Preparation and spectral characterization of fluorescence probes based on 4-N,N-dimethylamino benzoic acid and sterically hindered amines

Article abstract of DOI:10.1007/s10895-012-1076-7

The adducts of simple chromophore 4-N,N-dimethylamino benzoic acid with 2,2,6,6-tetrametyl-4-hydroxyor 4-amino-piperidine were examined as fluorescence probes (spin double sensors) to monitor radical processes. The links in the adducts were either an ester or amide group, and the sterically hindered amines were in the form of -NH, -NO? and -NOR. The spectral properties of the three related derivatives (esters or amides) were quite similar. The maxima of the absorption spectra were in the range of 295-315 nm, and the maximum of fluorescence was located in the range of 330-360 nm, depending on the polarity of the solvent. In polar solvents, a red-shifted fluorescence band at 460-475 nm was observed. The fluorescence of these derivatives was rather weak as compared to anthracene under the same conditions. The Stokes shift was large, as high as 6,000 cm^-1, indicating the formation of a twisted intramolecular charge transfer (TICT) state. No large differences in Stokes shifts were observed in polymer matrices of poly (methyl methacrylate), polystyrene and poly(vinyl chloride). The extent of intramolecular quenching was expressed as φ_NX/φ_NO (X=H, NOR) and was in the range of 1-3 in solution and as high as 8 in polymer matrices. The low efficiency of intramolecular quenching limits the application of these new adducts as fluorescence probes for the monitoring of radical processes in solution but favors their application in polymer matrices. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s10895-012-1076-7

J Fluoresc (2012) 22:1371–1381
DOI 10.1007/s10895-012-1076-7
ORIGINAL PAPER
Preparation and Spectral Characterization of Fluorescence
Probes Based on 4-N,N-Dimethylamino Benzoic Acid
and Sterically Hindered Amines
Csaba Kósa & Martin Danko & Pavol Hrdlovič
Received: 29 November 2011 /Accepted: 11 June 2012 /Published online: 24 June 2012
#
Springer Science+Business Media, LLC 2012
.
Keyword Fluorescence probes 4-N,N-
Abstract The adducts of simple chromophore 4-N,N-dime-
thylamino benzoic acid with 2,2,6,6-tetrametyl-4-hydroxy-
or 4-amino-piperidine were examined as fluorescence
probes (spin double sensors) to monitor radical processes.
The links in the adducts were either an ester or amide group,
and the sterically hindered amines were in the form of -NH,
-NO• and –NOR. The spectral properties of the three related
derivatives (esters or amides) were quite similar. The max-
ima of the absorption spectra were in the range of 295–
315 nm, and the maximum of fluorescence was located in
the range of 330–360 nm, depending on the polarity of the
solvent. In polar solvents, a red-shifted fluorescence band at
460–475 nm was observed. The fluorescence of these deriv-
atives was rather weak as compared to anthracene under the
same conditions. The Stokes shift was large, as high as
6,000 cm−1, indicating the formation of a twisted intra-
molecular charge transfer (TICT) state. No large differences
in Stokes shifts were observed in polymer matrices of poly
(methyl methacrylate), polystyrene and poly(vinyl chloride).
The extent of intramolecular quenching was expressed as
.
.
dimethylaminobenzoates Intramolecular quenching
1-R-2,2,6,6-tetramethyl-4-hydroxy -4-aminopiperidine
Introduction
Various fluorescence probes with different structures are
used to monitor processes in chemistry and especially biol-
ogy, where monitoring is required in different media, in-
cluding micelles, membranes, solid amorphous matrices and
other complex mixtures [1–5]. The spectral parameter of
choice for monitoring is fluorescence, because it exhibits a
strong dependence on medium. The advantages of fluores-
cence include high sensitivity, simple detection, easy quan-
titative evaluation of the selected chromophore and a
distinct medium effect. Simple chromophores like substitut-
ed aromatic hydrocarbons are widely used for this purpose.
The simplest chromophore of this type is a molecule
having strong electron donor-acceptor substituents. A typi-
cal example is a benzene ring substituted with a strong
electron donating group like dimethylamino, and opposite
to this group, an electron withdrawing group such as a
nitrile, carbonyl, carboxyl or ester group. A typical repre-
sentative of this class of compounds is 4-(N,N-dimethyla-
mino)benzonitrile. Often, these compounds exhibit dual
emission – normal (mirror-like fluorescence to the absorp-
tion) or local (LE) and red-shifted fluorescence due to
intramolecular charge transfer (ICT) or twisted intramolec-
ular charge transfer fluorescence (TICT). This type of excit-
ed state is non-emissive but photochemically active.
Φ
NX/ΦNO (X0H, NOR) and was in the range of 1–3 in
solution and as high as 8 in polymer matrices. The low
efficiency of intramolecular quenching limits the application
of these new adducts as fluorescence probes for the moni-
toring of radical processes in solution but favors their appli-
cation in polymer matrices.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-012-1076-7) contains supplementary material,
which is available to authorized users.
:
:
C. Kósa (*) M. Danko P. Hrdlovič
Polymer Institute, Center of excellence GLYCOMED,
Slovak Academy of Sciences,
Since the late 1950s, considerable attention has been
given to the photophysical properties of derivatives of dime-
thylamino benzoic acid and dimethylaminobenzonitrile, due
to appearance of unusual dual fluorescence in polar solvents
Dubravska cesta 9,
845 41 Bratislava 45, Slovakia
e-mail: upolkosa@savba.sk

1372
J Fluoresc (2012) 22:1371–1381
[6]. These studies have focused on the influence of the
electron donor or electron acceptor substituents as well as
the polarity of the medium and concentration of the
chromophore.
was observed for a series of p-N,N-dimethylaminobenzoates
with alkyl substituents of various lengths. The fluorescence
intensity ratio of the CT band to the local band was decreased
with increasing carbon number in the range from 1 to 3 and
then slightly increased with longer alkyl chains. Conversely,
this ratio for longer alkyl substituents is increased in ionic
micelles [19].
Since the introduction of the TICT state by Grabowski
and Rettig [7, 8] there have been many studies with the aim
of confirming or disproving this phenomenon, especially
with compounds having a benzene ring with electron donat-
ing and withdrawing groups. A spectral study of methyl-4-
(N,N-dimethylaminobenzoate) in a variety solvents revealed
that normal fluorescence is observed in most solvents, but
the red-shifted fluorescence in acetonitrile has been ascribed
to an excited dimer [9]. Even more complex is the spectral
behavior of aminosalicylates like methyl-2-hydroxy-4-
dimethylaminobenzoate, which exhibits three competitive
emissions, including normal fluorescence, TICT state fluo-
rescence and intramolecular proton-transfer fluorescence
[10]. Extensive studies on jet-cooled aniline derivatives
confirmed the existence of a monomolecular mechanism
involving the adiabatic formation of a fluorescent TICT state
induced by water solvent molecules and a mechanism in-
volving self-cluster formation that induces excimer-like be-
havior [11]. A study confirmed that, in the presence of
colloidal SiO2, the formation of an excited TICT state is
enhanced by hydrogen bonding between the carboxylic
group of dimethylamino benzoic acid (DMABA) and col-
loidal SiO2 [12]. A similar enhancement of fluorescence of
DMABA was also observed with DMABA absorbed on
NaY zeolites due to the formation of hydrogen bonding
[13]. Systematic investigation of the CT dual fluorescence
of sodium p-dialkylaminobenzoates in ionic micelles dem-
onstrated that the CT process is affected by the electric field
at the ionic micelle-water interface and that the CT emission
is enhanced at higher electric field [14]. Contrary to its ICT
emission, that of DMABA at the sodium bis(2-ethylhexyl)
sulfosuccinate (AOT) reverse micelle-water pool interface is
much weaker [15]. The same probe, DMABA, exhibited
concentration-dependent dual fluorescence in chloroform.
It was concluded that the ICT process and related ICT
fluorescence are facilitated by intramolecular H-bonding
mediated by excited state proton transfer and that this effect
is weaker than that of solvent polarity [16]. For series of
substituted phenyl p-dimethylaminobenzoates, it was ob-
served that the CT emission in the same solvent shifts to
lower energy (red-shift) with increasing electron-withdrawing
ability of the substituent on the phenyl ring, whereas the
locally excited (LE) shows no change. This shift was de-
creased in polar solvents [17]. When the dimethylamino sub-
stituent was changed to phenylamino, the ICT character of the
emissive state of 4-(N-phenylamino) benzoic acid showed a
single band emission with a sluggish response to solvent
polarity. The ICT state occurring in this probe is due to the
N-phenyl/amino conjugation effect [18]. CT dual fluorescence
Stable, free aminoxyl radicals are widely utilized in var-
ious fields of chemical and biochemical science, mainly as
probes to monitor radical reactions [20–25]. An important
role for nitroxide free radicals in biological systems is spin
labeling for the exploration of protein structure, dynamics
and function [26–28]. Another use employs their ability to
easily react with free radicals and significant stability in
biological environments to follow oxidative damage in bio-
logical systems. In this case, reaction of nitroxide with
radicals leads to the formation of diamagnetic N-hydroxyl-
amines or N-alkoxyamines, which exhibit increased fluores-
cence if the reduced species exhibits fluorescence. In
another method, a fluorophore is attached to a nitrone used
for spin trapping [29]. In polymer chemistry, stable ami-
noxyl radicals are used as mediating radicals in radical
polymerization, which provides control of the molecular
weight and molecular weight distribution (polydispersity)
of the polymer as well as the possibility of preparing
designed polymers [30–34]. Moreover, the adducts of ami-
noxyls with fluorescence probes can provide a deeper un-
derstanding of radical polymerizations by using sensitive
fluorescence spectroscopy [35].
Over the past decades, sterically hindered amines (HAS),
especially derivatives of 2,2,6,6-tetramethylpiperidine, have
become the most effective catalysts and stabilizers, especial-
ly in the preparation of polyolefins [36–38]. Their stabiliza-
tion action is based on the formation and regeneration of N-
oxyl radicals.
A significant amount of attention is often directed at the
initial stage (the so-called induction period) of polymer
photodegradation. In these studies, fluorescence spectrosco-
py was utilized in thermooxidative studies of polypropylene
[39, 40]. Fluorophores attached to HAS in the form of
fluorophore-spacer-receptor were used as fluorescence
probes to exploit the changes of probe fluorescence intensity
during the polymer degradation process. To find the ideal
system, various fluorophore-spacer-HAS systems were syn-
thesized and spectrally characterized [23–25, 41–46].
In this paper, we used the simple chromophore 4-
dimethylaminobenzoic acid (DMABA) and prepared
corresponding esters and amides with 4-amino- and 4-
hydroxy-2,2,6,6-tetramethyl piperidine, where the sterically
hindered centers were in the form of the parent amine, N-
oxyl or N-alkoxy group (Scheme 1). Detailed spectral char-
acterization was performed in both solution and polymer
matrices.

J Fluoresc (2012) 22:1371–1381
1373
R
R
N
O
C
O
-H
1
O
Cl
R
N
N
N
C
N
2
N R
-O
pyridine
+
3
O
X
o
C
N
50, C 6h
r.t., 18h
O
XH
O
C
NH
-H
-
4
5
N R
6
2
4
5
X = -O;
X = -NH; R = -H
X = -NH; R =
R = -O
O
-
Scheme 1 Structure of the studied compounds
Scheme 3 Synthesis of derivatives 2, 4 and 5
Experimental
1-(2,2,6,6-Tetramethyl Piperidine-4-yl)-4-N,N-
Dimethylaminobenzoate (1) (Scheme 2)
Synthesis
The synthesis of 1 was performed through the catalytic
reaction of methyl-4-N,N-dimethylamino benzoate (1 g,
5.6 mmol) with 2,2,6,6-tetramethyl-4-hydroxypiperidine
(TEMPOL) (0.704 g, 5.6 mmol) in dry toluene over
20 h at reflux with a liquid-liquid separator (a decanter)
using tetrabutyl orthotitanate (TBOT) as the catalyst.
The solvent was evaporated on a rotary evaporator,
and the solid residue was separated on a silica column
using ethyl acetate, methylene chloride and methanol as
the eluents. Brownish crystals of 1 with a melting point
of 144–145 °C were obtained from the elution solution
in 26 % yield (0.46 g). The product was analyzed by
HPLC-MS, FTIR and NMR spectroscopy.
The synthesis of the ester derivatives shown in Scheme 1
was performed according to Schemes 2, 3 and 4. To prepare
the required compounds, methyl 4-N,N-dimethylamino ben-
zoate and 4-N,N-dimethylamino benzoic acid chloride were
synthesized. The NMR chemical shifts and FTIR absorption
bands of the prepared derivatives are summarized in the
Supplementary material.
Methyl 4-N,N-Dimethylamino Benzoate
The methyl ester was prepared by refluxing 4-N,N-dimethy-
lamino benzoic acid in methanol in the presence of sulfuric
acid as a catalyst. Molecular sieves were added to absorb the
water by-product and shift the reaction toward the formation
of the methyl ester. The crude methyl ester was obtained
after evaporation of the solvent following neutralization of
the acid with sodium carbonate. The pure product was
crystallized from ethanol.
1-(1-Oxy-2,2,6,6-Tetramethyl Piperidine-4-yl)-4-N,N-
Dimethylamino Benzoate (2)
The synthesis of 2 was performed according to the meth-
od described by Vamecq et al. [49] by dropwise addition
of a solution of acyl chloride in dry pyridine (20 ml) to a
4-N,N-Dimethylamino Benzoic Acid Chloride
4-N,N-dimethylamino benzoic acid chloride was synthe-
sized from 4-N,N-dimethylamino benzoic acid by reaction
with oxalyl chloride [47] or SOCl2 [48] and was used
without further purification.
O
N
O
N
anisole
PMDETA
O
X
C
N
Br
H
N
+
CH3
Cu, CuBr
80 oC, 8h
Ar atm.
O
O
X
HO
NH
COOH
N
C
N
O C
O
C
O
MeOH
H2SO4
reflux
TBOT
toluene
reflux 20h
N
3
6
X = -O
X = -NH
N
Scheme 2 Synthesis of derivative 1
Scheme 4 Synthesis of derivatives 3 and 6

1374
J Fluoresc (2012) 22:1371–1381
solution of 1-oxy-2,2,6,6-tetramethyl 4-hydroxypiperidine
(3 g; 0.018 mol) in dry THF (20 ml) and dry pyridine
(15 ml). The resulting mixture was stirred under argon
atmosphere at 50 °C for 6 h and at room temperature for
18 h. The solvent was evaporated, and the solid residue
was separated on a silica column using methylene chlo-
ride and a mixture of ethyl acetate and i-hexane as elu-
ents. Orange crystals of 2 with a melting point of 134–
137 °C were obtained from the elution solution in 23 %
yield (1.3 g). The product was analyzed by HPLC-MS,
1-(2,2,6,6-Tetramethyl Piperidine-4-yl)-4-N,N-
Dimethylamino Benzamide (4). (Scheme 3 )
The synthesis of 4 was performed according to Vamecq et
al. [49] by the dropwise addition of a solution of acyl
chloride in dry pyridine (20 ml) to a solution of 2,2,6,6-
tertamethyl 4-aminopiperidine (1.1 ml, 6.04 mmol) in dry
THF (20 ml) and dry pyridine (15 ml). The reaction mixture
was stirred under argon at 50 °C for 6 h and at room
temperature for 18 h. The reaction mixture was quenched
with 100 ml of 1 M hydrochloric acid (with ice), and the
solvents were evaporated. The resulting yellow viscous
liquid was separated on a silica column using ethyl acetate
and methanol as eluents. The methanol fraction was con-
densed, dissolved in water, neutralized with sodium carbon-
ate solution and extracted with methylene chloride. White
crystals of 4 with a melting point of 59–61 °C were obtained
from methylene chloride in 68 % yield (1.25 g). The product
was analyzed by HPLC-MS, FTIR and NMR spectroscopy.
1
FTIR and H NMR spectroscopy in the presence of pen-
tafluorophenyl hydrazine as well as by ESR (Fig. 1).
1-((1-Phenylethoxy)-2,2,6,6-Tetramethylpiperidine-4-yl)-4-
N,N-Dimethylamino Benzoate (3)
Compound 2 (0.384 g, 1.2 mmol), bromoethyl benzene
(0.148 g, 0.8 mmol), PMDETA (pentamethyldiethylene
triamine) (0.244 g 1.4 mmol) and anisole (15 ml) were
added to a Schlenk flask, which was deareated with three
freeze-pump-thaw cycles and then filled with argon. Next,
CuBr (0.058 g, 0.4 mmol) and Cu (0.076 g, 1.2 mmol)
were quickly added to the reaction mixture, which was
stirred at 80 °C for 18 h. After cooling to room temper-
ature, the volume was reduced, and the residue was sep-
arated on basic alumina (Al2O3) using THF as an eluent
to remove the copper complex. The solvent was evapo-
rated, and the solid residue was separated on a silica
column using gradient elution with i-hexane/ethyl acetate
mixtures, beginning with i-hexane. White crystals of 3
with a melting point of 80–82 °C were obtained in 62 %
yield. The product was analyzed by HPLC-MS, FTIR and
NMR spectroscopy.
1-(1-Oxy-2,2,6,6-Tetramethylpiperidine-4-yl)-4-N,N-
Dimethylamino Benzamide (5)
The synthesis of 5 was performed by the dropwise addition of
a solution of acyl chloride in dry pyridine (10 ml) to a solution
of 1-oxy-2,2,6,6-tetramethyl 4-aminopiperidine in dry THF
(20 ml) and dry pyridine (15 ml). The reaction mixture was
stirred under an argon atmosphere at 50 °C for 6 h and at room
temperature for 18 h. The solvent was evaporated, and the
solid residue was separated on a silica column using gradient
elution with i-hexane/ethyl acetate mixtures, beginning with i-
hexane. Orange crystals of 5 with a melting point of 204–
212 °C were obtained in 21 % yield. The product was ana-
lyzed by HPLC-MS, FTIR and 1H NMR spectroscopy (in the
presence of pentafluorophenyl hydrazine). Additionally, the
ESR spectrum was compared to that of TEMPO (Fig. 1).
1-((1-Phenylethoxy)-2,2,6,6-Tetramethylpiperidine-4-yl)-4-
N,N-Dimethylamino Benzamide (6)
Compound 5 (0.382 g, 1.2 mmol), bromoethyl benzene
(0.148 g, 0.8 mmol), PMDETA (pentamethyldiethylene tria-
mine) (0.244 g 1.4 mmol) and anisole (15 ml) were added to a
Schlenk flask, which was deareated with three freeze-pump-
thaw cycles and filled with argon. Next, CuBr (0.058 g,
0.4 mmol) and Cu (0.076 g, 1.2 mmol) were quickly added
to the reaction mixture, which was stirred at 80 °C for 18 h.
After cooling to room temperature, the volume was reduced
on a rotary evaporator and the residue was separated on basic
alumina (Al2O3) using THF as an eluent to remove the copper
complex. The solvent was evaporated, and the solid residue
was separated on a silica column using gradient elution with i-
hexane/ethyl acetate mixtures, beginning with a 5/1 mixture
Fig. 1 ESR spectra of 2 and 5 compared with TEMPOL in benzene
solution (c010−3 mol L−1
)

J Fluoresc (2012) 22:1371–1381
1375
(v/v). White crystals of 6 with a melting point of 164–167 °C
were obtained in 31 % yield. The product was analyzed by
HPLC-MS, FTIR and NMR spectroscopy.
digitized and transferred to a PC using a home-made program.
The fluorescence decay curves were evaluated by a simple
phase plane method [52] using J. Snyder’s program based on
Ref. [53]. The standard deviation G1/20Σ((Iexp - Icalc)2/n)1/2
,
where Iexp and Icalc are the intensity of the experimental and
calculated emissions, respectively, was used to judge whether
the decay was mono-exponential. The decay curve was as-
sumed to be mono-exponential when G1/2 was less than 5 %.
Alternatively, the fitting of fluorescence decay curves for a bi-
exponential decay model was performed using an adapted
FluoFit MatLab package [54].
Preparation of Polymer Films
Polymer films doped with fluorescence probes were prepared
by casting from solution. Films of polystyrene (PS) (Chemi-
sche Werke Huels, Germany) and poly(methyl methacrylate)
(PMMA) (Diacon, ICI, England) were prepared by casting
0.002 or 0.02 mg of probe in 1 ml of a chloroform solution of
polymer (5 g/100 ml) on a glass plate (25×40 mm). To ensure
slow evaporation of the solvent, the glass plates were covered
with Petri dishes. Films of poly(vinylchloride) (PVC) (Neralit,
Spolana Neratovice s.e., CR) were prepared in a similar fash-
ion by casting from tetrahydrofuran solution (5 g/100 ml). The
concentration of the probe was 0.002 and 0.02 mol kg−1.
The quantum yields in solution and of the doped polymer
films were determined relative to anthracene in the respec-
tive medium according to relation (1) [50]:
Results and Discussion
Synthesis of Probes
Although 4-N,N-dimethylamino benzoic acid is a simple
molecule, its reactivity is strongly influenced by the pres-
ence of the electron donating dimethylamino group. A series
of simple alkyl ester derivatives of 4-N,N-dimethylamino
benzoic acid were prepared by refluxing the acid in the
corresponding alcohol [19]. The substituted phenylamino
benzoates were obtained by reaction of 4-N,N-dimethyla-
mino benzoic acid with substituted phenols in the presence
of POCl3 [17]. For the synthesis of alkanolamine esters or
benzamides, reaction of 4-N,N-dimethylamino benzoyl
chloride with the corresponding alkanolamines or amines
was applied [55].
The synthesis of 2,2,6,6-tetramethyl piperidine (TMP)
ester and amide derivatives was influenced by both the
reactivity of the acid and the reactivity of the 4-hydroxy-
or 4-amino group of the piperidine. Due to the lower reac-
tivity of TMP-type compounds, their synthesis through re-
action of the corresponding 4-hydroxy- or 4-amino
derivative with an ester was not successful. For synthesis
of the ester derivatives (1–3), two possible methods were
examined. In the first method, the synthesis was done
through a re-esterification reaction from the methyl ester
(Scheme 2). This method is relatively well suited for the
synthesis of TMP esters. The 1-oxy-4-hydroxy-TMP
exhibited lower reactivity in the re-esterification reaction
than the parent amine. Therefore, the second method was
employed for the preparation of its ester derivative (2). In
this method, the 4-hydroxy-TMP derivative was reacted
with 4-N,N-dimethylamino benzoyl chloride to give the
corresponding ester (Scheme 3). Similarly, the TMP amide
derivatives (4 and 5) were prepared by the reaction of the 4-
amino-TMP compounds with 4-N,N-dimethylamino benzo-
yl chloride (Scheme 3). In Fig. 1, the ESR spectra of
derivatives 2 and 5 are compared to TEMPOL as a model
compound. The shapes (typical triplet due to interaction of
an electron with a nitrogen atom) of the observed ESR
1
R
!
IFðnÞdn
IFSðnÞdn
S
1 ꢀ 10ꢀA
0
ΦF ¼ ΦSF
;
ð1Þ
1
1 ꢀ 10ꢀA
R
0
where ΦSF is the quantum yield of anthracene, which was
assumed to be 0.25 for a non-polar medium, the integrals
1
1
R
R
IFðnÞdnand
IFSðnÞdnare the areas under the emission
0
0
curves of the probe and standard, respectively, and A and AS
are absorptions of the probe and standard at the excitation
wavelength. Anthracene was excited at λex 355–365 nm,
depending on the medium.
The quantum yield of doped polymer films was determined
with anthracene as the standard in the respective medium
using the quantum yield of anthracene in cyclohexane 0.25
[51]. The quantum yields of anthracene fluorescence in dif-
ferent media, which were determined by comparison with the
fluorescence of anthracene in cyclohexane, were found to be
0.20 in methanol and acetonitrile, and 0.11 in chloroform. In
polymer matrices, the quantum yields were assumed to be
0.20 in PMMA, 0.16 in PS and 0.10 in PVC. The quantum
yields in the solutions and films were corrected to different
absorptions at the excitation wavelength [50]. The fluores-
cence spectra were taken using excitation at the maximum of
the longest wavelength absorption band.
The fluorescence lifetime measurements were performed
on a LIF 200 (Lasertechnik Ltd., Berlin, F.R.G.), which oper-
ates as a stroboscope. The excitation source was a nitrogen
laser emitting at 337 nm, and the emission was selected by a
cut-off filter. The output signal of the boxcar integrator was

1376
J Fluoresc (2012) 22:1371–1381
signals were similar to TEMPOL, indicating that derivatives
2 and 5 possessed the same type of paramagnetic center. The
comparable integral intensities of the signals also confirmed
the purity of these radical derivatives.
The high molar decadic extinction coefficient (>4.1) indicated
a strongly allowed transition that was not significantly influ-
enced by solvent polarity [55]. The absorption maxima of the
amide derivatives (4, 5, 6) in all of the studied media showed a
larger red-shift as compared to the esters (1, 2, 3) (Tables 1 and
2, Figs. 3 and 4, S1a-d and S2a-c). This was due to more
prevalent electron donation and conjugation in the case of the
amides, as observed by Allen et al [55].
Synthesis of the alkoxyamine derivatives (3, 6) was car-
ried out by reaction of the corresponding N-oxy derivative
with bromoethyl benzene in the presence of Cu and CuBr.
Using this method, a 60 % yield of ester (3) and 30 % yield
of amide (6) were achieved. Usually, N-alkoxyamines can
be prepared in a two-step synthesis by the radical reaction of
N-oxyl with vinyl (styrene) derivatives and subsequent re-
duction with NaBH4 [56, 57]. However, the synthesis of
derivatives 3 and 6 employing the approach given in the
literature [57, 58] was unsuccessful. Less than 5 % yield was
obtained for 3, together with side-products.
The fluorescence values of the parent amine and alkoxy
derivatives of 1-(R-2,2,6,6-tetramethylpiperidin-4-yl)-4-N,N-
dimethylamino benzoate, expressed as quantum yields in non-
polar and polar solvents as well as polymer matrices, were
rather low (Tables 1 and 2). The Stokes shifts of these deriv-
atives indicated a difference in geometry between the ground
and excited states. In cyclohexane solution, the studied deriv-
atives exhibited one broad emission band with a maximum at
330–340 nm (Table 1a, Fig. 3 and Fig. S1a). The maxima of
fluorescence of the benzamide derivatives (4, 5, 6) were red-
shifted in comparison to the ester derivatives (1, 2, 3). The
hindered amine moiety may have been protonated in the
parent amine (1, 4), oxidized form (2, 5) or substituted N-O-
R form (3, 6). The presence of a free radical center usually
leads to intramolecular quenching, which is caused by an
increase in the rate of radiationless processes due to the para-
magnetism of the free electron at the sterically hindered nitro-
gen position [20–23, 41–46]. The extent of intra-molecular
fluorescence quenching was expressed as the ratio of fluores-
cence quantum yields ΦNX/ΦNO of the parent amine NH or N-
O-R species and the oxidized N-O• species (Table 1). Com-
parison of the observed ΦNX/ΦNO ratios indicated that the
fluorescence behavior of the studied derivatives was influ-
enced by the type of chemical bond between the chromophore
Spectral Measurements
The comprehensive spectral characteristics of the fluores-
cence probes based on 4-N,N-dimethylaminobenzoic acid
were performed in solvents of various polarities as well as in
several polymer matrices.
All compounds exhibited strong absorption bands in the
near-UV region around 300 nm. The longest wavelength
absorption band was influenced by the solvent polarity. It
was shifted bathochromically 10 to 15 nm with increasing
polarity from cyclohexane to methanol (Fig. 2, Table 1), in-
dependent of the oxidation state of the sterically hindered
nitrogen. There was a sign of vibrational structure in this
absorption band in non-polar cyclohexane (Fig. 2). The effect
of the surrounding environment on this absorption band was
more pronounced in liquid solvents than in polymer matrices
(Table 2). This red shift of the absorption maxima with envi-
ronment polarity corresponded to a strongly mixed nπ/ππ*
transition, with the latter dominating as the lowest state [55].
and nitroxide. For the series of ester derivatives (1, 2, 3), ΦNX
/
ΦNO values around 2.5 corresponded to relatively low intramo-
lecular quenching (Table 1). ΦNX/ΦNO values of as high as 1.5
for the amide derivatives indicated poorly efficient or no intra-
molecular fluorescence quenching (Table 1). It should be noted
that the solubility of 4 in cyclohexane was very low. Therefore,
the quantum yield of 4 in cyclohexane might have a large error.
In the fluorescence spectra of benzoate derivatives 1–3 in
methanol, acetonitrile and chloroform, broad emission bands
with maxima at 350–360 nm were observed. For amide deriv-
atives 4–6, a second broad fluorescence band with a maxi-
mum at 460–475 nm was found (Fig. 3 and Fig. S1) in
methanol solution. This band was not detected in cyclohex-
ane. On the other hand, in acetonitrile and chloroform, dual
fluorescence was observed for the ester derivatives too (Fig. 3
and Fig. S1c,d). The fluorescence intensities of the studied
compounds in polar methanol, acetonitrile and chloroform
were approximately two orders of magnitude lower compared
to non-polar cyclohexane (Fig. 3). The values of ΦNX/ΦNO
observed for both the ester and amide derivatives in MeOH
were approximately 1, which indicated a low efficiency of
Fig. 2 UV absorption spectra of 1-(2,2,6,6-tetramethylpiperidin-4-yl)-
4-N,N-dimethylaminobenzoate (1) in various media

J Fluoresc (2012) 22:1371–1381
1377
Table 1 Spectral characteristics of derivatives of 1-(R-2,2,6,6-tetramethylpiperidin-4-yl)-4-N,N-dimethylamino benzoate (1, 2, 3) or benzamide (4,
5, 6) in solvents
Samplea
λA b nm
log εb mol−1 L cm−1
λF c nm
Δνe cm.−1
Φr(A) f
ΦNX/ΦNO
max
max.
(X0H or O-R)
a/ cyclohexane
1
2
300
303
4.324
4.494
332
334
3212
3063
0.158
0.064
2.5
–
3
300
284
286
284
4.344
4.275
4.359
4.369
332
336
338
336
3212
5449
5379
5449
0.173
0.043
0.0529
0.079
2.7
1.0
–
4
5
6
1.5
b/ methanol
1
2
313
311
4.484
4.608
340; 357
339;355
3937
3985
0.0015
0.0013
1.2
–
3
310
305
304
303
4.391
4.347
4.390
4.356
338, 357
359; 471
358; 465
358,470
4247
4931
4961
5070
0.0023
0.0054
0.0099
0.0111
1.8
0.5
–
4
5
6
1.1
c/Acetonitrile
1
2
309
311
4.466
4.559
346; 481
360; 485
3460
4376
0.029
0.011
2.6
–
3
310
293
296
293
4.495
4.347
4.458
4.441
358; 474
355; 450
339; 454
355; 452
4325
5960
4285
5960
0.029
0.069
0.023
0.066
2.6
3.0
–
4
5
6
2.9
d/chloroform
τh ns
1
2
3
4
5
6
314
304
309
293
297
295
4.513
4.374
4.450
4.459
4.515
4.517
353; 426
356; 427
350; 417
361; 425
378; 421
378; 421
3502
4789
3791
6429
7187
7471
0.036
0.044
0.039
0.004
0.007
0.005
0.82
–
1.40
1.20
0.96
3.10
1.97
2.66
0.98
0.57
–
0.71
a Structure of sample according to Scheme 1. b Maximum of the longest wavelength absorption band. c Log of the molar extinction coefficient.
d Maximum of the fluorescence band. e Stokes shift in cm−1 . f Relative quantum yield to anthracene. Absolute quantum yield might be estimated
taking Φ00.25 for anthracene in non polar cyclohexane. g Extent of intramolecular quenching. h Life-time in ns calculated by fitting to mono-
exponential or bi-exponential
intramolecular quenching. The values of ΦNX/ΦNO in aceto-
nitrile were in the range of 2.6–3.0 (Table 1a). Such dual
fluorescence of the benzene derivatives with strong electron
donating groups stems from the twisted intramolecular charge
transfer (TICT) state, as suggested by Grabowski [7] and
Rettig [8]. This state depends on the conformational freedom
of the dimethylamino group [58]. The solvent plays a decisive
role in stabilizing the highly dipolar TICT state with partial or
complete electron transfer. Therefore, it is formed in polar
media only [59]. Similar dual fluorescence was observed for
4-N,N-dimethyl amino benzoic acid and some of its esters in
polar solutions [16, 17, 19].
to cyclohexane. The addition of different concentrations of
methanol to a cyclohexane solution of derivative 6 showed an
effective quenching of fluorescence by increasing the metha-
nol concentration to >0.5 mol L−1. As shown in Fig. 5,
quenching of the fluorescence of 6 with methanol did not
obey the Stern – Volmer equation. The observed nonlinear
dependence can be interpreted as a preferential solvation of
the dipolar ground state by polar solvent molecules [60]. As
shown in Fig. 6, the observed solvatation effect was higher in
the case of amide derivatives compared to ester derivatives.
An unusual result was observed upon comparison of the
fluorescence intensity of the amide derivatives (4, 5 and 6)
in methanol. Instead of the expected decrease of fluores-
cence intensity in the case of N-oxyl derivative 5 by
As mentioned above, the fluorescence intensity of the
studied derivatives in methanol was very low in comparison

1378
J Fluoresc (2012) 22:1371–1381
Table 2 Spectral characteristics of derivatives of 1-(R-2,2,6,6-tetramethylpiperidin-4-yl)-4-N,N-dimethylamino benzoate (1, 2, 3) or benzamide (4,
5, 6) in polymer matrices
b
max
Samplea
λA
nm
log εb mol−1 L cm−1
λF c nm
Δνe cm.−1
Φr(A)f
Φ
NX/ΦNO (X0H or O-R)
τh ns
max.
a/ Polystyrene (PS)
1
2
3
4
5
6
312
311
308
296
297
292
4.314
4.46
344
2981
2999
2969
5772
5580
5428
0.065
0.033
0.071
0.092
0.055
0.088
2.0
–
2.61
343
0.8(79 %) 3.9(21 %)
4.31
339
2.2
1.7
–
2.20
4.024
4.205
4.099
357; 382
356; 382
347
3.60
0.9(68 %) 3.8(32 %)
2.0
1.6
b/ Polymethylmethacrylate (PMMA)
1
2
3
4
5
6
312
311
307
298
298
291
4.31
358; 411
353
4118
3825
3920
5067
5308
6431
0.445
0.055
0.425
0.099
0.017
0.056
8.1
–
2.20
4.421
4.464
4.219
4.258
4.374
0.3(89 %) 3.3(11 %)
349
7.8
5.8
–
2.90
351
2.70
354
0.7(78 %) 3.9(22 %)
2.60
358
3.3
c/ Poly(vinylchloride) (PVC)
1
2
3
4
5
6
315
314
311
306
301
299
4.164
4.239
4.404
4.109
4.274
4.417
365
370
357
371
370
372
4348
4820
4143
5941
6195
6563
0.254
0.103
0.150
0.249
0.080
0.11
2.5
–
2.40
0.13 (81 %) 2.48 (19 %)
1.5
3.1
–
2.5
2.4
1.90
2.0
1.4
a Structure of sample according to Scheme 1. b Maximum of the longest wavelength absorption band. c Log of the molar extinction coefficient.
f
d Maximum of the fluorescence band.e Stokes shift in cm−1 . Relative quantum yield to anthracene. Absolute quantum yield might be estimated
taking Φ00.25 for anthracene in non polar cyclohexane. g Extent of intramolecular quenching where NR is either NH (parent amine) or OR
(alkoxy). h Life-time in ns calculated by fitting to mono-exponential or bi-exponential
intramolecular quenching, a higher intensity of fluorescence
than in the case of parent amine 4 was observed. We com-
pared the fluorescence of N-oxyl derivatives 2 and 5 and
alkoxyamine derivatives 3 and 6 with and without an equi-
molar concentration of pentafluorophenyl hydrazine (PFFH)
(reduction agent for aminooxyl radicals). The fluorescence
spectra of derivatives 3 and 6 were not influenced by the
addition of PFFH. The fluorescence intensity of derivative 2
increased as expected through reduction of the paramagnetic
center. Surprisingly, the fluorescence intensity of derivative
5 decreased to the level observed for derivatives of the
parent amine of amide type 4 upon the addition of an
Fig. 3 UV absorption and fluorescence emission spectra of 6 in
Fig. 4 UV absorption and fluorescence emission spectra of 1 in
cyclohexane, methanol and acetonitrile solution (c010−5 mol L−1
)
PMMA, PVC and PS matrices (c00,002 mol kg−1
)

J Fluoresc (2012) 22:1371–1381
1379
as is evident in Fig. 3 (see also Figs. S1b and S1c in
supplementary material).
In the PS, PMMA and PVC matrices, the studied deriv-
atives exhibited broad fluorescence bands with maxima
between 345 and 375 nm, depending on the derivative and
polymer matrix (Fig. 4, Table 2). In some cases, dual fluo-
rescence similar to that in methanol solution was observed.
The twisted intramolecular charge transfer (TICT) fluores-
cence in polymer matrices was less intense than in polar
methanol or acetonitrile due to the frozen rotation of the
dimethylamino moiety in the rigid polymer.
Comparison of the fluorescence quantum yield of deriv-
atives 1, 2 and 3 (ΦNX/ΦNO ratio) in PMMA showed more
efficient intramolecular fluorescence quenching due to the
presence of the N-oxyl group than was observed in cyclo-
hexane and methanol (Table 2b). An effective intramolecu-
lar fluorescence quenching by the N-oxyl radical in PMMA
versus the solution state was also observed for derivative 5
in comparison to parent amine 4, as well as N-O-R deriva-
tive 6 (Table 2). A similar trend was observed in the PVC
and PS matrices. In these cases, the quenching effect was
lower than in PMMA (Table 2). Intramolecular energy
transfer (quenching) is more pronounced in polymer matri-
ces, while the TICT state plays an important role in the
spectral characterization of the studied probes in polar sol-
utions. Solid polymer matrices do not allow free rotation of
the dimethylamino group; thus, CT states do not play such
an important role in the spectral properties of the studied
probes in polymer matrices.
The lifetimes of fluorescence of the esters and amides
were determined in chloroform and polymer matrices. The
lifetimes of derivatives 1–6 in chloroform were in the range
of 1–4 ns, and intermolecular quenching in 2 and 5 due to
the paramagnetic center did not influence the fluorescence
decay. Contrary to what was observed in the PS and PMMA
matrices, the decay of fluorescence of 2 and 5 was biexpo-
nential, with a short lifetime of less than 1 ns. In PVC, the
fluorescence decay was monoexponential, as in chloroform.
Because the lifetimes of derivatives 1–6 were short, we were
unable to perform more detailed analyses.
Fig. 5 Quenching of fluorescence and shift of emission maxima of
derivative 6 in cyclohexane (c010−5 mol L−1) by addition of methanol
equimolar concentration of PFFH (Figs. S3, S4). Therefore,
the fluorescence of 4 was strongly influenced by solute –
solvent interactions in methanol solution. These types of
interactions were more efficient than in the case of N-oxyl
derivative 5, where intramolecular quenching should be
operating. The primary pathway of these interactions could
be through intermolecular hydrogen bonding with methanol.
The possibility of methanol forming hydrogen bonds with
the piperidine NH group is lower in the case of N-oxyl
derivative 5 than in the parent amine or hydroxylamine
(formed after reduction of N-oxy radical with PFFH). The
dependence of fluorescence properties of the amide-type
derivatives was also evident from a comparison of the
results observed in protic methanol and aprotic acetonitrile.
In acetonitrile, the CT emission was more intense than in
methanol, where the locally excited state is more favorable,
Conclusion
Probes based on intramolecular quenching of the fluores-
cence of both the chromophore and paramagnetic center
using the 4-N,N-dimethylaminophenyl chromophore with
an ester or amide linkage to sterically hindered amines did
not exhibit large operating ranges, especially in polar sol-
utions. For amide derivatives 4 and 5, an opposite effect was
observed due to the strong hydrogen bonding of 4 with
methanol, which is a more efficient quenching agent than
the paramagnetic group (N-O). A larger operation range of
Fig. 6 Dependence of the position of absorption maxima of the
derivatives 1-(1-R-2,2,6,6-tetramethylpiperidin -4-yl)-4-N,N-dimethy-
laminobenzoate (1, 2, 3) and –benzamide (4, 5, 6) in cyclohexane, (c0
10−5 mol L−1) in cyclohexane) on added methanol

1380
J Fluoresc (2012) 22:1371–1381
13. Kim Y, Lee BI, Yoon M (1998) Excited-state intramolecular
charge transfer of p-N,N-dimethylaminobenzoic acid in Y zeolites:
hydrogen bonding effects. Chem Phys Lett 286:466–472.
doi:10.1016/S0009-2614(98)00125-0
14. Jiang Y-B, Wang X-J, Jin M-G, Lin L-R (1999) The effect of
micelle-water interface electric field on the intramolecular charge
transfer within ionic micelles. Dual fluorescence of sodium p-
dialkylaminobenzoates in cetylamonium micelles. J Photochem
Photobiol Chem 126:125–133. doi:10.1016/S1010-6030(99)
00100-8
15. Jiang Y-B, Jin M-G (2000) Intramolecular charge transfer at re-
verse micelle-water pool interface: p-N, N- dimethylaminobenzoic
acid in AOT/cyclohexane/water reverse micelle. Spectrochim Acta
Part A 56:623–627. doi:10.1016/S1386-1425(99)00171-7
16. Jiang Y-B (1999) Concentration dependent dual fluorescence of 4-
(N, N-dimethylamino) bnzoic acid in chloroform. Spectrochim
Acta Part A 55:833–837. doi:10.1016/S1386-1425(98)00229-7
17. Huang W, Zhang X, Ma L-H, Wang Ch-J, Jiang Y-B (2002)
Intramolecular charge transfer dual fluorescence of substituted-
phenyl p-dimethylaminobenzoates with comparable electron
acceptors. Chem Phys Lett 352:401–407. doi:10.1016/S0009-
2614(01)01491-9
18. Ma L-H, Chen Z-B, Jiang Y-B (2003) Intramolecular charge trans-
fer with 4(N-ohenylamino)benzoic acid. The N-phenyl conjuga-
tion effect. Chem Phys Lett 372:104–113. doi:10.1016/S0009-
2614(03)00372-5
19. Zhang C-H, Chen Z-B, Jiang Y-B (2004) Intramolecular charge
transfer dual fluorescence p-methylaminobenzoates. Spectrochim
Acta Part A 60:2729–2732. doi:10.1016/j.saa.2004.01.011
20. Blough NV, Simpson DJ (1988) Chemically mediated fluorescence
yield switching in nitroxide –fluorophore adducts: optical sensors
of radical/redox reaction. J Am Chem Soc 110:1915–1917.
doi:10.1021/ja00214a041
21. Gerlock JL, Zacmanidis PJ, Bauer DR, Simpson DJ, Blough NV,
Solmeen IT (1990) Fluorescence detection of free radicals by
nitroxide scavenging. Free Rad Res Communs 10:119–121
22. Török M, Hideg K, Dux L, Horváth LI (1997) Accessibility of
protein sulfhydryl groups to nitroxyl spin labels. J Mol Struct 408/
409:177–180. doi:10.1016/S0022-2860(96)09634-2
23. Hrdlovič P, Chmela Š, Búcsiová L (1996) Spectral characteristics
of bifunctional fluorescence probe based on naphthalene: compar-
ison in solution and in polymer matrix. Chem Papers 50:271–278
24. Hrdlovič P, Chmela Š (1997) Spectral characteristics of multifunc-
tional probes based on pyrene in solution and in polymer matrices.
J PhotochemPhotobiol A Chem 105:83–88. doi:10.1016/S1010-
6030(97)00008-7
intramolecular quenching of as high as 8 was observed in
polar matrices for the ester derivatives compared to the
amide series. Therefore, these probes might be useful for
following radical processes in rigid polar media but within a
limited range.
Acknowledgments The authors are greatly acknowledged to Grant
agency VEGA for financial support through the project 2/0074/10 and
Slovak Research and Development Agency through projects APVV-
0109-10 and 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: Centre for materials, layers and systems for applica-
tions and chemical processes under extreme conditions supported by
the Research & Development Operational Programme funded by the
ERDF.
References
1. Lakowicz JR (1999) Principles of fluorescence spectroscopy, 2nd
edn. Kluwer Academic/Plenum Publisher, New York
2. Valuer B (2001) Molecular fluorescence: principles and applica-
tions. Wiley-VCH Verlag GmbH, Weinheim
3. Albani JR (2005) Structure and dynamics of macromoleules; ab-
sorption and fluorescence studies. Elsevier, Amsterdam
4. Winnik FM, Regismond STA (1996) Fluorescence methods in the
study of the interaction of surfactant with polymers. Colloid Sur-
face Physicochem Eng Aspects 118:1–39. doi:10.1016/0927-7757
(96)03733-8
5. Capek I (2002) Fate of excited probes in micellar systems. Adv
Colloid Interface Sci 97:91–149. doi:10.1016/S0001-8686(01)
00049-5
6. Lippert E, Luder W, Boos H(1962) In: Mangini A (ed) Advances
in molecular spectroscopy Pergamon, Oxford pp 443
7. Grabowski ZR, Rotkiewicz K, Siemiarczuk A, Cowley DJ,
Baumann W (1979) Twisted Intramolecular Charge Transfer
States (TICT) - new class of excited-states with a full charge
separation. Nouv J Chim 3:443–454
8. Rettig W (1986) Charge separation in excited states of decoupled
systems—TICT compounds and implications regarding the devel-
opment of new laser dyes and the primary process of vision and
photosynthesis. Angew Chem Intern Ed 25:971–988. doi:10.1002/
anie.198609711
9. Revill JAT, Brown RG (1992) Excimer versus TICT state forma-
tion in polar solutions of methyl 4-(N, N-demethylamino)benzoate.
Chem Phys Lett 188:433–438. doi:10.1016/0009-2614(92)80843-
Z
10. Gormin D, Kasha M (1988) Triple fluorescence in aminosalicy-
lates. Modulaton of normal proton transfer, and twisted intramo-
lecular charge transfer (TICT) fluorescence by physical and
chemical perturbations. Chem Phys Lett 153:574–576.
doi:10.1016/0009-2614(88)85263-1
11. Dendonder-Lardeux C, Jouvet C, Martrenchard S, Solgadi D, Mc
Combie J, Howells HD, Palmers TF, Subaric-Leitis A, Monte C,
Rettig W, Zimmermann P (1995) Photochemistry in jet cooled
aniline derivatives. Chem Phys 191:271–287. doi:10.1016/0301-
0104(94)00334-7
25. Búcsiová L, Hrdlovič P, Chmela Š (2001) Spectral characteristics
of fluorescence probes based on pyrene in solution and in polymer
matrix. J Photochem Photobiol A Chem 143:59–68. doi:10.1016/
S1010-6030(01)00483-X
26. Hrdlovič P, Chmela Š, Danko M (1998) Spectral characteristics and
photochemical stability of fluorescence probes based on 1,8-naph-
thaleneimide in solution and in polymer matrix. JPhotochem Photo-
biol A Chem 112:197–203. doi:10.1016/S1010-6030(97)00277-3
27. Hrdlovič P, Danko M, Chmela Š (2002) Spectral properties and
photolytical stability of anthracene-hindered amine probes: influ-
ence of the medium. J Photochem Photobiol A Chem 149:207–
216. doi:10.1016/S1010-6030(02)00011-4
28. Hrdlovič P, Kollár J, Chmela Š (2004) Novel fluorescence probe
based on pyrene and piperazine; spectral properties in solution and
in polymer matrices. J Photochem Photobiol A Chem 163:289–
296. doi:10.1016/j.jphotochem.2003.12.002
29. Hrdlovič P, Chmela Š (1998) Spectral and photostabilizing char-
acteristics of adducts of aromatic hydrocarbons and hindered
amine stabilizers. Polym Degrad Stab 61:177–182. doi:10.1016/
S0141-3910(97)00067-0
12. Kim Y, Cheon HW, Yoon M, Song NW, Kim D (1997) SiO2
Colloidal effects on the twisted intramolecular charge transfer of
p-N,N-dimethylaminobenzoic acid in acetonitrile. Chem Phys Lett
264:673–679. doi:10.1016/S0009-2614(96)01388-7

J Fluoresc (2012) 22:1371–1381
1381
30. Bucsiová L, Chmela Š, Hrdlovič P (2001) Preparation, photochem-
ical stability and stabilising efficiency of adducts of pyrene and
hindered amine stabilisers in iPP matrix. Polym Degrad Stab
71:135–145. doi:10.1016/S0141-3910(00)00164-6
31. Danko M, Chmela Š, Hrdlovič P (2003) Photochemical stability
and photostabilizing efficiency of anthracene/hindered amine sta-
bilizer probes in polymer matrices. Polym Degrad Stab 79:333–
343. doi:10.1016/S0141-3910(02)00297-5
32. Hawker CJ (1997) Living free radical polymerization: a unique
technique for the preparation of controlled macromolecular archi-
tecture. Acc Chem Res 30:373–382. doi:10.1021/ar960248m
33. Matyjaszewski K (1998) Controlled radical polymerization, Vol.
685. Matyjaszewski K (ed) ACS, Washington, p 2
34. Georges MK, Veregin RPN, Kazmaier PM, Hamer GK (1994)
Polydispersity polystyrene by a free-radical polymerization
process-rate enhancement. Macromolecules 27:7228–7229.
doi:10.1021/ma00102a039
35. Li I, Howell BA, Matyjaszewski K, Shigemoto T, Smith PB,
Priddy DB (1995) Kinetics of decomposition of 2,2,6,6-tetra-
methyl-1-(1-phenylethoxy)piperidine and its implications on
nitroxyl-mediated styrene polymerization. Macromolecules
28:6692–6693. doi:10.1021/ma00123a044
36. Fukuda T, Terauchi T, Goto A, Tsujii Y, Miyamoto T, Shimizu Y
(1996) Well-defined block copolymers comprising styrene −
acrylonitrile random copolymer sequences synthesized by “liv-
ing” radical polymerization. Macromolecules 29:3050–3052.
doi:10.1021/ma951792y
37. Bálint J, Kiss V, Egri G, Kálai T, Demeter Á, Balog M, Fogassy E,
Hideg K (2004) Kinetic resolution of 1-oxyl-3-hydroxymethyl-
2,2,5,5- tetramethylpyrrolidine derivatives by lipase-catalyzed en-
antiomer selective acylation. Tetrahedron-Asymmetry 15:671–
679. doi:10.1016/j.tetasy.2003.12.026
polypropylene Polym. Degrad Stab 89:427–435. doi:10.1016/
j.polymdegradstab.2005.01.030
46. Fairfull-Smith KE, Blinco JP, Keddie DJ, George GA, Bottle SE
(2008) A novel profluorescent dinitroxide for imaging polypropyl-
ene degradation. Macromolecules 41:1577–1580. doi:10.1021/
ma701944p
47. Carver FJ, Hunter CA, Livingstone DJ, McCabe JF, Seward EM
(2002) Substituent effects on edge-to-face aromatic interactions.
Chem Europ J 8:2847–2859. doi:10.1002/1521-3765(20020703)
8:13<2847::AID-CHEM2847>3.0.CO;2-M
48. Takashi H, Yoshihiko I, Kiyotaka K, Shinsaku N, Kazuko I et al
(1990) The effects of new cytochalasins from phomopsis sp. and
the derivatives on cellular structure and actin polymerization.
Chem Pharm Bull 38:971–974
49. Vamecq J, Lambert D, Poupaert JH, Masereel B, Stables J (1998)
Anticonvulsant activity and interactions with neuronal voltage-
dependent sodium channel of analogues of ameltolide. J Med
Chem 41:3307–3313. doi:10.1021/jm9608772
50. Kawski A, Kubicki A, Kuklinski B, Gryczynski I (1993) Unusual
absorption and fluorescence properties of 1,6-diphenyl-1,3,5-hex-
atriene in poly(vinyl alcohol) film. J Photochem Photobiol A
Chem 71:161–167. doi:10.1016/1010-6030(93)85068-J
51. Birks JB (1968) Photophysics of aromatic molecules. Willey-
Interscience a Division of John Wiley and Sons Ltd, New York,
London, Toronto, Sidney, Ch 4. p 121–127
52. Adamson AW, Demas JN (1971) Evaluation of photoluminescence
lifetimes. J Phys Chem 75:2463–2466. doi:10.1021/j100685a009
53. Demas JN (1973) Excited state lifetime measurements. Appendix
E. Academic, New York, p 245
54. Enderlein J, Erdmann R (1997) Fast fitting of multi-exponential
decay curves. Opt Commun 134:371–378. doi:10.1016/S0030-
4018(96)00384-7
38. Essman M, Sár PC, Hideg K, Marsh D (1993) Iodoacetamide,
indanedione, and chloromercuric spin label reagents with derivat-
ized nitroxide rings as ESR Reporter Groups for Protein Confor-
mation and Dynamics. Anal Biochem 213:336–348. doi:10.1006/
abio.1993.1430
55. Allen NS, Edge M, Sethi S, Catalina F, Corrales T (2000) Photo-
chemistry and photo-induced co-synergistic polymerization acti-
vites of novel N,N-dimethylaminobenzoates and benzamides. J
Photochem Photobiology A Chem 137:169–176. doi:10.1016/
S1010-6030(00)00364-6
39. Qi C, Kusnetzow AK, Hideg K, Price EA, Haworth IS, Qin PZ
(2007) Nanometer distance measurements in RNA using site-
directed spin labeling. Biophys J 93:2110–2117. doi:10.1529/
biophysj.107.109439
40. Kálai T, Hideg É, Vass I, Hideg K (1998) Double (Fluorescent and
Spin) sensors for detection of reactive oxygen species in the
thylakoid membrane. Free Radical Biol Med 24:649–652.
doi:10.1016/S0891-5849(97)00339-0
41. Chmela Š, Hrčková L (2009) Nitroxide mediated styrene radical
polymerization using a fluorescence marked mediator. Eur Polym J
45:2580–2586. doi:10.1016/j.eurpolymj.2009.06.019
42. Step EN, Turro NJ, Klemchuk PP, Gande ME (1995) Model
studies on the mechanism of hals stabilization. Angew Makromol
Chem 232:65–83. doi:10.1002/apmc.1995.052320105
56. Danko M, Chmela Š, Hrdlovič P (2006) Synthesis, photo-
chemical stability and photo-stabilizing efficiency of probes
based on benzothioxanthene chromophore and hindered amine
stabilizer. Polym Degrad Stab 91:1045–1051. doi:10.1016/
j.polymdegradstab.2005.07.009
57. Kollár J, Hrdlovič P, Chmela Š (2008) Synthesis and spectral
characteristics of di-substituted 1,8-naphthalimides; bi-radical for-
mation. J Photochem Photobiology A Chem 195:64–71.
doi:10.1016/j.jphotochem.2007.09.008
58. Rettig W, Lapouyade R (1994) Fluorescent probes based on twist-
ed intramolecular charge transfer (TICT) states and other adiabatic
photoreactions. In: Lakowicz JR (ed) Topics in fluorescence spec-
troscopy, Vol. 4.: probe design and chemical sensing. Plenum
Press, New York, pp 109–149
43. Chirinos-Padrón AJ (1989) Mechanistic aspects of polymer photo-
stabilization. J Photochem Photobiol A Chem 49:1–39.
doi:10.1016/1010-6030(89)87103-5
44. Pospıšil J (1991) Photo-oxidation degradation of polymers: causes,
consequences and protection. Chem Listy 85:904–923
45. Micallef AS, Blinco JP, George GA, Reid DA, Rizzardo E,
Thang SH, Bottle SE (2005) The application of a novel
profluorescent nitroxide to monitor thermo-oxidative degradation of
59. Kaholek M, Hrdlovič P (1999) Characteristics of the excited states
of 3-substituted coumarin derivatives and transfer of electronic
energy to N-oxyl radicals. J Photochem Photobiol A Chem
127:45–55. doi:10.1016/S1010-6030(99)00133-1
60. Köhler G, Rechthaler K, Rotkiewicz K, Rettig W (1996) Forma-
tion and stabilization of twisted intramolecular charge transfer
states in binary mixed solvents. Chem Phys 207:85–101.
doi:10.1016/0301-0104(96)00054-7