Diaryltrisphaeridine derivatives: Synthesis, experimental electrochemical and photophysical properties, and theoretical studies

Article abstract of DOI:10.1002/ejoc.201201051

A series of diaryltrisphaeridines have been synthesized by iodination of trisphaeridine and subsequent Suzuki coupling reactions. The photophysical and electrochemical properties of these compounds have been investigated and it has been found that they ar

Full text of DOI:10.1002/ejoc.201201051

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FULL PAPER
DOI: 10.1002/ejoc.201201051
Diaryltrisphaeridine Derivatives: Synthesis, Experimental Electrochemical and
Photophysical Properties, and Theoretical Studies
Alegría Caballero,[a] Pedro J. Campos,[a] and Miguel A. Rodríguez*[a]
Keywords: Heterocycles / Alkaloids / Luminescence / Sensors / Electrochemistry / Density functional theory
A series of diaryltrisphaeridines have been synthesized by
iodination of trisphaeridine and subsequent Suzuki coupling
reactions. The photophysical and electrochemical properties
of these compounds have been investigated and it has been
found that they are dependent on the aryl substituents. In
particular, the presence of dimethylamino groups induces a
bright-green emission, thus making this compound a promis-
ing candidate for fluorescence devices and molecular sen-
sors. Moreover, the green emission disappears on addition of
HCl and this material can therefore also be used as a sensor
for protons. The results have been rationalized by DFT and
TD-DFT theoretical studies.
found it of interest to assess the electrophilic aromatic io-
dination of trisphaeridine because this approach would
open the door to subsequent functionalization and the
preparation of compounds with enhanced π conjugation
and/or pharmacological activity.
Introduction
Polycyclic aromatic compounds that possess extended π
conjugation have been extensively used in optical and elec-
tronic devices, such as light-emitting diodes, photovoltaic
cells, and thin-film transistors, due to their optoelectronic
properties.[1] As a result, in recent years interest in this kind
of compound has risen markedly.[2] In the course of our
investigations we have developed a new method for the
preparation of several polycyclic heteroaromatic com-
pounds[3] based on the ring closure of photochemically gen-
erated iminyl radicals.[4] In particular, we have described an
alternative synthesis of alkaloid trisphaeridine ([1,3]di-
oxolo[4,5-j]phenanthridine, 1).[3] The structure of this poly-
cycle (Figure 1) is quite remarkable because the three aro-
matic rings have different electron densities, which makes
them more or less susceptible to electrophilic attack and
facilitates regioselective substitution. The ring that is fused
to the dioxolo group should have higher reactivity in such
reactions. Astonishingly, to the best of our knowledge, ex-
amples of trisphaeridine derivatives substituted at both the
7- and 11-positions of the phenanthridine skeleton have not
been described to date. This fact is remarkable if one con-
siders that phenanthridine alkaloids exhibit diverse and sig-
nificant pharmacological and biological activities.[5] In par-
ticular, trisphaeridine, which is found in the plants of the
family Amaryllidaceae, displays excellent antiproliferative
effects on both human and mouse cell lines.[6] Considering
our experience in the synthesis of iodo derivatives,[7] we
Figure 1. [1,3]Dioxolo[4,5-j]phenanthridine, trisphaeridine.
Results and Discussion
Iodination of Trisphaeridine
First, we addressed the preparation of the iodo deriva-
tives. The synthesis of 7,11-diiodo-[1,3]dioxolo[4,5-j]phen-
anthridine (2) is outlined in Scheme 1. The iodination reac-
tion of trisphaeridine with bis(pyridine)iodonium(I) tetra-
fluoroborate (Ipy2BF4) in the presence of CF3SO3H[7e] at
room temperature for 14 hours yielded the desired product
2 in 50% yield. The best result was obtained with 4 equiv.
of Ipy2BF4. The use of a smaller proportion of iodinating
reagent led to a mixture of mono- and diiodinated com-
pounds along with the starting trisphaeridine. Compound
[a] Departamento de Química, Centro de Investigación en Síntesis
Química, Universidad de La Rioja
c/. Madre de Dios, 51, 26006 Logroño, Spain
Fax: +34-941299621
E-mail: miguelangel.rodriguez@unirioja.es
Homepage: http://cisq.unirioja.es/grufor.php
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201051.
Scheme 1. Iodination of trisphaeridine.
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A. Caballero, P. J. Campos, M. A. Rodríguez
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2 is insoluble in chloroform and it was separated from the to –1.20 V for 4, but a correlation between the reduction
byproducts by washing with this solvent.
potentials and the electron-donating or -withdrawing
strengths of the substituents was not observed. The inten-
sities are very low in all cases except for compound 6. This
Synthesis of Diaryltrisphaeridine Derivatives
Table 1. Electrochemical properties of diaryltrisphaeridines in solu-
tion.[a,b]
Once the iodinated product had been obtained, we pro-
ceeded to test various coupling reactions. First, we at-
tempted to couple 2 with 1-ethynylbenzene under Sonoga-
shira reaction conditions.[8] Unfortunately, the reaction led
to an intractable mixture in which the mono- and disubsti-
tuted compounds were detected in very low yields. We next
tested the reaction of 2 under Suzuki coupling conditions.[9]
The optimized reaction conditions allowed the preparation
of 3 in 90% yield by heating a solution of 2, phenylboronic
acid (4 equiv.), [Pd(PPh3)4] (0.2 equiv.), and potassium
carbonate (2 equiv.) at reflux in dimethoxyethane for 5 days
(Scheme 2).
Compound
Ep [V]/I [μA/cm2]
red
Ep [V]/I [μA/cm2]
ox
3
4
5
6
7
8
–0.93/–15
–1.20/–26
–0.90/–19
–1.00/–103
–1.01/–34
–0.80/–25
–
–
–
–
–
0.98/36
[a] All potentials vs. Ag/AgCl. [b] 1 mm in CH2Cl2, 0.1 m TBAPF6.
Scan rate: 200 mV/s.
Scheme 2. Synthesis of diphenyltrisphaeridine 3.
In a similar way, more complex diaryltrisphaeridine de-
rivatives 4–8 were synthesized in an effort to explore the
influence of phenyl substituents (Figure 2). By using the
corresponding arylboronic acid, it was possible to incorpo-
rate phenyl (4), chloro (5), cyano (6), methoxy (7), or di-
methylamino (8) substituents into the structure.
Figure 2. Diaryltrisphaeridine derivatives 4–8 (isolated yields are
given in parentheses).
Electrochemical Properties
To elucidate the effect of the substituents on the electro-
chemical properties, cyclic voltammetry experiments were
performed on diaryltrisphaeridines 3–8 in solution in anhy-
drous CH2Cl2 (Table 1). The cyclic voltammograms (CVs)
of the compounds 6 and 8 are shown in Figure 3. Com-
pounds 3–7 exhibited irreversible single-electron reduction
peaks and oxidation did not occur, which implies a large
activation barrier to electron transfer.[10] In contrast, 8 ex-
hibited both reduction and oxidation peaks. The reduction
Figure 3. Cyclic voltammograms for the reduction of a) 6 and b) 8,
and c) the oxidation of 8 (1 mm in CH2Cl2, 0.1 m TBAPF6; scan
potentials (Epred) varied from –0.80 V (vs. Ag/AgCl) for 8 rate: 200 mV/s).
2
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Diaryltrisphaeridine Derivatives
can be accounted for by considering the large electron-with- Once again, the influence of the dimethylamino moiety
drawing effect of the cyano group, which should stabilize should be noted as this shifts the maximum absorption
the anion. On the other hand, an oxidation potential was wavelength to 507 nm for 8, that is, around 100 nm higher
only observed for 8, which has dimethylamino groups in its than those of compounds 3–7, and increases the quantum
structure, and it is reasonable to conclude that the amine efficiency to 40%, twice or more than the values obtained
nitrogen atoms are responsible for this behavior.
for the other diaryltrisphaeridines. This bright-green emis-
sion makes trisphaeridine derivative 8 a promising candi-
date for use in fluorescence devices and molecular sensors.
Photophysical Properties
The absorption spectra of 5, 7, and 8 in acetonitrile are
shown in Figure 4 (see the Supporting Information for the
spectra of 1, 3, 4, and 6) and the absorption maxima and
the corresponding molar extinction coefficients for 1 and
3–8 are given in Table 2. The maximum absorption wave-
length of trisphaeridine (1; 251 nm) is redshifted by intro-
ducing phenyl or biphenyl groups (269 or 294 nm for 3 or
4, respectively), whereas the introduction of substituents
onto the phenyl group causes a blueshift of the maximum
absorption wavelength of 5 (201 nm), 6 (235 nm), 7
(195 nm), or 8 (200 nm).
Figure 5. Emission spectra of 1 and 3–8 in acetonitrile; all the spec-
tra were recorded at a concentration of (1–2)ϫ10–5 m. Inset: photo
of the luminescence of 8.
Figure 4. Electronic absorption spectra of compounds 5, 7, and 8
in acetonitrile; all the spectra were recorded at a concentration of
(1–2)ϫ10–5 m.
Figure 6. Photograph showing the luminescence of compounds 1
and 3–8 in acetonitrile.
Table 2. Absorption and emission data for 1 and 3–8.
Compound
λabs [nm][a] log(ε/mcm–1) λem [nm][a]
Φf [%]
Theoretical Calculations
1
3
4
5
6
7
8
251
269
294
201
235
195
200
4.77
4.57
4.46
5.41
4.38
4.65
4.59
371
395
392
396
409
412
507
Ͻ0.1
15
18
12
16
20
40
In an effort to understand the experimental results, we
performed DFT and time-dependent DFT (TD-DFT) cal-
culations at the B3LYP level of theory with the 6-31G* ba-
sis set[11,12] on diaryltrisphaeridines 3–8 to determine the
effect that the substitution of 3 has on the HOMO and
LUMO energy levels (Table 3). The HOMO and LUMO
molecular orbital diagrams of derivatives 3–8 are shown in
[a] Absorption and emission maxima.
The emission spectra of 1 and 3–8 are shown in Figure 5 Figure 7.
and the emission maxima and fluorescence quantum yields
Our calculations gave values of –5.69 (HOMO), –1.28
are given in Table 2. The luminescence of compounds 1 and (LUMO), and 4.41 eV (energy gap) for 3. The presence of
3–8 in acetonitrile can be seen in Figure 6. The fluorescence extra conjugation (phenyl group, 4) leads to a slight in-
data demonstrate that the introduction of aryl groups crease in the energy of the HOMO (–5.66) and lowers the
strongly affects the quantum efficiencies of the compounds. energy of the LUMO (–1.39) to give an energy gap of
In fact, the value of Φf for 1 was determined to be Ͻ0.1%, 4.27 eV. The inclusion of an electron-withdrawing Cl (5) or
whereas for compounds 3–8 Φf varied from 12 to 40%. CN (6) group significantly lowers the energies of both the
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Table 3. Calculated HOMO and LUMO energies for 3–8.[a]
Compound
EHOMO [eV]
ELUMO [eV]
ΔE [eV]
3
4
5
6
7
8
–5.69
–5.66
–5.90
–6.20
–5.47
–4.87
–1.28
–1.39
–1.55
–2.12
–1.17
–0.95
4.41
4.27
4.35
4.08
4.30
3.92
[a] Optimized at the B3LYP/6-31G* level.
HOMO (–5.90 or –6.20, respectively) and the LUMO
(–1.55 or –2.12, respectively), which leads to a net decrease
in the energy gap of 0.06 or 0.33 eV, respectively. The effect
of an electron-donating OMe (7) or NMe2 (8) group is pre-
cisely the opposite in that it significantly raises the energies
of both the HOMO (–5.47 or –4.87 eV, respectively) and
the LUMO (–1.17 or –0.95 eV, respectively), although the
energy gaps are also decreased by 0.11 and 0.49 eV.[13]
Therefore the presence of NMe2 decreases the HOMO–
LUMO energy gap markedly and this implies that it is
easier to promote an electron from the HOMO to the
LUMO. These calculations clearly indicate that the in-
clusion of substituents on the phenyl ring significantly af-
fects the electronic and photophysical properties.
Furthermore, to better illustrate the calculated effect of
substituents on the HOMO and LUMO energies, the energy
values were plotted against the Hammett substituent con-
stants (σp;[14] Figure 8). It can be seen that the HOMO and
LUMO energies of 3–8 have a linear correlation with the σp
of the substituents on the phenyl groups; both have a nega-
tive slope. Thus, as mentioned previously, the electron-do-
nating groups cause an increase in orbital energy whereas
the electron-withdrawing groups cause a decrease in orbital
energy. The correlation coefficients of σp with the HOMO
and LUMO energies are 0.994 and 0.930, respectively.
As shown in Figure 7, in 3–6 the HOMO orbital is lo-
cated mainly on the phenanthridine skeleton, whereas the
density of the LUMO orbitals is distributed throughout al-
most the entire structure. The presence of methoxy groups
in 7 leads to a distribution of the HOMO density through-
out the structure and removes part of the LUMO density
from the phenanthridine skeleton. This effect is enhanced
in 8, in which the dimethylamino groups cause the HOMO
orbital to be mainly located on the dimethylaminophenyl
moiety and the majority of the density of the LUMO re-
sides on the phenanthridine skeleton, which indicates that
the low-lying transition involves redistribution of charge
from the nitrogen lone-pair to the phenanthridine skeleton.
Such a charge transfer should account for the different elec-
trochemical and photophysical properties of 8 observed ex-
perimentally.
The involvement of frontier orbitals in the photolumines-
cent properties of 8 was confirmed by TD-DFT calcula-
tions. The most significant theoretical excitations, oscillator
strengths (intensity), and orbital contributions are shown in
Table 4 (see the Supporting Information for the calculated
Figure 7. HOMO and LUMO molecular orbital diagrams of com-
pounds 3–8 calculated at the B3LYP/6-31G* level of theory.
values for 3–7). The calculated π–π* S0 ǞS1 excitation has most important contribution to this excitation is provided
an oscillator strength f of 0.24 with a λcalcd. of 364 nm. The by the transition between the HOMO (122) and LUMO
4
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Diaryltrisphaeridine Derivatives
should have a noticeable effect on the luminescence. With
this in mind, a series of luminescence assays were carried
out on 8 in the presence of varying amounts of HCl. The
results are shown in Figure 9. It can be seen that the pres-
ence of 0.4 equiv. of HCl reduces the luminescence by ap-
proximately half, whereas the luminescence virtually disap-
pears upon the addition of 0.8 equiv. of HCl, which indi-
cates that 8 could be used for the detection of protons.
Interestingly, 1 equiv. of HCl seems to be sufficient to block
the emission at 507 nm. To obtain a greater understanding
of the protonated species, we also performed a series of
DFT calculations on 8 with a proton added to each of the
nitrogen atoms. In the most stable structure the proton was
located on the phenanthridine nitrogen (9). TD-DFT calcu-
lations showed an excitation from the HOMO–1 to the
LUMO+2 (f = 0.25) with a λcalcd. of 313 nm (Figure 10).
Figure 9. Emission spectra of 8 in acetonitrile in the presence of
0.0, 0.4, and 0.8 equiv. of HCl.
Figure 8. Correlation of a) the HOMO and b) the LUMO energies
with the Hammett constants σp.
(123), which clearly matches the charge transfer. The calcu-
lated excitation centered at 337 nm (f = 0.20) also has a high
involvement of the HOMO and LUMO orbitals (HOMO–
2ǞLUMO and HOMOǞLUMO+1).
Table 4. Singlet excitations for 8 calculated by TD-DFT.
λcalcd. [nm]
f [a]
Contributions[b]
364
337
275
264
238
0.24
0.20
0.37
0.23
0.32
122Ǟ123 (45)
120Ǟ123 (10); 122Ǟ124 (31)
119Ǟ123 (15); 120Ǟ124 (14); 122Ǟ126 (6)
119Ǟ124 (12); 122Ǟ125 (7); 122Ǟ127 (10)
114Ǟ123 (7); 120Ǟ127 (13); 122Ǟ128 (6)
[a] Oscillator strength. Only the main intensities (Ն0.20) are shown.
[b] Value is [coeff]2 ϫ100. Only the main contributions (Ն5) are
shown.
Figure 10. HOMO–1 and LUMO+2 molecular orbitals (B3LYP/
6-31G*) and single-excitation calculations (TD-DFT/6-31G*) for
model compound 9.
Proton Sensor
The results suggest that the phenanthridine unit must be
Given that the electronic characteristics of the dimeth- the fluorophore whereas the presence of the different sub-
ylamino group on the phenyl ring confer a special lumines- stituents modulates the intensity and wavelength of the
cent behavior, blocking of the nitrogen atom electron pair emission band.
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A. Caballero, P. J. Campos, M. A. Rodríguez
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Conclusions
We have prepared a series of diaryltrisphaeridines by
iodination of trisphaeridine and subsequent Suzuki cou-
pling reaction. These compounds were investigated by pho-
tophysical and electrochemical methods. The properties of
the materials depend on the aryl substituent. In particular,
the presence of dimethylamino moieties induces a bright-
green emission, which makes this a promising compound
for use in fluorescence devices and molecular sensors.
Moreover, the green emission disappears on addition of
HCl and this material can therefore also be used as a proton
sensor. The results have been rationalized by DFT and TD-
DFT theoretical studies.
[2]
[3]
[4]
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7,11-Diiodo[1,3]dioxolo[4,5-j]phenanthridine (2): Bis(pyridine)-
iodonium(I) tetrafluoroborate (Ipy2BF4; 17 mmol, 6.5 g) was dis-
solved in dry CH2Cl2 (50 mL) in an oven-dried flask at room tem-
perature under argon. Trisphaeridine (1; 4.4 mmol, 0.9 g) was dis-
solved in dry CH2Cl2 in another flask under argon and this solu-
tion was added to the Ipy2BF4 solution. A solution of CF3SO3H
(35 mmol, 3.1 mL) in CH2Cl2 (5 mL) was added over a period of
3 min to the magnetically stirred mixture. Finally, the reaction mix-
ture was stirred overnight at room temperature. The reaction was
treated with aqueous sodium thiosulfate, extracted with CH2Cl2,
washed with brine, and dried (Na2SO4). The solvent was removed
under reduced pressure, the residue was washed with chloroform,
and the resulting solid was filtered off to give compound 2 as a
brown solid.
[6]
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General Procedure for the Preparation of Diaryltrisphaeridines: The
diiodinated trisphaeridine 2 (0.2 mmol, 0.1 g) and tetrakis(triphen-
ylphosphane)palladium(0) (0.05 mmol, 50 mg) were dissolved in
1,2-dimethoxyethane (50 mL) and the mixture was stirred for
20 min at room temperature. A solution of potassium carbonate
(0.7 mmol, 0.1 g) and phenylboronic acid (0.9 mmol, 0.1 g) in water
(8 mL) was then added and the resulting mixture was heated at
92 °C for 3–7 d. The mixture was extracted with diethyl ether (3ϫ
50 mL) and the organic layers were dried (Na2SO4), filtered, and
evaporated under reduced pressure. The resulting diaryltrisphaerid-
ine was purified by column chromatography (silica gel, hexane/
EtOAc, 7:3). The coupling reaction should be repeated twice for
the complete conversion of 2 into diaryltrisphaeridines 5–8.
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Theory, Practice and Application, Royal Society of Chemistry,
Cambridge, 2003.
All calculations were carried out by using the Gaussian 03 pro-
gram package. The geometry was fully optimized in the ground
state without any symmetry constraint for all model com-
pounds at the B3LYP/6-31G* level of theory. Subsequent cal-
culations of the orbitals were performed.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T.
Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar,
J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N.
Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pom-
elli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Supporting Information (see footnote on the first page of this arti-
cle): Complete experimental procedures, characterization data for
new compounds, and Cartesian coordinates and single-excitation
calculations for the geometries discussed in the text.
[11]
[12]
Acknowledgments
This work was partially supported by the Spanish Ministerio de
Economía y Competitividad (grant number CTQ2011-24800).
A. C. thanks the (CSIC) for a fellowship.
[1] a) P. M. Beaujuge, J. R. Reynolds, Chem. Rev. 2010, 110, 268;
b) T. J. J. Müller, U. H. F. Bunz (Eds.), Functional Organic Ma-
terials, Wiley-VCH, Weinheim, Germany, 2007; c) K. Mullen,
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Job/Unit: O21051
/KAP1
Date: 27-09-12 10:29:44
Pages: 8
Diaryltrisphaeridine Derivatives
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J.
Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W.
Wong, C. Gonzalez, J. A. Pople, Gaussian 03, revision C.02,
Gaussian, Inc., Wallingford, CT, 2004.
[13]
A similar effect on the energy of the frontier orbitals depending
on the type of substituent has been observed in other unsatu-
rated systems, see, for example: I. Fleming, Frontier Orbitals
and Organic Chemical Reactions, Wiley, Chichester, UK, 1976.
[14] C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165.
Received: August 3, 2012
Published Online: ᭿
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Job/Unit: O21051
/KAP1
Date: 27-09-12 10:29:44
Pages: 8
A. Caballero, P. J. Campos, M. A. Rodríguez
FULL PAPER
Diaryltrisphaeridine Derivatives
The properties of diaryltrisphaeridines de-
pend on the aryl substituents. In particular,
dimethylamino groups induce a bright-
green emission, which makes this a promis-
ing compound for use in fluorescence de-
vices and molecular sensors. The green em-
ission disappears on addition of HCl and
this material can therefore also be used as
a proton sensor. The results are rational-
ized by theoretical studies.
A. Caballero, P. J. Campos,
M. A. Rodríguez* .............................. 1–8
Diaryltrisphaeridine Derivatives: Synthesis,
Experimental Electrochemical and Photo-
physical Properties, and Theoretical Stud-
ies
Keywords: Heterocycles / Alkaloids / Lumi-
nescence / Sensors / Electrochemistry /
Density functional theory
8
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