190
I. Grabchev et al. / Dyes and Pigments 85 (2010) 189e193
emission spectra were recorded on a JASCO FP-6500 spectrofluorim-
eter at concentrations of 10ꢀ6 mol lꢀ1; fluorescence quantum yield fL
were determined, according to equation (1),
FT-IR (KBr) cmꢀ1: 3068, 2939, 1700, 1658, 1538, 1365, 1345, 1234,
781; 1H-NMR (CDCl3, 300 MHz ppm): 8.36 (d, J ¼ 8.8 Hz, 16H, AreH),
8.08 (d, J ¼ 8.0 Hz, 16H, AreH), 7.65 (t, 16H, AreH), 4.10 (t, 16H,
(OC)2NCH2), 2.74e2.35 (m, 32H, CH2N< þ 4H, >NCH2CH2CH2CH2N<),
2.08e1,34 (m, 24H, >NCH2CH2CH2N< þ 4H, >NCH2CH2CH2CH2N<).
Analysis: C136H128N14O16 (2212.9): Calcd. C 73.76, H 5.83, N 8.85;
Found C 73.49, H 5.79, N 8.80.
À
Á
Ref
ILS 1 ꢀ 10ꢀOD
nS2
fLS
¼
fRLef
(1)
À
Á
ꢀODS n2Ref
ILRef
1 ꢀ 10
where IL, the emission intensity, was calculated from the spectrum
area !I(l)dl, OD represents the optical density at the excitation
3. Results and discussion
wavelength and n the optical density of each solvent. The superscripts
“S” and “Ref” refer to the sample and to the standard, respectively.
Fluorescence quantum yield was determined on the basis of the
absorption and fluorescence spectra. Anthracene was used as a refer-
ence (F0 ¼ 0.26).
Novel, 1,8-naphthalimide-labelled poly(propylenamine) den-
drimers D1 and D2 were prepared using commercial poly(propy-
lenamine) dendrimers from first and second generation which
possess four and eight primary amine groups in their periphery,
respectively. 1,8-naphthalic anhydride reacts with the primary
amine groups in ethanol solution by the condensation reaction
shown in Scheme 1 thus providing the novel compound comprising
1,8-naphthalimide units in its structure (D1). The same procedure
was used to obtain dendrimer (D2) having eight 1,8-naphtalimide
units in its periphery (Scheme 2). Both D1 and D2 were obtained as
solids by filtration. FT-IR and 1H spectra obtained for D1 and D2 in
chloroform solutions are presented in the Experimental section
above.
The UVeVis spectra of the poly(propylenamine) dendrimers D1
and D2 have been investigated in acetonitrile solution at concen-
tration 10ꢀ6 mol lꢀ1. The basic photophysical characteristics of the
dendrimers depend on the polarization of 1,8-naphthalimide units. It
is well known that the polarization of 1,8-naphthalimide chromo-
phoric system depends on the electron donor-acceptor interaction
between the C-4 substituent and the carbonyl groups. In the case
when the substituent is a hydrogen atom which does not posses
electron donor ability, the respective 1,8-naphthalimide absorbs in
the UV region and emits very weak fluorescence [23].
NMR spectra were obtained using an Oxford AS operating at
300 MHz for 1H; measurements were carried out in CDCl3 solution
at ambient temperature and chemical shifts were referenced to tetra-
methylsilane (TMS) standard. IR spectra of both 1,8-naphthalimide-
labelled dendrimersweremeasuredona Bruker IFS-113vspectrometer
at a 2 cmꢀ1 resolution using KBr pellets. The effect of the metal cations
and protons upon the fluorescence intensity was examined by adding
a few ml of stock solution of the metal cations to a known volume of the
dendrimer solution (3 ml). The addition was limited to 0.08 ml so that
dilution remains insignificant [22]. The pH was adjusted by addition
of HCl and NaOH into the 100 ml of watereacetonitrile (4:1) solvent,
and was measured on a Meterlab pHM240 pH meter from Radiometer
Copenhagen. All spectral measurements in this study were performed
at room temperature.
2.2. Synthesis of 1,8-naphthalimide-labelled poly(propylenamine)
from first generation (D1)
First generation 0.001 M of poly(propylenamine) and 0.004 M
of 1,8-naphthalic anhydride were dissolved in 50 ml ethanol. The
solution was refluxed and the reaction was monitored using TLC.
After 6 h 200 ml water was added and the resulting precipitate was
filtered and dried in vacuum. Yield: 54%.
Table 1 summarizes the basic spectral characteristics of den-
drimers D1 and D2: absorption (labs) and fluorescence (lfl) maxima,
molar extinction coefficient (
fluorescence yield (FF).
3), Stokes shift (nA
ꢀ
nF) and quantum
FT-IR (KBr) cmꢀ1: 3066, 2956, 2792,1698,1658,1589,1441,1383,
1347,1234, 778; 1H-NMR (CDCl3, 300 MHz ppm): 8.48 (d, J ¼ 8.3 Hz,
8H, AreH), 8.08 (d, J ¼ 8.0 Hz, 8H, AreH), 7.65 (t, 8H, AreH), 4.19 (t,
8H, (OC)2NCH2), 2.62 (t, 8H, CH2N<), e2.48 (br. s, 4H,
>NCH2CH2CH2CH2N<), 1,88 (Kv, 8H, >NCH2CH2CH2N<), 1.47 (br. s,
4H, >NCH2CH2CH2CH2N<).
The absorption spectra of both dendrimers are approximately
identical and have a maximum at 326 nm. The molar extinction
coefficients (
l
3) of the absorption maxima are higher than 10,000 mol
ꢀ1 cmꢀ1, indicating that this is a charge transfer (CT) band, due to the
S0 / S1 transition. On the other hand the molar extinction coefficient
for both dendrimers is approximately four- (D1) and eight-fold (D2)
larger than that of the monomeric 1,8-naphthalimide derivatives
having a hydrogen atom as a substituents at C-4 position [23a], sug-
gesting no ground state interaction between the 1,8-naphthalimide
chromophoric units [24]. The studies on 1,8-naphthalimide-labelled
PAMAM derivatives of zero and second generation gave similar results
[10,11]. The Stokes' shift which is a parameter indicating the difference
in the properties and structure of the fluorophores in the ground (S0)
and first excited singlet state (S1) is 4010 cmꢀ1 for both dendrimers.
Analysis: C64H56O6N8 (1036.4): Calcd. C 74.11, H 5.44, N 8.01;
Found C 73.88, H 5.38, N 8.10.
2.3. Synthesis of 1,8-naphthalimide-labelled poly(propylenamine)
from second generation (D2)
The synthesis was carried out according to that for D1 using poly
(propylenamine) second generation dendrimer. Yield: 65%.
O
O
O
O
O
NH
N
2
H N
2
N
O
O
N
N
N
N
O
C H OH
5
O
2
NH
N
2
H N
2
N
O
O
Scheme 1. Synthesis of poly(propylenamine) dendrimer D1.