Synthesis and spectral characteristics of fluorescent dyes based on coumarin fluorophore and hindered amine stabilizer in solution and polymer matrices

Article abstract of DOI:10.1016/j.dyepig.2010.12.006

The spectral properties of novel bifunctional dyes based on - coumarin and diethylamino-coumarin and piperidine parent amine and N-oxyl and N-alkoxy derivatives were compared in cyclohexane, methanol, diethylene glycol, acetonitrile and chloroform solvents and in polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) polymer matrices. The fluorescence of the derivatives excited at the longest-wavelength band around 295 nm is very low for coumarin-based dyes, whereas the fluorescence of the 7-diethylamino-3- carboxy coumarin dyes excited at 420 nm is as intense as that of anthracene in comparable conditions. Intramolecular quenching on the singlet level, monitored by fluorescence and expressed as the ratio of Φ_NH/Φ _NO for the parent amine and N-oxyl and Φ_NOR/ Φ_NO for N-alkyloxy and N-oxyl, is more efficient in polymer matrices than in solvents. The spectral properties of 7-diethylamino-3-carboxy coumarin fluorophores depend on the polarity and viscosity of the environment. Highest values of quantum yield ratio were observed for 7-diethylamino-3-carboxy coumarin dyes in polystyrene and poly(vinylchloride). Though the fluorescence intensity is higher in chloroform or in poly(vinyl chloride) matrices, the intramolecular quenching efficiency of a given fluorophore by nitroxide is higher in low polarity media, such as cyclohexane and polystyrene.

Full text of DOI:10.1016/j.dyepig.2010.12.006

Dyes and Pigments 90 (2011) 129e138
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and spectral characteristics of fluorescent dyes based on coumarin
fluorophore and hindered amine stabilizer in solution and polymer matrices
*
Martin Danko , Erik Szabo, Pavol Hrdlovic
Polymer Institute, Center of Excellence GLYCOMED, Slovak Academy of Sciences, 845 41 Bratislava, Dubravska cesta 9, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history:
The spectral properties of novel bifunctional dyes based on e coumarin and diethylamino-coumarin and
piperidine parent amine and N-oxyl and N-alkoxy derivatives were compared in cyclohexane, methanol,
diethylene glycol, acetonitrile and chloroform solvents and in polystyrene, poly(methyl methacrylate)
and poly(vinyl chloride) polymer matrices. The fluorescence of the derivatives excited at the longest-
wavelength band around 295 nm is very low for coumarin-based dyes, whereas the fluorescence of the
7-diethylamino-3-carboxy coumarin dyes excited at 420 nm is as intense as that of anthracene in
comparable conditions. Intramolecular quenching on the singlet level, monitored by fluorescence and
Received 1 October 2010
Received in revised form
8 December 2010
Accepted 9 December 2010
Available online 17 December 2010
Keywords:
Fluorescence dye
Coumarin
Sterically hindered amine
N-oxyl
Intramolecular quenching
Solvent effect
expressed as the ratio of FNH/FNO for the parent amine and N-oxyl and FNOR/FNO for N-alkyloxy and
N-oxyl, is more efficient in polymer matrices than in solvents. The spectral properties of 7-diethylamino-
3-carboxy coumarin fluorophores depend on the polarity and viscosity of the environment. Highest
values of quantum yield ratio were observed for 7-diethylamino-3-carboxy coumarin dyes in polystyrene
and poly(vinylchloride). Though the fluorescence intensity is higher in chloroform or in poly(vinyl
chloride) matrices, the intramolecular quenching efficiency of a given fluorophore by nitroxide is higher
in low polarity media, such as cyclohexane and polystyrene.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
paramagnetic properties [7,8]. Fluorescence switch-off or switch-
on occurs as a result of a chemical reaction on this reaction center.
Fluorescence spectroscopy attracts considerable attention
because of its sensitivity in systems containing intrinsic fluo-
rophores and systems in which they might be introduced [1,2].
Biopolymers and synthetic polymers are used in numerous studies
in which fluorescence plays an important role [2,3].
Various fluorescence probes based on fluorophores of different
structures are used to monitor processes in environments such as
solutions, micelles and solid amorphous matrices [4e6]. The
spectral parameters of fluorescence that exhibit strong dependence
on medium, such as the position of the maximum, its intensity and
enhancement or quenching are exploited in these probes. The
advantages of parameters connected with fluorescence are high
sensitivity, simple detection and easy quantitative evaluation for
selected fluorophores.
Other fluorophores of varying complexity were tested for the
construction of this probe: 1-naphthoic, 1-naphthylacetic [9], 4-(1-
pyrene) alcanoic acids [10,11], 1,8-naphthaleneimide [12,13] and
anthracene [14]. These fluorophores were linked with sterically
hindered amines (known piperidine structure used for foto- and
thermal stabilization of polymers e hindered amine stabilizers
(HAS)), such as 2,2,6,6-tetramethyl-4-hydroxypiperidine, 2,2,6,6-
tetramethyl-4-aminopiperidine in the form of the parent amine,
nitroxyl radicals and alkoxy derivatives.
The efficiency of intramolecular quenching in these dyes
depends on the size of the linker as well as on the medium. The
stability of fluorophores under conditions of radical reactions is also
an issue. The linkage of the above-mentioned fluorophores with
sterically hindered amines does not seem to protect the fluo-
rophore from radical (photo-oxidative) damage in the solid state
[15e17].
In the 1990s, a specific group of probes (dyes) was developed
that are based on intramolecular fluorescence quenching. In these
probes, the fluorophores are linked with a structural unit easily
oxidized on the aminooxide of the N-oxyl type, which exhibits
Dyes based on intramolecular quenching that link suitable flu-
orophores with N-oxyl have been prepared and employed for
various applications. Hideg and co-workers [18e20] prepared and
optimized synthesis of double-sensor (fluorescent and spin)
molecules with dansyl, aminophthalimide, coumarins, pyrene and
4-nitrobenzofurazan fluorophores attached to nitroxide, inspired
* Corresponding author. Tel.: þ421 2 54777404; fax: þ421 2 54775923.
E-mail addresses: upoldan@savba.sk (M. Danko), upolhrdl@savba.sk (P. Hrdlovic).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.12.006

130
M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
by tetramethylpiperidine or tetramethyl-2,5-dihydro-1H-pyrrole,
which monitors reactive oxygen species in thylakoid membranes
and the side-specific labeling of proteins. Dual fluorophore-nitro-
xide probes were used for the determination of vitamin C in bio-
logical liquids [21]. Nitroxide-linked naphthalene was used as
a fluorescent dye for hydroxyl radicals [22]. A novel naphthalene-
hindered amine linked with a long alkyl chain was tested as
a thermo- and photo-stabilizer [23]. To gain some insight into the
radical processes occurring during the induction period in the
thermal degradation of polyolefins, a novel pro-fluorescent probe
based on phenanthrene and diphenyl-anthracene were used
[24,25].
coumarin-3-carboxylic acid (1) is
a
commercial product
(SigmaeAldrich, Slovakia) and was used as received. The synthesis of
7-N,N-diethylaminocoumarin-3-carboxylic acid (2) was performed
according to the synthetic details described in the work of Song et al.
[37]. The linkage of fluorophores with 4-amino-2,2,6,6-tetrame-
thylpiperidine and its N-oxyl and N-alkoxyderivatives wasperformed
by standard organic synthesis procedures. The preparation of 4-
Amino-2,2,6,6-tetramethylpiperidine-N-oxyl from 4-amino-2,2,6,6-
tetramethylpiperidine was performed in three steps by the acetic
amideprotection of eNH2, oxidation of piperidinenitrogenwith H2O2
and deprotection of the amine by hydrolysis according to procedure
described in Ref. [32]. For the amide formation of 1 with 4-amino-
A new approach for the detection of carbon-centered radicals in
enzymatic processes was developed using pre-fluorescent dyes
with quinoline as the fluorophore, which was linked with TEMPO
[26]. Fluorescent imaging using a pre-fluorescent radical probe was
employed for the mapping of photo-generated radicals in thin
polymer films [27]. Fluorescent sensor applications, including
detection of DNA damage, free radical formation and micro-
lithography, have been recently reviewed [28].
2,2,6,6-tetramethylpiperidine,
a
2,6-dimethoxy-1,3,5-triazine-N-
methylmorpholinium chloride (DMT-MM, 7) was used as a mediator
[33,34]. DMT-MM was prepared from 4-chloro-2,6-dimethoxy-1,3,5-
triazine and N-methylmorpholine in tetrahydrofuran (THF) solution
according to Ref. [35].
2.1.1. Coumarin-3-carboxy-(2,2,6,6-tetramethylpiperidine-4-yl)
amide (1a)
Adducts of 1,8-naphthaleneimide and sterically hindered amine
were used as ‘one-step’ brighteners and stabilizers [29].
In our laboratory, the fluorophore benzothioxanthone, which
yields bright fluorescence, was used for the construction of this
type of probe, and its stability in polymer matrices was recently
tested [30,31].
In this study, dyes based on simple fluorophores, derivatives of
coumarin and sterically hindered amine were prepared, and their
spectral properties were compared in solution and in polymer
matrices. A new approach for carboxyamide formation of electron-
rich aromatic chromophores was employed using a triazine-based
mediator. The goal of the spectroscopic study was to gain better
understanding of the photophysical processes leading to intra-
molecular fluorescence quenching.
Compound 1 (2 g, 10.5 mmol) was dissolved in 30 ml of
commercial THF in a 100-ml round-bottomed flask. The 4-amino-
2,2,6,6-tetramethyl-piperidine 4 (10% molar excess) solution in THF
(20 ml) was added to this solution. A few minutes after the addi-
tion, rapid precipitation occurred in the reaction mixture. After
stirring for 10 min, the dry powder of 7 (10% molar excess of 1) was
added to the reaction mixture and stirring was continued at r.t. for
approx. 18 h. The reaction was followed on TLC chromatography
(SiO2 plate) using methanol as the eluent. The reaction mixture was
poured into water and extracted three times with ether. The 1H
NMR analysis of the organic layer after evaporation of solution did
not confirm the presence of products. The white precipitate that
formed between the ether and water layers was separated by
filtration and was identified as the product in the form of an amino-
hydrochloride salt. The parent amine was obtained from methanol
solution by the addition of 30% of ammonium hydroxide (5 ml) in
55 ml of water. The white needles that formed were filtered and
dried under vacuum at 80 ꢀC. The yield of pure product was 1.15 g
(32%) of white needles with a m.p. of 190e192 ꢀC.
2. Experimental
2.1. Synthesis of dyes
1H NMR (CDCl3)
(s, 6H, eCH3 equatorial); 1.31 (s, 6H, eCH3 axial); 2.02 (dd, 2H,
d(ppm): 1.1 (d, 2H, eCHeCH2 equatorial); 1.18
The structures and preparation methods of reactants and dyes
under study are shown in Schemes 1, 2 and 3. The fluorophore
Scheme 1. The synthetic scheme for preparation of coumarin and diethylamino-coumarin derivatives (1aec and 2aec).

M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
131
Scheme 2. The synthetic scheme for preparation of the model diethylamino-coumarin alkylamide (2d).
eCHeCH2 axial); 4.46 (tt, 1H, eCHe); 7.37e7.45 (m, 2H, coumarin);
7.63e7.75 (m, 2H, coumarin); 8.66 (d, 1H, eNHe); 8.9 (s, 1H, >C]
CHemethylene).
1H NMR (CDCl3, reduced by pentafluorophenyl hydrazine)
d
(ppm): 1.25; (s, 6H, eCH3 equatorial); 1.28 (s, 6H, eCH3 axial); 1.57 (t,
2H, eCHeCH2 equatorial); 2.01 (2H, eCHeCH2 axial); 1.96 and 2.07
(2 strong singlets-residual pentafluorophenyl hydrazine), 3.92
(wide, 1H, >NeOH), 4.46 (m, 1H, eCHe); 5.11 (wide singlet -
residual pentafluorophenyl hydrazine); 6.38 (wide singlet - residual
pentafluorophenyl hydrazine); 7.36e7.46 (m, 2H, coumarin);
7.63e7.76 (m, 2H, coumarin); 8.66 (d, 1H, eNHe); 8.9 (s, 1H, >C]
CHemethylene).
13C NMR (CDCl3)
d
(ppm): 28.5 (2 ꢁ eCH3 equat); 35.02
(2 ꢁ eCH3 axial); 43.0 (CHeN); 45.03 (2 ꢁ CH2eCHeN); 51.5
(CeNH); 115.4 (CH-8 coumarin); 118.1 (C-3 coumarin); 118.2 (C-10
coumarin) 125.1 (CH-6 coumarine); 129.7 (CH-5 coumarin); 133.9
(CH-7 coumarin); 148.1 (eCH]methylene); 154.6 (C-10 coumar-
ine); 161.1 (C]O lactone); 162.2 (C]O amide).
FTIR (KBr)
n
(cmꢂ1): 3435, 3318, 3050, 2960, 2925, 1705, 1659,
FTIR (KBr) n
(cmꢂ1): 3454, 3322, 2972, 2940, 1696, 1652, 1619,
1612, 1569, 1546, 1456, 1364, 1248, 1161, 799, 748, 640.
Anal. Calcd. for C19H24N2O3: C69.49, H7.37, N8.53. Found:
C69.24, H7.42, N7.75.
1589, 1532, 1516, 1417, 1355, 1232, 1192, 1134, 1079, 792, 700.
2.1.3. 4-Amino-1-(10-phenylethyl)oxy-2,2,6,6-
tetramethylpiperidine (6)
2.1.2. Coumarin-3-carboxy-(N-oxy-2,2,6,6-tetramethylpiperidine-
4-yl)amide (1b)
The protection and deprotection of the amino group was per-
formed according to the method described in Ref. [36] by di-tert-
butyldicarbonate (Boc2O) and hydrolysis with tri-fluoro-acetic acid
(CF3COOH).
Three molar equivalents of NaHCO3 and a 20% molar excess of
Boc2O were consecutively added to the stirred solution of 5 (1 g,
5.85 mmol) in a THF/water mixture (1/1, 60 ml) in portions at 0 ꢀC
over 45 min. The reaction mixture was then stirred overnight at r.t.
The solution was then extracted with three portion of ether, and the
organic phases were washed with water and dried over Na2SO4.
After the evaporation of ether and drying, that formed orange
needles (1.5 g, 95%) were used further reaction.
The protected N-oxyl (BOC-NO, 1.5 g, 5.53 mmol) was dissolved
in a 150-ml mixture of toluene/ethanol (1/1), and a ten-fold molar
excess of styrene (5.1 g, freshly distilled) and Mn(OAc)3 2H2O (14.8 g)
were added to the solution. The mixture was stirred in an open 250-
ml round-bottomed flask. Three loads of NaBH4 (15-fold molar
excess with respect to BOC-NO, 3.15 g) were added portionwise in
15-min intervals to avoid the boiling of the reaction mixture. After
The synthesis of 1b was carried out using 7 as mediator, similar
to 1a. 1 (1.5 g, 7.9 mmol) reacted with 4-amino-1-oxo-2,2,6,6-tet-
ramethylpiperidine 5 (1.49 g, 10% molar excess) and 7 (2.4 g, 10%
molar excess) in THF solution. The reaction was stopped when the
TLC analysis (SiO2 plates, mixture ethylacetate/i-hexane 1/5 eluent)
of the reaction mixture showed no signs of any further changes in
its composition (w18 h). The formed precipitate was separated by
filtration and thoroughly washed on the funnel with THF
(6 ꢁ 15 ml), leaving the precipitate completely white and accu-
mulating approx. 120 ml of orange filtrate. The filtrate was reduced
in volume on a rotary evaporator to approx. 25 ml and cooled in the
freezer. Formed orange crystals were collected by filtration and
dried, yielding 2.4 g of crude product (86%). The product was
recrystallized from THF, yielding pale orange crystals with a m.p. of
222e227 ꢀC. The purity of product (100%) was confirmed by ESR
analysis comparing with 4-hydroxy-1-oxy-2,2,6,6-tetramethylpi-
peridine in toluene solution (1 ꢁ 10ꢂ3 mol Lꢂ1).
Scheme 3. The synthetic scheme for the preparation of the 4-amino-2,2,6,6-tetramethylpiperidine-N-alkoxyamine via protection and deprotection of the free amino group.

132
M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
the filtration of the inorganics, the solvents were evaporated and the
product was separated on a silica column using i-hexane/ethyl-
acetate (5/1, v/v) as the eluent, yielding 1.56 g (75%) of white powder.
BOC-NOR (1.55 g) was treated with CF3COOH (8 ml, 26 molar
excess) in dichloromethane at r.t. for the deprotection of amine.
Product 6 was formed as a white precipitate after the addition of
a water solution of NaHCO3 to the reaction mixture and was
separated by filtration (yield 1.01 g, 89%). FTIR analysis (KBr pellet)
of the washed and dried white powder did not show carboxyamide
absorption. For further reactions, 6 was used as the crude product.
All protection, N-oxyl formation and deprotection products
were analyzed by FTIR analysis in chloroform solution.
a hydrochloride salt. Another portion of the product was formed
during the evaporation of the THF solvent from the reaction mixture.
The pure parent amine was separated by extraction of a CHCl3
solution of hydrochloride with a water solution of NaHCO3, followed
by extraction with NaCl and water. The total yield of the parent
amine of 2a was 27% of white crystals, with a m.p. of 228e230 ꢀC.
1H NMR (CDCl3)
d(ppm): 1.07e1.11 (2H, eCHeCH2 equat.); 1.17
(s, 6H, eCH3 equat.); 1.24 (t, 6H, 2 ꢁ CH3eCH2eNe, J ¼ 7.14 Hz);
1.31 (s, 6H, eCH3 axial); 2.0 (dd, 2H, eCHeCH2 axial); 3.45 (q, 4H,
2 ꢁ CH3eCH2eNe, J ¼ 7.12 Hz); 4.36e4.52 (tm, 1H, eCHe); 6.50 (d,
1H, coumarin H-8); 6.65 (dd, 1H, coumarin H-6, J ¼ 8.95 Hz); 7.43
(d, 1H, coumarin H-5, J ¼ 8.96 Hz); 8.64 (d, 1H, eNHe); 8.68 (s, 1H,
>C]CHemethylene).
2.1.4. Coumarin-3-carboxy-(1-(10-phenylethyl)oxy-2,2,6,6-
tetramethylpiperidine-4-yl)amide (1c)
13C NMR (CDCl3)
d(ppm): 12.4 (CH3eCH2e); 28.5 (CH3 axial);
34.9 (CH3 equat.); 42.7 (CHeN); 45.1 (CH2eCHeN); 45.2
(CH3eCH2e); 51.1 (CeN); 96.5 (CH-8 coumarin); 108.4 (C-10
coumarin); 109.9 (C-5 naphthalene); 110.4 (C-3 lactone); 131.1
(eCH]methylene); 147.9 (CH-6 coumarin); 152.4 (C-9 coumarin);
157.6 (C-7 coumarin); 162.2 (C]O lactone); 162.7 (C]O amide).
The synthesis of 1c was carried out using mediator 7, similar to
that of 1b. 1 (0.63 g, 3.3 mmol), reacted with 6 (1.01 g, 10% molar
excess) and 7 (1.01 g,10% molar excess) inTHF solution. Because only
small changes were observed using TLC analysis SiO2 plate, elution
ethylacetate/i-hexane (1/5, v/v) of the reaction mixture stirred at r.t.,
the reaction was continued at 55 ꢀC for 8 h. The formed precipitate
was separated by filtration and thoroughly washed on the funnel
with THF (6 ꢁ 10 ml). The solvent was evaporated on a rotary
evaporator, and the solid residue was separated (chromatographed)
on the silica column using gradient elution of ethylacetate/i-hexane
mixtures, starting with a 1/5 mixture and finishing with pure eth-
ylacetate. Only the last fraction was identified by NMR analysis as
a product; preceding fractions did not contain NMR signals related
to the coumarin fluorophore. The yield of coumarin e N-alkoxy-
amine was 28% of theoretical amount of small white crystals with
a m.p. of <120 ꢀC (carbonization).
FTIR (KBr)
n
(cmꢂ1): 3434, 3315, 2974, 1692, 1648, 1618, 1585,
1537, 1513, 1413, 1355, 1244, 1191, 1136, 798.
Anal. Calcd. for C23H33N3O3: C69.14, H8.33, N10.52. Found:
C68.55, H8.25, N10.25.
2.1.7. 7-(N,N-diethylamino)coumarin-3-carboxy-(1-oxo-2,2,6,6-
tetramethylpiperidine-4-yl)amide (2b)
The synthesis of 2b was carried out using a DMT-MM mediator,
similar to that of 1b, using compounds 2 (1.92 mmol), 5 (2.11 mmol)
and 7 (2.11 mmol) in THF solution over 18 h. The FTIR analysis of the
reaction mixture showed formation of eC]O vibration related to
the amide. After the THF evaporation, the residue was dissolved in
chloroform, extracted with brine solution and water and dried over
anhydrous Na2SO4. The product was obtained by chromatographic
separation on a SiO2 column using a CHCl3/MeOH mixture (15/1, v/v)
as the first fraction. Yellow-orange crystals of 2b, with a m.p. of
263e267 ꢀC, were obtained by crystallization from the elution
solution during evaporation in 50% yield. The purity of product (98%)
was confirmed by ESR analysis comparing with 4-hydroxy-1-oxy-
2,2,6,6-tetramethylpiperidine in toluene solution (1 ꢁ10ꢂ3 mol Lꢂ1).
1H NMR (CDCl3)
d(ppm): 0.68 (s, 3H, 1 ꢁ CH3 equatorial); 1.17 (s,
6H, 1 ꢁ CH3 equatorial); 1.32e1.35 (2 ꢁ s, 6H, 2 ꢁ eCH3 axial); 1.5
(d, 3H, eCHeCH3, J ¼ 6.68 Hz); 1.57 (t, 2H, eCHeCH2 equatorial);
1.80 (td,1H, eCHeCH2 axial); 1.93 (td,1H, eCHeCH2 axial); 4.31 (m,
1H, eCHeNHe); 4.79 (q, 1H, eCHeCH3, J ¼ 6.68 Hz) 7.20e7.32 (m,
5H, 1-Phenyl); 7.34e7.43 (m, 2H, coumarin); 7.62e7.72 (m, 2H,
coumarin); 8.59 (d, 1H, eNHeCHe); 8.87 (s, 1H, >C]
CHemethylene).
13C NMR (CDCl3)
d(ppm): 20.9 (CH3eCHePh); 23.3 (CH3 axial);
1H NMR (CDCl3, reduced by pentafluorophenyl hydrazine)
d
30.9 (>CHeCH3); 34.0 and 34.3 (CH3 equat.); 41.7 (CHeNH); 45.9
(CH2eCHeN); 59.8 and 60.1 (2 ꢁ CeN); 83.3 (CH3eCHePh); 116.6
(CH-8 coumarin); 118.6 (C-3 lactone); 118.7 (C-10 coumarin); 125.2
(CH-6 coumarin); 126.7 (CH-2,6 phenyl); 126.9 (CH-4 phenyl);
128.0 (CH-3,5 phenyl); 129.8 (C-5 coumarin); 133.9 (eCH]meth-
ylene); 145.4 (C-1 phenyl); 148.1 (C-7 coumarin); 154.4 (C-9
coumarin); 160.7 (C]O lactone); 161.4 (C]O amide).
(ppm): 1.25 (t, 6H, 2 ꢁ CH3eCH2eNe); 1.44; (s, 6H, eCH3 equat.);
1.49 (s, 6H, eCH3 axial); 2.04 (t, 2H, eCHeCH2 equat.); 2.19 (dd, 2H,
eCHeCH2 axial); 3.46 (q, 4H, 2 ꢁ CH3eCH2eNe); 4.01 (strong
singlet, residual from pentafluorophenyl hydrazine); 4.45e4.62 (m,
1H, eCHe); 5.16 (wide, 1H, >NeOH); 6.50 (d, 1H, coumarin); 6.67
(dd, 1H, coumarin); 7.44 (d, 1H, coumarin); 8.65 (s, 1H, >C]
CHemethylene); 8.82 (d, 1H, eNHe).
FTIR (KBr)
n
(cmꢂ1): 3451, 3328, 2974, 2934, 1714, 1655, 1613,
FTIR (KBr) n
(cmꢂ1): 3439, 3301, 3040, 2975, 2935, 1706, 1659,
1570, 1539, 1457, 1365, 1241, 1078, 948, 799, 759, 699, 633.
Anal. Calcd. for C27H32N2O4: 72.30, H7.19, N6.25. Found: C72.18,
H7.22, N5.44.
1609, 1565, 1543, 1452, 1362, 1238, 1145, 798, 768, 642.
2.1.8. 7-(N,N-diethylamino)coumarin-3-carboxy-(1-(10-
phenylethyl)oxy-2,2,6,6-tetramethylpiperidine-4-yl)amide (2c)
The synthesis of 2c was carried out using DMT-MM mediator,
similar to that of 1c, using compounds 2 (1.16 mmol), 6 (1.27 mmol)
and 7 (1.27 mmol) in THF solution over 48 h. The formed precipitate
was separated by filtration and thoroughly washed on the funnel
with THF (6 ꢁ 10 ml). The solvent was evaporated on a rotary
evaporator, and the solid residue was chromatographed on a silica
column using gradient elution of i-hexane/ethylacetate mixtures,
beginning with a 1/1 mixture (v/v) and finishing with a 1/2 mixture
(v/v). Orange crystals of 2c with a m.p. of 230e232 ꢀC were
obtained by crystallization from the elution solution during evap-
oration in 27% yield.
2.1.5. 7-N,N-diethylaminocoumarin-3-carboxylic acid (2)
Product 2 was prepared according to the procedure of Song et al.
[37] in 63% yield. The product was crystallized from the reaction
mixture after cooling to ꢂ25 ꢀC as orange crystals with a m.p. of
230e232 ꢀC (ref. [37] m.p. 222e224 ꢀC) and was used for next step
without further purification. The chemical structure and purity of
this compound were proven by 1H NMR.
2.1.6. 7-(N,N-diethylamino)coumarin-3-carboxy-(2,2,6,6-
tetramethylpiperidine-4-yl)amide (2a)
The synthesis of 2a was performed as for 1a, using compounds 2
(1.16 mmol), 4 (1.27 mmol) and 7 (1.27 mmol) in THF solution for
18 h. The product was crystallized from the reaction mixture as
1H NMR (CDCl3)
3H, 1 ꢁ CH3 equat.); 1.26 (t, 6H, 2 ꢁ CH3eCH2eNe); 1.31e1.34
d(ppm): 0.67 (s, 3H, 1 ꢁ CH3 equat.); 1.17 (s,

M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
133
(2 ꢁ s, 6H, 2 ꢁ eCH3 axial); 1.49 (d, 3H, eCHeCH3, J ¼ 6.69 Hz);
1.54 (t, 2H, eCHeCH2 equat.); 1.79 (td, 1H, eCHeCH2 axial); 1.93
(td, 1H, eCHeCH2 axial); 3.45 (q, 4H, 2 ꢁ CH3eCH2eNe); 4.29 (m,
1H, eCHeNHe); 4.79 (q, 1H, eCHeCH3, J ¼ 6.69 Hz); 6.49 (s, 1H,
coumarin); 6.63 (dd, 1H, coumarin); 7.20e7.32 (m, 5H, 1-Phenyl.);
7.415 (d, 1H, coumarin); 8.58 (d, 1H, eNHeCHe); 8.86 (s, 1H, >C]
CHemethylene).
instrument (Thermo, USA) and ESR spectra on X-band spectrom-
eter E-4 Varian (USA) interfaced with PC using program Symphonia
Bruker.
UV-VIS absorption spectra were taken on a spectrometer UV
1650 PC (Shimadzu, Japan).
Fluorescence spectra were recorded on a RF-5301PC spectro-
fluorophotometer (Shimadzu, Japan) and on a Perkin-Elmer MPF-4
spectrofluorimeter (Perkin-Elmer, Norfolk, Conn. U.S.A.), which was
connected through interface and A/D convertors to the ISA slot of
a PC using a homemade data collection program. The program
Origin 6.1 (Microsoft) was used for data plotting. The fluorescence
of the solution was measured in a 1 cm cuvette in the right-angle
arrangement. The fluorescence of the polymer films was taken in
a front-face arrangement on the solid sample holder.
The quantumyield of the probes (1aec and 2aed) in solution and
doped in polymer films was determined using anthracene as the
standard in the respective medium, taking the quantum yield of
anthracene in cyclohexane 0.25 [38]. The quantum yields of
anthracene fluorescence in all other media were determined by the
comparison of the fluorescence of anthracene in cyclohexane and in
respective medium. It was found to be 0.20 in methanol and 0.11 in
chloroform. In polymer matrices, the quantum yields were assumed
to be 0.20 in PMMA, 0.16 in PS and 0.11 in PVC. The fluorescence
spectra were taken using excitation into the maximum of the
longest-wavelength absorption band. The anthracene concentra-
tionwas the same in all media. The quantumyields in solution and in
film were corrected for different absorption at the excitation
wavelength [39]. The fluorescence spectra were taken by the exci-
tation into the maximum of the longest-wavelength absorption
band; the fluorescence spectra of anthracene were excited at
355 nm.
13C NMR (CDCl3)
d(ppm): 12.6 (CH3eCH2e); 21.1
(CH3eCHePh); 23.5 (CH3 axial); 34.1 and 34.8 (CH3 equat.); 41.4
(CHeN); 45.3 (CH3eCH2e); 46.3 (CH2eCHeN); 60.1 and 60.3 (2 ꢁ
CeN); 83.4 (CH3eCHePh); 96.7 (CH-8 coumarin); 108.6 (C-10
coumarin); 110.1 (C-5 coumarin); 110.6 (C-3 lactone); 126.9 (CH-
2,6 phenyl); 127.1 (CH-4 phenyl); 128.2 (CH-3,5 phenyl); 131.3
(eCH]methylene); 145.7 (C-1 phenyl); 148.1 (CH-6 coumarin);
152.6 (C-9 coumarin); 157.8 (C-7 coumarin); 162.6 (C]O lactone);
162.9 (C]O amide).
FTIR (KBr)
n
(cmꢂ1): 3430, 3313, 2970, 1690, 1647, 1617, 1584,
1536, 1512, 1412, 1354, 1230, 1135, 797, 702.
Anal. Calcd. for C31H41N3O4: C71.45, H7.95, N8.09. Found:
C70.81, H7.85, N7.85.
2.1.9. 7-(N,N-diethylamino)coumarin-3-carboxy-(1-methyl-propyl)
amide (2d)
The synthesis of 2d was carried out using a DMT-MM mediator,
similar to other derivatives, using 2 (0.46 mmol), sec-butylamine 8
(0.85 mmol) and 7 (0.85 mmol) in THF solution over 18 h. The
solvent was evaporated on a rotary evaporator from a light yellow
reaction solution, and the solid yellow residue was dissolved in
chloroform. The organic layer was washed with brine solution
(twice) and distilled water (three times) and then dried over
anhydrous Na2SO4. The product was obtained by chromatographic
separation on a SiO2 column using an i-hexane/ethylacetate mixture
(1/2, v/v) as the second fraction. A yellow oily residue was identified
as the product according to NMR analysis in 40% total yield.
The calculated FNR
/FNO ratio of quantum yields of reduced and
oxidized forms of piperidine amine moiety reports the extent of
intramolecular quenching or energy transfer from fluorophore to
paramagnetic center.
1H NMR (CDCl3)
d(ppm): 0.96 (t, 3H, CH3eCH2eCHe); 1.21e1.27
(m, 2 ꢁ 3H, CH3eCHe þ CH3eCH2eN); 1.54e1.64 (d-qui, 2H,
CH3eCH2eCHe); 3.45 (q, 4H, 2 ꢁ CH3eCH2eNe); 4.07e4.15 (m,1H,
CH3eCH2eCHe); 6.52 (s, 1H, coumarin); 6.63 (dd, 1H, coumarin);
7.44 (d, 1H, coumarin); 8.64 (d, 1H, eNHeCHe); 8.71 (s, 1H, >C]
CHemethylene).
The fluorescence lifetime measurements were performed on
a LIF 200 (Lasertechnik Ltd., Berlin, F.R.G.), which operates as
a stroboscope. The excitation source is a nitrogen laser emitting at
337 nm, and the emission is selected by a cut-off filter. The output
signal of the boxcar integrator was digitized and transferred to
the PC using a homemade program. The fluorescence decay curves
were evaluated by a simple phase plane method [40] using
J. Snyder’s program based on [41]. The standard deviation
G1/2 ¼ S((Iexp ꢂ Icalc)2/n)1/2, where Iexp and Icalc are the experimental
and calculated emission intensity, respectively, is used to judge
whether the decay is mono-exponential. The decay curve is
assumed to be mono-exponential when G1/2 is less than 5%. The
fitting of the fluorescence decay curves for a model of bi-expo-
nential decay was performed using the adapted FluoFit MatLab
package [42].
13C NMR (CDCl3)
d(ppm): 10.4 (CH3eCH2eCHe); 12.4
(CH3eCH2eN); 20.4 (CH3eCHe); 29.7 (CH3eCH2eCHe); 45.0
(CH3eCH2e); 46.8 (CH3eCH2eCHe); 96.6 (CH-8 coumarin); 108.5
(C-10 coumarin); 109.9 (C-5 coumarin); 110.7 (C-3 lactone); 131.0
(eCH]methylene); 148.0 (CH-6 coumarin); 152.4 (C-9 coumarin);
157.6 (C-7 coumarin); 162.6 (C]O lactone); 162.9 (C]O amide).
FTIR (KBr)
n
(cmꢂ1): 3356, 2967, 1686, 1640, 1608, 1584, 1577,
1507, 1456, 1420, 1353, 1260, 1232, 1192, 1136, 1075, 819, 799, 684.
Anal. Calcd. for C18H24N2O3: C68.33, H7.65, N8.85. Found:
C68.55, H7.55, N8.90.
Polymer films doped with dyes were prepared by casting from
solution. Films of polystyrene (PS) (Chemische Werke Huels, F.R.G.)
and poly(methyl methacrylate) (PMMA) (Diacon, ICI, England) were
prepared by the casting of 1 ml of chloroform solution of the poly-
mer (5 g/100 ml) containing the desired amount of dye on a glass
plate (28 ꢁ 35 mm). The solvent was evaporated slowly. Films of poly
(vinylchloride) (PVC) (Neralit, Spolana Neratovice s.e., CR) were
prepared in a similar way by casting from tetrahydrofuran solution.
The steady-state and time-resolved fluorescence measurements
of solutions and polymer films were performed in the presence of
air at 1 ꢁ 10ꢂ5 mol Lꢂ1 concentrations in liquid media and at
0.002 mol kgꢂ1 in polymer matrices.
3. Results and discussion
The dye concentration in the polymer films was 0.002 mol kgꢂ1
.
3.1. Synthesis of dyes
2.2. Spectral measurements
7-(N,N-diethylamino)coumarin-3-carboxylic acid
2
was
synthesized by the Knoevenagel condensation of 3-(N,N-dieth-
ylamino)salicylaldehyde with Meldrum’s acid, catalyzed with
piperidinium acetate in ethanol, according to a known proce-
dure [37].
1H NMR spectra were recorded in solution on a Bruker AC-300P
(300.1 MHz) spectrometer, with the TMS proton signal as an
internal standard. FTIR spectra were measured on Nicolet 8700

134
M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
Esterification or amide formation of acids with directly con-
3.2. Spectral characterization
nected carboxyl groups containing conjugated or aromatic
carbohydrates or rings with 4-hydroxy- or 4-amino-2,2,6,6-
tetramethylpiperidine derivatives is often problematic due to
the lower reactivity of piperidine derivatives. Aromatic acids are
also known to have low reactivity, especially for the substitution
of aromatic rings with electron-donating substituents. Kunish-
ima et al. [33,34] recently published the synthesis, stability
studies and utilization of a triazine-adduct-based ring with N-
methylmorpholine, originally for low-molecular weight deriva-
tives. The reaction of acids with amines or alcohols mediated
with DMT-MM (7) can be performed in commercial, non-dried,
tetrahydrofuran solution or directly in water. DMT-MM displays
high solubility in water and alcohols without any detectable
decomposition. The triazine-based activation of carboxyls is an
example of a non-carbodiimide mediator. Currently carbodii-
mide mediators are often used for such reactions. One of the
reported experimental disadvantages of carbodiimides, apart
from the high cost, is the contamination of product with stable
N-acylurea groups or, in the case of 1,3-dicyclohexyl-
carbodiimide (DCC), the small dissolution of urea and the
necessity of dry conditions. On the other hand, triazine-based
mediators are frequently used for the activation of the carboxyls
of such biopolymers as enzymes, proteins [43] or poly-
saccharides [35] and synthetic polymers [44].
In the present work, we used 7 for condensation of the
coumarin-3-carboxylic acid derivatives 1 and 2 with parent 4-
amino-2,2,6,6-tetramethylpiperidine 4, N-oxyl 5 and N-alkoxy-
amine 6 (Schemes 1 and 2). For 1b, the reaction gave a high product
yield. In the cases of 1a and 2a, products were achieved in the form
of the hydrochloride salt of piperidine because of the higher
basicity of the TMP ring compared to N-methylmorpholine. Though
crude hydrochloride was achieved from the reaction in a high yield
(w70%), isolation of the free amine was rather inefficient. In the
synthesis of 1c, the reaction derivative conversion that followed on
TLC chromatography was low, and the reaction required increased
reaction temperature. However, the yields of pure 1c and 2c were
lower than 30%. Usually, N-alkoxyamine can be prepared in a two-
step synthesis by the radical reaction of N-oxyl with vinyl (styrene)
derivatives and subsequent reduction with NaBH4, as was
mentioned here for BOC-NOR and is described in [13,30]. Here, less
than 5% yield was obtained for 1c, together with different side-
products, which, according to NMR analysis, did not contain
coumarin moieties. As is shown in the Experimental section, such
problems were not observed for BOC-protected 4-amino-tetrame-
thylpiperidine-N-oxyl (Scheme 3).
3.2.1. Dyes based on coumarin; spectral data in solution and in
polymer films
Coumarins 1aec exhibit absorption bands with fine resolved
structures in non-polar cyclohexane at 287, 321 and 329 nm,
a shoulder at 295 nm and bands in chloroform and polar methanol
centered at 295 and 329 (330) nm without distinguished band at
287 and 321 nm (Table 1). The main emission maxima appear at
307e308 nm in cyclohexane for this type of fluorophore. Emission
in chloroform and methanol is redshifted by about 90e100 nm. This
shift is quantitatively expressed in large Stokes shift values of over
9500 cmꢂ1 in polar methanol and over 9000 cmꢂ1 in chloroform in
comparison to that in non-polar cyclohexane (about 2300 cmꢂ1).
Very low fluorescence intensity, expressed as quantum yield was
observed for all coumarin dyes 1aec in cyclohexane. In MeOH and
chloroform, the quantum yields are slightly higher but still low
(Table 1). As was reported previously [45] coumarine derivatives
substituted by electron-donating groups in position 3 form intra-
molecular charge-transfer (ICT) or twisted ICT state (TICT). Such
dipolar arrangement of molecule with partial positive charge on
piperidine amide substituent is effective radiationless route
competing with fluorescence. Some comparison with our deriva-
tives 1aec with carboxyamide group in position 3 could be only
speculation. However, large Stokes shift in polar protic methanol is
probably a consequence of stabilization of TICT state in this solvent.
Bognar et al. [20] reported similar weak fluorescence for this type of
fluorophore in polar and aprotic acetonitrile. The intramolecular
electron quenching of fluorescence by N-oxyl (ratio FNR/FNO) is
practically impossible to determine in cyclohexane due to very low
fluorescence intensity; in MeOH and CHCl3, these values lie in the
range of 3.0e5.0.
All relevant spectral data for the coumarin-type probe in poly-
mer matrices are given in Table 4. In the absorption spectra of these
dyes (parent amine 1a, N-oxyl 1b and N-alkoxy 1c), there is a broad
absorption band at 284e291 nm in PMMA and PS and at
296e299 nm in PVC, with a fairly distinct band or shoulder at
around 325e331 nm. This absorption band is not sensitive to the
environment. There is no distinct shift in the position of this band
or its overall features between the non-polar PS matrix and the
more polar PMMA or chlorinated PVC matrices (Fig. 1).
The fluorescence of these dyes is redshifted to 390e400 nm,
exhibiting a broad band with a small but distinguished redshifted
band at 465 nm, which is visible mainly for 1b. The Stokes shift is
rather large (around 5000 cmꢂ1). The overall fluorescence of the
dyes referenced with anthracene under the same conditions is
Table 1
Spectral characteristics of probes based on coumarin and HAS (1aec) in solutions at concentration of probes 1 ꢁ 10ꢂ5 mol Lꢂ1
.
Dnc cmꢂ1
Fd
FNR/FNO
a
e
Dye
Medium
lA (log
3) nm
lFb nm (relative intensity)
1a
1b
1c
cyclohexane
287 (4.09), 295sh, 321, 329
287 (4.14), 295sh, 321, 329
287 (4.1), 297sh, 321, 328
307(1), 327(0.55), 368(0.37)
308 (1), 334(0.32), 374(0.32)
308 (1), 332(0.36), 370(0.41)
2331
2315
2376
0.0008
0.001
0.001
w1.0
e
1.0
1a
1b
1c
MeOH
CHCl3
295 (4.13), 329
295 (4.17), 330
295 (4.07), 330
393sh, 414
394sh, 409
393sh, 412
9744
9564
9742
0.02
0.004
0.015
5.0
e
3.8
1a
1b
1c
295 (4.19), 329
295 (4.17) 330
295 (4.18), 330
363sh, 386(1), 399(0.99)
363sh, 385(1), 404(0.99)
363sh, 385(1), 404(0.99)
8836
9146
9146
0.0056
0.0017
0.0074
3.3
e
4.4
a
Maxima of the absorption wavelength bands with intensity of the highest wavelength band expressed as log
Maxima of the fluorescence (lexc ¼ 287 nm in cyclohexane, lexc ¼ 295 nm in MeOH and CHCl3).
Stokes shift.
Quantum yield of fluorescence based on anthracene.
Extent of intramolecular quenching.
3.
b
c
d
e

M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
135
Table 2
Main spectral parameters of dyes based on diethylamino-coumarin and HAS (2aed)
in solutions at concentration of probes 1 ꢁ 10ꢂ5 mol Lꢂ1
.
Mediuma
Dnb cmꢂ1
Fc
FNR/FNO
se ns
d
Dye
2d
Cy
1620
0.19
e
2.2
2a
2b
2c
2d
MeOH
DEG
2397
2305
2361
2260
0.009
0.009
0.0085
0.018
1
e
1
e
1.7
2a
2b
2c
2d
2152
2207
2207
2283
0.26
0.38
0.80
0.29
0.68
e
e
e
e
e
2.1
e
2a
2b
2c
2d
AcCN
CHCl3
2416
2404
2416
2427
0.14
0.064
0.15
0.15
2.2
e
e
e
e
e
2.3
e
2a
2b
1602
1587
0.41
0.062
6.6
e
2.6
Fig. 1. Absorption and fluorescence spectra of coumarin-3-carboxy-(2,2,6,6-tetrame-
thylpiperidine-4-yl)amide (1a) in PVC and in PMMA matrices at 0.002 mol kgꢂ1
(excitation wavelength 330 nm).
7.6 ps (95%)
2.1 ns (5%)
2.5
2c
2d
1502
1509
0.39
0.82
6.3
e
2.7
a
Solvent: Cy-cyclohexane, CHCl3-chloroform, DEG-diethylene glycol, AcCN-ace-
tonitril, MeOH-methanol.
rather weak, as in liquid media. The absolute quantum yields lie in
b
Stokes shift.
the range of 0.003e0.05 in all polymer media. The ratio FNR
reporting the extent of intramolecular quenching is in the range of
w0.64e2.3 and is rather low. Calculated ratio FNR FNO with value
/FNO
c
Quantum yield of fluorescence based on anthracene quantum yields of
anthracene in DEG and AcCN were assumed as in methanol (0.2).
d
/
Extent of intramolecular quenching.
Lifetime.
e
below 1 for 1a/1b in PMMA matrix is due to low intensity of fluo-
rescence rather than some physical phenomenon. In the polymer
matrices, this ratio has a large error, as is demonstrated by the
spectral data obtained in polar aprotic acetonitrile solution shows
slightly different spectral behavior. Quantum yields with more than
ten-fold enhancement are observed for all bifunctional dyes and for
model 2d in AcCN. The electron-donating group is one of the
components of the entire fluorophore, which could belong to the
fluorophore group with a twisted intramolecular charge transfer
(TICT) mechanism [46]. Generally, polarity-induced modulation of
non-radiative rates (rotation) in polar solvents leads to a decrease
of fluorescence intensity. However, the obtained data indicate that
hydrogen bonds are eventually responsible for the interaction of
the excited carbonyl with not only the amide groups in dyes 2aec
but also the carbonyl of the coumarin lactone ring. The nitrogen
atom in the pendant groups may also play some role. To confirm
this observation, the spectral characteristics of dyes 2aed in
solvents like toluene and ethylacetate were observed. Data are
listed in Table 3. The LipperteMataga plot of the fluorescence-
rather wide range of values for the ratio FNR/FNO, which is 3.5e10
in PS matrix and is listed in Table 4. Therefore, the application of
this type of dye for monitoring or imaging radical processes in the
solid state and in solutions is rather limited.
3.2.2. Dyes based on diethylamino-coumarin; spectral data in
solution and in polymer films
The spectral properties of the coumarin fluorophore with an
electron-donating diethylamino group (2aed) follow a similar
trend as the unsubstituted coumarin 1aec in terms of used media/
matrix but with one significant feature (Tables 2 and 3). The fluo-
rescence intensity is much higher and is accompanied by higher
intramolecular quenching, mainly in chloroform. The solubility of
tetramethylpiperidine-based dyes in cyclohexane is limited, so only
the spectral properties of the alkylamide 2d model dye are
presented.
maxima wavenumber dependency on the solvent parameter Df was
The absorption bands are redshifted to maxima at 417e420 nm
in all liquid media except 2d in cyclohexane, which exhibits three
bands at 381, 392 and 402 nm, indicating some vibrational struc-
ture. The emission spectra of the parent amine 2a, n-oxyl 2b, and
n-alkoxy amine 2c exhibit broad bands in range of 446e450 nm in
chloroform and 465e467 nm in MeOH (Fig. 2). Comparison with
model 2d shows the slight influence of the tetramethylpiperidine
moiety on the spectral properties of the coumarin fluorophore
because the absorption and emission maxima of this dye are shifted
by 1e4 nm to lower wavelengths in some solvents. Stokes shifts
derived from absorption and emission parameters for these type of
dyes are much lower, about 1500 cmꢂ1 in chloroform and
2200e2400 cmꢂ1 in polar solvents for 1aec (Table 3). Substantially
higher quantum yields of such dyes were calculated in chloroform.
The intramolecular quenching values in this solvent were more
efficient (about 6e7). However, these dyes (2aec) exhibit low
fluorescence quantum yield and low or negligible intramolecular
quenching in polar methanol. This behavior indicates the strong
interaction of the protic polar solvent with the electronic excited
state of diethylamino-coumarin fluorophore. Comparison with
constructed (Fig. 3). Solvent parameters were calculated according
to the following equation:
3
ꢂ 1
n2 ꢂ 1
D
f ¼
ꢂ
2n2 þ 1
2
3
þ 1
where 3 is the dielectric permittivity, and n is the solvent refractive
index [47,48]. The LipperteMataga model is useful for fluorescent
dyes with an intramolecular charge-transfer states that depend on
environment polarity. This simple model is valid for many solvents,
with the exception of protic solvents with specific interactions like
hydrogen-bonding [48]. The diethylamino-3-carboxy-coumarin
fluorophore in dye 2d does not show strong dependence on solvent
polarity because the slope of the linear regression is relatively small
in comparison with other typical ICT naphthalene-based dyes like
6-propanoyl-2-(N,N-dimethylamino)naphthalene (PRODAN) or
6-anilino-2-naphthalene sulfonic acid (ANS) [48,49] (Fig. 3B).
According to this assumption, protic polar solvents MeOH and DEG
should not fit this equation, and points for MeOH and DEG solvents
can be omitted from the linear dependency of other solvents. This

136
M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
Table 3
Absorption, emission maxima and calculated Stokes shifts of 2aed dyes in different liquid media at concentration of dyes 1 ꢁ 10ꢂ5 mol Lꢂ1
.
Mediuma
(Df)
Parameter
Dye
2a
2b
2c
2d
b
Cy (0.001)
lA [nm] (log
3)
3)
3)
3)
3)
3)
3)
e
e
e
e
e
e
e
e
e
381,392,402 (6.59)
409, 430
1620
lFc [nm]
Dnd [cmꢂ1
]
b
Toluene (0.013)
CHCl3 (0.153)
EtAc (0.201)
DEG (0.266)
lA [nm] (log
lFc [nm]
399sh, 411 (4.60)
434
1289
398sh, 414 (4.68)
439
1376
397sh, 412 (4.58)
435
1283
399sh, 410 (4.51)
434
1349
Dnd [cmꢂ1
]
b
lA [nm] (log
lFc [nm]
418 (4.75)
448
1602
420 (4.76)
450
1587
418 (4.74)
446
1502
417 (4.59)
445
1509
Dnd [cmꢂ1
]
b
lA [nm] (log
lFc [nm]
409 (4.54)
446
2028
411 (4.46)
447
1959
410 (4.58)
446
1969
408 (4.52)
445
2038
Dnd [cmꢂ1
]
b
lA [nm] (log
lFc [nm]
426 (4.16)
469
2152
425 (4.02)
469
2207
425 (3.36)
469
2207
422 (4.50)
467
2283
Dnd [cmꢂ1
]
b
AcCN (0.305)
lA [nm] (log
lFc [nm]
414 (4.65)
460
2416
415 (4.57)
461
2404
414 (4.50)
460
2416
413 (4.47)
459
2427
Dnd [cmꢂ1
]
b
MeOH (0.3093)
lA [nm] (log
lFc [nm]
420 (4.72)
467
2397
420 (4.37)
465
2305
419 (4.64)
465
2361
418 (4.52)
461
2260
Dnd [cmꢂ1
]
a
Solvent: Cy-cyclohexane, CHCl3-chloroform, EtAc-ethylacetate, DEG-diethylene glycol, AcCN-acetonitril, MeOH-methanol with calculated parameter Df for construction of
LipperteMataga plot.
b
Maxima of the absorption wavelength bands with intensity of the longest-wavelength band expressed as log
Maxima of the fluorescence (lexc ¼ 420 nm in all media except in cyclohexane).
Stokes shift.
3.
c
d
dependency then fits the LipperteMataga equation. These results
positions of the absorption and emission maxima of these dyes.
However, quantum yields are substantially higher in all polymer
matrices (both polar and non-polar) than in liquid media (Tables 2
and 4).
also explain observed low fluorescence intensity in MeOH. The
behavior of the bifunctional diethylamino-3-carboxy-coumarin
fluorophore linked with a HAS structure in 2a-c dyes is similar to
that of the 2d dyes (Fig. 3A). A slightly lower dependency was
observed for nitroxide 2b than for the parent amine 2a and
alkoxide 2c.
The spectral parameters of 2aed in polymer matrices are listed
in Table 4. The absorption and emission spectra of 2c are shown in
Fig. 4. Different polymer matrices do not significantly influence the
The calculated FNR/FNO values are also higher (7e12) in poly-
mer matrices. This phenomenon is most likely related to the
hindered rotation of diethylamino group in solid matrices. In low-
viscosity liquid media, the rotation of this group is free, which leads
to the dissipation of excitation energy and the formation of a TICT
state. However, in solid polymer matrices, rotations are completely
Table 4
Comparison of the main spectral parameters of fluorophore-HAS dyes 1aec and 2aec in polymer matrices at concentration 0.002 mol kgꢂ1
.
Mediuma
Parameter
Dye
1a
1b
1c
2a
2b
2c
2d
PS
Dnb [cmꢂ1
]
8802
0.012
3.8
8685
0.0032
e
8983
0.032
10
2310
0.474
18.2
2.6
2252
0.026
2177
0.275
10.6
2.4
2158
0.52
e
Fc
d
FNR
/FNO
e
se [ns]
e
e
e
1.1 ps (95%)
1.6 ns (5%)
2.4
PMMA
Dnb [cmꢂ1
]
9175
0.013
1.63
e
8457
0.008
e
9112
0.0064
0.64
e
2647
0.42
7.5
2704
0.056
2695
0.59
10.5
3.0
2221
0.42
e
Fc
d
FNR
/FNO
e
se [ns]
e
3.3
0.1 ps (93%)
2.5 ns (7%)
3.2
PVC
Dnb [cmꢂ1
]
8532
0.048
2.3
8319
0.021
e
8784
0.032
1.5
2656
0.44
12.6
2.8
2656
0.035
2578
0.304
8.6
2787
0.83
e
Fc
d
FNR
/FNO
e
se [ns]
e
e
e
9 ps (83%)
1.5 ns (17%)
2.8
3.3
a
Matrix: PS-polystyrene, PMMA-poly(methyl methacrylate), PVC-poly(vinyl chloride).
Stokes shift.
Quantum yield of fluorescence based on anthracene.
Extent of intramolecular quenching.
Lifetime.
b
c
d
e

M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
137
Fig. 2. Comparison of fluorescence spectra of 7-(N,N-diethylamino)coumarin-3-car-
boxy-(1-(10-phenylethyl)oxy-2,2,6,6-tetramethylpiperidine-4-yl)amide (2c) in CHCl3
and MeOH solutions at 1 ꢁ 10ꢂ5 mol Lꢂ1 (excitation wavelength 420 nm in both
media).
Fig. 4. Absorption and fluorescence spectra of 7-(N,N-diethylamino)coumarin-3-car-
boxy-(1-oxo-2,2,6,6-tetramethylpiperidine-4-yl)amide (2b) in PS and PMMA polymer
matrices at concentration 0.002 mol kg1 (excitation wavelength 420 nm in both
media).
hindered. Between these two extremes lie spectral properties in
more viscous protic polar diethylene glycol solution (Tables 2 and
4), in which the quantum yield is comparable with values
obtained for polymer matrices. These results also suggest that the
effect of the hindered rotation of the diethylamino group in DEG is
more important for the spectral properties of diethylamino-
coumarin fluorophores than the hydrogen-bonding interaction of
protic polar solvents.
A
Concerning the easy conversion of the parent amine of the
tetramethylpiperidine moiety to its N-oxy radical in the presence of
oxygen and the reactivity of this radical with other radical species
in photooxidation or thermooxidation processes, these types of
bifunctional probes (2aec adducts) are applicable as a radical
sensor in polymer matrices. However, the 7-diethylamino-3-car-
boxy coumarin fluorophore can be used alone as a polarity probe in
liquid media.
The lifetimes of the excited states of both coumarin-based dye
series were short. The lifetimes of dyes 2aed lie in range of 2e3 ns
in chloroform and polymer matrices and are charged with high
error (Tables 2 and 4). Our LIF 200 instrument for lifetime
measurements is not appropriate to measure short lifetimes below
5 ns. The fluorescence decay profiles of nitroxide 2b were reason-
B
ably fitted by
a bi-exponential function. This mathematical
expression gives two components with very short lifetimes, in
range of picoseconds, with a factor higher than 90% and longer
lifetimes in range of nanoseconds. Time-resolved fluorescence
measurements in methanol were not successful using our instru-
ment set-up due to the low fluorescence intensity and the solvent
interaction with the fluorophore excited state mentioned above.
The same problem occurred with the lifetimes of 1aec dyes as
a consequence of its low fluorescence intensity. Fluorescence life-
times are too short to be quenched by oxygen in diffusion-
controlled process.
4. Conclusions
There is a constant search for new, efficient probes that have
simple structures, are easy to prepare and exhibit interesting
spectral properties distinctly for at least two different spectroscopic
states (shining or quenched) for use in monitoring radical oxidation
and/or reduction and other processes. Therefore, in this article, we
described the detailed synthesis and spectral characterization of
Fig. 3. LipperteMataga plots of fluorescence wavenumber maxima of 7-(N,N-dieth-
ylamino)coumarin-3-carboxy-tetramethylpiperidine derivatives 2aec (A) (2a-rings,
linear fit-dashed line; 2b-triangles, linear fit-dotted line; 2c-squers, linear fit-solid line)
and model amide 2d (B) for different solutions. (Solvents and its
Df: cyclohexane (0),
toluene (0.013), chloroform (0.153), ethylacetate (0.201), diethylene glycol (0.266),
acetonitril (0.305), methanol (0.309)).

138
M. Danko et al. / Dyes and Pigments 90 (2011) 129e138
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The authors are greatly acknowledged to Grant agency VEGA for
financial support through the project 2/0097/09 and Slovak
Research and Development Agency through project APVV-0562-07.
For NMR measurements provided by the Slovak State Program
Project No. 2003SP200280203 are gratefully acknowledged as well.
This publication is the result of the project implementation: Center
for materials, layers and systems for applications and chemical
processes under extreme conditions Stage II supported by the
Research & Development Operational Programme funded by the
ERDF.
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