Alkyl-substituted dipyrrylmethenes and their oxa- and thia-analogs: Structure - Solvation properties correlations

Article abstract of DOI:10.1023/A:1026064923515

New data on the spectral properties and solution enthalpies of unsymmetrically substituted 2-(alkyl-2-pyrrolylmethylidene)methylpyrrolium bromides (or α,α-dipyrrylmethene hydrobromides), their α,β-, β,β-isomers, as well as their oxa and thia analogs, that is, 2-(2-furylmethylidene)- and 2-(2-thienylmethylidene)-3,4,5-trimethyl- 1H-pyrrolium bromides, in solutions of organic solvents of different nature are presented. A decrease in the number of substituents, as well as replacement of the heteroatom (N) in one five-membered ring of the dipyrrylmetnehe by oxygen or sulfur atoms cause a monotonic hypsochromic shift of absorption bands in the electronic absorption spectrum and weakening of the chromophore properties of the compounds. The chromophore properties of isomers are weakened from the α,α- to α,β- and β,β-dipyrrylmethenes. Main trends in the influence of structural factors on the specific features of thermooxidative destruction of the above-mentioned compounds were analyzed.

Full text of DOI:10.1023/A:1026064923515

Russian Chemical Bulletin, International Edition, Vol. 52, No. 8, pp. 1807—1813, August, 2003
1807
Alkylꢀsubstituted dipyrrylmethenes and their oxaꢀ and thiaꢀanalogs:
"structure—solvation properties" correlations
A. S. Semeikin,aꢀ M. B. Berezin,b O. M. Chernova,a E. V. Antina,b S. A. Syrbu,a T. V. Lyubimova,b and A. M. Kutepovb
aIvanovo State University of Chemical Technology,
7 prosp. F. Engel´sa, 153460 Ivanovo, Russian Federation.
Fax: +7 (0932) 32 9502. Eꢀmail: s_serg@icti.ivanovo.su
bInstutute of Chemistry of Solutions, Russian Academy of Sciences,
1 ul. Akademicheskaya, 153045 Ivanovo, Russian Federation.
Fax: +7 (0932) 37 8509. Eꢀmail: dbb@ihnr.polytech.ivanovo.su
New data on the spectral properties and solution enthalpies of unsymmetrically substituted
2ꢀ(alkylꢀ2ꢀpyrrolylmethylidene)methylpyrrolium bromides (or α,αꢀdipyrrylmethene hydroꢀ
bromides), their α,βꢀ, β,βꢀisomers, as well as their oxa and thia analogs, that is, 2ꢀ(2ꢀfurylꢀ
methylidene)ꢀ and 2ꢀ(2ꢀthienylmethylidene)ꢀ3,4,5ꢀtrimethylꢀ1Hꢀpyrrolium bromides, in soꢀ
lutions of organic solvents of different nature are presented. A decrease in the number of
substituents, as well as replacement of the heteroatom (N) in one fiveꢀmembered ring of the
dipyrrylmetnehe by oxygen or sulfur atoms cause a monotonic hypsochromic shift of absorpꢀ
tion bands in the electronic absorption spectrum and weakening of the chromophore properties
of the compounds. The chromophore properties of isomers are weakened from the α,αꢀ to
α,βꢀ and β,βꢀdipyrrylmethenes. Main trends in the influence of structural factors on the
specific features of thermooxidative destruction of the aboveꢀmentioned compounds were
analyzed.
Key words: pyrroles, dipyrrylmethenes, solutions, spectrophotometry, calorimetry,
thermogravimetry, synthesis.
The soꢀcalled dipyrrylmethenes are intermediates in
the synthesis of porphyrins in the laboratory experiments
and in nature.1,2 The physicochemical properties of the
classical porphyrins and metal porphyrins have been the
subject of numerous studies. The physicochemical propꢀ
erties of dipyrrylmethenes (precursors of porphyrins) have
been investigated to a much lesser extent; only the elecꢀ
tronic absorption spectra (EAS) of these compounds, reꢀ
corded in a few solvents are usually available. Taking into
account the fact that most processes involving porphyrins
proceed in the liquid media, the necessity of investigaꢀ
tions on the solvation properties, coordination ability,
and other properties of not only porphyrins but also simꢀ
pler fragments the porphyrins are built of (in particular,
dipyrrylmethenes and their structural analogs) is obvious.
Earlier,3,4 we have reported the results of calorimetric
measurements of the enthalpies of the interactions beꢀ
tween substituted monopyrroles and dipyrroles (dipyrꢀ
rolylmethanes) and organic solvents of different nature. It
was shown that the contribution of solvation to the change
in the enthalpy of intermolecular solute—solvent interacꢀ
tions for monopyrroles is primarily determined by the
state of the N—H bond. In dipyrrolylmethanes, we deal
with the mutual influence of the pyrrole fragments and
with their effect on the solvation properties; however,
steric factors can also play some role depending on
the chain length and nature of substituents in posiꢀ
tions 3 and 3´.
Dipyrrylmethenes are conjugated linear chromoꢀ
phores. The field of their applications is more extended
compared to dipyrrolylmethanes.5—7
The aim of this work was to establish main trends in
the influence of successive alkyl substitution on the physiꢀ
cochemical properties of both methylꢀsubstituted α,αꢀ,
α,βꢀ and β,βꢀisomers of dipyrrylmethene and their strucꢀ
tural oxaꢀ and thiaꢀanalogs in solutions in various organic
solvents and in the solid phase.
To this end, we synthesized hydrobromides of various
dipyrrylmethenes (structures 1—7), the oxaꢀ and thiaꢀ
analogs of compound 6 (structures 8 and 9, respectively),
and hydrobromides of the α,βꢀ and β,βꢀisomers of comꢀ
pound 1 (structures 10 and 11, respectively). The spectral
characteristics and solvation properties of these comꢀ
pounds in organic solvents of different nature were studꢀ
ied by electronic spectroscopy and solution calorimetry.
Dipyrrylmethene hydrobromides were synthesized by conꢀ
densation of alkylpyrroles having no substituent at one of
the four ring carbon atoms with formylpyrroles in methaꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1712—1718, August, 2003.
1066ꢀ5285/03/5208ꢀ1807 $25.00 © 2003 Plenum Publishing Corporation

1808
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
Semeikin et al.
Scheme 1
Experimental
3,3´,4,4´,5,5´ꢀHexamethylꢀ2,2´ꢀdipyrrylmethene hydroꢀ
bromide (1). To a solution of 2,3,4ꢀtrimethylpyrrole (0.75 g,
6.9 mmol) and 2ꢀformylꢀ3,4,5ꢀtrimethylpyrrole (0.95 g,
6.9 mmol) in methanol (15 mL), conc. HBr (1 mL) was added
and the mixture was stirred for 1 h. The precipitate was filtered
off, successively washed with methanol and ether, dried, and
recrystallized from a chloroform—methanol mixture to which
HBr was added. Hydrobromides 2—9 were obtained analogously.
The yields and characteristics of the compounds synthesized are
listed in Tables 1 and 2.
2´,3,4,4´,5,5´ꢀHexamethylꢀ2,3´ꢀdipyrrylmethene hydroꢀ
bromide (10). To a solution of 2,3,4ꢀtrimethylpyrrole (0.80 g,
7.3 mmol) and 3ꢀformylꢀ2,4,5ꢀtrimethylpyrrole (1.00 g,
7.3 mmol) in methanol (10 mL), conc. HBr (1 mL) was added
and the mixture was stirred for 2 h. The reaction mixture was
diluted with water (1 : 1 v/v), the precipitate was filtered off,
dried, boiled with acetone (50 mL) for 15 min, filtered off, and
washed with acetone.
2,2´,4,4´,5,5´ꢀHexamethylꢀ3,3´ꢀdipyrrylmethene hydroꢀ
bromide (11). To a solution of 2,3,5ꢀtrimethylpyrrole (0.95 g,
8.7 mmol) and 3ꢀformylꢀ2,4,5ꢀtrimethylpyrrole (1.20 g,
8.7 mmol) in methanol (5 mL), conc. HBr (2 mL) was added
and the mixture was stirred for 2 h. Then ether (50 mL) was
added dropwise. The solution was decanted, the precipitate was
boiled with benzene (50 mL), filtered off, and dried.
Compound
R1
Me
Me
Me
H
Me
H
H
R2
Me
Et
Bu
Me
H
H
H
H
H
R3
Me
Me
Me
Me
Me
H
H
H
H
R4
Me
Et
Bu
Me
Me
Me
Me
Me
Me
X
NH
NH
NH
NH
NH
NH
NMe
O
1
2
3
4
5
6
7
8
9
H
H
S
The initial pyrroles and formylpyrroles were obtained folꢀ
lowing the known procedures.3
1
The H NMR spectra of pyrroles were recorded on a Tesla
BSꢀ497 spectrometer while those of dipyrrylmethenes were reꢀ
corded on a Bruker ACꢀ200 spectrometer with hexamethylꢀ
disiloxane as internal reference. Electronic absorption spectra
were recorded on a Specord Mꢀ40 spectrophotometer. The solꢀ
vents for calorimetric measurements were prepared following
the known procedures.8 The experimental data obtained using a
precision isothermicꢀshell calorimeter at 298.15 K 9 are listed in
Table 3.
Results and Discussion
Compounds 1—11 are complex chromophores, as inꢀ
dicated by the absorption of their solutions in the visible
region (see Tables 1 and 4). They differ from one another
in position, number, and volume of substituents, as well
as in position and nature of the heteroatom and, hence, in
the effect of these fragments on the centers of nonꢀspeꢀ
cific and specific solvation of the molecules of dipyrrylꢀ
methenes and their furan and thiophene analogs. The
presence of NH groups in the dipyrrylmethene molecules
implies solvation by electronꢀdonor solvents. The tertiary
nitrogen atoms (—N=) favor the interaction of the
dipyrrylmethene molecules with protonꢀdonor solvents,
while the presence of a conjugated system of πꢀbonds
must favor solvation by aromatic solvents. On this basis
we chose benzene as inert aprotic solvent, DMF and pyriꢀ
dine as electronꢀdonor solvents, and propanol and chloꢀ
roform as protonꢀdonor solvents.
nol in the presence of concentrated hydrobromic acid
(Scheme 1).
The yields, elemental analyses data, positions of bands
in the EAS, and signal assignment in the 1H NMR spectra
are listed in Tables 1 and 2.

Dipyrrylmethenes and their oxa and thia analogs
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
1809
Table 1. Yields and elemental analysis data for compounds 1—11 and their electron absorption spectra (EAS) in methylene chloride
Comꢀ
pound
Yield
(%)
Found
Calculated
Empirical
formula
EAS,
max/nm (lgε)
(%)
N
λ
С
Н
Br
I
II
1
70
69
58
70
79
68
59
63
48
70
56
58.26
58.15
60.54
61.00
64.11
64.18
56.96
56.38
56.96
56.33
53.95
53.20
55.53
55.12
53.75
53.28
50.71
51.15
58.26
58.25
58.26
58.90
6.84
6.10
7.47
7.53
8.45
8.95
6.49
6.29
6.49
6.28
5.66
5.01
6.09
5.85
5.26
5.98
4.96
5.41
6.84
6.15
6.84
6.09
9.06
9.00
8.31
8.25
7.12
7.00
9.49
9.01
9.49
9.78
10.49
10.99
9.96
9.43
5.22
4.81
4.93
5.25
9.06
9.15
9.06
9.23
26.84
26.53
23.69
23.72
22.31
22.02
27.07
27.58
27.07
28.00
29.91
29.41
28.42
28.91
29.80
29.30
28.11
27.85
26.84
26.82
26.84
26.15
C15H21BrN2
C17H25BrN2
C21H33BrN2
C14H19BrN2
C14H19BrN2
C12H15BrN2
C13H17BrN2
C12H14BrNO
C12H14BrNS
C15H21BrN2
C15H21BrN2
486
(4.99)
488
(5.03)
490
(5.05)
480
(4.88)
478
362
(4.02)
370
2
3
354
374
4
5
373
(4.98)
378
6
465
(4.88)
479
(4.58)
425
(4.45)
429
(4.46)
477
7
378
—
8
9*
10
11
—
403
(4.10)
376
(4.36)
475
(4.02)
(4.39)
* S — 11.28/11.34.
1
Table 2. H NMR spectra (δ, J/Hz) of the compounds synthesized (deuterated chloroform as solvent, with hexamethyldisiloxane as
internal reference)
Comꢀ
pound
NH
(s)
mesoꢀH
(s)
R1
R2
R3
3ꢀMe
(s)
4ꢀMe
5ꢀMe
(s)
1
2
12.92
12.48
7.02
7.32
2.64 s
2.64 s
1.97 s
2.25 s
2.38 s
2.25
2.38
1.97 s
2.64
2.64
2.52 (q, J = 7.6);
1.13 (t, J = 7.6)
2.35 (t, J = 7.1);
1.30 m;
2.52 (q, J = 7.6)
1.13 (t, J = 7.6)
2.35 (t, J = 7.1);
1.30 m;
3
12.88
7.02
2.65 s
2.26 s
2.26
2.65
0.93 (t, J = 7.1)
2.00 s
0.93 (t, J = 7.1)
2.06 s
4
5
6
7
12.80;
12.93
12.94;
13.08
13.35;
13.53
2.83;
7.15
7.05
7.20
7.30
7.52 s
2.67 s
2.28 s
2.35 s
7.10 s
7.19 m
2.29
2.26
2.29
2.35
2.70
2.67
2.75
2.83
6.13 s
6.50 m
6.52 s
1.99 s
2.02 s
1.97 s
7.70 m
8.81 m
13.02
13.76
13.96
11.73;
11.92
12.39
8
9
10
7.17
7.52
7.50
8.33 s
9.16 s
2.71 s
(5´ꢀMe)
2.28 s
6.73 s
7.45 s
2.02 s
(4´ꢀMe)
2.03 s
7.63 s
7.96 s
2.14 s
(3´ꢀMe)
2.45 s
2.31
2.35
2.28
(2ꢀMe)
2.45
2.07 s
2.06 s
2.01 s
3.04
3.01
2.74
11
8.15
2.03 s
2.28
(5´ꢀMe)
(4´ꢀMe)
(2´ꢀMe)
(2ꢀMe)

1810
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
Semeikin et al.
Table 3. Changes in the standard solution enthalpies (∆solH°) and enthalpies of transfer (∆ H°) of dipyrrylmethene
tr
hydrobromides and their structural analogs (kJ mol–1
)
Comꢀ
pound
PrOH
solН°
CHCl3
DMF
solН°
Py
∆ Н°
C6H6
solН°
20.5
25.3 0.8
32.8 0.8
—
30.3 3.0
4.0 0.4
6.7 0.5
—
5.7 1.6
—
∆
∆
solН°
∆ Н°
∆
∆ Н°
∆
solН°
∆
∆ Н°
tr
tr
tr
tr
1
2
3
4
5
6
7
8
35.1 1.1 4.8 0.3
40.9 1.5 2.5 0.2
30.6 1.5 6.2 1.8
–30.3
–38.4
–24.4
—
–13.3
–8.6
18.5 0.5
19.3 0.5
19.2 0.9
9.2 1.0
3.5 0.3
5.3 0.5
7.1 0.5
7.8 0.7
–16.6
–21.6
–11.4
—
–19.8
–11.6
10.7 1.8 –24.7 0.7
–14.6
–15.6
2.2
—
7.0
–12.9
–15.0
—
–16.4
—
—
—
–11.9
—
–4.4
1.8
18.7 0.5
13.5 1.4
18.9 1.3
18.7 0.5
—
4.7 0.7
23.3 1.5 10.0 1.2
16.9 1.1 8.3 0.8
21.7 2.0
32.1 1.5 14.5 1.2
22.1 0.5 1.9 0.2
25.0 1.5 3.2 0.3
5.2 0.1 –9.2 0.9
0
1.5
–21.7
–17.6
–14.6 –16.7 1.2 –38.4
–24.3 –19.7 1.6 –51.8
9
10
11
–20.2 –40.3 1.5 –62.4 –17.6 0.5 –39.7
–21.8
–14.4 –16.7 0.9 –21.9
11.8 0.5
–13.2
—
—
—
—
—
—
Table 4. Positions (λ/nm) and intensities (lg ε) of absorption bands in the electronic spectra of
dipyrrylmethenes and their structural analogs
Compounds
PrOH
CHCl3
DMF
Py
C6H6
1
478 (4.61)
361 (3.75)
479
483 (4.75)
362 (4.02)
488
435 (4.67)
448 (4.59)
484 (4.66)
364 (3.41)
494
2
3
4
5
6
7
8
438
451
362
480
363
363
490
362
364
498
368
440
456
463 (4.63)
371
468 (4.42)
356
463 (4.58)
401
474 (4.42)
376
477 (4.56)
370
472 (4.64)
354
465 (4.62)
374
476 (4.44)
378
415 (4.28)
428 (4.33)
390 (4.07)
402 (4.01)
368 (3.31)
421 (4.24)
434 (4.35)
400 (4.09)
433 (3.63)
380 (3.77)
479 (3.75)
425
474 (4.61)
361
462 (4.58)
485 (3.57)
382
408 (3.70)
370
424 (4.44)
LS*
9
10
412 (3.40)
473 (4.34)
412 (4.07)
476 (3.98)
378 (3.91)
427 (4.35)
481 (4.48)
403 (4.10)
472 (4.43)
376 (4.39)
359 (4.06)
402 (4.08)
378 (3.40)
406
429 (3.54)
LS*
уш.
407
11
400 (3.86)
LS*
* LS is low soluble.
The proton delocalized over the aza groups in the
pyrrole rings of the molecules of dipyrrylmethene hydroꢀ
bromides acts as an auxochrome, which induces polarizaꢀ
tion of the chromophore molecule.10 Changes in the volꢀ
ume, number, and positions of substituents in dipyrrylꢀ
methenes strongly affects their EAS and other physicoꢀ
chemical properties.
Compounds 1—3 have symmetrical structures in which
the excess positive charge of the H atom of the HBr molꢀ
ecule is uniformly delocalized over both pyrrole rings.
Replacement of methyl substituent by ethyl and butyl
groups (compounds 2 and 3, respectively) causes a minor
improvement of the chromophore properties of dipyrrylꢀ
methenes (see Table 1). A decrease in the number of
methyl groups in the molecules (compounds 4—6) is reꢀ
sponsible for both the hypsochromic shift (by 21 nm) of
the absorption band in the EAS and the changes in the
band intensity (given are the data for solutions in chloroꢀ
form with a concentration of nearly 10–5 mol L–1). The
lack of substituent in αꢀ or βꢀposition in one pyrrole ring
(compounds 4 and 5) has little effect on the position but
strongly affects the intensity of the absorption band. This

Dipyrrylmethenes and their oxa and thia analogs
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
1811
is indicative of nonequivalence of the αꢀ and βꢀpositions
in the pyrrole moieties, involved in the common conjugaꢀ
tion system of dipyrrylmethene.
shift of the first absorption band, which is indeed obꢀ
served experimentally.
It is assumed10 that the change in solvation of the
excited state makes the major contribution to the changes
in the electronic spectrum. Comparison of positions of
the first absorption bands in the nonꢀpolar solvent and in
polar solvents (see Table 4) shows that, depending on the
structure of the dipyrrolylmethene molecule, transition to
the excited state is almost without exception accompaꢀ
nied by a decrease in the dipole moment of its πꢀelectron
cloud. Dissolution of dipyrrylmethene hydrobromides in
protonꢀacceptor solvents (pyridine, DMF) is accompaꢀ
nied by elimination of HBr molecule. As a result, the
electronic spectra observed are characteristic of free
dipyrrylmethene bases.11
The presence of methyl group at the N atom in molꢀ
ecule 7 has virtually no effect on the energy of absorbed
light quantum. At the same time it causes a significant
decrease in the absorption probability, thus indicating that
the charge of H+ is still delocalized over the Nꢀmethylꢀ
substituted dipyrrylmethene. Replacement of the heteroꢀ
atom (nitrogen) in one pyrrole moiety by oxygen or sulfur
atoms (compounds 8 and 9, respectively) causes a shift of
the maximum of the absorption band by 40 nm toward the
boundary of the visible region compared to dipyrrylꢀ
methene 6 and a strong decrease in the band intensity. So
significant changes in the EAS are likely due to a decrease
in the extent of delocalization of the positive charge over
these atoms.
Thus, the absorption band in the EAS experiences a
bathochromic shift as the number of methyl groups inꢀ
creases in the order 6 < 7, 5, 4 < 1. Replacement of the
H atom by a methyl group at the nitrogen atom in one
pyrrole ring causes a larger shift of the absorption band as
compared to the effect of methyl substituent at the C atom.
Replacement of the pyrrole ring by the furan or thiophene
ring weakens the chromophore properties. Weakening of
πꢀconjugation in these compounds is likely due to elecꢀ
tronic rather than steric factors.
Analysis of the EAS recorded in DMF and pyridine
revealed a characteristic feature of dipyrrylmethene bases
and their analogs, namely, a marked solvatochromic efꢀ
fect even in the electronꢀdonor solvents of similar nature.
In pyridine solutions, the absorption bands experience a
bathochromic shift as compared to DMF, which is probꢀ
ably due to π,π*ꢀsolvation interactions between dipyrrylꢀ
methenes and their analogs and πꢀdeficient pyridine.
By and large, positions of absorption bands in the EAS
of compounds 1—11 depend on the molecular structure
in a complex manner due to superimposition of the elecꢀ
tronic and steric effects of functional substituents and to
the effect of the nature of the heteroatom. This requires
further investigations.
Our study of changes in the molal solution enthalꢀ
pies, ∆solHm = f(m), of compounds 1—11 showed that,
within the limits of experimental error, the ∆solHm valꢀ
ues are independent of the solute concentration (all
compounds possess the properties of very weak electroꢀ
lytes in the chosen range of operating concentrations,
10–5—10–4 mol kg–1 12).
In the α,αꢀ, α,βꢀ, and β,βꢀisomers, the effect of plaꢀ
narity distortion of dipyrrylmethene molecules (estimated
by calculations) changes as follows: 1 < 10 < 11. This
structural distortion has little effect on the position of the
spectral band (hypsochromic shift is only ∼10 nm) but
strongly affects the absorption probability, which subꢀ
stantially decreases (see Table 1). Besides, the absorption
band is broadened (probably, due to activation of vibraꢀ
tional states).
An analogous, though less pronounced, effect is obꢀ
served in the case of Nꢀmethyl substitution in dipyrrylꢀ
methene 7. Here, the absorption band experiences a
bathochromic shift relative to dipyrrylmethene 6 and its
intensity substantially decreases, which is probably due to
steric hindrances produced by the methyl group at the
nitrogen atom.
No less specific is the effect of the solvent nature on
the EAS of compounds studied (see Table 4). A characꢀ
teristic feature of the EAS of hydrobromides is the presꢀ
ence of two absorption bands. The first, longꢀwavelength,
band is intense while the second band is weak and, as a
rule, disappears in the protonꢀacceptor solvents (DMF,
pyridine) responsible for (this is our hypothesis) eliminaꢀ
tion and solvation of HBr molecule. The lastꢀmentioned
process must cause a decrease in the polarity of the
dipyrrylmethene molecule, weakening of its interaction
with the solvent and, as a consequence, a hypsochromic
Since compounds 4, 8, 10, and 11 are virtually inꢀ
soluble in benzene, we calculated the enthalpies of transꢀ
fer (∆trH°) from 1ꢀpropanol (see Table 3). The results
obtained show a strong dependence of the change in the
enthalpy of the solute—solvent interaction on the strucꢀ
ture, up to insolubility of some compounds in benzene
(see above). Apparently, a nearly 100% insolubility of
dipyrrylmethenes 10 and 11 in benzene is due to distorꢀ
tion of planarity in the molecules of these compounds. As
a result, the π—πꢀsolvation interactions between benzene
and dipyrrylmethene become sterically hindered by subꢀ
stituents.
Propanol possesses the weakest solvation properties
toward most of the compounds studied; therefore, the
corresponding solution enthalpies have the largest posiꢀ
tive values. Among methylꢀsubstituted dipyrrylmethenes
(1 and 4—7), the largest and smallest effective molecular
volumes have hexamethylꢀsubstituted dipyrrylmethene 1

1812
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
Semeikin et al.
and compound 6, respectively, the latter having one
unsubstituted pyrrole moiety. In this connection the
endothermicity of solution of α,αꢀdipyrrylmethenes in
propanol decreases as follows: 1 > 5 > 7 > 6. The hypsoꢀ
chromic shift of λ1 in the EAS increases (i.e., the chroꢀ
mophore properties of the molecules weaken) in the same
order. Loosening of the lattice in the molecular crystals of
the α,αꢀ, α,βꢀ, and β,βꢀisomers of hexamethylꢀsubstiꢀ
tuted dipyrrylmethenes (1, 10, 11) due to increasing moꢀ
lecular distortion (α,α < α,β < β,β) causes a decrease in
the endothermicity of solution in the order 1 > 10 > 11 in
all the protonꢀdonor and protonꢀacceptor solvents.
Thanks to the protonꢀdonor nature of chloroform,
additional solvation of dipyrrylmethene hydrobromide
molecules due to the formation of hydrogen bonds,
Br–...HCCl3, is possible in this case. This requires the
bromide ion and proton to be delocalized over the pyrrole
rings. Analysis of the enthalpies of transfer to chloroform
(see Table 3) suggests that the degree of proton delocalꢀ
ization over the pyrrole rings in the α,αꢀdipyrrylmethene
molecules decreases in the order 1 > 7 > 5 > 6.
As mentioned above, the interaction of hydrobromides
of dipyrrylmethenes and their analogs with protonꢀaccepꢀ
tor solvents is accompanied by elimination of HBr molꢀ
ecule and by π—πꢀinteractions in the case of pyridine.
High enthalpy of transfer of Nꢀmethylꢀsubstituted diꢀ
pyrrylmethene 7 to pyridine is likely due to electronꢀ
donor effect of the methyl group bound to the nitrogen
atom and to strengthening of the π—πꢀinteractions beꢀ
tween 7 and pyridine.
thermicity of solution of 8 in propanol is nearly doubled
as compared to the corresponding value for dipyrrylꢀ
methene 6. Simultaneously, solvation by chloroform enꢀ
hances (∆trH°, see Table 3), which is likely due to the
formation of hydrogen bonds between HCCl3 and the
oxygen atom. At the same time a dramatic enhancement
of solvation by electronꢀdonor solvents (especially pyriꢀ
dine) occurs because of an increase in the positive charge
of the adjacent pyrrolenine moiety.
The endothermicity of solution of thienyl derivative 9
in propanol is intermediate between the corresponding
values for compounds 6 and 8. The electronegativity of
sulfur is lower than that of oxygen; however, the 3dꢀorbital
of sulfur atom is a kind of electron density "reservoir."
The enthalpy of transfer to chloroform (∆trH°) for 9 is
close to that of compound 8, while the enthalpy of transꢀ
fer to DMF for 9 is the largest (∆trH° = –62 kJ mol–1).
The reason is still to be clarified.
By and large, changes in the nature of heteroatom
manifest themselves as enhancement of solvation of comꢀ
pounds by organic solvents in the order 6 < 8 < 9.
The results of our study13 on thermooxidative deꢀ
struction of dipyrrylmethenes and their hydrobromides
(Table 5) are also of interest. The nature and arrangement
of substituents were found to have a strong effect on the
thermal stability of these compounds. Among α,αꢀisoꢀ
mers, hexamethylꢀsubstituted dipyrrylmethene in the form
of both hydrobromide 1 and free base is the most stable
toward thermooxidative destruction. Pyrolysis of hydroꢀ
Structurally similar compounds 6, 8, and 9 differ in
heteroatom (N, O, S) in one fiveꢀmembered ring. The
corresponding changes in the solution enthalpies depend
on the solvent nature in a complex manner. As follows
from the EAS, the degree of delocalization of the positive
charge over the fiveꢀmembered heterocycles in these comꢀ
pounds appreciably decreases due to weakening of conjuꢀ
gation. As a result, the molecular fragments become much
less conjugated than the symmetrically substituted diꢀ
pyrrylmethene 1. The endothermicity of solution in proꢀ
panol decreases in the order 8 > 9 > 6. One can assume
that solvation of these compounds in propanol enhances
in the same order.
Three methyl substituents in the same ring of diꢀ
pyrrylmethene 6 to some extent compensate the effect of
positive charge on the nitrogen atom in aprotic and proꢀ
tonꢀdonor solvents. In DMF and pyridine, where elimiꢀ
nation of HBr occurs, the pyrrolenine ring acts as accepꢀ
tor toward the adjacent ring. This causes weakening of
solvation of dipyrrylmethene 6 in electronꢀdonor solvents
and especially pyridine (positive enthalpy of transfer).
In compound 8, the electron density is shifted to the
furan ring due to high electronegativity of oxygen atom.
This should cause enhancement of solvation by protonꢀ
donor solvents (propanol and chloroform). The endoꢀ
Table 5. Parameters of thermooxidative destruction of dipyrrylꢀ
methene hydrobromides, their structural analogs, and some free
bases
0
0
Compound
Te
Td
Tmax
Ttd
°C
1
2
3
4
5
6
7
8
255
210
200
240
200
130
195
127
168
195
145
310
318
300
290
320
300
265
208
320
305
300
190
160
140
490
541
460
520
552
510
290
306
570
520
630
500
535
520
565
663
570
610
651
550
675
637
660
635
727
560
580
600
9
10
11
1•HBr
2•HBr
3•HBr
Note. Te0 is the temperature at which elimination of HBr begins,
0
Td is the temperature at which thermooxidative destruction of
the free base begins, Tmax is the temperature corresponding to
the maximum exothermic effect, and Tt is the temperature at
which the process is completed.

Dipyrrylmethenes and their oxa and thia analogs
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
1813
2. A. F. Mironov, in Porfiriny: Struktura, svoistva, sintez [The
Porphyrins.: Structure, Properties, Synthesis], Nauka, Mosꢀ
cow, 1985, p. 282 (in Russian).
3. M. B. Berezin, A. S. Semeikin, A. I. V´yugin, and G. A.
Krestov, Izv. Akad. Nauk, Ser. Khim., 1993, 495 [Russ. Chem.
Bull., 1993, 42, 449 (Engl. Transl.)].
4. M. B. Berezin, A. S. Semeikin, and A. I. V´yugin, Zh. Fiz.
Khim., 1996, 70, 1364 [Russ. J. Phys. Chem., 1996, 70 (Engl.
Transl.)].
5. Ch. Kratky, C. Jorde, H. Falk, and K. Thirring, Tetrahedron,
1983, 39, 1859.
6. V. A. Ganzha, G. P. Gurinovich, and B. M. Dzhagarov,
Zh. Prikl. Spektrosk., 1987, 47, 84 [J. Appl. Spectr., 1987, 47
(Engl. Transl.)].
7. K. Venkataraman, The Chemistry of Synthetic Dyes, Vol. 2,
Acad. Press, New York, 1952.
8. A. J. Gordon and R. A. Ford, The Chemist´s Companion,
A Handbook of Practical Data, Techniques, and References,
J. Wiley, New York—London—Sydney—Toronto, 1972,
541 pp..
bromide 1 involves two stages in the temperature interval
190—560 °C with the maximum exothermic peak at 560 °C
and consists in elimination of HBr (first stage) followed
by oxidation of the free base, which is accompanied by a
100% loss of the sample mass. As the chain length of alkyl
substituents in positions 4 and 4´ increases, the thermal
stability of the compounds is markedly reduced in the
order 1 > 2 > 3. Successive decrease in the number of
methyl groups in one pyrrole ring of the molecule also
reduces the stabilities of the corresponding salts as folꢀ
lows: 1 > 4 > 5 > 6.
Among isomeric hexamethylꢀsubstituted dipyrrylꢀ
methenes, the stability of hydrobromides toward thermoꢀ
oxidative destruction (elimination of HBr) is signifiꢀ
cantly (by 100 °C on the average) reduced in the order
1 > 10 > 11. This is likely due to distortion of the planar
structure of the α,βꢀ and β,βꢀisomers (10 and 11, respecꢀ
tively) compared to dipyrrylmethene 1.
9. V. K. Abrosimov and V. V. Korolev, in Eksperimental´nye
metody khimii rastvorov: Spektroskopiya i kalorimetriya
[Experimental Methods of Solution Chemistry: Spectroscopy
and Calorimetry], Nauka, Moscow, 1995, p. 239 (in
Russian).a
10. A. N. Terenin, Fotonika molekul krasitelei i rodstvennykh
soedinenii [Photonics of Dyes and Related Compounds], Nauka,
Leningrad, 1967, 616 pp. (in Russian).
The second factor affecting the stability of compounds
toward thermooxidative destruction is the nature of the
heteroatom in one fragment. A decrease in aromaticity
and in the degree of delocalization of positive charge
over the fiveꢀmembered rings as compared to dipyrrylꢀ
methene 1 reduces the thermal stability of compounds in
the order 9 > 6 > 8.
Thus, solvation of the compounds synthesized in this
work is determined by the solvent nature, positions, numꢀ
ber, and size of alkyl substituents, as well as by the nature
and position of the heteroatom in the chromophore molꢀ
ecule. The stability of the aboveꢀmentioned compounds
toward thermooxidative destruction is due to the same
factors, except for the solvent effect.
11. N. A. Pashanova, A. S. Semeikin, and M. B. Berezin,
Zh. Obshch. Khim., 1999, 69, 1537 [Russ. J. Gen. Chem.,
1999, 69 (Engl. Transl.)].
12. N. Sh. Lebedeva, A. S. Semeikin, E. V. Antina, M. B.
Berezin, and K. A. Mikhailovskii, Konduktometricheskoe
issledovanie rastvorov ryada dipirrolilmetenov i ikh gidroꢀ
bromidov v nꢀpropanole [Conductometric Study of Soluꢀ
tions of Some Dipyrrolilmetenes and Their Hydrobromides
in nꢀPropanol], Deposited in and available from
VINITI, No. 509ꢀB00 (23.01.00), Moscow, 2000, 4 pp. (in
Russian).
This work was carried out with the financial support of
the Ministry of Education of the Russian Federation
(Project No. T00ꢀ9.1ꢀ2050)
13. N. Sh. Lebedeva, E. V. Antina, M. B. Berezin, and A. S.
Semeikin, G. B. Bukushina, Zh. Fiz. Khim., 2000, 74, 1165
[Russ. J. Phys. Chem., 2000, 74 (Engl. Transl.)].
References
1. V. Ya. Bykhovskii, in Uspekhi khimii porfirinov [Adv. in Porꢀ
phyrin Chemistry], Izdꢀvo NIIKhimii SPbGU, St. Petersꢀ
burg, 1997, 1, p. 21 (in Russian).
Received February 26, 2002;
in revised form April 4, 2003