Unsymmetrical dyes derived from 7,8-dihydrobenzo[c,d]furo[2,3-f]indole

Article abstract of DOI:10.1002/poc.1582

On the basis of the 7,8-dihydrobenzo[c,d]furo[2,3-f]indole nucleus, a number of unsymmetrical carbocyanines as well as styryl dyes have been synthesized and their absorption spectra have been measured. Starting from the deviations of long-wavelength maxima, the value of electron-donor ability D has been estimated for the heterocycle under study and a number of dye end groups have been ranked by their electron-donor properties. Experimental inferences are supported by the quantum chemically calculated bond length alternations and energy levels for the dyes concerned. Copyright

Full text of DOI:10.1002/poc.1582

Research Article
Received: 10 February 2009,
Accepted: 25 May 2009,
Published online in Wiley InterScience: 3 August 2009
(www.interscience.wiley.com) DOI 10.1002/poc.1582
Unsymmetrical dyes derived from
7,8-dihydrobenzo[c,d]furo[2,3-f]indole
a
a
a
*
Iryna G. Davydenko , Aleksei D. Kachkovsky , Marina L. Dekhtyar ,
Yurii L. Slominskiia and Aleksei I. Tolmacheva
On the basis of the 7,8-dihydrobenzo[c,d]furo[2,3-f]indole nucleus, a number of unsymmetrical carbocyanines as well
as styryl dyes have been synthesized and their absorption spectra have been measured. Starting from the deviations
of long-wavelength maxima, the value of electron-donor ability D has been estimated for the heterocycle under study
and a number of dye end groups have been ranked by their electron-donor properties. Experimental inferences are
supported by the quantum chemically calculated bond length alternations and energy levels for the dyes concerned.
Copyright ß 2009 John Wiley & Sons, Ltd.
Keywords: 7,8-Dihydrobenzo[c,d]furo[2,3-f]indole; deviation; electron-donor ability of heterocycles; polymethine dyes;
styryl dyes
deviation value[6] from the arithmetic mean for the corresponding
band maximum positions of two parent symmetrical dyes.
INTRODUCTION
Near-infrared absorbing dyes have been widely used in advanced
engineering applications. Many such dyes belong to a broad
class of cyanines known as photosensitizers, photodynamic
therapeutic agents, fluorescent probes for biopolymers, non-
linear optical materials, etc.[1,2] Cyanine dyes, also called poly-
methine dyes (PDs), often exhibit intense and selective near-
infrared absorption,[3,4] thus showing promise in the generation of
third-order nonlinear optical effects.[5] Long-wavelength PDs can
be designed by the lengthening of the polymethine chain or,
alternatively, by the introduction of end groups having their own
extensive conjugated system.[3,4] From the practical point of view,
it is indispensable that PDs be sufficiently stable. Dye stability
essentially depends on the equalization of p-bond orders in the
chromophore, which in turn is governed by ‘Brooker’s basicity’[6]
or electron-donor ability[7,8] of heterocyclic end residues. Aiming
at novel long-wavelength stable PDs, we previously synthesized[9,10]
symmetrical dyes 1a–c and 2 (Fig. 1) derived from 7,8-dihydro-
benzo[c,d]furo[2,3-f]indole, a nucleus with large effective length.[7]
Spectroscopic and quantum chemical methods were used to
determine the relative positions of molecular energy levels, the
corresponding molecular orbital (MO) shapes, and the nature of
electronic transitions in the PDs obtained as well as their linear[11]
and nonlinear optical properties.[12]
Accordingly, the widely applied series of Brooker’s basicities for
PD end groups has been based on the deviations of unsym-
metrical PDs, with one end group represented by the least
electron-donor 4-dimethylaminophenyl residue and the other
end group varied.[14,15] Moreover, Brooker’s sensitivity rule states
that the spectral effects of structural alterations in unsymmetrical
PDs are proportional to their deviations and hence related to the
relative basicities (electron-donor abilities) of their end groups.[14]
Interestingly, negative deviations are sometimes observed
which can be rationalized in terms of a simple topological
approach.[16] A dye deviation is contributed by the so-called
topological part determined by end-group structures (which can
be negative as well as positive) and by the part arising from bond
alternation in the chromophore (which is always positive). As a
rule, the latter contribution predominates, giving rise to positive
deviations but if it is small enough (bond alternation is very slight)
a negative deviation can appear. Therefore, negative deviations
are only possible for PDs with very close end-group basicities and
a sufficiently short polymethine chain.
In this context, it is clear that a series of unsymmetrical PDs
containing the 7,8-dihydrobenzo[c,d]furo[2,3-f]indol-4-yl residue
is required to get an insight into its electron-donor properties
and, in particular, to establish the effect of dihydrofuran
annulation on the basicity of the 2-benz[c,d]indolyl residue.
As shown, the dihydrofuran ring annulated to the benz[c,d]-
indolium nucleus at the 6,7-positions causes a much larger
bathochromic shift of dye absorption (about 50 nm) than an
alkoxy group introduced at the 6-position. The respective
absorption maxima of dyes 1a, 1b, 1c and 2 are found at 892,
993, 1112 and 1232 nm in methylene chloride.
The present study addresses the electron-donor ability of the
7,8-dihydrobenzo[c,d]furo[2,3-f]indol-4-yl residue. It can be
estimated from the spectra of appropriate unsymmetrical PDs
using the simple and long-known deviation method suggested
first by Kiprianov[13] and then independently by Brooker.[6] They
noticed that the long-wavelength band maximum of an
unsymmetrical PD was as a rule shifted to blue by the so-called
*
Correspondence to: I. G. Davydenko, Institute of Organic Chemistry, National
Academy of Sciences of Ukraine, Murmanskaya Str. 5, Kiev-02094, Ukraine.
E-mail: ira_davidenko@ukr.net
a
I. G. Davydenko, A. D. Kachkovsky, M. L. Dekhtyar, Y. L. Slominskii, A. I.
Tolmachev
Institute of Organic Chemistry, National Academy of Sciences of Ukraine,
Murmanskaya Str. 5, Kiev-02094, Ukraine
J. Phys. Org. Chem. 2010, 23 96–104
Copyright ß 2009 John Wiley & Sons, Ltd.

UNSYMMETRICAL DYES
large amounts. To prepare carbocyanines 12–14, aldehyde 8 was
obtained and reacted with the quaternary salts of 2- and
4-methylquinolinium and the salt of 4-methyl-2,6-diphenyl-
thiopyrylium. All the compounds synthesized have been structu-
1
rally determined by H NMR spectroscopy.
To calculate the deviations and to find the features of the
electronic structure for the unsymmetrical PDs under study, we
have used the data for their parent symmetrical dyes, namely,
monomethines 15 and 16,[17] dicarbocyanine 17[18] and
carbocyanines 1a, 18[19] and 19–23[20] (Fig. 2).
The structural and electronic characteristics of PDs have been
studied quantum chemically. The ground-state equilibrium geometry
of dye molecules was optimized in the semiempirical AM1
approximation by the HyperChem package; the structures were
refined until the energy gradient was less than 0.01 kcal/mol. The
energy spectra of the dyes were calculated by the same method.
PDs 11 and 20 containing the 2-benzothiazolyl residue have been
excluded from computational consideration because the para-
metrization of the AM1 method does not enable their energy
spectra to be calculated correctly (the energies of the MOs localized
on the two-coordinated sulphur atom are much overestimated).
Figure 1. Symmetrical polymethine dyes derived from 7,8-dihydro-
benzo[c,d]furo[2,3-f]indole
OBJECTS AND METHODOLOGY
We have synthesized styryl, hemicyanine and unsymmetrical
carbocyanine dyes 4–6 and 9–14 containing heterocyclic resi-
dues with various electron-donor abilities (Scheme 1). Quaternary
salts 3a,b were condensed with anisaldehyde, p-dimethylamino-
benzaldehyde and 3-dimethylaminoacrolein in acetic anhydride
to provide styryls 4 and 5 and hemicyanine 6, respectively.
Condensation of salt 3a with the respective v-aldehydes of
benz[c,d]indole and indolenine in acetic anhydride provided the
carbocyanines 9 and 10. Dye 11 was obtained by the reaction of
salt 3a with 3-ethyl-2-(2-acetanilidovinyl)benzothiazolium tosy-
late in acetic anhydride in the presence of triethylamine. How-
ever, this method failed to produce good yields of PDs 12–14 as it
caused the formation of the corresponding symmetrical dyes in
RESULTS AND DISCUSSION
Deviations
The spectral properties of the unsymmetrical PDs prepared were
studied in media of various polarity, namely in dichloromethane,
ethanol and methanol, with their respective dielectric constants
9.08, 24.30 and 32.66. The first of the three solvents was selected
so as to rule out hydrogen bonding to the oxygen atom of the
7,8-dihydrobenzo[c,d]furo[2,3-f]indole nucleus.
Scheme 1.
J. Phys. Org. Chem. 2010, 23 96–104
Copyright ß 2009 John Wiley & Sons, Ltd.
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I. G. DAVYDENKO ET AL.
Figure 2. Parent symmetrical dyes
Table 1 lists the positions of the long-wavelength absorption
maxima of the dyes as well as their deviations (Dl). As seen, going
from dichloromethane to alcoholic solutions causes hypso- and
hypochromic effects for most of the dyes concerned. This is
probably due both to enhanced intermolecular universal solute–
solvent interactions in media with larger dielectric constants and
to stronger nucleophilic solvation of PD molecules by alcohols,
the two factors mainly stabilizing the more polar ground state of
cationic dyes.[21]
The largest positive deviations observed for styryls 4 and 5
(Table 1) suggest the most significant asymmetry of the electron
density distribution in their molecules and the drastically higher
electron-donor ability of the 7,8-dihydrobenzo[c,d]furo[2,3-f]
indol-4-yl nucleus as compared to the very low-basic 4-dimethyl-
aminophenyl and 4-methoxyphenyl residues. On the other hand,
a significant positive deviation of carbocyanine 13 implies that
the 7,8-dihydrobenzo[c,d]furo[2,3-f]indol-4-yl nucleus is a much
weaker electron donor than the highly basic 4-quinolyl group. At
the same time, carbocyanines 9 and 12 containing the 2-benz-
[c,d]indolyl and 2-quinolyl residues as end groups, respectively,
exhibit rather small positive deviations in dichloromethane
(which considerably increase when passing to alcoholic
solutions). According to the deviation theory of Kiprianov and
Brooker, the two above-mentioned end groups should slightly
differ in their electron-donor properties from the 7,8-dihydro-
benzo[c,d]furo[2,3-f]indol-4-yl nucleus under study, though this
difference rises with solvent polarity. By chemical intuition, one
can assume that 7,8-dihydrobenzo[c,d]furo[2,3-f]indol-4-yl as an
electron donor falls in between the second end groups of PDs 9
and 12. At the same time, it is the end groups of PDs 10, 11 and
14 with negative deviations that are closest to the 7,8-dihydro-
benzo[c,d]furo[2,3-f]indol-4-yl residue in electron-donor ability.
Table 1. Characteristics of the long-wavelength absorption bands for the dyes synthesized, along with their deviations (Dl) and
Ilchenko‘s basicity values (D) determined for the 7,8-dihydrobenzo[c,d]furo[2,3-f]indolium nucleus from spectral experiments in
ethanolic solutions
Dichloromethane
Ethanol
Dl (nm)
Methanol
Dye
lmax (eꢀ10ꢁ4 a
)
Dl (nm)
lmax (nm)
D
lmax (eꢀ10ꢁ4 a
)
Dl (nm)
b
4
5
6
9c
10
11
12
13
14
594 (3.07) 630 (2.76)
674 (5.54) 721 (6.88)
626 (4.87) 676 (5.28)
827
670 (6.38) 733 (9.81)
675 (5.89) 738 (8.95)
687 (6.33) 748 (10.36)
724 (5.07) 794 (9.59)
789 (7.58) 835 (8.68)
109
30
—
1.5
ꢁ14
ꢁ11.5
1
586
697.5
660
813
724
726
731
771
809
—
44
—
6
—
1.24
—
583 (3.01)
686 (6.09)
613 (5.00) 658 (5.20)
807
662 (6.33) 720 (8.73)
664 (5.97) 721 (7.95)
675 (6.16) 727 (8.37)
709 (5.25) 765 (7.23)
768 (7.41) 805 (7.28)
103
46
b
b
ꢁ16
1.26
d
5
ꢁ14
ꢁ9
9
22
7
—
—
ꢁ15.5
ꢁ10
7
18
4
d
1.29
1.28
9
ꢁ9
e
—
a lmax measured in nanometres (nm) and e in l/mol ꢀ cm.
b Experimental data for parent symmetrical dyes are lacking so that the corresponding deviations and D values cannot be
determined.
c The dye is sparingly soluble.
d D values are not defined for negative deviations.
e The reference point for the second end group is absent on the D scale.
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 96–104

UNSYMMETRICAL DYES
(As already explained, a negative deviation can occur provided if
there is negligible bond alternation.) Thus, the three correspond-
ing end groups and the residue of interest somehow surrounded
by them should be placed between the 2-benz[c,d]indolyl and
2-quinolyl groups.
methanol, whereas PD 14 exhibits it only in the least polar aprotic
dichloromethane). A weaker tendency of PD 14 than of PD 11
towards negative deviations just indicates that the 4-thiopyranyl
end group behaves as if it were a somewhat stronger electron
donor than the 2-benzothiazolyl residue, contrary to what was
calculated previously in the topological approximation[23] and
found in spectroscopic experiments.[24]
It is seen from Table 1 that the deviations of most dyes increase
algebraically (so that negative values become smaller and posi-
tive larger in absolute magnitude) on passing from dichlor-
omethane to alcoholic solutions. This might be expected in view
of the stronger negative solvatochromism of unsymmetrical PDs
in comparison with their symmetrical counterparts (the former
are characterized by a larger decrease in the dipole moment on
excitation). At the same time, some of the dyes exhibit the
opposite trend, thus suggesting interference of another factor.
Indeed, hydrogen bonding between an alcoholic solvent and
7,8-dihydrobenzo[c,d]furo[2,3-f]indole should lower its electron-
donor ability because one of the two lone electron pairs of the
heterocyclic oxygen atom is protonated and thus removed from
conjugation.[22] As a result, going from aprotic to protic solvents
makes a dye more electronically symmetric if the second end
groupislesselectron-donorthan7,8-dihydrobenzo[c,d]furo[2,3-f]-
indol-4-yl and more electronically asymmetric in the opposite
case. Evidently, such an effect may diminish, neutralize or even
overcompensate the mentioned (algebraic) increase in devi-
ations with solvent polarity for PDs like 4, 5 or 9 (with the weakly
basic second end group) and vice versa, make it more
pronounced for PDs like 12 or 13 (with the strongly basic
second end group).
An empirical quantitative measure to estimate the electron-
donor ability (D) of PD end nuclei was previously suggested by
Ilchenko.[8] The values of D for the 4-dimethylaminophenyl and
2-(1,3,3-trimethylindolenyl) residues were put equal to 0 and 1,
respectively, and the (positive) deviation Dl of an unsymmetrical
PD was related to the difference in electron-donor ability jDDj
2
3
between two end nuclei: Dl ¼ 36jDDj – 6jDDj . Starting from the
positive deviations of PDs 5, 9, 12 and 13 in alcoholic solutions
(Table 1), the electron-donor ability D for the 7,8-dihydrobenzo-
[c,d]furo[2,3-f]indol-4-yl heterocyclic residue has been found by
the above equation. It varies from 1.24 to 1.29, with the average
value of 1.27, thus occupying an intermediate position between
the 2-indolenyl and 2-benzothiazolyl nuclei on the Ilchenko scale
shown in Fig. 3. As seen, annulation of the dihydrofuran ring
significantly (by 0.43 units of D) increases the electron-donor
properties of the 2-benz[c,d]indolyl nucleus. It is also notable that
the qualitative assessment of relative end-group electron-donor
abilities is fairly consistent with the empirically derived scale[8]
(cf. above series and Fig. 3).
The role of hydrogen bonding appears to be significant
enough, since it reverses the trend of deviation change with
solvent polarity even for highly electronically asymmetric PD 4.
Thus, we can invoke this effect to classify the dye end groups as
less or more electron-donor than 7,8-dihydrobenzo[c,d]furo[2,3-
f]indol-4-yl according to whether the corresponding PDs exhibit
an algebraic decrease or increase in deviations on passing from
dichloromethane to alcoholic solutions. Taking into account
the deviation values and the scales of their change (Table 1), the
heterocyclic residues contained as the second end groups in the
unsymmetrical dyes under study can be ranked in the increasing
order of their electron-donor abilities: 4 < 5 < 9 < 10 < 1 < 11 <
14 < 12 < 13. (Here the end groups are denoted as the
corresponding PDs and 1 refers to 7,8-dihydrobenzo[c,d]furo[2,3-
f]indol-4-yl).
As regards negative deviations, they can hardly occur,
irrespective of the solvent nature, for the terminal members of
the series (like 4, 5 or 12, 13) but are highly probable in any
solvents for the central members in the vicinity of 1 (as is the case
with 10 and 11) – refer to Table 1. If a heterocyclic residue
moderately differs from 7,8-dihydrobenzo[c,d]furo[2,3-f]indol-4-yl
in electron-donor properties, the situation can be governed by the
protic/aprotic nature of the solvent: hydrogen bonding reinforces
the tendency to negative deviations for weaker electron donors
than 1 and suppresses it for stronger electron donors (accordingly,
negative deviation is possible for PD 6 in the most polar protic
Alternation of bond lengths in the polymethine
chromophore
Brooker,[6] Platt,[25] Kiprianov[13] and Dyadyusha[16,26] attributed the
occurrence of positive deviations to the C—C bond length alter-
nation in the polymethine chain of unsymmetrical PDs (whereas
symmetrical PDs are characterized by essentially equalized
bonds, particularly in the central part of the chromophore).
We have optimized the molecular geometry of unsymmetrical
PDs 9, 10, 12–14 and their symmetrical counterparts 1a and 18,
19, 21–23 in the AM1 approximation. As demonstrated by Fig. 4,
the symmetrical PDs have two equalized bonds in the middle of
the polymethine chromophore and two terminal bonds which
are a little longer than the central ones. The slight difference in
length between the two bond types is due to the effect of end
groups and implies that their electron-donor abilities are not
equal to the middle value: if it were the case, the corresponding
end groups would be neither donors nor acceptors with respect
to the polymethine chain and the so-called ideal polymethine
state with fully equalized chromophore bonds would result.[27]
The elongated bonds adjacent to the end groups point to an
increased ability of the heterocyclic residues to donate electron
density to the polymethine chain (i.e. the resonance structure
with the positively charged end groups should greatly contribute
to the ground state of the PDs in question). Thus, as far as the
AM1 approximation is concerned, any of the heterocyclic
Figure 3. The empirically calculated scale of electron-donor ability (D)[8] for PD end nuclei
J. Phys. Org. Chem. 2010, 23 96–104
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I. G. DAVYDENKO ET AL.
Figure 4. Bond lengths in the chromophore of unsymmetrical PDs 9, 10, 12–14 and the corresponding parent symmetrical PDs 1a and 18, 19, 21–23.
Four bonds of the polymethine chain are numbered from the 7,8-dihydrobenzo[c,d]furo[2,3-f]indolium nucleus
residues considered here appears more donor than an end group
with the middle electron-donor ability.
goes, at the same time, some distortion; the terminal bond is
longer at the former than at the latter nucleus. These effects arise
from a much higher electron-donor ability of the 2-quinolyl than
of the 7,8-dihydrobenzo[c,d]furo[2,3-f]indol-4-yl end group.
Dye 13 is similar to symmetrical PDs in that its chromophore
bond lengths are essentially equalized (Fig. 4). This feature could
hardly be expected (since 4-quinolyl is known to be a even more
electron-donor group than 2-quinolyl) and is at variance with the
large positive deviation of the dye. It may be attributable to the
fact that the distortion of molecular planarity has been
exaggerated by the AM1 optimization procedure thus causing
too pronounced deconjugation of the 4-quinolyl group and
hence an underestimation of its electron-donor ability.
Regarding PD 14, its bond-alternation pattern agrees with the
experimental inference that 4-thiopyranyl has a higher electron-
donor ability than 7,8-dihydrobenzo[c,d]furo[2,3-f]indolium-4-yl.
Indeed, it is in the vicinity of the former end group that bonds are
longer and alternate stronger. At the same time, the difference in
electron donor ability between the two end groups should be
Only the chromophores of unsymmetrical PDs 9 and 10 exhibit
a noticeable degree of bond length alternation (Fig. 4). It is more
pronounced in the vicinity of the 7,8-dihydrobenzo[c,d]-
furo[2,3-f]indolium-4-yl nucleus and decays gradually towards
the second heterocyclic nucleus, 2-benz[c,d]indolyl or 2-indolenyl,
thus suggesting that the former end group is farther from the
middle electron-donor ability than either of the latter two. Since
7,8-dihydrobenzo[c,d]furo[2,3-f]indole is a stronger electron
donor, its residue is bound to the polymethine chain by a
longer bond than the two above-mentioned heterocyclic nuclei.
Interestingly, PD 10 manifests a negative deviation in spite of the
sufficiently strong bond alternation in the chromophore. One can
assume that the deviation of this dye is mainly contributed by the
topological part dictated by the end-group specificity.
It is also evident from Fig. 4 that the bond alternation in the
chromophore of PD 12 decays from the 2-quinolyl to the
7,8-dihydrobenzo[c,d]furo[2,3-f]indolium-4-yl residue and under-
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 96–104

UNSYMMETRICAL DYES
negligible, as indicated by only slightly differing terminal bond
lengths and smoothed bond alternation. This evidence is
consistent with the negative deviation exhibited by PD 14 in
dichloromethane which implies that the topological part of the
deviation probably predominates over the contribution from
alternating bonds.
It is seen that the computationally estimated C—C bond length
alternation in the PDs concerned mostly supports the positions of
the corresponding end groups in the deviation-based relative
basicity series (discussed above) as well as on the Ilchenko scale
(Fig. 3).
compounds (except for PD 4). Thus, the difference in first
transition energies is mainly determined by the positions of the
highest occupied MOs (HOMOs). In the general case, they should
shift downward from the arithmetic mean positions, for positive
deviations to occur. This effect is evident only for PD 5 giving rise
to a visible increase in the energy gap. Dye 4 is also characterized
by the increased gap, since its LUMO is shifted upward much
more than HOMO. The large positive deviations of PDs 4 and 5
can thus be understood.
For the rest of the unsymmetrical dyes, the energy gap appears
the same as (PDs 9 and 13) or smaller than (PDs 6, 10, 12 and 14)
that calculated from averaged HOMO and LUMO energies of the
corresponding parent dyes. This result apparently accounts for
the rather small positive deviations of PDs 9 and 12 and is
consistent with the negative deviations observed for PDs 6, 10
and 14.
Thus, the rough one-configuration treatment proves sufficient
to distinguish between the PDs exhibiting large positive
deviations and those for which deviations vanish or even
become negative. The only contradiction between the significant
positive deviations of PD 13 and its energy gap perfectly
coinciding with the averaged one of the corresponding parent
dyes is probably due to the already mentioned artifact of the AM1
technique (the underestimated p-coupling of the 4-quinolyl
group with the rest of the molecule).
Relative positions of electron energy levels
Dye deviations can be considered in terms of the relative
positions of the frontier energy levels in unsymmetrical and
parent symmetrical PDs. This approach can sometimes shed light
on the potentially negative topological part of deviation which
arises from the interaction in dye molecules between frontier
levels localized on the polymethine chain and others in the
vicinity localized on end groups. The greater the role of
the topological contribution, the higher the level density in
the vicinity of the Fermi level in parent symmetrical dyes.
Figure 5a demonstrates the part of the energy spectrum
including the frontier and close-to-frontier levels for the
symmetrical PDs involved as parent compounds in the present
study. As seen, the PDs significantly differ both in the positions
and near-energy-gap densities of their occupied and vacant
levels. Note that the dyes presented in the rising order of their
energy gap positions (i.e. ranked by their electron-donor
properties) again reproduce, though roughly, the empirical
basicity series given above. The same is true for the
corresponding unsymmetrical PDs, with their analogous energy
diagrams shown in Fig. 5b. Based on the level systems of
symmetrical and unsymmetrical PDs, one can qualitatively
interpret the occurrence of positive and negative deviations.
As seen, the energies of the lowest unoccupied MOs (LUMOs)
are rather close for all unsymmetrical PDs and very slightly differ
from the arithmetic mean values for the corresponding parent
CONCLUSIONS
Combined spectroscopic and quantum chemical study of
unsymmetrical PDs derived from 7,8-dihydrobenzo[c,d]furo-
[2,3-f]indole demonstrates that dihydrofuran annulation signifi-
cantly increases the electron-donor ability of the 2-benz[c,d]indolyl
nucleus. It has been found that the hemicyanine obtained from
the new heterocyclic nucleus and the PDs containing, as the
second end group, the residues of indolenine, benzothiazole and
thiopyran exhibit negative deviations in spite of regular C—C bond
alternation in the polymethine chromophore. This phenomenon
suggests that the mentioned dyes are characterized by the
Figure 5. Electron energy levels of symmetrical parent (a) and unsymmetrical (b) PDs. Frontier MOs are represented by bold lines and their arithmetic
mean positions for the corresponding parent dyes are shown by dashed lines. The energy diagrams are placed in the rising order of molecular Fermi
levels, i.e. ranked by dye electron-donor properties
J. Phys. Org. Chem. 2010, 23 96–104
Copyright ß 2009 John Wiley & Sons, Ltd.
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I. G. DAVYDENKO ET AL.
predomination of the topological part of the deviation over the
contribution from alternating bonds. Importantly, negative
deviations which are often regarded as an obscuring factor in
studying dye end-group basicities are invoked here, as well as
positive deviations, to assess the electron-donor properties of
various residues. On this basis, the deviation-based basicity series
has been constructed which agrees fairly well both with the
previously known empirical scale and with the computational
data on bond alternation and energy levels in the dyes concerned.
(3 ml) was heated for 30 min (bath temperature 1208C) and then
cooled to room temperature. After adding diethyl ether (70 ml),
the reaction mixture was allowed to stand for 3 h and the
resulting precipitate was filtered off. Then the product (0.61 g)
was dissolved on heating in methanol (15 ml) and filtered into a
solution of sodium perchlorate (0.8 g) in methanol (5 ml). After
2 h, the precipitate of the hemicyanine was filtered off and
washed with methanol (1 ꢂ 1 ml), hot water (3 ꢂ 5 ml) and diethyl
ether (3 ꢂ 5 ml). Yield 0.22 g (24%).
4-[2-Dimethylaminoethenyl]-5,8-dimethyl-7,8-
dihydrobenzo[c,d]furo[2,3-f]indol-5-ium tetrafluoroborate (7)
EXPERIMENTAL
1H NMR spectra were recorded on a Varian VXR-300 spectrometer
at 300 MHz using TMS as an internal standard. Electronic
absorption spectra were measured on a Shimadzu UV-3100
spectrophotometer. The yields, melting points, molecular
formulae and elemental analytical data are summarized in
A solution of quaternary salt 3a (1.63g, 5mmol) and DMF (2ml, 2.1g,
29 mmol) in acetic anhydride (5 ml) was boiled for 45 min and then
cooled to room temperature. After adding diethyl ether (100 ml), the
reaction mixture was allowed to stand for 3 h. The resulting
precipitate of the hemicyanine (1.6 g) was filtered off, washed with
diethyl ether (3 ꢂ 2 ml), boiled with benzene (30 ml) for 3 min and
hot-filtered. Then the product was dissolved in acetonitrile (30 ml)
and treated with activated charcoal, followed by filtering the
solution and evaporating the solvent in vacuo. Yield 1.44 g (76%).
1
Table 2 and H NMR spectral data in Table 3.
5-Butyl-4-[2-(4-methoxyphenyl)ethenyl]-8-methyl-7,8-
dihydrobenzo[c,d]furo[2,3-f]indol-5-ium perchlorate (4)
A
mixture of quaternary salt 3b (0.55 g, 1.5 mmol) and
2-[5,8-Dimethyl-7,8-dihydrobenzo[c,d]furo[2,3-f]indol-
4(5H)-ylidene]acetaldehyde (8)
4-methoxybenzaldehyde (0.19 ml, 0.24 g, 1.7 mmol) in acetic
anhydride (1 ml) was boiled for 4 min and then cooled to room
temperature; 2 h later, the precipitate was filtered off and washed
with a small amount of acetic anhydride (2 ꢂ 0.5 ml) and diethyl
ether (3 ꢂ 2 ml). Then the product (0.2 g) was dissolved in
dichloromethane (80 ml), followed by filtering the solution and
evaporating 2/3 of the solvent volume. The resulting precipitate
of the dye was filtered off and washed with diethyl ether. Yield
0.17 g (23%).
Dye 5 was obtained likewise from the corresponding salt 3a
and 4-(dimethylamino)benzaldehyde. The 1H NMR spectra
indicated that the dye precipitated in a pure state and required
no additional purification.
Hemicyanine 7 (1.52 g, 4 mmol) was added to a solution of
potassium hydroxide (1.23 g, 22 mmol) in methanol (60 ml) and
the mixture was boiled under reflux for 45 min. After cooling,
water (500 ml) was added and the product was extracted with
chloroform (4 ꢂ 35 ml). The combined extracts were dried over
potassium carbonate and evaporated in vacuo. The product was
purified by recrystallization from a 1:2 mixture of benzene and
hexane with activated charcoal added. Yield 0.72 g (68%).
5,8-Dimethyl-4-(3-[1-methylbenz[c,d]indol-2(1H)-ylidene]-
1-propenyl)-7,8-dihydrobenzo[c,d]furo[2,3-f]indol-5-ium
tetrafluoroborate (9)
5-Butyl-4-[4-dimethylamino-1,3-butadienyl]-8-methyl-7,8-
dihydrobenzo[c,d]furo[2,3-f]indol-5-ium perchlorate (6)
A mixture of quaternary salt 3a (0.33 g, 1 mmol) and 2-[1-methyl-
benz[c,d]indol-2(1H)-ylidene]acetaldehyde (0.21 g, 1 mmol) in
acetic anhydride (4 ml) was boiled for 1 min. The product
precipitating from the boiling solution was filtered off 3 h later
A solution of quaternary salt 3b (0.74 g, 2 mmol) and
3-dimethylamino-2-propenal (0.4 g, 4 mmol) in acetic anhydride
Table 2. Characterization data for the compounds synthesized
Elemental analysis (%) calcd./found
Compd.
M.p. (8C)
Yield (%)
Molecular formula
C
H
N (I or S)
4
5
6
7
8
>250
>250
183–185
>250
170–171
>250
173–176
>250
23
75
24
76
68
73
71
53
50
21
52
C27H28ClNO6
C25H25BF4N2O
C24H29ClN2O5
C19H21BF4N2O
C17H15NO2
C30H25BF4N2O
C29H29BF4N2O
C26H23BF4N2OS
C35H32N2O4S
C31H31IN2O
65.12 65.14
65.81 65.77
62.54 62.56
60.02 60.00
76.96 76.99
69.78 69.61
68.52 68.42
62.66 62.69
72.89 73.06
64.81
5.67 5.75
5.52 5.59
6.34 6.35
5.57 5.55
5.70 5.60
4.88 4.92
5.75 5.80
4.65 4.70
5.59 5.62
5.44
2.81 2.87
6.14 6.26
6.08 6.12
7.37 7.45
5.28 5.25
5.43 5.46
5.51 5.52
5.62 5.75
4.86 4.93
9
10
11
12
13
14
>250
>250
>250
(22.09) (21.88)
2.34 (5.37) 2.45 (5.42)
C35H28BF4NOS
70.36 70.45
4.72 4.85
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 96–104

UNSYMMETRICAL DYES
Table 3. 1H NMR spectral data for the compounds synthesized (in DMSO-d6 solutions)
Compd.
1H NMR (d ppm, J Hz)
3
3
4
0.93 (t, 3H, JHH ¼ 7.3, N(CH2)3CH3) 1.40–1.49 (m, 2H, CH2), 1.59 (d, 3H, JHH ¼ 6.3, CH3), 1.85–1.91 (m, 2H, CH2),
3.08–3.16 (m, 1H, C7H), 3.62–3.72 (m, 1H, C7H), 3.90 (s, 3H, OCH3), 4.78 (t, 2H, JHH ¼ 6.9, NCH2), 5.43–5.51
3
(m, 1H, C8H), 7.16 (d, 2H, JHH ¼ 8.9, ArH), 7.79 (d, 1H, JHH ¼ 16.5, CHchain), 8.07–8.16 (m, 3H, ArH), 8.34
3
3
(s, 1H, C6H), 8.58 (d, 1H, JHH ¼ 16.8, CHchain), 8.62 (d, 1H, JHH ¼ 8.1, ArH), 9.31 (d, 1H, JHH ¼ 7.5, ArH)
3
3
3
3
5
6
1.56 (d, 3H, JHH ¼ 6.3, CH3), 3.02–3.11 (m, 1H, C7H), 3.14 (s, 6H, N(CH3)2), 3.57–3.63 (m, 1H, C7H), 5.33–5.43
(m, 1H, C8H), 6.86 (d, 2H, JHH ¼ 9.1, ArH), 7.50 (d, 1H, JHH ¼ 15.6, CHchain), 7.61–8.00 (m, 3H, ArH), 8.02
3
3
(s, 1H, C6H), 8.44 (d, 1H, JHH ¼ 8.1, ArH), 8.51 (d, 1H, JHH ¼ 15.9, CHchain), 9.14 (d, 1H, JHH ¼ 7.5, ArH)
3
3
3
3
3
0.94 (t, 3H, JHH ¼ 7.3, N(CH2)3CH3) 1.35–1.45 (m, 2H, CH2), 1.52 (d, 3H, JHH ¼ 6.3, CH3), 1.69–1.80 (m, 2H, CH2),
2.95–3.03 (m, 1H, C7H), 3.24 (s, 3H, NCH3), 3.40 (s, 3H, NCH3), 3.48–3.56 (m, 1H, C7H), 4.13 (t, 2H, JHH ¼ 7.2,
3
NCH2), 5.18–5.26 (m, 1H, C8H), 6.37 (t, 1H, JHH ¼ 12.0, CHchain), 6.50 (d, 1H, JHH ¼ 13.2, CHchain), 7.42
3
3
(s, 1H, C6H), 7.82 (dd, 1H, JHH ¼ 7.3, JHH ¼ 7.8, ArH), 8.09 (d, 1H, JHH ¼ 7.8, ArH), 8.27 (d, 1H, JHH ¼ 12.0,
3
3
3
3
3
3
CHchain), 8.50 (t, 1H, JHH ¼ 13.2, CHchain), 8.63 (d, 1H, JHH ¼ 7.3, ArH)
3
7
1.52 (d, 3H, JHH ¼ 6.3, CH3), 2.98–3.05 (m, 1H, C7H), 3.31 (s, 3H, NCH3), 3.50–3.56 (m, 1H, C7H), 3.55
(s, 3H, NCH3), 3.72 (s, 3H, NCH3), 5.21–5.31 (m, 1H, C8H), 5.89 (d, 1H, JHH ¼ 13.5, CHchain), 7.51 (s, 1H, C6H),
3
3
3
3
3
7.84 (dd, 1H, JHH ¼ 7.5, JHH ¼ 8.1, ArH), 8.17 (d, 1H, JHH ¼ 8.1, ArH), 8.66 (d, 1H, JHH ¼ 13.2, CHchain),
3
8.74 (d, 1H, JHH ¼ 7.5, ArH)
3
8a
1.57 (d, 3H, JHH ¼ 6.3, CH3), 2.95–3.01 (m, 1H, C7H), 3.37 (s, 3H, NCH3), 3.38–3.43 (m, 1H, C7H), 5.14–5.17
(m, 1H, C8H), 5.67 (d, 1H, JHH ¼ 7.8, CHCHO), 6.67 (s, 1H, C6H), 7.60 (t, 1H, JHH ¼ 7.8, ArH), 7.94
3
3
3
3
3
(d, 1H, JHH ¼ 8.1, ArH), 8.38 (d, 1H, JHH ¼ 7.8, ArH), 10.27 (1H, d, JHH ¼ 7.5, CHCHO)
3
9b
1.55 (d, 3H, JHH ¼ 6.1, CH3), 2.92–3.01 (m, 1H, C7H), 3.44–3.53 (m, 1H, C7H), 3.65 (s, 3H, NCH3),
3.86 (s, 3H, NCH3), 5.22–5.31 (m, 1H, C8H), 6.78 (d, 1H, JHH ¼ 12.5, CHchain), 7.04 (d, 1H, JHH ¼ 13.5, CHchain),
3
3
3
7.34 (d, 1H, JHH ¼ 7.5, ArH), 7.54–7.62 (m, 2H, ArH), 7.64 (s, 1H, C6H), 7.86–7.92 (m, 1H, ArH), 7.96–8.02
3
3
3
(m, 1H, ArH), 8.13 (d, 1H, JHH ¼ 8.1, ArH), 8.22 (d, 1H, JHH ¼ 8.1, ArH), 8.32 (d, 1H, JHH ¼ 7.8, ArH),
3
3
8.61 (d, 1H, JHH ¼ 7.8, ArH), 8.83 (t, 1H, JHH ¼ 13.5, CHchain
)
3
10
1.54 (d, 3H, JHH ¼ 6.3, CH3), 1.73 (s, 6H, C(CH3)2), 2.98–3.05 (m, 1H, C7H), 3.53–3.60 (m, 1H, C7H), 3.70
(s, 3H, NCH3), 3.83 (s, 3H, NCH3), 5.23–5.33 (m, 1H, C8H), 6.58 (d, 1H, JHH ¼ 13.5, CHchain),
3
3
6.80 (d, 1H, JHH ¼ 13.5, CHchain), 7.28–7.35 (m, 1H, ArH), 7.45–7.48 (m, 2H, ArH), 7.63–7.66 (m, 2H, ArH),
3
3
3
3
7.96 (dd, 1H, JHH ¼ 7.5, JHH ¼ 8.1, ArH), 8.21 (d, 1H, JHH ¼ 8.1, ArH), 8.39 (d, 1H, JHH ¼ 7.5, ArH),
3
8.58 (t, 1H, JHH ¼ 13.5, CHchain
)
3
11
12
1.50 (d, 3H, JHH ¼ 6.3, CH3), 2.86–2.96 (m, 1H, C7H), 3.39–3.48 (m, 1H, C7H), 3.66 (s, 3H, NCH3),
3.86 (s, 3H, NCH3), 5.10–5.23 (m, 1H, C8H), 6.52 (d, 1H, JHH ¼ 13.2, CHchain), 6.83 (d, 1H, JHH ¼ 13.2,
3
3
CHchain), 7.33 (s, 1H, C6H), 7.44 (t, 1H, JHH ¼ 7.5, ArH), 7.58 (t, 1H, JHH ¼ 7.8, ArH), 7.78–7.83 (m, 2H, ArH),
3
3
3
3
3
8.02 (t, 2H, JHH ¼ 8.1, ArH), 8.29 (t, 1H, JHH ¼ 13.2, CHchain), 8.34 (d, 1H, JHH ¼ 7.3, ArH)
3
1.50 (d, 3H, JHH ¼ 6.3, CH3), 2.28 (s, 3H, ArCH3), 2.83–2.89 (m, 1H, C7H), 3.31–3.41 (m, 1H, C7H), 3.57
(s, 3H, NCH3), 3.93 (s, 3H, NCH3), 5.08–5.15 (m, 1H, C8H), 6.52 (d, 1H, JHH ¼ 13.2, CHchain),
3
3
6.77 (d, 1H, JHH ¼ 13.2, CHchain), 7.09–7.12 (m, 3H, ArH), 7.47–7.55 (m, 3H, ArH), 7.73–7.81 (m, 2H, ArH),
3
3
7.90–7.95 (m, 3H, ArH), 8.18 (d, 1H, JHH ¼ 8.3, ArH), 8.29 (d, 1H, JHH ¼ 8.3, ArH), 8.52
3
3
(t, 1H, JHH ¼ 13.2, CHchain), 8.63 (d, 1H, JHH ¼ 7.1, ArH)
3
3
13
0.94 (t, 3H, JHH ¼ 7.3, N(CH2)3CH3) 1.38–1.42 (m, 2H, CH2), 1.52 (d, 3H, JHH ¼ 6.3, CH3), 1.82–1.88 (m, 2H, CH2),
2.88–3.00 (m, 1H, C7H), 3.40–3.44 (m, 1H, C7H), 3.60 (s, 3H, NCH3), 4.68 (t, 2H, JHH ¼ 7.2, NCH2), 5.15–5.18
3
(m, 1H, C8H)), 6.58 (d, 1H, JHH ¼ 12.5, CHchain), 7.14 (s, 1H, C6H), 7.49 (d, 1H, JHH ¼ 12.5, CHchain), 7.78–7.84
3
3
3
3
(m, 2H, ArH), 7.91 (d, 1H, JHH ¼ 7.8, ArH), 8.02 (t, 1H, JHH ¼ 7.8, ArH), 8.15–8.19 (m, 2H, ArH), 8.62–8.75
(m, 4H, ArH þ CHchain
)
3
14b
1.53 (d, 3H, JHH ¼ 6.1, CH3), 2.95–3.01 (m, 1H, C7H), 3.40–3.53 (m, 1H, C7H), 3.95 (s, 3H, NCH3), 5.26–5.34
(m, 1H, C8H), 6.71 (d, 1H, JHH ¼ 13.3, CHchain), 6.95 (d, 1H, JHH ¼ 13.5, CHchain), 7.52–7.62 (m, 7H, ArH),
3
3
3
7.74–7.76 (m, 2H, ArH), 7.85–7.95 (m, 4H, ArH), 8.19 (s, 1H, ArH), 8.29 (d, 1H, JHH ¼ 8.1, ArH), 8.72
3
3
(t, 1H, JHH ¼ 13.5, CHchain), 8.99 (d, 1H, JHH ¼ 7.9, ArH)
a In CDCl3 solutions.
b In DMSO-d6 solutions with the addition of CF3COOD.
J. Phys. Org. Chem. 2010, 23 96–104
Copyright ß 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

I. G. DAVYDENKO ET AL.
and washed with a small amount of acetic anhydride (2 ꢂ 0.5 ml)
and diethyl ether (2 ꢂ 1 ml). Then the dye (0.41 g) was dissolved in
nitromethane (75 ml) and precipitated from the hot-filtered
solution with isopropanol (120 ml). Yield 0.38 g (73%).
Dye 10 was obtained likewise from salt 3a and 2-(1,3,3-
trimethyl-1,3-dihydro-2H-indol-2-ylidene)acetaldehyde and pur-
ified by reprecipitation from acetonitrile into isopropanol. Yield
71%.
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A solution of quaternary salt 3a (0.33 g, 1 mmol) and 2-[2-acetani-
lidoethenyl]-3-ethyl-1,3-benzothiazol-3-ium4-methylbenzenesul-
fonate (0.45 g, 1 mmol) in acetic anhydride (3 ml) was boiled in
the presence of triethylamine (0.28 ml, 2 mmol) for 2 min (bath
temperature 1008C and inside temperature 90–958C). After 1 h,
the resulting precipitate was filtered off and washed with acetic
anhydride (2 ꢂ 1 ml) and diethyl ether (2 ꢂ 3 ml). Then the
product (0.31 g) was dissolved in nitromethane (7 ml) and filtered
into a solution of benzyltrimethylammonium tetrafluoroborate
(0.3 g) in ethanol (10 ml). After 1 h, the precipitate of the dye was
filtered off and washed with ethanol (1 ꢂ 1 ml), hot water
(3 ꢂ 5 ml) and diethyl ether (3 ꢂ 5 ml). Yield 0.26 g (53%).
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5,8-Dimethyl-4-(3-[1-methylquinolin-2(1H)-ylidene]-1-
propenyl)-7,8-dihydrobenzo[c,d]furo[2,3-f]indol-5-ium
4-methylbenzenesulfonate (12)
A solution of aldehyde 8 (0.40 g, 1.5 mmol) and 1,2-dimethyl-
quinolinium 4-methylbenzenesulfonate (0.49 g, 1.5 mmol) in
acetic anhydride (4 ml) was heated for 20 min (bath temperature
1008C and inside temperature 90–958C). After 2 h, the resulting
precipitate was filtered off and washed with acetic anhydride
(2 ꢂ 0.5 ml) and diethyl ether (3 ꢂ 2 ml). Then the product (0.46 g)
was dissolved in methanol (3 ml) and precipitated from the
hot-filtered solution with isopropanol (9 ml). After 4 h, the dye
was filtered off and washed with isopropanol (2 ꢂ 0.5 ml) and
diethyl ether (3 ꢂ 2 ml). Yield 0.43 g (50%).
Dyes 13 and 14 were obtained likewise by reacting aldehyde 8
with 1-butyl-4-methylquinolinium iodide and 4-methyl-2,6-
diphenylthiopyrylium tetrafluoroborate, respectively. Dye 13
was purified by column chromatography on alumina using chloroform
as eluent. Yield 21%. Dye 14 was purified by recrystallization from a 2:1
mixture of acetonitrile and methanol. Yield 52%.
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Copyright ß 2009 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 96–104