Electrochemistry
Electrochemical studies were performed using DCM as a
solvent and N,N,N,N-tetrabutylammonium hexafluorophosphate
(TBAFP) as the supporting electrolyte. The substrate concen-
tration was ca. 5 mM. A 1 mm diameter Pt or glassy carbon
electrode was used as the working electrode, along with a
Ag+/Ag (10ꢀ2 M) reference electrode and a Pt wire counter
electrode. The cell was connected to a CH Instruments 600B
potentiostat monitored by a PC computer. The reference
electrode was checked vs. ferrocene as recommended by
IUPAC. In our case, E0(Fc+/Fc) = 0.097 V. All solutions
were degassed by argon bubbling prior to each experiment.
Fig. 1 Formula of NITZ.
the excited state of the tetrazine and the naphthalimide, since
the redox potential of the former has been estimated between
1.2 V and 1.4 V, based on the optical gap.5a The photochemical
behaviour of this new original molecule, along with the
demonstration of its improved brilliance (about 10 times the
one of a standard tetrazine) is presented along with its
electrochemical properties. Also, we show that the compound
can be inserted into a polymer and lead to a transparent and
yellow fluorescent object, a unique feature that should find
application in the realization of decorative objects.
Photophysical measurements
Steady-state spectroscopy. All the spectroscopic experiments
were carried out in DCM and at concentrations ca. 10 mmol Lꢀ1
for absorption spectra and ca. 1 mmol Lꢀ1 for fluorescence
spectra. UV-vis absorption spectra were recorded on a Varian
Cary 500 spectrophotometer. Fluorescence emission and
excitation spectra were measured on a SPEX fluorolog-3
(Horiba-Jobin-Yvon). For emission fluorescence spectra, the
excitation wavelengths were set equal to the maximum of the
corresponding absorption spectra. Only dilute solutions with
an absorbance below 0.1 at the excitation wavelength lex were
used. For the determination of the relative fluorescence
quantum yields (fF), sulforhodamine 101 in ethanol (fF = 0.9)
was used as a fluorescence standard.
Experimental
Materials and methods
All reagents were purchased from Sigma-Aldrich or Fluka and
used as received. All solvents were obtained from Carlo-Erba.
Synthesis grade ones have been dried prior to use according to
standard literature procedures. All reactions were carried out
under an inert argon atmosphere. Photophysical and electro-
chemical studies have been done in spectroscopic grade
solvents. Solution NMR spectroscopy was performed on a
Bruker AMX 500 MHz instrument. Mass spectrometric
analyses were carried out on an Agilent 5973N apparatus.
Dichloro-s-tetrazine was prepared as previously described.5b
Time-resolved spectroscopy. The fluorescence decay curves
were obtained with a time-correlated single-photon-counting
method using a titanium–sapphire laser (82 MHz, repetition
rate lowered to 4 MHz thanks to a pulse-peaker, 1 ps pulse
width, a doubling crystal is used to reach 495 and 355 nm
excitations) pumped by an argon ion laser. Data were analyzed
by a nonlinear least-squares method (Levenberg–Marquardt
algorithm) with the aid of Globals software (Globals Unlimited,
University of Illinois at Urbana-Champaign, Laboratory of
Fluorescence Dynamics). Pulse deconvolution was performed
from the time profile of the exciting pulse recorded under the
same conditions by using a Ludox solution. In order to
estimate the quality of the fit, the weighted residuals were
calculated.
Synthesis
Synthesis of N-(2-hydroxyethyl)-1,8-naphthalimide.11 1,8-
Naphthalimide (0.2g, 1 mmol) was reacted with 2-bromoethanol
(0.125g, 1 mmol) in dimethylformamide (DMF, 15 ml) in the
presence of potassium carbonate for 10 h (previous workers
used acetonitrile but in our hands the yields were unsatisfactory).
Then the resulting solution was poured in water (10 ml) and
extracted with ethyl acetate; the product was purified by
chromatography on silica gel using dichloromethane (DCM)
as an eluant to give N-(2-hydroxyethyl)-1,8-naphthalimide
(yield: 73%). 1H NMR (400 MHz, CDCl3, ppm): 2.36
(t, 1H, J = 5.5 Hz, OH), 3.98 (dt, 2H, J1 = 5.5 Hz, J2 =
5 Hz, CH2-OH), 4.47 (t, 2H, J = 5 Hz, CH2-N), 7.76
(t, 2H, J = 7.5 Hz), 8.23 (d, 2H, J = 7.5 Hz), 8.62 (d, 2H,
J = 7.5 Hz).
Results and discussion
The following synthetic route (Scheme 1) was used for the
preparation of NITZ.
The synthesis is relatively straightforward and allows the
preparation of appreciable quantities of compound if needed.
We have performed the spectroscopic and electrochemical
study of both N-(2-hydroxyethyl)-1,8-naphthalimide and
NITZ in order to evaluate the properties of the imide alone,
and further to be able to analyze the energy transfer in the
bichromophoric NITZ. Table 1 gathers the physicochemical
characteristics of N-(2-hydroxyethyl)-1,8-naphthalimide and
NITZ, in the latter case focusing, respectively, on the imide
and the tetrazine characteristics.
Synthesis of NITZ. The reaction of dichloro-s-tetrazine and
N-(2-hydroxyethyl)-1,8-naphthalimide was conducted under
previously described standard conditions12 (1 eq. dichloro-s-
tetrazine, 1 eq. alcohol, 2 eq. collidine in DCM, RT, 2 h) to
1
give NITZ in 56% yield. H NMR (400 MHz, CDCl3, ppm):
4.75 (t, 2H, J = 5 Hz), 5.06 (t, 2H, J = 5 Hz), 7.75 (t, 2H, J =
7.5 Hz), 8.23 (d, 2H, J = 7.5 Hz), 8.56 (d, 2H, J = 7.5 Hz).
13C NMR (100 MHz, CDCl3, ppm): 38.1 (CH2-O), 67.6
(CH2-N), 122.3, 127.1, 128.9, 131.8, 132.3, 134.5 (naphthalene
core), 163.7, 164.5, 166.7 (CQO and tetrazine). High-res ESI
MS (positive ion): 355, 357 m/z (M+).
Regarding absorption and fluorescence, the N-(2-hydroxy-
ethyl)-1,8-naphthalimide behaves like a standard naphthalimide,
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
New J. Chem., 2011, 35, 1678–1682 1679