A new fluorescent H+ sensor based on core-substituted naphthalene diimide

Article abstract of DOI:10.1016/j.cplett.2011.11.028

The synthesis and spectroscopic characterisation of a new amino core-substituted naphthalene diimide is reported. The compound exhibits H ⁺ sensing properties leading to a change in optical output through both absorption and emission. On addition of trifluoroacetic acid, an increase in the fluorescence quantum yield from 0.5 to 0.67 and a ～500-550 cm ^-1 blue shift of absorption and emission maxima are observed. Addition of triethylamine fully reverses these effects and the cycle is repeatable many times. The increase in fluorescence emission is attributed to protonation blocking weakly competitive photoinduced electron transfer operative in the neutral form of the molecule.

Full text of DOI:10.1016/j.cplett.2011.11.028

Chemical Physics Letters 521 (2012) 59–63
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
A new fluorescent H+ sensor based on core-substituted naphthalene diimide
Rosalind P. Cox, Heather F. Higginbotham, Brenton A. Graystone, Saman Sandanayake, Steven J. Langford,
⇑
Toby D.M. Bell
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and spectroscopic characterisation of a new amino core-substituted naphthalene diimide is
reported. The compound exhibits H+ sensing properties leading to a change in optical output through
both absorption and emission. On addition of trifluoroacetic acid, an increase in the fluorescence quan-
tum yield from 0.5 to 0.67 and a ꢀ500–550 cmꢁ1 blue shift of absorption and emission maxima are
observed. Addition of triethylamine fully reverses these effects and the cycle is repeatable many times.
The increase in fluorescence emission is attributed to protonation blocking weakly competitive photoin-
duced electron transfer operative in the neutral form of the molecule.
Received 13 September 2011
In final form 10 November 2011
Available online 19 November 2011
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
groups such as alkyl amines [14–16]. Substituted amino-NDIs
(SANDIs) have strong photoluminescence and are brightly col-
Fluorescent chemosensors have been widely reported because
of the advantages they possess over non-fluorescent chemosensors
such as high sensitivity, real-time visualisation and a rapid re-
sponse time [1]. Observable changes in absorption or fluorescence
(or both) at a particular wavelength based on one or more inputs
(protonic, ionic, electronic) has led to the development of commer-
cial sensors and has contributed to the more futuristic area of
molecular logic [2–4]. Fluorescent pH sensors have been especially
useful in the areas of biology, medicine and the environment [5].
For example, changes in intracellular pH (pHi) are important in a
number of biochemical processes and in determining disease
states. There are two broad methods of studying pHi using ion-
sensitive microelectrodes and dyes [6]. Fluorescent pH-sensitive
dyes, e.g. fluoresceins, xanthenes and pyrenes have a range of dif-
ferent properties including pKa, excitation and emission wave-
lengths, lipid solubility, and photostability making them suitable
for various applications. Naturally, the use of dyes to measure
pHi is strongly influenced by the ability of target cells to retain
the dye and that measuring pH is strongly affected by temperature,
cell type, indicator dye and use of transport inhibitors to prevent
dye export [7]. Hence new dye systems capable of systematic
change of properties through functionalisation are always in need.
Naphthalene diimides (NDIs) are particularly desirable sensor
candidates because they are small, easily functionalisable mole-
cules with relatively broad solubility in a range of solvents and
tunable fluorescent properties [8–13]. Although NDIs themselves
have very weak emissions, core substituted NDIs (cNDIs) are able
to fluoresce, especially with the introduction of electron donor
oured, ranging from red through to blue [17]. Additionally, chang-
ing the substituents of a cNDI can greatly affect the absorbance and
fluorescence of the NDI [5,7] allowing good, measurable photo-
physical changes as required for an effective sensor. In recent work
[18], a DBU-annulated SANDI exhibited changes in both absor-
bance and fluorescence in different pH environments which were
attributed to internal charge transfer (ICT). Incorporation of addi-
tional amine groups in the substituents offers a way for the mole-
cule to respond to environmental changes, particularly pH, whilst
retaining the key features of the alkylated NDI system. With this
in mind, we developed 1 with side groups containing two amine
groups.
Many chemosensors operate using one of two basic systems:
internal charge transfer (ICT) and photoinduced electron transfer
(PET) [1]. Chemosensors designed for PET processes involve a fluo-
rophore–spacer–receptor system that contains a PET acceptor (typ-
ically a fluorophore) and a PET donor (typically an amine) [19,20].
The PET process between N,N0-disubstituted naphthalene diimide
[21,22], naphthalimide [23–26] or related perrylene diimide [27]
based fluorophores have been studied [21] and PET has been
reported in a SANDI based molecular system for sensing Zn2+ [28].
Herein we report on the synthesis and spectroscopic analysis of a
protic switch, disubstituted SANDI (DiSANDI) 1, which exhibits
PET properties and sensitivity to H+ in both chloroform and toluene.
2. Materials and methods
The synthesis of the dark blue pH responsive compound 1 [29]
was achieved (Scheme 1) by reacting N,N0-bis(n-octyl)-2,6-dibromo
naphthalene diimide 2 [30,31] with commercially available N-ben-
zyl-4-aminopiperidine 3 under relatively mild conditions and stan-
⇑
Corresponding author. Fax: +61 3 9905 4597.
E-mail address: toby.bell@monash.edu (T.D.M. Bell).
0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.11.028

60
R.P. Cox et al. / Chemical Physics Letters 521 (2012) 59–63
Scheme 1. Synthesis of H+ responsive compound 1, and the resultant compound 1.2H+ following protonation at the N atom in the piperidine ring of each core substituent.
dard work-up and purification procedures. As is also shown in
Scheme 1, compound 1 is able to accept a proton at the each of
the piperyl N atoms in the side groups (see below) and this form
of the molecule is referred to as 1.2H+. Full synthetic details are gi-
ven in the Supplementary Material along with characterisation
spectra in Figures S9–16.
UV–Vis spectra were obtained using a Varian Cary 100 Bio UV–
Visible Spectrophotometer. Emission spectra were recorded using a
Varian Cary Eclipse fluorescence spectrophotometer in 1 cm quartz
cuvettes. Acidification and basification in chloroform (Aldrich, spectro-
photometric grade) was achieved by adding aliquots of trifluoroacetic
water) had a full width half maximum of ꢀ90 ps. Prior to measure-
ment all samples were degassed either through several freeze–
pump–thaw cycles or by bubbling with nitrogen for 20 min immedi-
ately before measurement. Fluorescence decay times were obtained
by fitting the data with a single exponential decay function con-
volved with the IRF using an iterative least-squares routine based
on the Levenberg–Marquardt algorithm. Goodness of fit was judged
by the chi-squared parameter (v
2) and by inspection of the residu-
als. Decay histograms and fit functions are given in the Supporting
Information.
acid (TFA, 0.6 mM, 20
lL, Aldrich AR grade) and triethylamine (TEA,
0.7 mM, 40 L, Aldrich AR grade), respectively, to a solution of 1
l
3. Results and discussion
(2.5 mL). Fluorescence quantum yields were determined by comparing
areas under corrected emission spectra recorded under identical con-
ditions with that of Rhodamine 101 (Uf = 1.00 in 0.01% HCl in EtOH
[32] (Aldrich spectrophotometric grade)). Solutions of Rhodamine
101, 1 and 1.2H+ were prepared in quartz cuvettes, with their absor-
bance at 550 nm being less than 0.1 in order to avoid inner filter effects.
They were deoxygenated by bubbling with N2 gas for 20 min immedi-
ately prior to measurement. Corrections were made for the different
refractive indexes of the solvents and for any differences in the fraction
of light absorbed at the excitation wavelength.
UV–Vis absorption spectra of 1 in CHCl3 ([1] = 10 lM) with
varying amounts of trifluoroacetic acid (TFA) are shown in Figure
1. The spectrum in neat CHCl3 shows characteristic peaks between
330 and 400 nm corresponding to the p–
p⁄ transitions of the NDI
Fluorescence decay histograms were obtained using the method
of Time Correlated Single Photon Counting (TCSPC) with excitation
from a picosecond pulsed supercontinuum fiber laser (Fianium, SC
400-4-pp) providing ꢀ40 ps pulses across the visible and near IR
at 5 MHz repetition rate. Excitation wavelength selection was
achieved using a 700 nm shortpass filter and 10 nm bandpass filter
centred at 594 nm (Chroma). Emission from the sample was col-
lected at 90° to excitation and passed through a monochromator
(CVI, dk480) and focused onto a fast response avalanche photodiode
detector (APD, Id-Quantique, Id-100). Photon emission times were
recorded by a photon counting module (Picoquant, PicoHarp 300)
with start signal provided by a sync out from the excitation laser
and stop signal from the APD detector. An instrument response func-
tion (IRF) recorded from a scattering solution (dilute milk powder in
Figure 1. Normalised UV–Vis absorption spectra of 1 in CHCl3 ([1] = 10
different concentrations of TFA ([TFA] = 0, 2.5, 5.0, 7.5, 10, 12.5, 15, 17.5, 20
l
M) with
lM).

R.P. Cox et al. / Chemical Physics Letters 521 (2012) 59–63
61
Table 1
core. A strong, broad absorption at a longer wavelength is also
Optical properties for 1 and 1.2H+ in CHCl3 and toluene.
present with a maximum at 618 nm and a molar absorption coef-
ficient of 16700 Mꢁ1 cmꢁ1 at this wavelength (See Figure S1 in the
Supplementary Material). This band is characteristic of n–p⁄ tran-
sitions made available by donation of electron density into the NDI
core primarily from the lone pairs on the N atoms immediately
adjacent to the NDI core [23]. Upon addition of TFA, the major
absorption peak at 618 nm begins to blue shift (Figure 1, spectra
DiSANDI
Solvent
kabs [nm]
kem [nm]
Uf
sflu [ns]
1
CHCl3
CHCl3
Toluene
Toluene
364, 618
363, 598
365, 612
363, 593
644
622
639
619
0.50
0.67
0.54
0.70
10.2
11.9
10.5
11.3
1.2H+
1
1.2H+
where [TFA] = 2.5, 5.0, 7.5, 10, 12.5, 15, 17.5, 20
lM) ultimately
by 515 cmꢁ1 to 598 nm ([TFA] = 20
lM) with no significant change
e.g. those of Abad et al. ꢀ12ꢂ [23], Daffy et al., ꢀ12ꢂ [24] and Zhou
et al., ꢀ24ꢂ [18], however, it could be useful, with appropriate
thresholds, for pH sensor applications [34]. A possible advantage
of the molecule having significant emission both when neutral
and protonated, is that it can be used simply as a fluorescent label
under a wide range of H+ concentrations.
in absorptivity. The addition of acid does not alter the NDI peaks
between 330 and 400 nm, implying that the protonation does not
affect transitions relating to the NDI core. Fluorescence excitation
spectra of 1 and 1.2H+ are very similar in shape to their absorption
spectra (see Figure S2 in the Supplementary Material).
Fluorescence emission spectrum of 1 in CHCl3 is shown in Fig-
ure 2 along with the titration of TFA to form 1.2H+. Clearly evident
is an increase in the total fluorescence with an unusual concomi-
tant blue shift of 22 nm (549 cmꢁ1) in the maxima as a protic re-
sponse [33]. The fluorescence quantum yields of the neutral and
protonated species were calculated in CHCl3 to be 0.50 and 0.67,
respectively, using Rhodamine 101 as a reference [32]. The stoichi-
ometry of binding was determined from fluorescence titration data
to be 1:2 (see Figure S3 in the Supplementary Material). Steady
state UV–Vis and fluorescence spectra indicate that protonation
occurs at both sites at essentially the same pH. This implies that
addition of a proton at one site has negligible effect on the pKa
of the other protonation site which is reasonable given their place-
ment out on the side arms of the molecule.
The blue shift that arises from the protonation of 1 is unusual for
a pH sensor [21,27] and is an advantageous property for such appli-
cations. Sensors with only an intensity change response need to be
effectively ‘ON–OFF’ systems in order to provide a clear ‘read-out’. A
sensor showing a spectral shift enables a ratiometric approach to be
taken by comparing the intensities at the respective maxima of the
bound and free states. The difference of 549 cmꢁ1 in the emission
maxima and the change in quantum yield of the protonated and
neutral forms of the molecule, yield an approximately eightfold
emission output difference (for the same fraction of light absorbed)
in the spectral region of 600–620 nm while still retaining 20% of the
total emission output of the protonated system. An approximately
8ꢂ output change is not as pronounced as other PET based sensors,
The optical properties the protic response of 1 was also exam-
ined in toluene. The compound displays only very marginally
blue-shifted (ꢀ100 cmꢁ1) absorption and emission in the main
transition in the visible spectrum and responds in a very similar
fashion to the addition of TFA with an increase in fluorescence
quantum yield from 0.54 to 0.70 and a blue shift of the emission
of 506 cmꢁ1. This is probably a result of the molecule being highly
symmetrical and change in solvent polarity has little effect on the
dipole of the molecule in both neutral and protonated forms. Data
for both solvents are given in Table 1.
The reversibility of the acid-induced fluorescence increase and
blue shift were tested by deprotonating 1.2H+ through addition
of triethylamine (TEA). Addition of TEA to 1.2H+ in CHCl3 led to a
decrease in fluorescence and concomitant red shifting of the emis-
sion maximum back to 644 nm, implying a complete switching to
1. Repeatability of the switching was tested by alternately adding
TFA and TEA to 1 in CHCl3 over nine cycles as shown in Figure 3.
Similar results were obtained in toluene. Control experiments
using N,N0-(dibutyl)-2,6-(dibutylamino)-1,4,5,8-naphthalene dii-
mide which contains simple alkylamino core substituents. No
changes in absorption or emission of this compound on addition
of TFA or TEA were observed indicating that the N atoms in the
piperidine rings of 1 are the protonation sites.
Time-resolved fluorescence profiles were recorded by TCSPC for
1 and 1.2H+ in toluene and CHCl3. Decay histograms could be well
fitted in all cases by a single exponential decay function convolved
with an instrument response function. TCSPS data and fitting de-
Figure 2. Corrected fluorescence emission spectra (kex = 550 nm) of 1 in CHCl3
([1] = 10
12.5, 15, 17.5, 20
lM) with the different concentrations of TFA ([TFA] = 0, 2.5, 5.0, 7.5, 10,
lM). The vertical dashed lines are a guide to the eye indicating the
Figure 3. Fluorescence emission spectra of 1 and 1.2H+ in CHCl3 solution with
alternating addition of TFA and TEA. Inset: Fluorescence intensity vs. number of
additions at 622 nm (kex = 550 nm). Note: the absorbance of 1.2H+ at 550 nm is
approximately 1.6 times that of 1 in CHCl3.
region of greatest intensity difference for 1 and 1.2H+ (approximately eightfold
when adjusted for the different fractions of light absorbed at the excitation
wavelength).

62
R.P. Cox et al. / Chemical Physics Letters 521 (2012) 59–63
tails are included in the Supporting Information. Fluorescence life-
times, flu (decay constant reciprocals) are given in Table 1 and are
facilitated by the hydrogen bond between the amine hydrogen
and the imide oxygen yielding a planar geometry. The lone pairs
of the N atoms of the piperidine rings were found to be slightly
angled towards the NDI core with a clear line of sight to the N
atoms immediately adjacent to the core. There is a small but dis-
cernable (at 99% contour) amount of electron density in the HOMO
of 1 present on the N atoms in the piperidine rings while none is
seen for 1.2H+. This suggests that additional electron donation
from the piperyl N atoms into the core is possible for 1 and pre-
sumably this would lower the energy of the transition, red-shifting
it. This effect would be lost on protonation thus the transition in
1.2H+ is blue-shifted with respect to 1.
s
relatively long, ranging between 10 and 12 ns. Such lifetimes are
similar to those of another di-allyl core substituted SANDI [13].
There is little variation with solvent, however the lifetime of 1 in
both solvents is reduced by about 10–15% compared to 1.2H+ con-
sistent with the reductions in QY also observed. These differences
suggest the presence of a weakly competitive quenching mecha-
nism operative in the neutral form of the molecule. Given the elec-
tron accepting properties of NDIs and electron donating properties
of amines, this is attributable to photoinduced electron transfer
(PET) from the piperidine on one of the core substituents to the
NDI core. Protonation of the piperyl N atoms would block this
mechanism, thus removing its quenching effect on QY and lifetime.
PET has been observed in many systems, usually being far more
efficient and leading to an ‘ON–OFF’ system [1].
The presence of PET can account for the differences in fluores-
cence lifetime and quantum yield of 1 and 1.2H+, however, does
not in itself, explain the blue shift on protonation. The origin of this
shift may arise, however, as a result of protonation by causing the
loss of a small amount of electron donation from the N atoms in
the piperidine rings of the substituents into the NDI core. It is well
established that the fluorescence band of SANDIs in the visible re-
gion is due to electron donation from N atoms immediately adjacent
to the core [15] and that increasing the number of substituents on
the core (hence the amount of electron donation into the core) leads
to further red shifts, i.e. kem tetra > di > mono [17]. The piperyl N
atoms may contribute some additional electron density via overlap
of their lone pair with that of the N atoms adjacent to the core add-
ing to the red shift of the transition. This effect would be lost when
the piperyl N atoms are protonated. Interaction between adjacent
piperidine rings in a donor–acceptor triad system was previously
seen by Brouwer et al. [35].
4. Conclusion
The synthesis of a new SANDI derivate substituted with two
core substituents each containing a piperidine ring has been
achieved. The compound has been fully characterised and found
to exhibit a reversible protic response through both colour and
emission. There is an increase in fluorescence quantum yield and
emission lifetime of 10–15% in the presence of H+ which is accom-
panied by a blue-shift of ꢀ500–550 cmꢁ1 in both absorption and
emission maxima. These effects are fully reversible on addition of
base and can be repeated many times. Protonation is occurring at
piperyl N atoms and this blocks a weakly competitive PET process
which leads to the emission increase. While the design is appropri-
ate to exhibit control, future work will be aimed at making the sys-
tem more optically sensitive to protonation and to diversify this
research into the sensing of exogenous anions.
Acknowledgement
Financial support from the Australian Research Council through the
Discovery Grant Scheme (DP1093337) is gratefully acknowledged.
To investigate this possibility, the ground state geometry and
HOMO orbitals of 1 and 1.2H+in the gas phase were modelled at
the B3LYP/6-31(d) level of theory [36] and are shown in Figure 4.
The distance between the two nitrogen atoms in the side groups
was found to be 4.1 Å. The lone pairs of the N atoms adjacent to
the core are conjugated with the electron cloud of the NDI core
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2011.11.028.
References
[1] N.I. Georgiev, V.B. Bojinov, Dyes Pigm. 84 (2010) 249.
[2] A.P. de Silva et al., J. Am. Chem. Soc. 129 (2007) 3050.
[3] A.P. de Silva, M.R. James, B.O.F. McKinney, D.A. Pears, S.M. Weir, Nat. Mater. 5
(2006) 787.
[4] S.J. Langford, T. Yann, J. Am. Chem. Soc. 125 (2003) 11198.
[5] Z.Z. Li, C.G. Niu, G.M. Zeng, Y.G. Liu, P.F. Gao, G.H. Huang, Y.A. Mao, Sens.
Actuators B 114 (2006) 308.
[6] A. Salvi, J.M. Quillan, W. Sadee, AAPS PharmSci. 4 (2002).
[7] N.A. Ritucci, J.S. Erlichman, J.B. Dean, R.W. Putnam, J. Neurosci. Methods 68
(1996) 149.
[8] S.V. Bhosale, S.V. Bhosale, M.B. Kalyankar, S.J. Langford, Org. Lett. 11 (2009)
5418.
[9] S.V. Bhosale, C.H. Jani, S.J. Langford, Chem. Soc. Rev. 37 (2008) 331.
[10] S.V. Bhosale, M.B. Kalyankar, S.V. Bhosale, S.J. Langford, E.F. Reid, C.F. Hogan,
New J. Chem. 33 (2009) 2409.
[11] D. Buckland, S.V. Bhosale, S.J. Langford, Tet. Lett. 52 (2011) 1990.
[12] K.P. Ghiggino et al., Adv. Funct. Mater. 17 (2007) 805.
[13] T.D.M. Bell et al., Chem. – Asian J. 4 (2009) 1542–1550.
[14] N. Sakai, J. Mareda, E. Vauthey, S. Matile, Chem. Commun. 46 (2010) 4225.
[15] C. Thalacker, C. Röger, F. Würthner, J. Org. Chem. 71 (2006) 8098.
[16] F. Würthner, S. Ahmed, C. Thalacker, T. Debaerdemaeker, Chem. – Eur. J. 8
(2002) 4742.
[17] R.S.K. Kishore et al., J. Am. Chem. Soc. 131 (2009) 11106.
[18] C. Zhou, Y. Li, Y. Zhao, J. Zhang, W. Yang, Y. Li, Org. Lett. 13 (2010) 292.
[19] R. Ameloot, M. Roeffaers, M. Baruah, G. De Cremer, B. Sels, D. De Vos, J.
Hofkens, Photochem. Photobiol. 8 (2009) 453.
[20] A.P. de Silva, J. Eilers, G. Zlokarnik, Proc. Natl. Acad. Sci. 96 (1999) 8336.
[21] S. Alp, S. Erten, C. Karapire, B. Köz, A.O. Doroshenko, S. Içli, J. Photochem.
Photobiol. A Chem. 135 (2000) 103.
[22] S.J. Langford, M.J. Latter, C.P. Woodward, Photochem. Photobiol. 82 (2006)
1530.
Figure 4. Ground state geometry of 1 (upper panel), and 1.2H+ (lower panel) with
HOMO orbitals shown at 99% contour. There is a small amount of electron density is
present on N atoms in piperidine rings for 1 which is not present for 1.2H+.

R.P. Cox et al. / Chemical Physics Letters 521 (2012) 59–63
63
[23] S. Abad, M. Kluciar, M.A. Miranda, U. Pischel, J. Org. Chem. 70 (2005) 10565.
[24] L.M. Daffy, A.P. de Silva, H.Q.N. Gunaratne, C. Huber, P.L.M. Lynch, T. Werner,
O.S. Wolfbeis, Chem. – Eur. J. 4 (1998) 1810.
[30] F. Chaignon, M. Falkenstrom, S. Karlsson, E. Blart, F. Odobel, L. Hammarstrom,
Chem. Commun., (2007) 64–66.
[31] X. Gao et al., Org. Lett. 9 (2007) 3917.
[25] L. Shen, X. Lu, H. Tian, W. Zhu, Macromolecules 44 (2011) 5612.
[26] H. Wang, H. Wu, L. Xue, Y. Shi, X. Li, Org. Biomol. Chem. 9 (2011) 5436.
[27] L. Zang, R. Liu, M.W. Holman, K.T. Nguyen, D.M. Adams, J. Am. Chem. Soc. 124
(2002) 10640.
[28] X.Y. Lu, W.H. Zhu, Y.S. Xie, X. Li, Y.A. Gao, F.Y. Li, H. Tian, Chem. – Eur. J. 16
(2010) 8355.
[32] T. Karstens, K. Kobs, J. Phys. Chem. 84 (1980) 1871.
[33] R. Bissell, A. Prasanna de Silva, H. Nimal Gunaratne, P. Mark Lynch, G. Maguire,
C. McCoy, K. Samankumara Sandanayake, Fluorescent PET (photoinduced
electron transfer) sensors, in: J. Mattay (Ed.), Photoinduced Electron Transfer
V, Springer, Berlin/Heidelberg, 1993, pp. 223–264.
[34] The inset to Figure 3 shows the intensity at 625 nm varying repeatably over a
5-fold range. The greatest intensity ratio was found to be at 605 nm with a
fluorescence intensity of the protonated form 15 times greater than the
neutral form.
[29] Compound 1 was formed in 69% yield. 1H NMR (300 MHz, 300 K): 9.45 (d,
5.7 Hz, 1H, NH), 8.10 (s, 2H, ArH), 7.2–7.4 (m, 10H, ArH), 4.14 (t, 5.7 Hz, 4H,
NCH2), 3.69 (m, 2H, NHCH), 3.57 (s, 4H, CH2Ar), 2.89 (m, 4H), 2.30 (t, 4H), 2.12
(m, 4H), 1.7 (m, 8H), 1.2–1.5 (m, 20H), 0.8 (m, 6H). 13C NMR (100 MHz, CDCl3,
300 K): 166.48, 163.42, 148.59, 129.43, 128.60, 127.42, 126.12, 121.59, 118.79,
102.35, 63.38, 52.25, 40.84, 40.06, 32.76, 32.18, 32.12, 29.99, 29.69, 29.59,
29.58, 29.53, 28.47, 27.57, 27.27, 23.70, 22.98, 22.96, 14.42, 14.40. ESMS m/z
calcd for C54H71N6O4 867.5530, found 867.5530.
[35] R.J. Willemse, J.J. Piet, J.M. Warman, F. Hartl, J.W. Verhoeven, A.M. Brouwer, J.
Am. Chem. Soc. 122 (2000) 3721.
[36] M.J. Frisch et al., R.A. GAUSSIAN 09, GAUSSIAN, Inc., Wallingford CT, 2009.