1-3
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9
The relative intensity of the maxima corresponding to the between 15 and 16 lies in the chromophore valence angles,
View Article Online
longest-wavelength (0–0) and higher (0–1, 0–2) vibronic which are more distorted in the latter; forDeOxIa: m10p.1l0e3,9t/hCe9NmJ0e3th27in5De
transitions can be used as the characteristic of electronic CCC-angle is equal to 135° and 139° in 15 and 16 respectively. It
asymmetry in this case. If the donor group remains the same, should be assumed therefore that such distortions cause the
this asymmetry will be a criterion for the strength of the additional hypsochromic shift of the absorption band, which
acceptor. Estimated in such a way electron-withdrawing should obviously be greater in 16, resulting in an increase of its
abilities of the acceptor groups roughly correlate with the pKa deviation and, hence, comparative underestimation of the
of the corresponding CH-acids (see ESI, Table S2).
barbituric residue acceptor strength.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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Why does the order of the acceptor strength (acidity) presented 3.2.Electronic structure and fluorescent properties
in ref. 16 differ from that found in this study? Earlier,
L.G.S. Brooker suggested using the deviation Da of styryl dyes as
The signs of solvatofluorochromism of merocyanines 1–3 and
4–6 coincide with the signs of their solvatochromism (Table 1).
a criterion for donor-acceptor properties of the terminal
At that, the ranges of solvatofluorochromic shifts are smaller.
groups, since 4-dimethylaminophenyl was one of the least
This is especially noticeable for negatively solvatochromic
benzimidazole derivatives 4–6 in the series DCM–DMF–ethanol.
electron-donating (“weakly basic”) residues.36 There are
typically two methine groups in the cationic styryl dyes used to
Indeed, for polymethine dyes the solute–solvent interactions
evaluate the strength of donor groups and three methine
are usually diminished in the state S1, most probably due to the
groups in the corresponding parent dyes. Spatial hindrances in
reduction of charge alternation in the chromophore.44 For
such molecules are insignificant and cannot affect position of
negatively solvatochromic merocyanines 4–6, the dipolarity
the spectral band. On the contrary, in the case of the acceptor
should decrease in the excited state as well, so the solute–
groups, both the styryls and parent anionic dyes contain only
solvent interactions abate even more. As a result, the Stokes
one methine group (Scheme 4). Spectral properties of such dyes
shifts decrease for positively solvatochromic indole-based dyes
should be determined not only by the donor-acceptor
1–3 and increase for dyes 4–6 as the solvent polarity rises.
Growth of the VS and decrease of Brooker's deviations of the
fluorescence bands of merocyanines 4–6 in comparison with
those parameters of their absorption bands (Table 1) indicate
properties of the terminal groups, but also by the steric
hindrances which cause a violation of the chromophore
planarity and a distortion of the valence angles in it.
that in the fluorescent state S1 their electronic structure
O
O
becomes more symmetrical, i.e. the contribution of the ideal
structure A2 increases. This tendency is also visible from TD-DFT
calculations, which reveal greater electronic symmetry for the
S1FC excited state of merocyanines.
Me
O
N
Ph
N
N
Me2N
Me2N
O
N
O
Ph
Me
16
15
Note, however, that in the pair
4 - 5 the fluorescence VS are
O
O
O
O
Me
Me
O
O
O
O
smaller than the absorption VS in low- and medium-polarity
solvents. Probably, this is caused by an additional bathochromic
shift of the fluorescence bands of merocyanine
greater distortion of its chromophore planarity in the state S1.
This assumption is supported by the considerable broadening of
Ph
Ph
Ph
N
N
N
N
N
N
N
O
O
4 due to a
N
Ph
Me
Me
17
18
the fluorescence bands of dye
bands (cf. the σa and σf values in Table 1). For its indole analogue
the absorption and fluorescence bandwidths are rather close
while for higher vinylogues the fluorescence become
4 compared to its absorption
Scheme 4. Molecular structures of some styryl dyes (15, 16) and parent anionic dyes (17,
18; without counterions) used by L.G.S. Brooker and other authors to evaluate the
strength of acceptor residues.
1
2, 3, 5, 6
narrower than the absorption (Table 1, Fig. 4).
According to the PCMDCM/DFT/B3LYP/6-31G(d,p) calculations,
the angle between the mean planes of the terminal
heterocycles in molecules 17 and 18 is equal to 32° and 34°
respectively (see ESI, Fig. S28), i.e. their non-planar distortions
are nearly the same. The valence angle distortions in them are
also very close, for example, the CCC-angle at the central
methine atom of dyes 17 and 18 is equal to 132° and 133°
respectively. It can be assumed therefore that the error in
estimating the relative acceptor strength of 1,2-diphenyl-3,5-
pyrazolidinedione and 1,3-dimethyl-barbituric residues is
caused by the different degrees of distortion of chromophores
15 and 16. Their PCMDCM/DFT/B3LYP/6-31G(d,p) geometry
optimization results in nearly planar structures stabilized by the
intramolecular CH···O hydrogen bond. Note that the calculated
structure of 16 is very close to its X-ray data,43 which testifies in
favour of the chosen calculation method. The main difference
Fig. 4. Absorption (solid) and fluorescence (dash) spectra of dyes 4–6 in DCM.
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Benzo[cd]indole derivatives 7–9 have very low FQY, so their also some singular cases of the enhancement of competitive
fluorescence spectra were not detected in all solvents (ESI, Fig. non-radiative relaxation of the excited state S1. For example, for
View Article Online
DOI: 10.1039/C9NJ03275D
S19–S21). It is found that solvatofluorochromism of dyes 7–9 is 1,3-indanedione derivatives, whose FQY decrease sharply in
also weakened as compared to their solvatochromism. At that, protic solvents, the mechanism of the fast non-radiative decay
in the pair DMF–ethanol, their solvatofluorochromic shifts is proposed, based on the TD-DFT calculations, associated with
apparently depend mostly on the solvent refractive index the low-lying non-fluorescent ππ*-state, which mainly has
(Table 1). The fluorescence VS are much higher than the HOMO→LUMO+1 character with the LUMO+1 localized at the
5
6
7
8
9
absorption VS; for example, in DMF for the pair
8 - 9
they are 1,3-indanedione residue.50
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equal to 115 nm and 20 nm respectively. The fluorescence In the present case, reasons i–iv can be ruled out. Thus, as
bands are narrower than the corresponding absorption bands, shown hereinabove, the electronic structure of dyes 1–9 is
at least for vinylogues
deduced that the contribution of the structure A2 increases 1,3-dimethylbarbituric derivatives, hence, vibronic interactions
significantly for merocyanines 7–9 in the fluorescent state S1. should be comparable for them. The hydrodynamic volume of
8 and 9 (Table 1). Hence, it can be rather close on the A1–A2–A3 scale to that of the corresponding
The FQYs of positively solvatochromic dyes 1–3 and 7–9 the 1,2-diphenyl-3,5-pyrazolidinedione residue is not smaller
increase significantly upon the solvent polarity growth. For than those of the (thio)barbituric residues, i.e. there is no
negatively/reversely solvatochromic benzimidazole derivatives evident reason for enhancing trans-cis photoisomerization of
4–6, the maximum Φf values are reached in toluene or DCM and merocyanines based on the former. In addition, spectral-
then decrease in high-polarity DMF and ethanol (Table 1). The fluorescent properties of dye
FQYs increase also upon the polymethine chain lengthening. viscous ethylene glycol as the solvent (Fig. S24, S25). Going from
This regularity is however violated for the pair , whose ethanol to ethylene glycol, the FQY of dye increases from 1.4%
fluorescence bands lie in the longest-wavelength spectral range to 4.0%. For comparison, the 1,3-dimethylbarbituric and 1,3-
among the studied dyes. In accordance with the energy gap law, diethylthiobarbituric analogues of have the FQY of 14% and
2 has been studied in highly
8
-
9
2
2
the nonradiative fluorescent-state energy dissipation via 16% in ethanol, respectively.45,46 Hence, in the present case
internal conversion should be enhanced for them. Another should be other fast competitive processes besides
irregularity is observed in the series 1–3 in n-hexane, in which, isomerization, absent for the dyes with, e.g., the barbituric
as shown above, the contribution of the non-polar polyene acceptor groups. Then, molecules 1–9 do not comprise a
structure A1 increases drastically going from dye
1 to vinylogues common TICT-active group or a heavy spin-active atom.
2
and . An electronic asymmetry increase for positively Therefore, to identify the cause(s) of their fast non-radiative
3
solvatochromic merocyanines with the polymethine chain decay, the TD-DFT calculations were performed.
lengthening is most pronounced in low-polarity solvents. This 3.3.Quantum-chemical calculations
results in a greater dependence of FQY on the solvent polarity
for higher vinylogues. For example, going from n-hexane to
ethanol as the solvent, the Φf value increases 14 times for dye
The all-trans conformation of the polymethine chain was
chosen as the initial geometry for optimization of molecules 1–
9
. It is consistent with both their 1H NMR spectra and the X-ray
1, 280 times for 2, and 5400 times for 3.
structural data for similar merocyanines.51 According to the
PCM/DFT-B3LYP/6-31G(d,p) calculations, the chromophores of
dyes 1–9 are essentially planar in the ground state S0. The
maximum planarity violation is observed for benzimidazole
derivatives 4–6, owing mainly to a torsion around the bond
connecting the donor residue and the polymethine chain, which
still does not exceed 3°.
The obtained data are consistent with the regularities revealed
in our previous works, viz. the FQY of merocyanines increases
when their electronic structure approaches the structure A2,
which can be explained by both a decrease in vibronic
interactions and an increase in the oscillator strength of the
long-wavelength polymethine transition.2
Comparing the FQYs of dyes 1–9 and their analogues based on
The parameter of bond length alternation in the polymethine
chain (BLA) characterizes the position of merocyanines within
the A1–A2–A3 range: it is positive for the A1–A2 region, goes to
zero for the ideal polymethine A2, and becomes negative for the
A2–A3 region.52 It is found that the calculated BLA values of
molecules 1–9 (Table 2) are in good agreement with the
conclusions about their electronic structure inferred from the
study of their solvatochromism.
1,3-dimethylbarbituric
and
1,3-diethylthiorbarbarituric
acids,45,46 one can notice that for positively solvatochromic
merocyanines 1–3 and 7–9 the Φf values are much lower than
those for the corresponding (thio)barbiturates. As
abovementioned, the low fluorescence “brightness” of 1,2-
diphenyl-3,5-pyrazolidinedione derivatives was observed in ref.
25, however an explanation was not suggested for this effect.
The major competitive channels for fluorescence decay in
merocyanine dyes are: (i) electron-vibrational relaxation of the
polymethine excited state; (ii) twisting and trans-cis
photoisomerization around the polymethine chain bonds;40 (iii)
formation of the twisted intramolecular charge transfer (TICT)
states;47 (iv) spin–orbital coupling (heavy atom) effects;48 (v)
low-lying non-fluorescent nπ*-states associated with the lone
electron pairs on heteroatoms in the acceptor group, the
presence of which enhances internal conversion.49 There are
Thus, for positively solvatochromic merocyanines 1–3 and 7–9
,
the BLA values are positive, increase with the polymethine chain
lengthening, and decrease going from n-hexane to DCM and
ethanol as the solvent. For dye
BLA is equal to zero and becomes negative in more polar
solvents. Then, for its reverse-solvatochromic vinylogues and
, the BLA is positive in n-hexane and negative in DCM and
4 in n-hexane, the parameter
5
6
ethanol. An increase in dipolarity of molecules 1–9 with the
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solvent polarity growth is also reflected in their dipole moments fall within the ranges of −4.6 to −5.6 eV and −1.6 to −2.9 eV
View Article Online
(μD). For dyes with the same polymethine chain length, charge respectively (Table 2), which are typical DfoOrI:m10o.1s0t39p/oCl9yNmJ0e3th27in5De
alternation in the chromophore in the ground state S1 should be dyes.54 The highest levels of HOMO and LUMO are observed for
proportional to the dipolarity. Its magnitude can be estimated the derivatives of strongly electron-donating benzimidazole 4–
as the sum of the absolute values of charges in the polymethine
6. In all three vinylogues series, the values of EHOMO increase and
chain Σ|qi| (see Table S3 in ESI for more detail). The Σ|qi| values the values of ELUMO decrease upon the polymethine chain
are maximal for benzimidazole merocyanines 4–6 and minimal lengthening (Table 2), which results in a decrease of the energy
for weakly electron-donating benzo[cd]indole derivatives 7–9 gaps for higher vinylogues. The solvent dependence of EHOMO
(Table 2). Like dipole moments, they increase in more polar and ELUMO is most pronounced for highly dipolar merocyanines
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17
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solvents for all dyes 1–9
.
4–6 and diminishes going to indole and benzo[cd]indole based
The energies of frontier molecular orbitals, HOMO and LUMO, dyes 1–3 and 7–9, which indicates the stronger solute–solvent
are key parameters for organic light energy converters.53,54 For interactions for the former.
merocyanines 1–9, the DFT calculated EHOMO and ELUMO values
Table 2. Some results[a] of the PCM/(TD)DFT-B3LYP/6-31G(d,p) calculations for dyes 1–9.
*
Dye
PCM solvent
BLA
(Å)
μD
(D)
7.3
8.8
9.2
5.6
μD
Σ|qi|
Σ|qi|*
Δq
EHOMO
(eV)
ELUMO
(eV)
λcalc
(nm)
(D)
1
n-Hexane
DCM
0.023
0.015
0.012
−0.004
0.035
0.020
0.016
0.005
0.039
0.023
0.018
0.008
0.000
−0.016
−0.020
0.011
−0.010
−0.015
0.017
−0.006
−0.013
0.025
0.015
0.012
0.035
0.023
0.020
0.038
0.027
0.023
10.3
11.5
11.8
7.3
10.4
15.8
15.7
13.0
17.3
20.2
21.0
18.1
9.3
0.854
0.889
0.895
0.920
0.945
1.011
1.024
1.047
1.017
1.110
1.132
1.162
1.063
1.093
1.097
1.193
1.233
1.238
1.288
1.352
1.355
0.749
0.781
0.788
0.803
0.869
0.885
0.860
0.951
0.973
0.662
0.466
0.473
0.491
0.812
0.571
0.583
0.594
0.651
0.636
0.648
0.672
0.870
0.624
0.895
0.761
0.756
0.755
0.820
0.824
0.822
0.807
0.554
0.561
0.579
0.609
0.614
0.605
0.622
0.640
−0.342
−0.067
−0.064
−0.061
−0.413
−0.062
−0.067
−0.049
−0.064
−0.048
−0.046
−0.046
−0.287
0.007
−0.294
0.002
0.009
−0.001
0.000
0.004
−5.410
−5.461
−5.477
−5.574
−5.172
−5.196
−5.207
−5.294
−4.990
−4.997
−5.006
−5.086
−5.020
−5.187
−5.234
−4.781
−4.922
−4.964
−4.615
−4.732
−4.771
−5.353
−5.414
−5.433
−5.141
−5.175
−5.188
−4.987
−5.002
−5.011
−2.081
−2.150
−2.168
−2.268
−2.342
−2.416
−2.435
−2.534
−2.508
−2.590
−2.611
−2.707
−1.635
−1.740
−1.772
−1.955
−2.069
−2.096
−2.180
−2.297
−2.321
−2.737
−2.798
−2.816
−2.806
−2.871
−2.889
−2.861
−2.929
−2.949
492.2
401.2
401.9
401.4
553.2
456.5
454.9
458.5
501.9
512.0
512.1
511.9
452.1
375.0
389.2
442.7
434.3
436.5
499.6
491.6
493.8
658.8
495.5
493.4
535.7
541.0
541.8
577.0
588.2
589.0
EtOH
EtOH(2)[b]
n-Hexane
DCM
2
3
10.3
13.0
13.8
11.0
12.7
16.8
18.0
15.9
9.7
11.6
12.1
14.0
17.7
18.6
17.7
23.5
22.1
6.8
8.4
8.7
8.9
11.5
12.2
10.8
14.3
15.3
EtOH
EtOH(2)
n-Hexane
DCM
EtOH
EtOH(2)
n-Hexane
DCM
EtOH
n-Hexane
DCM
EtOH
n-Hexane
DCM
EtOH
n-Hexane
DCM
EtOH
n-Hexane
DCM
EtOH
n-Hexane
DCM
4
5
6
7
8
9
9.3
7.3
13.2
16.3
16.7
18.3
22.6
22.4
15.0
8.9
0.001
−0.262
−0.011
−0.011
−0.024
−0.010
−0.003
−0.065
−0.029
−0.021
9.1
10.7
12.7
13.5
13.7
16.8
17.4
EtOH
[a] BLA, bond length alternation in the polymethine chain; μD, dipole moment; Σ|qi|, sum of absolute values of charges in the polymethine chain (see ESI for more detail);
Δq, sum of charge alternations in the polymethine chain upon the S1FC←S0 transition; EHOMO, ELUMO, calculated energy levels of the frontier molecular orbitals; λcalc
,
calculated wavelength of the S1FC←S0 transition; the values corresponding to the state S1FC are marked with an asterisk. [b] Calculations marked as EtOH(2) were performed
with ethanol as the PCM solvent and with two molecules of methanol H-bonded to the two carbonyls of the acceptor moiety, the dipole moments μD for these tasks are
characteristics of the whole “cluster”, not of an individual merocyanine molecule.
The TD-DFT energy of the long-wavelength electronic transition the light-induced intramolecular charge transfer.13,57
A
in polymethine dyes is known to be systematically comparison of the experimental absorption maxima λa of dyes
overestimated.44,55,56 This is generally explained by the nature 1–9 (Table 1) and the PCM/TD-B3LYP calculated energies of the
of the polymethine ππ*-transition, which is accompanied by S1FC←S0 electronic transitions λcalc (Table 2) shows the same
both the light-induced charge resonance, i.e. the redistribution issue. There are, however, noticeable irregularities in the λcalc
of electron density between the even and odd positions array. The values of λcalc are higher than expected for
through the chromophore, and, in the case of asymmetric dyes, merocyanines 1, 2, 4, 7 in n-hexane and for dye 4 in ethanol. The
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in n-hexane even exceeds the experimental violated if the transition S1FC←S0 is not a polymethine-type
1
2
3
4
5
6
7
8
λcalc of dye
maximum of its absorption band in this solvent.
An analysis of the electronic transitions in molecules 1–9 has To study the states S1 and S2, we used the PCM/TD-DFT
shown that the non-fluorescent nπ*-states are higher than the calculations with a linear-response solvation. According to
polymethine ππ*-states even in low-polarity n-hexane, i.e. they them, in n-hexane for dyes
1, 2, 4, 7 the main contribution to
cannot possibly be the cause of their low fluorescence. Indeed, the state S1 is from the HOMO-1→LUMO excitation. In other
the FQYs of dyes 1–3 are comparatively low even in ethanol, in cases, the longest-wavelength transition is the polymethine
which the nπ*-excitation should be much suppressed due to the transition and comes mainly from the HOMO→LUMO
View Article Online
transition, for example, for dyes 1, 4, 7
inDnO-Ih: e10x.a10n3e9./C9NJ03275D
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solute–solvent hydrogen bond formation.
excitation. At that, the separation of the states S1 and S2
However, it has been found that there is a low-lying forbidden increases with the polymethine chain lengthening and the
ππ*-transition in molecules 1–9, the main contribution to which solvent polarity growth. The energy difference between these
gives the HOMO-1→LUMO excitation, from the MO located on states depends also on the donor group of merocyanines, being
the acceptor 1,2-diphenyl-3,5-pyrazolidinedione residue (Fig. 5, the smallest for indole-based merocyanines and the largest for
Fig. S29–S31 in ESI). Since the energy of this transition is very benzimidazole derivatives 5, 6.
close to the energy of the polymethine ππ*-transition, they Since TD-DFT approach has many limitations for large
become vibronically coupled (proximity effect).[58] Close excited conjugated systems, to verify the TD-DFT results, for
states of the same spin multiplicity S1 and S2 are connected merocyanine 2 the excitation energies has been calculated also
through a nonadiabaticity integral, and internal conversion of at the second-order approximate coupled-cluster (CC2) level59
the fluorescent state should be noticeably enhanced due to with the def2-TZVP basis set. This calculation was performed
nonadiabatic coupling interactions.
using the Turbomole program version 6.4.60 The CC2 was shown
to give from all second-order methods the smallest mean
absolute errors for adiabatic excitation energies.61
The molecular geometry of dye
PCMDCM/DFT-B3LYP/6-31G(d,p) optimization. It has been found
that the CC2 excitations in molecule are very similar to those,
2 was taken from the
2
obtained using the TD-DFT. Namely, the CC2 has also shown
that the longest-wavelength (2.648 eV, 468 nm) singlet
transition in dye
2 is the dipole forbidden ππ*-transition, the
main contribution to which gives the HOMO-1→LUMO
excitation, the same as in the TD-DFT calculation. The
polymethine HOMO→LUMO transition (2.687 eV, 461 nm) is
the second one. Again, the CC2 calculation shows intermixing of
these transitions, with the HOMO→LUMO giving 15.3%
contribution to the former and the HOMO-1→LUMO giving
10.4% contribution to the latter. Hence, our conclusions, based
on the TD-DFT calculations are supported by the advanced ab
initio second-order method.
Thus, the calculations have allowed us to find the probable
reason of the fast non-radiative decay in merocyanines based
on 1,2-diphenyl-3,5-pyrazolidinedione. Still, since in molecules
1–9 the HOMO-1 is partially localized on the phenyl substituents
of the acceptor group (Fig. 5), the questions arise as to what the
effect of these substituents is and whether this decay pathway
arises if phenyl substituents are introduced into the barbituric
acceptor group. In order to clarify these points, the
PCMDCM/(TD)DFT-B3LYP calculations were carried out for four
model molecules comprising the indole donor group and
different acceptor groups (Scheme 5, Fig. S32, S33 in ESI).
It is found that disruption of conjugation of the phenyl groups
with pyrazolidinedione (structure 19) or even their replacement
with the methyl groups (structure 20) does not increase
substantially the separation of the forbidden HOMO-1→LUMO
transition from the polymethine transition. Hence, such
chemical modifications should not enhance fluorescence of
Fig. 5. Molecular orbitals of dye 1 (PCMDCM/DFT-B3LYP/6-31G(d,p), contour value is 0.03
bohr−3/2).
The different nature of the long-wavelength transitions in
molecules 1–9 can be traced by the charge alternation in the
polymethine chain upon the S1FC←S0 transition. For
a
polymethine transition, the electron density should increase on
the even atoms of the chromophore and decrease on the odd
ones (light-induced charge resonance).13 The PCM/TD-B3LYP
calculations are consistent with this rule. As a result, the charge
alternation in the polymethine chain decreases in the excited
state (cf. the parameters Σ|qi| and Σ|qi|* in Table 2). More
significantly, the net charge change in the polymethine chain is
very small (see the parameter Δq in Table 2). This regularity is
merocyanines 19 and 20 in comparison with that of dye 2. For
barbituric derivatives 21 and 22, the next to the polymethine
transition is the forbidden nπ*-transition, which is separated
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from the former by a more than 1 eV gap and, consequently, light conversion materials. Information on the donor-acceptor
should not affects dyes' fluorescence ability. This result suggests properties of the terminal groups usedDiOnI:m10e.1r0o3c9y/aCn9iNnJe0s32a7n5Dd
that the group responsible for the fluorescence quenching of relative dyes is crucial in that respect. In this paper, we not only
View Article Online
pyrazolidinedione
merocyanines
is
the
fragment have shown that the acceptor strength of one of such groups
(O=C)N−N(C=O) by itself, rather than, for example, the azo- was estimated erroneously, but also have found the cause of
chromophore that can be discerned in molecules 1–9
.
this error. The probable reason of the unexpectedly low
fluorescence of the pyrazolidinedione-based merocyanines has
also been conjectured from the quantum-chemical calculations.
Therefore, the results of this paper should be useful and
appealing from both the practical and theoretical perspective.
Me
Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
A
N
Me
O
O
O
O
Me
Conflicts of interest
There are no conflicts to declare.
Ph
N
N
A:
N
Me
N
N
O
O
N
N
N
O
O
Me
O
O
Me
Ph
19
21
20
22
Acknowledgements
Scheme 5. Molecular structure of merocyanines 19–22.
The quantum-chemical calculations were performed using the
computational facilities of the joint computational cluster of
State Scientific Institution “Institute for Single Crystals” and
Institute for Scintillation Materials of NAS of Ukraine
incorporated into the Ukrainian National Grid.
4. Conclusions
A study of the absorption and fluorescence solvatochromism of
the three synthesized vinylogous series of merocyanines
comprising the 1,2-diphenyl-3,5-pyrazolidinedione residue as
the acceptor group has revealed that the latter, contrary to the
literature data,16 is not a record-strong acceptor. This residue is
shown to be comparable by an electron-withdrawing ability to
the barbituric or 1,3-dialkylbarbituric one, yielding to the
thiobarbituric acceptors. The inaccurate estimation of the
relative strength of some acceptor groups in classical studies is
rationalized by the steric hindrances in the monomethine
anionic dyes and monomethine styryl dyes, UV–Vis maxima of
which were used as the key experimental parameters and yet
depended unforeseeably on the spatial requirements of an
acceptor residue.
It is found that the fluorescence quantum yields of indole- and
benzo[cd]indole-based dyes 1–3 and 7–9 are significantly lower
than those of relative merocyanines with the barbituric and
thiobarbituric acceptor groups. The PCM/TD-B3LYP calculations
have revealed that the probable reason of this is the fast non-
radiative decay, caused by the presence of the low-lying ππ*-
state, associated with the forbidden transition from the
HOMO-1 localized on the acceptor 1,2-diphenyl-3,5-
pyrazolidinedione fragment. This result has been verified by the
second-order approximate coupled-cluster (CC2) calculation
We wish to acknowledge the help by Prof. Alexander B.
Rozhenko, who performed the Turbomole ADC(2) calculation.
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New Journal of Chemistry
Heorhii V. Humeniuk, Nadezhda A. Derevyanko, Alexander A. Ishchenko and Andrii V. Kulinich*
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Merocyanines based on 1,2-diphenyl-3,5-pyrazolidinedione
View Article Online
DOI: 10.1039/C9NJ03275D
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Acceptor strength of the 1,2-diphenyl-3,5-pyrazolidinedione residue is re-evaluated based on the spectral-
fluorescent properties of donor–acceptor dyes. The fluorescence of the title merocyanines is shown to be
impaired by the low-lying forbidden ππ*-state.