Photoinduced electron and energy transfer in aryldihydropyridines

Article abstract of DOI:10.1021/jo9010816

(Chemical Equation Presented) Dimethyl 2,6-dimethyl-1,4-dihydropyridine-3, 5-dicarboxylates (Hantzsch DHPs) fluoresce weakly in fluid solution. However, these compounds exhibit an efficient fluorescence both in a viscous medium (glycerin) at roomtemperatu

Full text of DOI:10.1021/jo9010816

pubs.acs.org/joc
Photoinduced Electron and Energy Transfer in Aryldihydropyridines
Angel J. Jimenez,§ Maurizio Fagnoni, Mariella Mella, and Angelo Albini*
Department of Organic Chemistry, University of Pavia, v. Taramelli 10, 27100 Pavia, Italy.
Present address: Department of Organic Chemistry, Universidad Autonoma de Madrid, Cantoblanco,
28049 Madrid, Spain
§
angelo.albini@unipv.it
Received May 22, 2009
Dimethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylates (Hantzsch DHPs) fluoresce weakly
in fluid solution. However, these compounds exhibit an efficient fluorescence both in a viscous
medium (glycerin) at room temperature and in a glassy matrix at 77 K (but no phosphorescence, since
ISC is negligible). DHPs bearing an aryl group in position 4 have been synthesized. These contain two
different π systems separated by an sp3 carbon (DHP-Ar dyads). The occurrence of energy and
electron transfer processes between the chromophores is investigated through luminescence mea-
surements. In particular, when 3Ar emits at a slow rate (e.g., Ar = phenanthryl) or not at all (Ar =
nitrophenyl) the intradyad forward/backward electron transfer sequence offers a path for arriving at
the DHP-localized triplet and the corresponding phosphorescence is observed. When 3Ar emits at a
faster rate (Ar = acylphenyl), the phosphorescence from either of the two localized triplets, 3Ar or
3DHP, can be observed depending on λexc. When the aryl group has a triplet energy lower than that of
3DHP, this functions as emitting (4-cyano-1-naphthyl) or nonemitting (MeO2CCHdCHC6H4)
energy sink. The results document the possibility of building tailor-made Hantzsch aryldihydropyr-
idines as versatile photoactivated dyads.
Introduction
based on the use of transition metal ions, and demonstrated
to be useful for a variety of light-induced applications, from
molecular machines to systems that mimic chlorophyll
photosynthesis.1 However, simple organic dyads contain-
ing two separated chromophores may likewise be useful in
this respect and have the advantage of possessing a strong
scaffold of covalent bonds that can be varied almost at will
in order to fit the desired properties by the synthetic
methods of organic chemistry.2 We recently pointed out a
simple system that seems to incorporate a number of
potentially useful characteristics, that of 4-aryl-1,4-dihy-
dropyridines.3 Dyads of this structure have been only
sparsely investigated up to now. A few photochemical
applications have been demonstrated, e.g., for photosensi-
tive polymers,4 and more can be envisaged, such as the use
of dihydropyridines of appropriate structure as biosensors
Supramolecular assemblies of increasing sophistication
have been developed over the last decades, in most cases
*To whom correspondence should be addressed. Fax: 39 0382 987323.
(1) Gust, D.; Moore, T. A. Science 1989, 244, 35. Schanze, K. S.; Walters,
K. A., Molecular and Supramolecular Photochemistry; Ramamurthy, V.,
Schanze, K. S., Eds.; M. Dekker: New York, 1998; Vol. 2, p 75. Gust, D.;
Moore, T. A.; Moore, A. L. Chem. Commun. 2006, 1169. Durrant, J. R.;
Haque, S. A.; Palomares, E. Chem. Commun. 2006, 3279. Lee, C. H.; Guo, J.;
Chen, L. X.; Mandal, B. K. J. Org. Chem. 2008, 73, 8219. Armaroli, N.
Photochem. Photobiol. Sci. 2003, 2, 73.
(2) Verhoeven, J. W.; van Ramesdonk, H. J.; Groeneveld, M. M.;
Benniston, A. C.; Harriman, A. ChemPhysChem 2005, 6, 2251. Fukuzumi,
S. Pure Appl. Chem. 2007, 79, 981. D’Souza, F.; Chitta, R.; Ohkubo, K.;
Tasior, M.; Subbaiyan, N. K.; Zandler, M. E.; Rogacki, M. K.; Gryko, D. T.;
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Fukuzumi, S. J. Am. Chem. Soc. 2008, 130, 14263. Jimenez, M. C.; Miranda,
M. A.; Termos, R. Chem. Soc. Rev. 2005, 34, 783. Gonzalez-Rodriguez, D.;
Torres, T.; Herranz, M. A.; Echegoyen, L.; Carbonell, E.; Guldi, D. M.
Chem.;Eur. J. 2008, 14, 7670. Kawauchi, H.; Suzuki, S.; Kozaki, M.;
Okada, K.; Islam, D. M. S.; Araki, Y.; Ito, O.; Yamanaka, K. J. Phys.
Chem. A 2008, 112, 5878. Nakamura, T.; Araki, Y.; Ito, O.; Takimiya, K.;
Otsubo, T. J. Phys. Chem. A 2008, 112, 1125. Fujitsuka, M.; Harada, K.;
Sugimoto, A.; Majima, T. J. Phys. Chem. A 2008, 112, 10193. Shibano, Y.;
Umeyama, T.; Matano, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Imahori,
H. Org. Lett. 2006, 8, 4425. Yongjun; Zheng, H.; Li, Y.; Wang, S.; Wu, Z.;
Liu, P.; Gao, Z.; Liu, H.; Zhu, D. J. Org. Chem. 2007, 72, 2878.
(3) Fasani, F.; Fagnoni, M.; Dondi, D.; Albini, A. J. Org. Chem. 2006, 71,
2037.
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DOI: 10.1021/jo9010816
r
Published on Web 07/31/2009
J. Org. Chem. 2009, 74, 6615–6622 6615
2009 American Chemical Society

Jimenez et al.
JOCArticle
SCHEME 1
CHART 1
or for the mapping of enzyme kinetics by fluorescence
similarly to NADH.5
A convenient basis structure is that of 1,4-dihydropyridine-
3,5-dicarboxylates that are available through the versatile
Hantzsch synthesis.6 One can imagine that the ester functions
can be elaborated in order to obtain molecules of different
chemical characteristics, solubility, and ability to form com-
plexes. Independently from this, the light-activated moiety
can be freely elaborated by choosing the aldehyde in the
Hantzsch synthesis. In particular, the dihydropyridine moiety
may be exploited for an intramolecular electron transfer
process by introducing a (substituted) aryl group in position
4 that functions as a reducible moiety (see Scheme 1).7
The dihydropyridine (DHP) chromophore (here and in the
following reference is always to Hantzsch dihydropyridines,
containing an enaminocarboxylate chromophore) is endowed
by a strongly allowed absorption with the maximum around
350 nm and extending to the border of the visible. Thus, all of
the UV is absorbed and when an appropriate electron-accept-
ing substituent is present inposition 4, intramolecular electron
transfer can be quantitative and convert light into charge
Results
In the present work, the luminescence of a series of DHP
derivatives bearing an aryl group in 4 (1-3) was examined.
The aryl group introduced was chosen in such a way that the
singlet excited state (1Ar) was in any case higher than 1DHP,
while the triplet (3Ar) could be either higher or lower than
3DHP; furthermore, structures where photoinduced electron
transfer between the two moieties was predicted (by the
Weller equation) to be either endo- or exoergonic were
included (see below). The compounds studied are shown in
Chart 1 and included phenyl, naphthyl, and phenanthryl
derivatives with various electron-withdrawing substituents.
Some of these compounds had been previously reported and
some were purposely synthesized; in every case, the samples
were prepared via the Hantzsch synthesis as detailed in the
Experimental Section.
In the series of phenyl derivatives 1 various substituents
were introduced in position 3 in order to modulate the
electron-accepting properties. Presumably the same results
would be obtained with the same substituents in position
4 and, in order to have an indication of this point, a
4-substituted derivative, compound 1e, was also examined.
For the sake of comparison, the 4-methyl-1,4-dihydropyr-
idine 4, lacking the aryl substituent, was examined under the
same conditions. In every case, the absorption spectrum was
˚
separation over a ca. 6 A distance. Indeed, an example was
previously found for the case of 4-(3-nitrophenyl)-1,4-dihy-
dropyridine and some related compounds.3 It was observed
that in the parent 4-phenyl derivative (Ph-DHP) there was no
interaction between the two moieties, as demonstrated by the
fact thatthe weak fluorescence ofthis compoundwas identical
to that of 4-unsubstituted DHP and was thus attributed to the
unperturbed localized singlet (Ph-1DHP). In contrast, with
the 4-(3-nitrophenyl) derivative no fluorescence was observed
at room temperature, a result attributed to intramolecular
electrontransfer (ET) between the two moieties present togive
the zwitterion 3-NO2Ph•--DHP•+. In a glassy matrix at 77 K,
a phosphorescence attributable to a localized triplet (3-NO2-
Ph-3DHP) was observed, despite the fact that ISC from 1DHP
to 3DHP was negligible (indeed no phosphorescence was
observed from 4-unsubstituted DHP). It was suggested that
this resulted from intramolecular ET to 3-NO2Ph•--DHP•+
followed by back electron transfer to the DHP localized
triplet, favored by the fact that in matrix the zwitterion was
higher in energy than in solution, due to the missing stabiliza-
tion by solvation.
This simple dyad seemed worth further examination in
view of the many variations that could be introduced by
taking advantage of the Hantzsch synthesis.
(5) Wang, H. W.; Gukassyan, V.; Chen, C. T.; Wei, Y. H.; Guo, H. W.;
Yu, J. S.; Kao, F. J. J. Biomed. Opt. 2008, 13, 054011/1. Ramanujan, V. K.;
Jo, J. A.; Cantu, G.; Herman, B. A. J. Microsc. 2008, 230, 329. Katz, E.;
Heleg-Shabtai, V.; Bardea, A.; Willner, I.; Rau, H. K.; Haehnel, W. Biosens.
Bioelectron. 1998, 13, 741.
(6) Hantzsch, A. Liebigs Ann. 1882, 215 (1), 72. Lavilla, R. J. Chem. Soc.,
Perkin Trans. 1 2002, 1141.
(7) Triggle, A. M.; Shefter, E.; Triggle, G. J. Med. Chem. 1980, 23, 1442.
Fossheim, R.; Joslyn, A.; Solo, A. J.; Luchowski, E.; Rutlege, A.; Triggle, D.
J. J. Med. Chem. 1988, 31, 300. Wang, S. D.; Herbette, L. G.; Rhodes, D. G.
Acta Crystallogr., Sect. C 1989, C45, 1748. Palmer, R. B.; Andersen, N. H.
Bioorg. Med. Chem. Lett. 1996, 6, 2173.
FIGURE 1. Absorption spectrum of dihydropyridine 2a in aceto-
nitrile (2 ꢀ 10-4 M, optical path 1 cm).
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Jimenez et al.
JOCArticle
TABLE 1. Prompt Emission for 4-Substituted 1,4-Dihydropyridines 1-4
MeCN, 293 K
glycerin,a 293 K
λem
EPA,b 77 K
compd
4-substituent
λem
Φf
Φf
λem
Φf
4
1a
Me
Ph
414, 436
414, 436
0.006
0.005
445
442
0.27
0.33
0.8
0.8
410, 435
1b
1c
1d
1e
1f
2a
2b
3
Ph-3-CHO
Ph-3-COPh
Ph-3-NO2
Ph-4-NO2
Ph-3-CHdCHCO2Me
1-Np
410, 436
410, 435
0.22
0.24
416, 434
very weak
very weak
413, 436
416, 436
473
0.003
442
0.07
0.004
0.008
0.043
0.003
410, 435
0.18
0.35
0.22
0.8
442
448
0.34
0.03
1-Np-4-CN
9-Phen
433, 452
407, 429
418, 438
aContaining 5% methanol. bDiethyl ether-pentane-ethyl alcohol 5-5-2.
FIGURE 2. Fluorescence spectra of (a) dihydropyridines 1a (blue), 1c (green), 2b (red), and 4 (magenta) in acetonitrile at 20 °C and
(b) dihydropyridine 2a 1 ꢀ 10-4 M in ethanol (green) and in glycerine (red) at 20 °C, λexc 295 nm.
identical with the sum of the spectra of suitable models with a
single chromophore, e.g., the spectrum of the naphthyl
derivative 2a (see Figure 1) was undistinguishable from
the point-to-point addition of those of compound 4 and
1-methylnaphthalene. This had been previously observed8
for the case of a pair of phenyl-DHP and is general for the
present compounds. Thus, it can be concluded that there is
no mutual interaction between the two chromophores in the
ground state. As a consequence of the choice of the aryl
substituents (high-lying 1Ar state), with all of the compounds
it was possible to distinguish at the red end of the spectrum a
region where the DHP chromophore was exclusively excited
(around 360 nm for 2a in Figure 1). On the other hand, DHP
has a minimum around 290 nm (ε290 < 0.1ε360) where most
of the Ar chromophores considered absorb intensively
FIGURE 3. Fluorescence spectra of dihydropyridines 2b (blue) and
3 (magenta) in ether-pentane-alcohol at 77 K, λexc 300 nm.
(see Figure 1). This makes it possible to guarantee excitation
of the last moiety.
A few 4-phenyl-1,4-dihydropyridines, including 1a, had
been previously examined and found to be weakly fluorescent
in solution,3,9 with emission maxima at ca. 415 and 435 nm
and emission quantum yield ,0.01. Excitation of the present
4-aryl derivatives at 360 nm, where only the DHP chromo-
phore absorbs, gave a similar result, with an even lower ΦF (see
Table 1) and a short lifetime, τF , 1 ns. With the nitrophenyl
derivatives 1d,e the emission was all but undetectable.
Irradiation at 290-300 nm or at 260 nm, where with all of
these compounds, except 1a, the aryl chromophores
contributed .50% to the absorption (e.g., benzoylphenyl,
naphthyl, phenanthryl), led to no different emission. The
only exception among the cases considered was that of the
(4-cyanonaphthyl) derivative 2b, where a more intense emis-
sion, with a profile different from that of both DHP and
1-cyanonaphthalene, was observed (see Figure 2a). This had
a longer lifetime, τF 1.3 ns.
The effect of the solvent was investigated. Apparently,
polarity or proticity had no major effect, with essentially the
same emission in cyclohexane, ethyl acetate, and ethanol.
However, viscous solvents changed the result and with
some of these compounds the fluorescence largely increased
in glycerin. In this solvent the emission was somewhat
€
(8) Kurfurst, A.; Kuthan, J. Collect. Czech., Chem. Commun. 1983, 43,
1422.
(9) Deme, A. K.; Lusis, V. K.; Dubur, G. Y. Khim. Geterotsikl. Soedin.
ꢁ
1987, 67.Pavez, P.; Encinas, M. V. Photochem. Photobiol. 2007, 83, 722.
J. Org. Chem. Vol. 74, No. 17, 2009 6617

Jimenez et al.
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FIGURE 4. Phosphorescence spectra of dihydropyridines (a) 1b by irradiation at 266 (red), 300 (blue), 320 (green), and 360 nm (violet) and
(b) 1c by irradiation at 266 (magenta), 300 (blue), and 360 nm (green) in ether-pentane-alcohol at 77 K.
red-shifted, more intensive (by a factor >30) (see Figure 2b),
and longer lived, e.g., τF increased from ,1 to 1.8 ns in the
case of 2a. The intensity increment was less marked for the
benzoylphenyl derivative 1c and did not take place with the
nitro compounds, where fluorescence remained almost un-
detectable. As for the cyanonaphthyl derivative 2b, its
peculiar emission was not strengthened in glycerin.
The luminescence in a glassy matrix, ether-pentane-
alcohol glass at 77 K, was then examined. As for the prompt
emission, this was more clearly vibrationalized under these
conditions, but had essentially the same shape as the blue
fluorescence in solution at 20 °C. What was different was the
intensity. This was much higher and indeed approached a
unitary quantum yield for some compounds, such as 1a and
3, while with others, in particular the nitrophenyl derivatives,
it remained quite low. The spectra reported in Figure 3 result
from irradiation at 360 nm, but again irradiation at 300 nm
produced no new emission. The cyanonaphthyl derivative 2b
exhibited an emission of its own, at shorter wavelength than
FIGURE 5. Phosphorescence spectra of nitrophenyldihydropyri-
dine 1d (red) and 1e (blue) in ether-pentane-alcohol at 77 K,
λ
exc 300 nm.
at room temperature, and likewise independent of λexc
.
The phosphorescence was likewise examined under the
same conditions and was rather varied among the com-
pounds examined. The phenyl derivative 1a exhibited only
a weak phosphorescence, practically the same as the methyl
derivative 4. When an aldehyde or ketone chromophore was
present in the acceptor moiety, as in 1b,c, the result depended
on the irradiation wavelength. At shorter λexc, where most of
the light was absorbed by the aromatic carbonyl, the typical
vibrationalized emission of the benzaldehyde or benzophe-
none chromophore was the main component. The decay of
such emission was similar to that measured with the corre-
sponding carbonyl under the same conditions. On the con-
trary, irradiation at a longer wavelength, where the DHP
chromophore was mainly or exclusively excited, generated
a different, and somewhat longer lived, emission at ca. 510,
540 nm (Figure 4a,b).
As for the nitrophenyl derivatives 1d,e, both of them
exhibited a marked phosphorescence that was similar in
shape to that obtained with 1b,c by 360 nm irradiation (see
Figure 5), but in this case was observed also by short
wavelength irradiation. With 1d,e the yellow phosphorescence
could be visually appreciated, since, different from the other
compounds, it was not mixed with the blue fluorescence.
FIGURE 6. Phosphorescence spectra of dihydropyridines 2a
(blue), 2b (green), and 3 (red) in ether-pentane-alcohol at 77 K,
λ
exc 300 nm.
The ethoxycarbonylvinylphenyl DHP 1f, on the other hand,
exhibited no phosphorescence at all.
A long-lived yellow emission with maxima around 500 and
540 nm, similar to that of 1b-e, was exhibited also by the
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Jimenez et al.
JOCArticle
TABLE 2. Delayed Emission for 4-Substituted 1,4-Dihydropyridines
SCHEME 2. Photoinduced Electron and Energy Transfer in
1-3 in Ether-Pentane-Alcohol at 77 Ka
DHP-Ar Dyadsa
compd
4-substituent
λem
ET,c kcal/mol
b
1b
Ph-3-CHO
508, 544d
59.7
412, 432,462e
514, 547d
1c
Ph-3-COPh
59.6
428, 446, 481, 521e
513, 547
520, 555
1d
1e
1f
2a
2b
3
Ph-3-NO2
Ph-4-NO2
Ph-3-CHdCHCO2Me none
1-Np
1-Np-4-CN
9-Phen
490, 504, 521
516, 562, 611
502, 540
59.7
56.6
60.2
aIn diethyl ether-pentane-ethyl alcohol 5-5-2. bWhen not other-
wise stated, independent of λexc. cThe energy of triplet 3ArH is taken as
an indication of that of the localized DHP-3Ar triplet. dBy irradiation at
360 nm. eBy irradiation at 300 nm.
aThe range of variation of the aryl localized triplets (DHP-3Ar) and radical ion
pairs (DHP•þ-Ar•-) is indicated.
phenanthryl derivative (3, see Figure 6; Table 2). With the
naphthyl derivative the emission was again similar, but
weaker and somewhat blue-shifted (see Figure 6). In con-
trast, with the cyanonaphthyl substituted derivative 2b a
different greenisch phosphorescence with a marked vibra-
tional strucutre was recorded. This was identical with that
obtained from an authentic sample of 1-cyanonaphthalene.
With all of these compounds, the observed emission did not
depend on the excitation wavelength.
Some ancillary experiments were carried out. In particu-
lar, it was tested whether the DHP and aryl moieties inter-
acted also intermolecularly, when the chromophores were in
separated molecules. Thus, it was observed that the fluores-
cence of dihydropyridine 4 was minimally quenched by
arenes, such as naphthalene or phenanthrene. A significant
quenching was observed with 1-cyanonaphthalene and
nitrobenzene (KSV = 3 and 32 M-1 respectively in ethanol
at 20 °C). A preliminary test on the photochemical reactivity
of these compounds was carried out and it was found that
none of them was highly reactive (Φr < 0.01), except for 1f,
which appeared to undergo geometric isomerization.
and lifetime in going to a viscous solvent as glycerin or to a
glassy state at 77 K. The two situations are not equivalent, as
is apparent when comparing the spectra. As an example
compound 1a (and similarly 4) exhibits a broadband begin-
ning at 395 nm and with a maximum around 442 nm in
glycerin, somewhat red-shifted with respect to nonviscous
liquids. In EPA glass at 77 K the spectrum is vibrationalized
(λmax = 410, 435 nm) but not red-shifted with respect to a
fluid solution at room temperature. These results suggest
that, besides the freezing of conformational equilibria due to
the high viscosity, there is also a specific stabilization in
glycerin, not in EPA glass. The large increase of fluorescence
is not accompanied by a significant phosphorescence in
viscous or glassy media with the above derivatives, in accord
with the notion that ISC within the DHP moiety (1DHP-Ar
to 3DHP-Ar) has no role.
As for the aryl moiety, this was chosen in such a way that in
every case the singlet lies higher in energy than 1DHP, thus
long-wavelength irradiation gave 1DHP-Ar. With all of the
derivatives tested, except 1a, irradiation at ca. 300 nm, where
the absorptivity of DHP is low (ε < 102), makes possible the
selective, or at least predominant, excitation of the Ar
chromophore. Thus, it is also possible to have DHP-1Ar as
the initial state. However, no fluorescence from such chro-
mophores is observed, neither in fluid solution nor in a rigid
matrix. This can be due either to an intrachromophore or to
an interchromophore process. In the first case the localized
singlet is rapidly depopulated, typically because of a fast ISC
Discussion
In the dyads we considered, an aromatic moiety is present
and separated from the DHP chromophore by a sp3 carbon
(DHP-Ar). In these compounds, donor and acceptor moi-
˚
eties are ca. 6 A apart and are held in a roughly perpendicular
position by the bulk of the two ester groups.3,7 There is no
significant interaction between the two moieties in the
ground state, and the absorption of each compound corre-
sponds to the sum of those of the two separated chromo-
phores. However, the data above show a quite varied
panorama in the luminescence of such aryldihydropyridines,
revealing that electronic excitation may lead to a different
end result.
1
3
from Ar to Ar, as one would expect with the aromatic
carbonyls 1b,c. As for the latter one, an expected process is
efficient singlet-singlet energy transfer (kEnT, see Scheme 2)
leading to the locally excited 1DHP-Ar state (the lowest lying
singlet). This does not lead to increased fluorescence, because,
as mentioned above, dihydropyridines emit poorly in fluid
solution. However, one canjudge whether1DHP-Aris formed
from the emission in viscous or frozen media, as is apparent in
the strong blue fluorescence observed with hydrocarbon-
substituted dyads 1a, 2a, and 3.
Considering first the DHP moiety, it is known that the
intense absorption by this chromophore at ca. 350 nm
(log ε ≈ 4) corresponds to an allowed transition with int-
ernal charge transfer character, with calculated f = 0.1.8 The
corresponding “natural” fluorescence lifetime is around
10 ns. However, as indicated above, the lifetime measured
in solution is well below 1 ns. This is due to internal
conversion promoted by rapid conformational equilibrium
of the nonrigid dihydropyridine skeleton. A clear support for
this rationalization comes from the increase of both intensity
Alternatively, electron transfer (kET, Scheme 2) between
the two moieties may have a role. For several of the aromatic
systems considered electron transfer (ET) from 1DHP to
the aromatics (eq 1) to form solvent separated radical
ion pairs is largely exoergonic in the monochromophoric
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Jimenez et al.
JOCArticle
TABLE 3. Calculated Free Energy Change for Electron Transfer Processes and Energy of the Solvent Separated Radical Ion Pair (SSRIP)a
ΔGET
compd
4-substituent
Ph-3-CHO
Ph-3-COPh
Ph-3(4)-NO2
Ph-3-CHdCHCO2Me
1-Np
ET(Ar)
1DHP f Ar
3DHP f Ar
3Ar f DHP
SSRIP, kcal/mol
1b
1c
1d/1e
1f
2a
2b
3
71.7
68.1
∼62
54.8
61.4
58.0
61.6
-11.6
-12.7
-26.1
-10.4
3.8
1.2
0
-13.3
2.4
16.7
1.2
13
-10.3
-8.3
15.9
7.8
15.5
3.4
61.3
60.2
46.8
62.5
76.8
61.3
73.1
1-Np-4-CN
9-Phen
-11.7
0.2
11.6
aThe required data were taken from the literature. ET(Ar) (in kcal/mol): Ar = 3-MePhCHO, 71.7;10a 3-MePhCOPh, 68.1;10a 3-MePhNO2, 62.8;10b
PhCHdCHCO2Me, 54.8;10c 1-MeNp, 61.4;10d Np-1-CN, 58.0;10d Phen, 61.6;10e DHP, 60.2.3 Ered(Ar) ( V vs. SCE): Ar = 3-MePhCHO, -1.88;10f
3-MePhCOPh, -1.83;10g PhNO2, -1.17;10h PhCHdCHCO2Me, -1.93;10i 1-MeNp, -2.55; 10j Np-4-CN, -1.88;10d Phen, -2.39.10k Eox(DHP), see refs 3
and 10l.
analogues ArH (see Table 3)
derivatives (see Table 3) and gives again the RIP (analogously
to eq 3). The RIP formed from any of the above paths may
undergo ISC to give, provided that this is thermodynamically
allowed, the lowest localized triplet (eq 4).
1DHPH þ ArH f DHPH•þ þ ArH• -
ð1Þ
Due to the short lifetime of the dihydropyridines singlet,
intermolecular ET occurs only at high concentrations of the
quencher, as shown by the low or moderate Stern-Volmer
constants. When the two moieties are held closely as in the
present dyads, however, intramolecular electron transfer is facile
and a tethered radical ion pair is formed (RIP, kET, see eq 2)
δ -
• -
3
ðDHPδþ -Ar Þ=DHP•þ -Ar f DHP -Ar or DHP- 3Ar
ð4Þ
Taking into account that the energy of the localized
dihydropyridine triplet 3DHP-Ar is ca. 60 kcal/mol (see
Table 2) and the variable energy of 3Ar and the RIP accord-
ing to the nature of Ar, different situations can be recog-
nized, as illustrated below (see Scheme 2 and Table 3; notice,
however, that the SSRIP energy values reported in the table
refer to fluid solution and are not exactly representative of
the situation in the glass).
1DHP -Ar f DHP•þ -Ar• -
ð2Þ
Thus, it is expected that ET has a large role when the aryl
group is easily reduced, that is when it bears a carbonyl,
nitro, cyano, or acrylate substituent. A particular case is that
of the cyanonaphthyl derivative 2b, with which a new emis-
sion is observed rather than more quenching. Apparently, an
exciplex is formed. Both the emission intensity and the
lifetime are larger than those of localized excited states that
are formed in other dyads.
With the naphthyl (2a) and phenanthryl (3) derivatives
1
3
both DHP fluorescence and DHP phosphorescence are
observed and are independent of the excitation wavelength.
The singlet excited states of aromatic hydrocarbons have a
lifetime on the order of tens of nanoseconds essentially
determined by ISC [kISC(Ar)]. This long lifetime makes
1DHP -Ar f ðDHPδþ -Arδ -
Þ
ð3Þ
The formation of such an exciplex must have geometric
requirements, as indicated by the blue-shifted emission in
glycerin and in EPA glass with respect to the fluid solution,
suggesting that the electronic structure is less perturbed with
1
quenching of Ar within the dyad complete. This involves
in part energy transfer (EnT), as evidenced by the 1DHP-Ar
fluorescence, in part ET, barely possible and thus ineffi-
cient with the naphthyl derivative, while kISC(Ar) has no
role [kISC(Ar) < kEnT(Ar), kET(Ar)]. Of the two localized
triplets, 3DHP-Ar is formed through back electron transfer
from the RIP [kBET(DHP)] as revealed by the phosphores-
cence. It is possible that the about isoenergetic DHP-3Ar
is likewise formed from the RIP and is in equilibrium, but
due to the small emission rate constant typical of hydro-
carbon triplets gives rise to no detectable emission (see
Table 4).
1
respect to localized DHP-Ar. Emitting exciplexes between
1-cyanonapthalene and aromatics are not known; however,
when the two moieties are tethered emission is favored,
as already demonstrated for 1-cyanonaphthalene-alkenes
exciplexes.11
The processes considered up to now (see Scheme 2) lead to
either 1DHP-Ar, the RIP (nonemitting except for the case of
2b), or DHP-3Ar. Subsequent processes can be envisaged. For
3
example, ET from Ar to DHP is possible for the carbonyl
With aromatic aldehydes and ketones the kISC(Ar) is
larger. Thus, when 1b,c are irradiated in the aryl chromo-
phore, the short lifetime of 1Ar allows only for partial energy
transfer in the singlet and the main process in the dyad
remains intersystem crossing as in the isolated ArH mole-
(10) (a) Lin, Z. P.; Aue, W. A. Spectrochim. Acta, Part A 2000, 56, 111.
(b) Takezaki, M.; Hirota, N.; Terazima, M. J. Phys. Chem. A 1997, 101, 3443.
(c) Herkstroeter, W. G.; Farid, S. J. Photochem. 1986, 36, 71. (d) Schweitzer,
C.; Mehrdad, Z.; Noll, A.; Grabner, E. W.; Schmidt, R. J. Phys. Chem. A
2003, 107, 2192. (e) Li, R.; Lim, E. C. J. Chem. Phys. 1972, 57, 605.
(f) Ishitani, O.; Yanagida, S.; Takamuku, S.; Pac, C. J. Org. Chem. 1987,
52, 2790. (g) Yanagida, S.; Ogata, T.; Shindo, A.; Hosokawa, H.; Mori, H.;
Sakata, T.; Wada, Y. Bull. Chem. Soc. Jpn. 1995, 68, 752. (h) Bento, M. F.;
Medeiros, M. J.; Montenegro, M. I.; Beriot, C.; Pletcher, D. J. Electroanal.
Chem. 1993, 345, 273. (i) Shibata, T.; Kabumoto, A.; Shiragami, T.; Ishitani,
O.; Pac, C.; Yanagida, S. J. Phys. Chem. 1990, 94, 2068. (j) Milne, P. H.;
Wayner, D. D. M.; DeCosta, D. P.; Pincock, J. A. Can. J. Chem. 1992, 70,
121. (k) Pedersen, S. U.; Lund, T.; Daasbjerg, K.; Pop, M.; Fussing, I.; Lund,
H. Acta Chem. Scand. 1993, 47, 398. (l) Lopez-Alarcon, C.; Squella, J. A.;
Mirando-Wilson, D.; Nunez-Veregara, L. Electroanalysis 2004, 16, 539.
(11) Albini, A.; Spreti, S. Tetrahedron 1984, 40, 2975.
cule. Thus, DHP-3Ar is formed [kISC(Ar) > kEnT(Ar), kET
-
(Ar)] and revealed by the phosphorescence (see experiments
by short wavelength irradiation). As shown in Table 3, the Ar
localized triplet could not be formed from other paths. On
the other hand, selective excitation in the DHP region is
followed by the greatly favored ET to the RIP [kET(1DHP)]
and back ET to give the localized 3DHP-Ar, the most stable
triplet.
6620 J. Org. Chem. Vol. 74, No. 17, 2009

Jimenez et al.
JOCArticle
TABLE 4. Paths Followed upon Photoexcitation of Dyads 1-3a,b
compd
4-substituent
processes occurring
1b,c
Ph-COR
(a) DHP-1Ar f DHP-3Ar (P)
(b) DHP-1Ar f DHP•þ-Ar•- f 3DHP-Ar (P)
(c) 1DHP-Ar f DHP•þ-Ar•- f 3DHP-Ar (P)
DHP-1Ar f DHP•þ-Ar•- f 3DHP-Ar (P)c,d
DHP-1Ar f DHP•þ-Ar•- f DHP-3Arc
DHP-1Ar f DHP•þ-Ar•- f 3DHP-Ar (P)c
DHP-1Ar f DHP•þ-Ar•- (F) f DHP-3Ar (P)c
1d
1f
2a, 3
2b
Ph-3-NO2
Ph-3-CHdCHCO2Me
1-Np, 9-Phen
1-Np-4-CN
aF, P: fluorescence or respectively phosphorescence detected from the species indicated. bBesides emission from directly excited 1DHP (F) and
the process DHP-1Ar f 1DHP-Ar (F), observed with all of these compounds, except where noted. cSame process occurring also starting from 1DHP-Ar.
dNo 1DHP fluorescence observed.
With the nitrophenyl derivatives 1d,e ET from 1,3DHP
to Ar is markedly exoergonic, although in matrix not as
much as in solution, and the 1,3Ar localized states are none-
mitting. Thus, emssion is due only to DHP localized states.
Fluorescence is detected neither in fluid solution nor in glass,
but in the latter case phosphorescence is apparent. Thus, ET
is the main process, both when the nitrobenzene chromo-
phore is irradiated [kISC(Ar)<kET(1Ar)] and when DHP is
irradiated [kIC(DHP)<kET(1DHP)]. In the glassy solution
where phosphorescence is measured the RIPs in the dyad are
not as stabilized as the free molecules in solution, and BET
gives 3DHP-Ar [kBET(DHP)], apparently the most stable
triplet (although formation of some DHP-3Ar cannot be
discounted, since this would not emit and thus would escape
detection). Notice that the phosphorescence spectra detected
with several of the above dyads and attributed to 3DHP are
similar, but not identical. Thus, the final state may be either
how the position of 3Ar and of the RIP (DHP•þ-Ar•-) vary
within the brackets indicated and correspondingly how vary
3
their position relative to DHP, governing the evolution of
the system. Splitting is possible, as illustrated by the case of
the fast ISC in carbonyl derivatives 1b,c that diminishes, but
does not completely hinder, ET from DHP.
Considered along with the easy and versatile synthesis of
dihydropyridines, these results suggest that such systems can
be considered among the most useful for the elaboration of
fully organic dyads for the vectorial transport of energy or
charge tranfer. In the present study, emission is used as a
diagnostic tool of the processes occurring. However, if the
limitation of the poor emission in fluid solution is overcome,
DHP-based dyads may be developed as versatile sensors too.
Experimental Section
Compound 4 was of commercial origin. Compounds 1a,d,e
were prepared and purified as previously reported.7 The other
1,4-dihydropyridines were prepared through the Hatzsch synth-
esis as detailed below. Chromatographic separations were car-
ried out on silica gel eluting with cyclohexane-ethyl acetate
mixtures. Samples for spectroscopic examination were repeat-
edly crystallized until a constant mp was reached.
3
“pure” DHP-Ar or an exciplex with the excitation mainly
on the DHP moiety.
As for cynnamate 1f, this has a low-lying, nonemissive
triplet (because of the competition with fast geometrical
isomerization). As a result, a little DHP fluorescence is
observed, but exoergonic ET is the main process from the
singlet [kISC(Ar), kEnT(1Ar) < kET(1Ar) and analogously for
1DHP]. As for the triplet multiplicity, the system inevitably
evolves to DHP-3Ar (kBET(Ar) predominating) and thus no
long-lived emission occurs. The situation of compound 2b
bearing a cyanonaphthalene substituent is in part similar,
again with ET predominating from both 1Ar and 1DHP and
BET leading to DHP-3Ar, with the important difference that
both the RIP (in fluid solution) and DHP-3Ar (in glass) emit
and are in fact directly detected.
Dimethyl 1,4-Dihydro-2,6-dimethyl-4-(3-formylphenyl)-3,5-di-
carboxylate (1b). Isophthaladehyde (1.99 g, 14.1 mmol) and
methyl acetacetate (3.36 mL, 3.11 mmol) were dissolved in 10
mL of methanol, 1.1 mL of 30% aqueous ammonia was added,
and the mixture was refluxed for 4 h, when TLC showed that the
starting material has been consumed. Separation by chromatog-
raphy and repeated crystallization gave the title compound as a
1
slightly yellow solid (0.075 g, 5% yield, mp 260 °C). H NMR
(CDCl3) δ 2.38 (s, 6H), 3.66 (s, 6H), 5.10 (s, 1H), 5.71 (s, 1H,
exch), 7.42 (t, 1H, J=7 Hz), 7.60 (d, 1H, J= 7 Hz), 7.67 (d, 1H,
J=7 Hz), 7.77 (br s, 1H), 9.98 (s, 1H); 13C (CDCl3) δ 19.6 (CH3),
39.4 (CH), 51.0 (CH3), 103.5, 128.0 (CH), 128.5 (CH), 128.7
(CH), 134.1 (CH), 136.4, 144.4, 148.6, 167.6, 192.6 (CH). IR
(KBr) 3220, 1700 cm-1. Anal. Calcd for C18H19NO5: C, 65.64; H,
5.81; N, 4.25. Found C, 65.4; H, 6.0; N, 4.1.
The general features of the light-induced processes of the
DHP-Ar dyads are summarized in Table 4, where the
processes occurring on irradiating either of the two chromo-
phores are indicated, along with the luminescence detected
1
(fluorescence from DHP, observed in most cases, is not
indicated for the sake of simplicity).
Dimethyl 1,4-Dihydro-2,6-dimethyl-4-(3-benzoylphenyl)-3,5-di-
carboxylate (1c).12 3-Benzoylbenzaldehyde (0.50 g, 2.35 mmol)
and methyl acetacetate (0.55 mL, 5.2 mmol) were disssolved in
methanol (1.4 mL), 30% aqueous ammonia was added (0.2 mL),
and the solution was refluxed for 4 h, when TLC showed that the
starting material had been consumed. Separation by column
chromatography and crystallization gave the title compound as
a colorless solid (0.41 g, 44% yield, mp 150-151 °C). 1H NMR
(CDCl3) δ 2.38 (s, 6H), 3.68 (s, 6H), 5.08 (s, 1H), 5.89 (s, 1H,
exch), 7.35 (t, 2H, J=7 Hz), 7.43-7.55 (m, 2H), 7.55-7.65 (m,
2H), 7.72 (br s, 1H), 7.8 (d, 2H, J=7 Hz); 13C (CDCl3) δ 19.4
(CH3), 39.3 (CH), 51.0 (CH3), 103.4, 127.9 (CH), 128.0 (2 CH),
Conclusion
The above data evidence the versatility of the DHP-based
dyads, in the sense that energy and electron transfer are
directed by the structure. Thus, one can engineer these
systems in such a way that upon excitation of either compo-
nent, the energy can be finally deposited on either compo-
nent, DHP or Ar, by the rational planning of the intradyad
steps, or at least this is possible for the triplet states. Key
parameters are triplet energy and reduction potential of the
chosen Ar group that must be compared with the fixed triplet
energy and oxidation potential of DHP. Scheme 2 illustrates
(12) Rampa, A.; Budriesi, R.; Bisi, A.; Valenti, P. Arzneim. Forsch. 1992,
42, 1284.
J. Org. Chem. Vol. 74, No. 17, 2009 6621

Jimenez et al.
JOCArticle
129.1 (CH), 129.5 (CH), 130.0 (2 CH), 132.0 (CH), 132.2 (CH),
137.1, 137.7, 144.5, 147.7, 167.7, 196.9. IR (KBr) 3240, 1690, 1665
cm-1. Anal. Calcd for C24H23NO5: C, 71.10; H, 5.72; N, 3.45.
Found: C, 70.9; H, 5.7; N, 3.3.
5 h, when TLC showed that the starting material had been
consumed. Separation by column chromatography and crystal-
lization gave the title compound as a yellow solid (0.18 g, 20%
yield, mp 218-220 °C). 1H NMR (CDCl3) δ 2.39 (s, 6H), 3.38 (s,
6H), 5.81 (s, 1H, exch), 5.89 (s, 1H), 7.65 (d, 1H, J = 8 Hz), 7.7
(Z)-Methyl 3-{3-[4-(1,4-Dihydro-2,6-dimethyl-3,5-dimethoxy-
carboxyl)]}propenoate (1f). Methyl 3-(3-formylphenyl)acrylate
(0.35 g, 2.0 mmol)13 and methyl acetacetate (0.45 mL, 4.2 mmol)
were dissolved in methanol (1.1 mL), 30% aqueous ammonia was
added (0.2 mL), and the solution was refluxed for 3 h, when TLC
showed that the starting material had been consumed. Separation
by column chromatography and crystallization gave the title
compound as a colorless solid (0.40 g, 82% yield, mp 155 °C).
1H NMR (CDCl3) δ 2.35(s, 6H), 3.66 (s, 6H), 3.82 (s, 3H), 5.03(s,
1H), 6.12 (s, 1H, exch), 6.39 (d, 1H, J=16 Hz), 7.20-7.35 (m,
3H), 7.44 (br s, 1H), 7.68 (d, 1H, J=16 Hz); 13C NMR (CDCl3) δ
29.6 (CH3), 39.3 (CH), 50.5 (CH3), 55.6 (CH3), 103.4, 116.0 (CH),
125.6 (CH), 127.8 (CH), 128.4 (CH), 129.9 (CH), 134.0, 144.6,
(m, 2H), 7.8 (d, 1H, J = 8 Hz), 8.1 (m, 1H), 8.75 (m, 1H); 13
C
NMR (CDCl3) δ 19.5 (2 CH3), 35.2 (CH), 50.7 (2 CH3), 104.7,
108.3, 118.3, 125.9 (CH), 126.2 (CH), 126.5 (CH), 127.7 (CH),
130.4, 132.2, 132.6 (CH), 144.0, 152.8, 167.5. IR (KBr) 3240,
2210, 1690 cm-1. Anal. Calcd for C22H20N2O4: C, 70.20; H,
5.57; N, 7.44. Found: C, 70.4; H, 5.7; N, 7.2.
Dimethyl 1,4-Dihydro-2,6-dimethyl-4-(9-phenanthryl)-3,5-di-
carboxylate (3).15 9-Phenanthenecarboxyaldehyde (0.56 g,
2.7 mmol) and methyl acetacetate (0.70 mL, 5.1 mmol) were
disssolved in methanol (1.6 mL), 30% aqueous ammonia was
added (0.2 mL), and the solution was refluxed for 4 h 30 min,
when TLC showed that the starting material had been con-
sumed. Separation by column chromatography and recrystalli-
zation from methanol gave the title compound as a colorless
solid (40 mg, 4%, mp 245 °C). 1H NMR (CDCl3) δ 2.42 (s, 6H),
3.40 (s, 6H), 5.65 (s, 1H, exch), 5.82 (s, 1H), 7.5-7.8 (m, 6H),
8.63-8.70 (m, 3H); 13C NMR (CDCl3) δ19.5 (CH3), 34.4 (CH),
50.5 (CH3), 105.6, 122.25 (CH), 122.3 (CH), 125.6 (2 CH), 125.9
(CH), 126.0 (CH), 127.9 (CH), 128.0 (CH), 129.6, 129.8, 130.3,
145.5 (CH), 148.2, 162.6, 162.8. IR (KBr) 3240, 1690, 1665 cm-1
.
Anal. Calcd for C21H23NO6: C, 65.44; H, 6.02; N, 3.63. Found: C,
65.7; H, 6.0; N, 3.4.
Dimethyl 1,4-Dihydro-2,6-dimethyl-4-(1-naphthyl)-3,5-dicar-
boxylate (2a).14 1-Naphthylaldehyde (2.87 g, 18.4 mmol) and
methyl acetacetate (4.4 mL, 40.5 mmol) were dissolved in metha-
nol (11 mL), 30% aqueous ammonia was added (1.5 mL), and the
solution was refluxed for 5 h, when TLC showed that the starting
material had been consumed. Separation by column chromatog-
raphy and crystallization gave the title compound as a colorless
solid (0.80 g, 13% yield, mp 248-249 °C). 1H NMR (CDCl3) δ
2.38 (s, 6H), 3.43 (s, 6H), 5.73 (s, 1H), 5.84 (s, 1H, exch), 7.35 (t,
2H), 7.45 (m, 3H), 7.55 (m 2H), 7.63 (d, 1H, J=8 Hz), 7.78 (d,
1H, J=8 Hz), 8.58 (d, 1H, J=8.5 Hz); 13C NMR (CDCl3) δ 19.4
(CH3), 39.3 (CH), 50.6 (CH3), 105.4, 124.9 (CH), 125.0 (CH),
125.05 (CH), 125.6 (CH), 126.8 (CH), 127.0 (CH), 127.9 (CH),
130.7, 133.1, 143.3, 146.5, 168.7. IR (KBr) 3240, 1670 cm-1. Anal.
Calcd for C21H21NO4; C 71.78; H, 6.02; N, 3.99. Found: C, 72.0;
H, 6.1; N, 3.8.
132.0, 143.0, 145.8, 168.0. IR (KBr) 3300, 2210, 1695 cm-1
.
Anal. Calcd for C25H23 NO4: C, 74.79; H, 5.77; N, 3.49. Found:
C, 74.7; H, 5.8; N, 3.4.
Measurements: Luminescence. The luminescence was mea-
sured by means of a thermostated pulsed fluorimeter either at
20 °C or at 77K by using the liquid nitrogen fitting. Quantum
yields of emission were measured taking quinine bisulfate (Φf =
0.546 at room temperature) or carbazole (in glass, Φp = 0.24)16
as standards. The fluorescence lifetime was measured through
the single photon counting technique.
Acknowledgment. Partial support of this work by the
Italian Department of Education, Rome, is gratefully ac-
knowledged.
Dimethyl 1,4-Dihydro-2,6-dimethyl-4-[1-(4-cyanonaphthyl)]-
3,5-dicarboxylate (2b). 4-Cyanonaphthalene-1-carboxyalde-
hyde (0.46 g, 2.5 mmol) and methyl acetacetate (0.65 mL,
5.6 mmol) were disssolved in methanol (1.5 mL), 30% aqueous
ammonia was added (0.2 mL), and the solution was refluxed for
1
Supporting Information Available: Copies of H and 13C
NMR spectra of compounds 1b, 1c, 1f, 2a, 2b, and 3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(13) Guengerich, F. P.;Brian, W. R.;Iwasaki, M.;Sari, M. A.;Baeaernhielm,
C.; Berntsson, P. J. Med. Chem. 1991, 34, 1838. Ohsumi, K.; Ohishi, K.;
Morinaga, Y.; Nakagawa, R.; Suga, Y.; Takashi, T.; Yukio, A.; Takashi, T.
Chem. Pharm. Bull. 1995, 43, 818. Gupta, A. K.; Song, C. H.; Oh, C. H.
Tetrahedron Lett. 2004, 45, 4113.
(15) Treibs, W.; Beger, J. Liebigs Ann. Chem. 1962, 652, 192.
(16) Bonesi, S. M.; Erra-Balsells, R. J. Lumin. 2001, 93, 51.
(14) Dewar, M. J. S.; Grisdale, P. J. J. Am. Chem. Soc. 1962, 84, 3541.
6622 J. Org. Chem. Vol. 74, No. 17, 2009