Synthesis, stability in solutions, and spectral and thermal properties of alkyl-substituted 3,3'-bis(dipyrromethene) hydrobromides

Article abstract of DOI:10.1134/S0036023610080036

3,3'-Bis(dipyrromethene) dihydrobromides (H₂L - 2HBr) with various types of alkylation have been synthesized. On the basis of IR, 1H NMR, and electronic absorption spectra, spectrophotometric titra- tion, and thermogravimetry, conclusions have been drawn about the influence of structural factors on the optical and thermal properties and salt stability in solutions. An increase in the degree of alkylation causes a significant high-frequency shift of the N-H stretching vibration band in the IR spectra, an upfield shift of the 1H NMR signals of NH protons, a decrease in the auxochromic effects of protons on the aromatic system of the chromophore, and an increase in the salt stability in solutions in the presence of nucleophilic reagents and in the thermal stability in air and argon. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0036023610080036

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 8, pp. 1172–1178. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.V. Antina, G.B. Guseva, N.A. Dudina, A.I. V’yugin, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 8, pp. 1246–1252.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Synthesis, Stability in Solutions, and Spectral
and Thermal Properties of AlkylꢀSubstituted
3,3'ꢀBis(dipyrromethene) Hydrobromides
E. V. Antina, G. B. Guseva, N. A. Dudina, and A. I. V’yugin
Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
eꢀmail: eva@iscꢀras.ru
Received March 20, 2009
Abstract—3,3'ꢀBis(dipyrromethene) dihydrobromides (H2L ⋅ 2HBr) with various types of alkylation have
been synthesized. On the basis of IR, 1H NMR, and electronic absorption spectra, spectrophotometric titraꢀ
tion, and thermogravimetry, conclusions have been drawn about the influence of structural factors on the
optical and thermal properties and salt stability in solutions. An increase in the degree of alkylation causes a
significant highꢀfrequency shift of the N–H stretching vibration band in the IR spectra, an upfield shift of the
1H NMR signals of NH protons, a decrease in the auxochromic effects of protons on the aromatic system of
the chromophore, and an increase in the salt stability in solutions in the presence of nucleophilic reagents and
in the thermal stability in air and argon.
DOI: 10.1134/S0036023610080036
The 3,3'ꢀbis(dipyrromethene) molecules are R3 = R4 = R5 = R6 = R7 = R8 = R9 = Me, R1 = R10 =
formed by the chromophore dipyrromethene domains R11 = H; VII: R4 = R5 = R6 = R7 = Me; R1 = R2 = R3 =
linked by a 3,3'ꢀmethylene spacer (H2L). These comꢀ R8 = R9 = R10 = R11 = H.
pounds are synthesized as salts with mineral acids
(
H2L 2HBr), which significantly enhances the resisꢀ
EXPERIMENTAL
I–VII were synthesized by known
tance of the ligands to photoꢀ and thermodestruction
[1] and, thus, favorably affects their useful properties.
The clearly pronounced chromophore and fluorescent
properties and high photoꢀ and thermostability of
3,3'ꢀbis(dipyrromethene) salts and complexes make
them promising for use as chemisensors, fluorescent
labels, selective analytical agents, laser dyes, and antiꢀ
mumor agents [2–4]. The present study deals with the
optical and thermal properties and stability in soluꢀ
tions of 3,3'ꢀbis(dipyrromethene) hydrobromides with
various types of alkylation.
Compounds
procedures [5–7].
Bis(1,2,3,7,9ꢀpentamethyldipyrrometheneꢀ8ꢀyl)methꢀ
ane dihydrobromide (I) = 602.46). IR: νNH
3480 cm–1. 1Н NMR (
, ppm; CDCl3, TMS internal
(
М
=
δ
reference): 13.11 (s, 2H, 2N+H), 12.99 (s, 2H, 2NH),
7.02 (s, 2H, 5ꢀCHmeso, 15ꢀCHmeso), 3.55 (s, 2H,
10ꢀCH2, spacer), 2.67 (s, 6H, 1ꢀCH3, 19ꢀCH3), 2.59
(s, 6H, 9ꢀCH3, 11ꢀCH3), 2.25 (s, 6H, 7ꢀCH3,
13ꢀCH3), 2.15 (s, 6H, 3ꢀCH3, 17ꢀCH3), 1.98 (s, 6H,
2ꢀCH3, 18ꢀCH3).
Br–
H
N
H
For C29H38Br2N4 anal. calcd. (%): C, 57.82; H,
6.36; N, 9.30.
R6
R10
R9
N
+
R11
3'
Found (%): C, 57.96; H, 6.40; N, 9.35.
Bis(1,3,7,9ꢀtetramethylꢀ2ꢀethyldipyrrometheneꢀ8ꢀ
R3
R2
R1
R4
C
R7
R8
H
3
yl)methane dihydrobromide (II)
(
M
= 630.51). IR:
+
νNH = 3460 cm–1. 1Н NMR (
δ, ppm; CDCl3, TMS
R5
N
H
N
H
internal reference): 13.09 (s, 2H, 2N+H), 13.00 (s,
2H, 2NH), 7.02 (s, 2H, 5ꢀCHmeso, 15ꢀCHmeso), 3.55
(s, 2H, 10ꢀCH2, spacer), 2.68 (s, 6H, 1ꢀCH3, 19ꢀ
Br–
I
: R1 = R2 = R3 = R4 = R5 = R6 = R7 = R8 = R9 =
CH3), 2.58 (s, 6H, 9ꢀCH3, 11ꢀCH3), 2.48 (q, 4H,
J =
R10 = Me; R11 = H; II: R1 = R3 = R4 = R5 = R6 = R7 =
R8 = R10 = Me; R2 = R9 = Et, R11 = H; III: R1 = R3 =
R4 = R5 = R6 = R7 = R8 = R10 = Me, R2 = R9 = Et,
R11 = Ph; IV: R1 = R2 = R3 = R5 = R6 = R8 = R9 = R10 =
8 Hz, 2, 18ꢀCH2CH3), 2.26 (s, 6H, 7ꢀCH3, 3CH3),
2.12 (s, 6H, 3ꢀCH3, 17ꢀCH3), 1.06 (t, 6H,
18ꢀCH2CH3).
J = 8 Hz, 2,
Me, R4 = R7 = Et, R11 = H;
V: R1 = R3 = R4 = R5 =
For C31H42N4Br2 anal. calcd. (%): C, 59.05; H,
R6 = R7 = R8 = R10 = Me, R2 = R9 = R11 = H; VI: R2 = 6.71; N, 9.14.
1172

SYNTHESIS, STABILITY IN SOLUTIONS
1173
1
Found (%): C, 59.25; H, 6.96; N, 9.30.
2.22 (s, 6H, 7ꢀCH3, 13ꢀCH3); H NMR (
δ, ppm;
DMSOꢀd6, TMS internal reference): 12.56 (s, 2H,
Bis(1,3,7,9ꢀtetramethylꢀ2ꢀethyldipyrromethenꢀ8ꢀ
N+H), 12.46 (s, 2H, NH), 7.93 (s, 2H, 1ꢀCH, 19ꢀ
CH), 7.83 (s, 2H, 2ꢀCH, 18ꢀCH), 7.71 (s, 2H, 3ꢀCH,
17ꢀCH), 6.69, 6.75 (s, 2H, 5ꢀCHmeso, 15ꢀ CHmeso),
3.75 (s, 2H, 10ꢀCH22 spacer), 2.83 (s, 6H, 9ꢀCH3, 11ꢀ
CH3), 2.34 (s, 6H, 7ꢀCH3, 13ꢀCH3).
yl)methane dihydrobromide (III)
νNH = 3455 cm–1. 1H NMR (
internal reference): 13.21 (s, 2H, 2 N+H), 13.02 (s,
2H, 2 NH), 7.45 (s, 2H, ꢀCHꢀPh), 7.38 (s, 2H,
ꢀCHꢀPh), 7.35 (s, 2H, ꢀCHꢀPh), 7.16 (s, H,
(
M
= 706.61). IR:
δ, ppm; CDCl3, TMS
p
m
o
15ꢀCHmeso), 7.12 (s, H, 5ꢀCHmeso), 5.41 (s, 2H,
10ꢀCHPh spacer), 2.70 (s, 6H, 1ꢀCH3, 19ꢀCH3), 2.44
For C23H26N4Br2 anal. calcd. (%): C, 53.30; H,
5.06; N, 10.81.
(q, 4H,
J = 8 Hz, 2, 18ꢀCH2CH3), 2.28 (s, 6H, 9ꢀCH3,
11ꢀCH3), 2.27 (s, 6H, 7ꢀCH3, 13ꢀCH3), 1.96 (s, 6H,
Found (%): C, 51.53; H, 4.85; N, 10.40.
3ꢀCH3, 17ꢀCH3), 1.08 (t, 6H,
18ꢀCH2CH3).
J = 8 Hz, 2,
The IR spectra of 3,3'ꢀbis(dipyrromethene) dihyꢀ
drobromides were recorded as KBr pellets on an Avaꢀ
tar 360 FTꢀIR ESP spectrophotometer. The 1H NMR
spectra were recorded as solutions in deuterated chloꢀ
roform on Bruker 200 and Bruker 500 spectrometers.
The electronic absorption spectra of the samples and
reaction mixtures in organic solvents were recorded in
the range 300–800 nm on an SFꢀ103 spectrophotoꢀ
meter (Akvilon, Russia) operated by a PC with the
Spectr 1.0 program package. Studies were carried out
in quartz cells with a path length of 2 and 10 mm,
which was thermostated at 298.15 K by means of a
Pettier cell.
For C37H46N4Br2 anal. calcd. (%):C, 62.90; H,
6.56,N 7.93.
Found (%):C, 62.50; H, 64.13; N, 7.39.
Bis(1,2,3,9ꢀtetramethylꢀ7ꢀethyldipyrrometheneꢀ8ꢀ
yl)methane dihydrobromide (IV)
(
M
= 630.51). IR:
1
νNH = 3450 cm–1. H NMR (
δ, ppm; CDCl3, TMS
internal reference): 13.15 (s, 2H, 2N+H), 12.98 (s,
2H, 2NH), 6.73 (s, 2H, 5ꢀCHmeso, 15ꢀCHmeso), 3.58
(s, 2H, 10ꢀCH2, spacer), 2.67 (s, 6H, 1ꢀCH3,
19ꢀCH3), 2.58 (s, 6H, 9ꢀCH3, 11ꢀCH3), 2.52 (q, 4H,
J
= 8 Hz, 7, 13ꢀCH2CH3), 2.24 (s, 6H, 3ꢀCH3,
17ꢀCH3), 1.98 (s, 6H, 2ꢀCH3, 18ꢀCH3), 1.04 (t, 6H,
J
= 8 Hz, 7, 13ꢀCH2CH3).
For C31H42N4Br2 anal. calcd. (%): C, 59.05; H,
Organic solvents of chemically pure grade were
additionally purified by common procedures [8]. The
water content of the solvents determined by the Karl
Fischer titration method did not exceed 0.02 wt %.
1ꢀPropanol (1ꢀProp) (UVꢀIRꢀHPLCꢀHPLC preparꢀ
ative) PAI and triethylamine (Pancreac, PA for analyꢀ
tics) were used as purchased.
6.71; N, 9.14.
Found (%): C, 59.19; H, 6.93; N, 9.31.
Bis(1,3,7,9ꢀtetramethyldipyrrometheneꢀ8ꢀyl)methane
dihydrobromide (V)
1H NMR (
, ppm; CDCl3, TMS internal reference):
13.17 (s, 4H, 2NH, 2N+H), 7.05 (s, 2H, 5ꢀCHmeso
15ꢀCHmeso), 6.18 (s, 2H, 2ꢀCH, 18ꢀCH), 3.57 (2H,
10ꢀCH2, spacer), 2.69 (6H, 9ꢀCH3, 11ꢀCH3), 2.60 (s,
6H, 3ꢀCH3, 17ꢀCH3), 2.35 (s, 6H, 9ꢀCH3, 11ꢀCH3),
2.15 (s, 6H, 7ꢀCH3, 13ꢀCH3).
(M
= 574.40). IR: νNH = 3442 cm–1.
δ
,
The equilibrium concentrations, reagent stoichioꢀ
metric ratio, and the constants of reactions of the salts
with amine were determined using the spectrophotoꢀ
metric titration and molar ratio methods. In spectroꢀ
photometric titration, solutions of salts
1ꢀProp ( = 2
10–5 mol/L) were titrated with a triethꢀ
ylamine (C2H5)3N) solution in the same solvent
= 2
10–4 mol/L) up to a fourfold molar excess of
I–VII in
For C27H34N4Br2 anal. calcd. (%): C, 56.46; H,
5.97; N. 9.75.
C
×
Found (%):C, 55.97; H, 6.11; N, 9.55.
(
с
×
Bis(2,3,7,9ꢀtetramethyldipyrrometheneꢀ8ꢀyl)methane
the titrant. The titration curves were obtained by plotꢀ
ting along the vertical axis the salt absorption at the
strong longꢀwavelength band minus the absorption at
the same wavelength of the ligand formed in the course
of the reaction. The molar ratio curves were obtained
dihydrobromide (VI)
(
M
= 574.40). IR: νNH
=
3438 cm–1. 1H NMR (
δ, ppm; CDCl3, TMS internal
reference): 13.32 (s, 2H, N+H), 13.25 (s, 2H, NH),
7.62 (s, 2H, 1ꢀCH, 19ꢀCH), 7.02 (s, 2H, 5ꢀCHmeso
,
at a constant salt concentration (2 ×
10–5 mol/L) while
15ꢀCHmeso), 3.59 (s, 2H, 10ꢀCH2, spacer), 2.71 (s, 6H,
9ꢀCH3, 11ꢀCH3), 2.28 (s, 6H, 7ꢀCH3, 13ꢀCH3), 2.19
(6H, 3ꢀCH3, 17ꢀCH3), 2.07 (s, 6H, 2ꢀCH3, 18ꢀCH3).
For C27H34N4Br2 anal. calcd.(%): C, 56.46; H,
5.97; N, 9.75.
the molar amine excess was varied from 0 to 5. The
error of calculation of equilibrium constants from
spectrophotometric data was no more than 2%. Therꢀ
mogravimetric measurements were carried out on an
MOM 1000D derivatograph in air. A weighed sample
of a compound (~20–30 mg) was placed in a platinum
crucible and heated at a rate of 2.5 K/min. Thermoꢀ
gravimetric studies were also carried out on a Netzsch
TG 209 F1 thermoꢀmicrobalance in an argon flow
Found (%):C, 54.81; H, 6.11; N, 9.08.
Bis(7,9ꢀdimethyldipyrromethenꢀ8ꢀyl)dihydrobromide
1
(VII)
(
M
= 518.29). IR: νNH = 3434 cm–1. H NMR
(
δ
, ppm; CDCl3, TMS internal reference): 13.86 (s,
2H, N+H), 13.70 (s, 2H, NH), 7.90 (s, 2H, 1ꢀCH, 19ꢀ
CH), 7.81 (s, 2H, 2ꢀCH, 18ꢀCH), 7.77 (s, 2H, 3ꢀCH, (a sample of ~3 mg, heating rate of 10 K/min). The
17ꢀCH), 6.73 (s, 2H, 5ꢀCHmeso, 15ꢀCHmeso), 3.63 (s, temperature range was 17–700
. the error of weight
10–4 mg.
°С
2H, 10ꢀCH2 spacer), 2.69 (s, 6H, 9ꢀCH3, 11ꢀCH3), loss determination was 1 ×
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

1174
ANTINA et al.
Table 1. Electronic absorption spectra (λ, nm (logε) of 3,3'ꢀbis(dipyrromethene) dihydrobromides in organic solvents
Compounds
Solven
t
I
II
III
IV
V
VI
370 ctb
458 sh, 4 91– 449 sh, 483
492 (4.96)
VII
370–380 ctb,
DMF c1 372–375 ctb
,
,
,
374–380 ctb
463 sh, 500
(5.01)
,
368–380 ctb
466sh,501–502 459 sh, 494
(5.06) (4.82)
,
370–376 ctb
,
360–385 ctb
455 sh, 492
(5.08)
,
,
456 sh, 496
(4.87)
c2 456 (4.67)
463 (4.79)
460–462 (4.74) 459 (4.73)
455 (4.73)
446 (4.65)
415
DMSO c1 367–380 ctb
465 sh, 503
370 ctb, 465 sh, 365–385 ctb
,
375ctb,462sh, 360–380 ctb
,
374–378 ctb
458 sh, 496
(5.12)
,
375–390 ctb
,
505 (5.18)
465 (4.81)
466 sh, 504
(5.12)
505 (5.14)
462 (4.78)
456 sh, 495
(5.14)
450 sh, 485
(5.09)
c2 463 (4.70)
464 (4.77)
458–460 (4.78) 452 (4.69)
417
–
C5H5N c1 373–375 ctb
462 sh, 499
373–375 ctb
467 sh, 500
(5.01)
,
370 ctb, 466 sh, 368–375 ctb
,
365–370 ctb
459 sh, 491
(4.87)
,
–
500 (4.92)
465 (4.73)
462 sh, 497
(4.89)
(4.87)
c2 462 (4.64)
467 (4.80)
462 (4.73)
460 (4.69)
449 (4.64)
419
CHCl3 c3 364 ctb, 461 sh, 363 ctb, 462 sh, 360–377 ctb
,
363 ctb, 463 350–365 ctb
sh, 504 (5.45) 456 sh, 496
(5.44)
,
373ctb,456sh, 355–362 ctb,
502 (5.43)
505 (5.45)
465 sh, 505
(5.45)
497 (5.38)
p.s.
445 sh, 483
p.s.
C6H6 c3 365 ctb
,
365–375 ctb
,
360–375 ctb
467 sh,508
(5.19)
,
370 ctb
466 sh
508 p.s.
,
360–380 ctb
460 sh
498 p.s.
,
466 sh
,
465 sh
,
,
,
507 p.s.
508 p.s.
–4
–6
–6
–4
Note: c = 1 × 10 mol/L, spectrum of the salt H L ⋅ 2HBr; c = 1 × 10 mol/L, spectrum of the free base H L; c = 1 × 10 –1 × 10 mol/L,
1
2
2
2
3
spectrum of the salt H L ⋅2HBr; p.s. means that a compound is poorly soluble; sh is shoulder; ctb is the charge transfer band. The data for comꢀ
pounds I, II, and IV were taken from [5].
2
RESULTS AND DISCUSSION
vents, especially in pyridine (the strongest electronꢀ
donor solvent among the studied ones), the ligand is
rapidly deprotonated: the spectrum of a fresh dilute
solution shows only the band due to the molecular ligand.
Basic challenges of molecular design of 3,3'ꢀ
bis(dipyrromethene)s [3] involve imparting to the
ligand and its salts useful optical and thermal properꢀ
ties and stability in organic solvents [5], which are in
many respects determined by specific features of alkyꢀ
lation of the oligopyrrole ligand.
In concentrated solutions (c1 > 1 ×
10–4 mol/L), the solꢀ
volytic dissociation rate decreases because of associaꢀ
tion processes and characteristic bands of the protoꢀ
nated form of the ligand H2L ⋅ 2HBr appear in the
Synthesized salts I–VII are insoluble in water and
moderately soluble (up to ~10–4–10–3 mol/L) in
organic solvents. In nonpolar and protonꢀdonor solꢀ
vents (С6Н6, СCl4, CH2Cl2, СHCl3, and others), the
salts are stable. In solvents with pronounced electronꢀ
spectrum. However, on longꢀterm storage, the specꢀ
trum of the solution gradually transforms to the specꢀ
trum of H2L. Comparative analysis of the hypsochroꢀ
ligand
salt
mic shift
=
is evidence of a sigꢀ
Δλmax λ m ax − λ m ax
nificant enhancement of the auxochromic effect of
protons on the dipyrromethene chromophore sysꢀ
donor properties (C5H5N, DMF, DMSO), the H2L
⋅
2HBr salts are subjected to solvolytic dissociation
π
[H2L ⋅ 2НBr]solv + 2Вsolv H2Lsolv + 2[В ⋅ HBr]solv,(1)
tems of the ligand with a decrease in the degree of its
alkylation. Because of the low inductive effect of subꢀ
stituents, the largest polarizability caused by the proꢀ
ton auxochromic effect is observed for the chroꢀ
mophore system of the ligand of compound VII, for
where B is an electronꢀdonor solvent.
The process is accompanied by typical transformaꢀ
tions of the spectrum of the salt to the spectrum of the
nonprotonated ligand: the first longꢀwavelength band
is hypsochromically shifted by 31–68 nm, and the
charge transfer band disappears (Table 1). In dilute
which
is as large as 68 nm. In the spectra of benꢀ
Δλmax
zene solutions, as distinct from other solvents, a
noticeable solvatochromic effect (up to 8–10 nm) is
solutions (c2 < 5 ×
10–5 mol/L) of electronꢀdonor solꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

SYNTHESIS, STABILITY IN SOLUTIONS
1175
ΔA
ΔA
(а)
(b)
1.0
0.8
λ
= 417 nm
λmax = 477 nm
0.5
0.4
λmax = 477 nm
A
A
1.0
λ
= 477 nm
1.0
λ
= 477 nm
0.2
0
λmax = 417 nm
λmax = 417 nm
1
(C H )
2
3
H L · 2HBr
0
1
(C H )
2
3
4
H L · 2HBr
c
/
c
c
/
c
N
N
2
5 3
2
2
5 3
2
0.5
0.5
0
0
350
400
450
500
350
400
450
500
λ, nm
λ, nm
Fig. 1. (a) Spectrophotometric titration and (b) molar ratio curves and evolution of the electronic absorption spectra for the sysꢀ
tem VII–(C H ) N–1ꢀProp.
2
5 3
observed, which can be caused by chromophore polarꢀ deprotonation of the ligand. However, the monoproꢀ
ization through stacking with solvent molecules. An tonated form H2L HBr is evidently less stable: it is not
π
⋅
increase in the degree of ligand alkylation leads to a
significant (up to 19 nm for the salt and 48 nm for the
spectrally revealed, and the spectral transformation
diagram obtained by the molar series method shows
only one family of isosbestic points, which is evidence
of the existence in solution of only two stable chroꢀ
mophore species, the salt and the molecular ligand.
Taking into account this fact, we can represent the
equation for the overall equilibrium process as follows:
free base) bathochromic shift of
of the first
Δλmax
strong band in the electronic absorption spectrum.
For comparative evaluation of the thermodynamic staꢀ
bilities of the salts, the H2L ⋅ 2НBr–(C2H5)3N–1ꢀProp
systems have been studied by the molar ratio and specꢀ
trophotometric titration methods. Typical spectroꢀ
photometric titration and molar ratio curves and the
corresponding spectral changes are exemplified by
compound VII in Fig. 1. At reagent molar ratios
[H2L ⋅ 2HBr]solv + 2(C2H5)3Nsolv
(2)
H2Lsolv + 2[(C2H5)3N ⋅ HBr]solv
.
The equilibrium constants calculated from the
spectrophotometric titration and molar ratio data for
the individual salts are consistent (with an error of less
than 2%) and are almost independent of the concenꢀ
tration ratio of the reagents, which, like the reaction
[(C2H5)3N]/[H2L ⋅ 2HBr] < 2, the protonated and
molecular ligand forms are at equilibrium in the sysꢀ
tem, which is spectrally detected. The dissociation of
H2L ⋅ 2HBr to H2L upon the reaction with amine, as
well as with basic solvents, is accompanied by striking
changes in the absorption spectrum (Figs. 1a, 1b). The
difference between the positions of the disappearing
salt
products, are nonelectrolytes. The average log
ues for process (2) were 0.68 for , 0.73 for II, 0.76 for
III, 0.80 for IV, 0.85 for , 0.91 for VI, and 1.14 for
VII. Thus, with a decrease in the number of alkyl substitꢀ
uents from 10 to 4 in the series of compounds II, …,
VII, the values increase almost threefold, which
K valꢀ
I
V
strong band of the salt
and the growing up band
λmax
lig a n d
of the molecular ligand
for compounds I–VII
λ m a x
I,
is within 20–60 nm. The spectrum of the system with
a large amine excess shows only the characteristic
band of H2L. The spectrophotometric titration curve
K
points to a noticeable decrease in the stability of the
dihydrobromides. Clearly pronounced changes in
absorption spectra (Figs. 1a, 1b) and color (from crimꢀ
sonꢀorange to yellow) of bis(dipyrromethene) dihyꢀ
drobromide solutions in the presence of nucleophilic
reagents and the high conditional sensitivity of analytꢀ
ical determination of (C2H5)3N in organic media (no
(Fig. 1a) reflects the decrease in the absorbance at
salt
max
due to the uniform increase in the (C2H5)3N
λ
concentration in the system. At the molar ratios
(C2H5)3N]/[H2L 2HBr] of 1 and 2, the titration
[
⋅
curves show equivalence points (as two smoothed
bends) and then there is an almost horizontal plateau
in the curve, the saturation curve. At the same molar
ratios, there are bends in the molar ratio curves plotted
on the basis of the absorbances at the salt and ligand
wavelengths. These data are evidence of the stepwise
less than 1.0 × ⋅ 2HBr salts
10–8 mol/L) make the H2L
good candidates for use as colorimetric (nakedꢀeye)
and optical chemosensors and in quantitative spectroꢀ
photometric determination of trace amounts of
(C2H5)3N and other amines in organic media.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

1176
ANTINA et al.
Table 2. Thermooxidative destruction of dihydrobromides
I–VII in air1 and argon2: Tb, Tm, and Te are the temperatures
of, respectively, the beginning, maximal exotherm, and end of
the destruction process (°C)
2'ꢀpositions, which enhance spiralization of the moleꢀ
cule and, thus, prevent the creation of optimal condiꢀ
tions for stabilization of the intermediate substrate
radical with a planar structure. An analogous tendency
was also reported for 2,2'ꢀ and 3,3'ꢀdipyrromethenes
[9], Taking into account this fact and the results of
studying the thermal properties, we can conclude that
the change in the position of attachment of the alkyl
spacer to pyrroles can significantly change the reactivꢀ
ity of oligopyrrole salts in redox reactions and their
thermal properties. Another distinctive feature of the
Process characteristics
Compound
Tb
Tm
Te
I1
260
300
590
I2
275.7
240
282.4
269
destruction of compounds I and VII is that it lacks the
initial stage characteristic of dipyrromethene salts and
biladienes [1] (Fig. 2c). This stage is accompanied by
endotherm and is caused by thermodissociation of a
salt, resulting in removal of HBr from a solid sample
without chemical modification of the ligand:
II1
II2
III1
IV1
IV2
V1
552
247.6
235
259.6
260
600
515
T
H2L
⋅
2HBrsolid
H2Lsolid + 2HBrgas.
(3)
219
255
In any oxidative medium, the destruction of comꢀ
pounds VII begins with the stage accompanied by a
I–
239.2
210
242.8
252
strong exotherm (Figs. 2a, 2b). The weight loss at this
stage is ~6 wt % lower than the theoretical value calcuꢀ
lated for process (3). Quenching the destruction of salt
570
560
500
VI1
VII1
196
261
V
at temperatures corresponding to the end of the first
stage (280–300 ) and elemental analysis of the
°С
cooled product showed that it contained 53.01 wt % C,
1.66 wt % H, 11.17 wt % N, 15.38 wt % O, and 18.78 wt %
Br. If the tetrapyrrole structure is assumed to persist,
the composition of the product can be described by the
formula С22H8N4O5Br. However, taking into account
that the product becomes insoluble in organic solvents
and that its IR spectrum has fundamental differences
from the spectrum of the initial compound, we can
assume that the sample is a mixture of tetrapyrrole oxiꢀ
dation products, including those with a lower oligoꢀ
merization degree, which we failed to identify. Neverꢀ
theless, our findings point to the oxidative character of
the first stage of 3,3'ꢀbis(dipyrromethene) destruction
accompanied by partial removal of carbon and broꢀ
mine and significant removal of hydrogen. The higher
resistance of 3,3'ꢀbis(dipyrromethene) dihydrobroꢀ
mides to thermodestruction, as compared with the
(2,2') analogue salts, is evidently due to an increase in
185
250
Information on the thermal stability of oligopyrꢀ
role salts is important for solving some preparative
(highꢀtemperature synthesis, purification and isolaꢀ
tion of compounds and their derivatives) and technical
problems. The first results of studying the thermal
properties of 3,3'ꢀbis(dipyrromethene)s are presented
in Table 2, and typical thermal destruction curves are
shown in Fig. 2 for compounds
I and IV. The transiꢀ
tion from (2,2') analogues—alkylated biladieneꢀа,с
derivatives [1]—to 3,3'ꢀbis(dipyrromethene)s is
accompanied by a considerable increase in the resisꢀ
tance of the compounds to thermooxidative destrucꢀ
tion. In particular, the difference between the destrucꢀ
tion onset temperatures of salt
I and the decamethylꢀ
substituted biladieneꢀ dihydrobromide (a (2,2')
а,с
the ≥N···HBr bond energy, which is indirectly indiꢀ
analogue) [1] in an oxidative atmosphere is 65 K. It is
worth noting that, in studying the antioxidant properꢀ
ties of linear oligopyrroles during initiated autoxidaꢀ
tion of styrene in chlorobenzene [9], it was found that
salt II was stable in the presence of alkyloxyl radicals
ROO• and was not an active antioxidant, as distinct
from its (2,2') analogue, which has a considerably
higher antioxidant activity than bilirubin and typical
antioxidants, phenol derivatives. Such an increase in
resistance to oxidation was attributed in [9] as a conseꢀ
cated by the increase in the νN–H stretching vibration
frequency in the series of compounds porphyrin <
2,2'ꢀbis(dipyrromethene) < dipyrromethene < dipyrꢀ
romethane < 3,3'ꢀbis(dipyrromethene) < pyrrole [5,
10]. The second stage of the thermodestruction of salts
I–VII is accompanied by a broad exotherm and, in an
oxidative atmosphere, ends with the complete weight
loss owing to the formation of gaseous products. The
solid product of destruction of compounds in argon
mainly (by 99 wt %) consists of carbon. For comꢀ
quence of steric effects of methyl groups in the 2ꢀ and pounds I, II and IV as an example, it was shown that
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

SYNTHESIS, STABILITY IN SOLUTIONS
1177
Δ
m
, %
TG
Δ
0
m
, %
ТГ
T, °C
T, °C
(a)
1000
(c)
0
800
600
800
DTG
DTA
DTA
600
50
50
400
200
400
200
Т
100
50
100
150
, min
Т
τ
100
50
100
150
, min
τ
Δ
m
, %
T,
°
C
(b)
ТГ
0
ДТГ
500
300
50
Т
100
100
0
20
40
τ, min
Fig. 2. Thermal curves of (a) compound
I
in air, (b) compound IV in argon, and (c) decamethylꢀsubstituted biladieneꢀ
a,c
in air [1].
the resistance of the salts to thermodestruction in features of alkylation of the ligands in salts
I
–
VII: in
(NH) and
(N+H); in IR spectra, the N–H stretching vibration
frequency (νN–H); in the electronic absorption spectra,
the values of the strong longꢀwavelength band.
1H NMR spectra, the chemical shifts
δ
argon increases by 16–19 K. An increase in the degree
of alkylation of the ligand leads to an increase in the
δ
thermal stability of the salts in the series
nearly 75 K (Table 2).
I, II, …, VII by
(ε)
λmax
A decrease in the degree of alkylation leads to an
It is worth noting that the quantitative characterisꢀ
tics of the salt stability in solutions and in the solid
phase change monotonically with a change in the
parameters of characteristic bands and signals in IR,
1H NMR, and electronic absorption spectra. The
results of spectral studies demonstrate that the followꢀ
ing parameters are the most sensitive to the specific
increase in the downfield shift of the 1H NMR signals
δ
(NH) and δ
(N+H), a decrease in the stretching vibraꢀ
tion frequency νN–H in the IR spectra of the salts, and
a decrease in the proton auxochromic effect of on
chromophore systems, which is manifested in the hypꢀ
sochromic shift of λmax of the first strong band in the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

1178
ANTINA et al.
νNH, cm–1
λmax, nm
log
K
Tb, °С
(c)
IV III II
(a)
(b)
I
VII VI V
I
3480
1.2
1.0
0.8
0.6
III
260
500
II
I
II
IV
3460
III
IV
V
220
180
VI
490
V
VII
VI
3440
VII
13.0
13.4
13.8
13.0
13.4
NH,
13.8
N+H), ppm
3440
3460
3480
νNH, cm–1
1/2Σ
(
δ
δ
1/2Σ(δNH, δ
N+H), ppm
Fig. 3. Plots of (a) stretching vibration frequencies
ν
in IR spectra and (b) the first absorption maximum (
λ
) in the elecꢀ
N–H
max
T
1
+
tronic absorption spectra in DMF vs. H NMR chemical shifts
compounds VII
δ(NH) and
δ
(N H), and (c) plots of log
K
and
vs.
ν
for
b
N–H
I–
.
electronic absorption spectra of H2L
solvents (Fig. 3). The observed correlation between the
logK, and b values and spectral characteristics is eviꢀ
dence of the decrease in the ligand basicity due to a
decrease in the overall +I inductive effect of substituꢀ
ents and makes it possible to qualitatively predict the
stability of new structurally related 3,3'ꢀbis(dipyrꢀ
romethene) dihydrobromides on the basis of 1H NMR
and IR spectra.
⋅
2HBr in organic
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T
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This work was supported by the Basic research proꢀ
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010