Photophysical properties of a series of 4-aryl substituted 1,4-dihydropyridines

Article abstract of DOI:10.1002/poc.2916

In this article, a series of Hantzsch 1,4-dihydropyridines with different substituted aryl groups were synthesized and its spectral data obtained by UV-Vis absorption and fluorescence emission spectroscopies in solution. The dihydropyridines present absor

Full text of DOI:10.1002/poc.2916

Research Article
Received: 5 July 2011,
Revised: 8 December 2011,
Accepted: 16 December 2011,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.2916
Photophysical properties of a series of 4-aryl
substituted 1,4-dihydropyridines
Ricardo Ferreira Affeldta, Rodrigo Sebastian Iglesiasb,
Fabiano Severo Rodembuscha* and Dennis Russowskya*
In this article, a series of Hantzsch 1,4-dihydropyridines with different substituted aryl groups were synthesized and
its spectral data obtained by UV–Vis absorption and fluorescence emission spectroscopies in solution. The dihydro-
pyridines present absorption located around 350 nm and fluorescence emission in the blue–green region. A higher
Stokes’ shift could be observed for the derivative 3b because of an intramolecular charge transfer in the excited
state from the dimethylaniline to the dihydropyridine chromophores, which was corroborated by a linear relation
of the fluorescence maxima (nmax) versus the solvent polarity function (Δf) from the Lippert–Mataga correlation.
A comparison between the experimental data and time-dependent density functional theory-polarizable contin-
uum model calculations of the vertical transitions was performed to help on the elucidation of the photophysics
of these compounds. For these calculations, the S0 and S1 states were optimized using Becke, three-parameter,
Lee–Yang–Parr/6-31 G* and Configuration Interaction Singles/6-31 G*, respectively. The predicted absorption max-
ima are in good agreement with the experimental; however, the theoretical fluorescence emission maxima do not
match the experimental, which means that the excited specie cannot be related to neither a locally excited state
nor to an aromatized structure. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: aromatization; charge transfer; fluorescence; Hantzsch; NADH biomimmetics; oxidation photophysics;
photophysics; time-dependent density functional theory; UV–Vis absorption; 1,4-dihydropyridines
metabolized by oxidative mechanisms to dehydro-derivatives
INTRODUCTION
in human liver.[14] However, these compounds are highly sensi-
tive to photo-oxidation and its photodegraded products may
possess none or little biological activity.[15–17] Nifedipine decom-
poses in UV light to give the aromatized 4-(2-nitrosophenyl)-
pyridine homologue and in presence of oxygen, the nitroso
group is reoxidized resulting in the 4-(2-nitrophenyl)pyridine ho-
mologue (Scheme 3).[18–20] It is worth to mention that the nitroso
product can be related to different biological applications.[21,22]
The photochemical characterization of bioactive molecules is
extremely relevant considering the light absorption and conse-
quent modified pharmaco-dynamics, which results in unexpected
efficiency of these drugs.[13] Although the fluorescence emission
from the DHPs derivatives is well known,[23–26] no agreement
The 1,4-dihydropyridines, so called Hantzsch’s esters are low
molecular weight heterocyclic compounds described more than a
century ago.[1] These compounds have been shown a wide-scope
of biological activity and present a recognized capacity as calcium
channel blockers, thus acting as vital drugs against heart diseases.[2,3]
The dihydropyridine moiety (1,4-DHP) is common in many commer-
cialized drugs (Scheme 1). Furthermore, the 1,4-dihydropyridines
show other properties such as antioxidant, antiatherosclerosis,
bronchodilator, antitumor, antidiabetic and neuroprotector
properties, and are promising drugs for Alzheimer’s disease
treatment.[4] The 1,4-dihydropyridines are also known as biomi-
metic analogues of the reduced form of nicotinamide adenine
dinucleotide (NADH) hydrogen donor coenzyme system, bearing
an oxidizable dihydropyridine core-based structure (DHP)
connected to a p aromatic (pAr) system through an sp3 carbon.[5]
The classical Hantzsch synthesis involves multicomponent
one-pot condensation of an aldehyde, ethyl acetoacetate and
ammonia under reflux in alcoholic solvent, as presented in
Scheme 2.[1,6] It is worth to mention that this methodology is
associated with some disadvantages, such as long reaction times,
harsh conditions and low product yields. However, because of its
medicinal importance, the search for novel derivatives, improved
reaction conditions and environmentally beneficial methods have
been widely developed.[7–12]
* Correspondence to: Fabiano Severo Rodembusch and Dennis Russowsky,
Instituto de Química, Departamento de Química Orgânica, Universidade Federal
do Rio Grande do Sul, Av. Bento Gonçalves 9500. CEP 91501-970, Porto Alegre-RS,
Brazil.
E-mail: dennis@iq.ufrgs.br and rodembusch@iq.ufrgs.br
a R. F. Affeldt, F. S. Rodembusch, D. Russowsky
Instituto de Química, Departamento de Química Orgânica, Universidade
Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, CEP 91501-970, Porto
Alegre, RS, Brazil
The photochemical oxidation and rearrangement mechanisms
of the 1,4-dihydropyridines have already been studied.[13] These
antihypertensive drugs, for example Nifedipine, are rapidly
b R. S. Iglesias
Faculdade de Engenharia, Pontifícia Universidade Católica do Rio Grande do
Sul, Av. Ipiranga 6681, CEP 90619-900, Porto Alegre, RS, Brazil
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

R. F. AFFELDT ET AL.
Scheme 1. Commercially available Hantzsch 1,4-dihydropyridines
Scheme 2. Multicomponent synthesis of 1,4-dihydropyridines
Scheme 3. Photochemical oxidation of Nifedipine
can be found about the excited species responsible for their
photophysical behaviour. Some authors describe the radiation
deactivation from locally excited species, which arise from an
irreversible photochemical aromatization of the DHP moiety.
On the other hand, several zwitterionic species are described
leading to fluorescence emission from a charge transfer mecha-
nism, where the aromatic DHP is not directly involved in the
fluorescence emission.[27–30] Recent results indicates that 2-
nitrophenyldihydropyridines, such as the well-known Nifedipine
and Nisoldipine, present a fast intramolecular electron transfer
when UV exposed in a solid matrix, followed by a proton transfer
leading to an irreversible zwitterionic intermediate.[27] Therefore,
it could be observed that the two chromophores (DHP-pAr) of
the derivative 3-nitrophenyldihydropyridine presents absorption
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PHOTOPHYSICAL PROPERTIES OF A SERIES OF 4-ARYL SUBSTITUTED 1,4-DIHYDROPYRIDINES
solvent was then evaporated affording a crude solid product, which was
finally purified by column chromatography (ethyl acetate : hexane
mixtures).
end emission independently, where the dihydropyridine decays
from a singlet and a triplet state while the 3-nitrophenyl moiety
decays by internal conversion.[28] Additionally, Jimenez et al.[30]
demonstrated by fluorescence and phosphorescence studies
that it is possible to excite the DHP-pAr systems independently
by choosing an appropriate wavelength as far as these systems
interact significantly in the excited state only, where energy
and electron transfer are strongly related to the substituent. It
could also be observed that the photochemical aromatization
of the DHP systems, although inefficient, takes place in the UV
exposure of solutions of nitro and chloro-phenyl substituted
compounds.[29] Nevertheless, there is no evidence for a revers-
ible aromatization in the system.
In the present work, we dedicate our efforts to synthesize and
study alternate substituted dihydropyridines in search for a bet-
ter understanding of the photophysics of these compounds in
the excited state comparing with theoretical studies by computa-
tional time-dependent density functional theory-polarizable con-
tinuum model (TDDFT-PCM) calculations.
Chemical aromatization
The chemical oxidation of the 1,4-DHP diethyl 1,4-dihydro-2,6-dimethyl-
4-phenylpyridine-3,5-dicarboxylate was performed in a 10 mL round-
bottom flask with a solution 1.0mmol of the 1,4-DHP in 5 mL of MeCN
and 2.0 mmols of I2. The mixture was sonicated for 10 min and then
refluxed for 5 h. After cooling the reaction mixture, a saturated solution
of sodium thiosulfate was added and extracted with dichloromethane
(3ꢁ10 mL). The organic phase was dried over sodium sulfate and concen-
trated affording the crude product. The crude product was purified in
column chromatography affording the pure pyridine derivative (86%).[33]
Computational details
Calculations were performed employing the GAUSSIAN 03 (Gaussian, Inc.,
Wallingford, Connecticut/USA) package.[34] A conformational analysis
was carried out at semi-empirical level (Parametric Method
3
parameterization) for the seven DHP derivatives in the ground and S1
state, and for the aromatized structure. This allowed establishing the most
stable orientation of the 4,5 substituents, which was used in all subse-
quent calculations. For the calculation of the vertical transitions, the
ground state was further optimized using DFT at Becke, three-parameter,
Lee–Yang–Parr (B3LYP)/6-31 G* level, while the excited S1 state was opti-
mized using CIS/6-31 G*. The conformations were also optimized at
higher level (DFT and CIS) to confirm the semi-empirical minimum-energy
structures. No symmetry restrictions were applied on these optimizations.
Franck–Condon transitions were obtained by TDDFT calculations (singlet
states only) on the previously minimized structures. Thus, absorption is
considered as the TDDFT electronic excitation from the B3LYP/6-31 G* op-
timized ground state, while fluorescence is taken as the electronic excita-
tion from the ground state, taking the CIS/6-31 G* excited-state optimized
geometry. For these calculations, the PBE0 exchange-correlation func-
tional was chosen. PBE0 is a parameter-free, 25% exact exchange-correla-
tion functional, built on Perdew–Burke–Erzenhof pure functional,[35,36]
and is known for giving results in good agreement with experimental ab-
sorption and fluorescence measurements for a wide variety of chromo-
phores.[37–44] All calculations were performed at 6-31-G + d level. Solvent
effects were evaluated by the PCM self-consistent reaction field model.
Two nonprotic solvents (dichloromethane and acetonitrile) were chosen,
for which experimental spectra are available for comparison.
EXPERIMENTAL
General information
All chemicals were purchased from commercial sources and used with-
out further purification. The products were purified by column chroma-
tography performed in silica gel (200–400 mesh) with different solvent
mixtures (hexane/ethyl acetate) and the reactions were monitored by
thin layer chromatography. Nuclear magnetic resonance (NMR) data
were recorded with Varian VNMRS (Agilent Technologies, Colorado
1
Springs, Colorado, USA) 300 MHz and 75 MHz spectrometers, for H and
13 C-NMR respectively. Chemical shifts were reported as d values (ppm)
relative to the internal reference Tetramethylsilane (TMS) peak set at
d = 0.0 ppm (1H NMR) and CDCl3 set at d = 77.0 ppm (13 C NMR). Coupling
constants (J) were expressed in Hz. Infrared spectra were obtained as KBR
pellets on a Perkin Elmer FTIR Spectrum 1000 (Perkin Elmer, Waltham,
Massachusetts/USA), with resolution of 4 cm–1 between 400 and
4000 cm–1. Melting points were determined with an Olympus BX41
(Olympus Optical do Brasil, Ltda., São Paulo, São Paulo/Brazil) optical mi-
croscope and a Mettler Toledo FP-90 F 982 T (Mettler-Toledo Ind. e Com.
Ltda., Barueri, São Paulo/Brazil) temperature-controlled furnace. An ultra-
sound bath Thornton T14 (THORNTON INPEC, Vinhedo, São Paulo/Brazil)
of 40 kHz (50 W) was used for the chemical aromatization reaction. Spec-
troscopic grade solvents dichloromethane, acetonitrile and 1,4-dioxane
(Merck) were used in fluorescence emission and UV–Vis absorption spec-
troscopy measurements. UV–Vis absorption spectra were recorded in a
spectrophotometer Shimadzu UV-2450PC (Shimadzu Corporation,
Kyoto/Japan). Fluorescence emission and excitation spectra were mea-
sured in spectrofluorometer Shimadzu, Model RF-5301PC. Spectrum cor-
rection was performed to enable measuring of a true spectrum by elimi-
nating instrumental response such as wavelength characteristics of the
monochromator or detector using Rhodamine B as an internal standard
(quantum counter). The Lippert–Mataga correlation was performed us-
ing DHP solutions of acetonitrile/1,4-dioxane (1:0, 1.0:0.5, 1.0:1.0, 1.0:1.5,
1.0:2.0, 1.0:2.5 v/v).[31,32] All experiments were performed at room temper-
ature in a concentration range of 10–6 M.
RESULTS AND DISCUSSION
Synthesis
The 1,4-dihydropyridines and the catalyst were synthesized
according to the method described in the literature (Scheme 4).[45]
The 1,4-dihydropyridines synthesis started with the condensation
of an aldehyde (1a–h), ethyl acetoacetate (2a) and ammonium
acetate with In/SiO2 composite as catalyst heated with no solvent
at 100 ꢀC affording the desired dihydropyridine (3a–h) within 30
min of reaction. We also performed a chemical oxidation of the
1,4-DHP derivative 3a to afford the aromatized derivative 4a
(Scheme 5).
Synthesis of the 1,4-DHPs
Diethyl 2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,
In a 25 mL round-bottom flask were added 2mmol of the ethyl acetoace-
tate and 1.0mmol of the corresponding aldehyde followed by the addition
of 2.0mmol of NH4OAc. The catalyst In/SiO2 (10 mol%) was added in a sin-
gle portion and the stirring mixture was heated at 100ꢀC for 30 min until
total consumption of the reagents as indicated by TLC (Thin layer chroma-
tography). The crude product was dissolved in ethyl acetate, washed and
the organic layer dried over magnesium sulfate. After the filtration, the
5-dicarboxylate (3a)
1
Yield: 75%. H NMR (300 MHz, CDCl3): d = 1.22 (t, J = 17.1, 6H);
2.33 (s, 6H); 4.04–4.13 (m, 4H); 4.99 (s, 1H); 5.64 (bs, 1H); 7.12–7.30
(m, 5H). 13 C NMR (75 MHz, CDCl3): d = 14.2; 19.6; 39.6; 59.7; 104.2;
126.1; 127.8; 128.0; 143.8; 147.7; 167.6. IR (KBr, υ = cm–1): 3342,
3060, 2982, 1687, 1652, 1488, 1211, 703. M.p.: 154–160 ꢀC.[46]
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R. F. AFFELDT ET AL.
Scheme 4. Synthesis of 1,4-dihydropyridines (3a–h) from different aldehydes (1a–h), where (a) R = Ph, (b) R = 4-N,N-(Me)2-C6H4, (c) R = 4-NO2–C6H4, (d)
R = 4-MeO–C6H4, (e) R = 3,4-(MeO)2–C6H3, (f) R = 1-Naphtyl, (g), R = 4-CN–C6H4 and (h) R = 2-Thiophenyl
Scheme 5. Chemical oxidation of 1,4-dihydropyridine 3a to pyridine 4a
Diethyl 2,6-dimethyl-4-(4-N,N-dimethylaminophenyl)-1,
4-dihydropyridine-3,5-dicarboxylate (3b)
CDCl3): d = 14.4; 19.5; 38.9; 55.7; 59.7; 104.1; 110.7; 111.6;
119.7; 140.7; 143.7; 147.2; 148.0; 167.7. IR (KBr, υ = cm–1):
3343, 3096, 2982, 1686, 1651, 1483, 1208. M.p.: 137–145 ꢀC.[50]
1
Yield: 50%. H NMR (300 MHz, CDCl3): d = 1.23 (t, J = 7.2 Hz, 6H);
2.26 (s, 6H); 2.86 (s, 6H); 4.04–4.13 (m, 4H); 4.89 (s, 1H); 6.43
(bs, 1H); 6.60 (d, J = 8.7 Hz, 2H); 7.15 (d, J = 8.7 Hz, 2H). 13 C
NMR (75 MHz, CDCl3): d = 14.1; 19.1; 38.2; 40.6; 59.4; 103.8;
112.2; 128.4; 136.6; 142.9; 148.8; 167.9. IR (KBr, υ = cm–1):
3354, 3095, 2976, 1693, 1652, 1488, 1212. M.p.: 158–162 ꢀC.[47]
Diethyl 2,6-dimethyl-4-(1-naphthyl)-1,4-dihydropyridine-3,
5-dicarboxylate (3f)
1
Yield: 84%. H NMR (300 MHz, CDCl3): d = 1.27 (t, J = 7.2 Hz, 6H);
2.33 (s, 6H); 4.09–4.22 (m, 4H); 5.34 (s, 1H); 5.98 (bs, 1H); 6.78–6.85
(m, 2H) 7.05 (d, J = 2.4Hz, 1H). 13 C NMR (75 MHz, CDCl3): d = 14.3;
19.4; 34.3; 59.9; 103.4; 123.0; 123.1; 126.3; 144.5; 151.5; 167.3. IR
(KBr, υ = cm–1): 3373, 3098, 2985, 1679, 1630, 1483, 1200, 793. M.
p.: 197–200 ꢀC.[3]
Diethyl 2,6-dimethyl-4-(4-nitrophenyl)-1,4-dihydropyridine-3,
5-dicarboxylate (3c)
1
Yield: 78%. H NMR (300 MHz, CDCl3): d = 1.22 (t, J = 7.2 Hz, 6H);
2.35 (s, 6H); 4.08–4.10 (m, 4H); 5.09 (s, 1H); 5.90 (bs, 1H); 7.45 (d,
J = 9.0 Hz, 2H); 8.08 (d, J = 9.0 Hz, 2H). 13 C NMR (75 MHz, CDCl3):
d = 14.2; 19.5; 40.1; 59.9; 103.0; 123.2; 128.9; 144.8; 146.2; 155.1;
167.1. IR (KBr, υ = cm–1): 3319, 3102, 2978, 1704, 1651, 1519,
1488, 1348, 1212. M.p.: 118–127 ꢀC.[48]
Diethyl 4-(4-cyanophenyl)-2,6-dimethyl-1,4-dihydropyridine-3,
5-dicarboxylate (3g)
1
Yield: 71%. H NMR (300 MHz, CDCl3): d = 1.21 (t, J = 7.2 Hz, 6H);
2.35 (s, 6H); 4.05–4.13 (m, 4H); 5.03 (s, 1H); 6.00 (bs, 1H); 7.39 (d,
J = 8.4 Hz, 2H) 7.51 (d, J = 8.1 Hz, 2H). 13 C NMR (75 MHz, CDCl3):
d = 14.2; 19.5; 40.4; 59.9; 103.1; 109.6; 119.3; 128.8; 131.8; 144.6;
153.1; 167.1. IR (KBr, υ = cm–1): 3344, 3091, 2981, 2228, 1682,
1489, 1214. M.p.: 196–199 ꢀC.[51]
Diethyl 2,6-dimethyl-4-(4-methoxyphenyl)-1,4-dihydropyridine-3,
5-dicarboxylate (3d)
1
Yield: 62%. H NMR (300 MHz, CDCl3): d = 1.23 (t, J = 7.2 Hz, 6H);
2.31 (s, 6H); 3.75 (s, 3H); 4.07–4.10 (m, 4H); 4.92 (s, 1H); 5.73
(bs, 1H); 6.74 (d, J = 7.5 Hz, 2H); 7.19 (d, J = 7.8 Hz, 2H). 13 C NMR
(75 MHz, CDCl3): d = 14.2; 19.6; 38.7; 55.1; 59.7; 104.3; 113.1;
128.9; 140.3; 143.5; 157.8; 167.7. IR (KBr, υ = cm–1): 3342, 3096,
2984, 1690, 1651, 1490, 1211. M.p.: 148–153 ꢀC.[49]
Diethyl 2,6-dimethyl-4-(2-thiophenyl)-1,4-dihydropyridine-3,5-dicar-
boxylate (3h)
1
Yield: 77%. H NMR (300 MHz, CDCl3): d = 1.21 (t, J = 7.2 Hz, 6H);
2.35 (s, 6H); 4.05–4.13 (m, 4H); 5.03 (s, 1H); 6.00 (broad s, 1H);
6.80–6.79 (m, 1H); 6.84 (dd, J = 4.2 Hz and J = 3.3 Hz, 1H); 7.05
(dd, J = 5.1 Hz and J = 1.2 Hz, 1H). 13 C NMR (75 MHz, CDCl3): d =
14.3; 19.4; 34.3; 59.9; 103.4; 123.0; 123.1; 126.3; 144.5; 151.5;
167.3. IR (KBr, υ = cm–1): 3345, 3098, 2979, 1692, 1658, 1486,
1211, 722. M.p.: 164–168 ꢀC.[46]
Diethyl 2,6-dimethyl-4-(3,4-dimethoxyphenyl)-1,4-dihydropyridine-3,
5-dicarboxylate (3e)
1
Yield: 69%. H NMR (300 MHz, CDCl3): d = 1.24 (t, J = 7.1 Hz, 6H);
2.33 (s, 6H); 3.82 (s, 3H); 3.84 (s, 3H); 4.09–4.122 (m. 4H); 4.94
(s, 1H); 5.81 (bs, 1H); 6.74–6.99 (m, 3H). 13 C NMR (75 MHz,
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PHOTOPHYSICAL PROPERTIES OF A SERIES OF 4-ARYL SUBSTITUTED 1,4-DIHYDROPYRIDINES
Photophysical characterization
difference of the absorption band location in the same solvent
can be associated with the different auxochrome groups pre-
sented in the DHP moiety. It could also be observed that the
absorption maxima in the more polar solvent acetonitrile are
red shifted in relation to the spectra in dichloromethane, as
expected.
The photophysical study was performed in dichloromethane and
acetonitrile only in the photoactive DHP derivatives and in
dichloromethane for the aromatized 3a. Considering that the
DHP bearing an electron-withdrawer nitro group at the para
position of the aromatic ring (3c) showed no visible fluorescence
in solid state when irradiated at 365 nm, as its synthesized analo-
gues, it has been excluded from the photophysical study. In the
Fig. 1 are presented the normalized UV–Vis absorption spectra of
these dyes. The relevant UV–Vis data are summarized in Table 1.
An absorption band maxima (labs) located around 326–352 nm
Figure 2 presents the normalized fluorescence emission spec-
tra of these dyes. The curves were obtained using the absorption
maxima as the excitation wavelengths. The relevant data are also
summarized in Table 1. A fluorescence emission maxima can be
observed located at 420 nm in both solvents, with a Stokes shift
at around 70 nm. The same red shift can be observed for the
DHPs in acetonitrile when compared with the less polar solvent
dichloromethane. Because the emission maxima location is not
shifted by the substituents (Ph, 4-MeO–C6H4, 3,4-(MeO)2–C6H3,
4-CN-C6H4, 1-Naphthyl and 2-Thiophenyl), which are electroni-
cally different, the emission can be attributed to the dihydropyr-
idine chromophore. Furthermore, the observed Stokes shift is
plausible to an energy loss in the excited state, which is usually
related to internal electron transfer and/or energy transfer char-
acter of the corresponding excited state.[29,30]
and 331–356 nm, with molar extinction coefficient values (emax
)
in agreement with p–p* transitions, could be observed with the
dyes in dichloromethane and acetonitrile, respectively. The
It is worth mentioning that the DHP 3b presents a particular
photophysical behaviour indicating a higher energy loss in the
excited state, which leads to a Stokes shift from 151 to 185 nm,
in dichloromethane and acetonitrile, respectively (9710.4 cm–1
and 10831.7 cm–1, respectively), which concerning the data
reported in the literature cannot be related to electron or energy
transfer character.[29,30] However, as already observed in similar
structures, the photophysical behaviour can be ascribed as a
charge transfer from the dimethylaniline to the dihydropyridine
moiety.[52] To support the affirmation that intramolecular charge
transfer (ICT) state takes place in the excited state of the DHP 3b,
we studied the linear relation of the solvatochromic shifts from
the fluorescence and absorption maxima (nmax) versus the sol-
vent polarity function (Δf) from the well-known Lippert–Mataga
correlation.[53,54] In this approach, it is supposed that a point
dipole is situated at the center of the spherical cavity and
Figure 1. Normalized absorption spectra of the photoactive DHPs in
dichloromethane (top) and acetonitrile (bottom)
Table 1. UV–Vis and fluorescence emission data of the
photoactive DHPs
Solvent DHP
emax ꢁ104
ΔlST
labs (nm)
lem (nm)
(lꢂmol–1ꢂcm–1)
(nm/cm–1)
CH2Cl2
3a
3b
3d
3e
3f
3g
3h
3a
3b
3d
3e
3f
344
326
343
342
351
352
338
347
331
347
344
356
356
340
0.50
1.44
1.15
1.10
0.94
0.50
1.38
0.97
1.11
0.90
0.97
0.88
0.94
0.75
417
477
415
416
418
413
402
420
73/5089.0
151/9710.4
72/5058.1
74/5201.3
67/4566.6
61/4196.0
64/4710.2
73/5008.9
CH3CN
516 185/10831.7
419
422
421
429
410
72/4952.1
78/5373.1
65/4336.9
73/4779.9
70/5021.6
3g
3h
Figure 2. Normalized fluorescence emission spectra of the photoactive
DHPs in dichloromethane (top) and acetonitrile (bottom)
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R. F. AFFELDT ET AL.
neglecting the mean solute polarizability (a) in the states in-
volved in the transition (a = ae = ag = 0, where e and g stand
the DHPs 3a and 3d, and even the aromatized 4a, because
the fluorescence emission maxima versus the solvent polarity
function could not be fitted by a linear plot (r2 = 0.352, 0.181,
0.674 to 3a, 3d and 4a, respectively). However, the DHP 3b fits a
linear plot (r2 = 0.996), which corroborates to an intramolecular
charge transfer mechanism in the excited state, as already
described.[55–58] The additional DHPs, and even the aromatized
4a, could not be fitted by a linear plot (r2 = 0.352, 0.181, 0.674 to
3a, 3d and 4a, respectively).
To investigate the electronic structure of the excited specie
responsible for the fluorescence emission in the DHPs, we also
performed a photophysical study in dichloromethane of the
DHP 3a and its synthesized aromatized analogue 4a (Fig. 4). At
this time, this comparison is relevant because several pub-
lications indicate irreversible photochemical aromatization of
1,4-dihydropyridines; however, no spectral comparison is found
between the reduced and oxidized forms.[27,59,60]
It could be observed that the DHP 3a absorbs at higher wave-
lengths (~340 nm) with extinction coefficient ascribed to p–p*
transitions. On the other hand, the aromatized 4a presents an in-
tense absorption band at 270 nm. These absorption spectra are
similar to the spectra of NADH/NAD+ system,[61] which indicates
that 3a/4a simulate very well the photophysical behaviour of
this coenzyme pair. Additionally, the emission spectra can also
be useful to discuss the electronic changes between the nonaro-
matized and aromatized structures, where the 3a presents an
emission located at 420 nm, as expected for a nonaromatized
dihydropyridine ring in the NADH. Moreover, it is well known
that the oxidized form NAD+ does not present fluorescence
emission.[62] The dye 4a shows fluorescence emission located
at 372 nm, which can be probably related to an energy or elec-
tron transfer[29,30] between the two aromatic systems, because
the pyridine moiety presents a fluorescence emission at
290 nm.[63]
for excited and ground states, respectively) one obtains[55,56]
:
h
ꢀ
ꢁ
i
hcvmax ¼ hcvmaxð0Þ ꢃ 2me me ꢃ mg =a3 ꢄ Δf;
(1)
Δf ¼ fð«Þ ꢃ ð1=2ÞfðnÞ;
(2)
(3)
ꢂ
ꢃꢂ
ꢃ
fð«Þ ¼ ð« ꢃ 1Þ=ð2« þ 1Þ; fðnÞ ¼ n2 ꢃ 1 2n2 þ 1 ;
where mg is the dipole moment of the solute in the ground state,
nmax is the solvent-equilibrated fluorescence maxima and nmax
(0) is the value of spectral position of the fluorescence maxima
extrapolated to the gas phase, a is the radius of the cavity in
which the molecule resides. Because charge transfer states can
be more stabilized increasing the solvent polarity, a linear relation
of the fluorescence maxima (nmax) versus the solvent polarity
function (Δf) indicates the occurrence of the internal charge
transfer state. For mixed solvents, as used in this work, the dielec-
tric constant (emix) and the refractive index (nmix) were calculated
as already presented in the literature, where fA and fB are the
volumetric fractions of the two solvents[31,57]
«
¼ fA«A þ fB«B
(4)
(5)
mix
n2mix ¼ fAn2A þ fBn2
B
Figure 3 shows the solvatochromic shifts from the absorption
and fluorescence emission curves versus the solvent polarity
function. It is worth mentioning that no linear correlation could
be observed for all studied DHPs when the solvatochromic shift
is calculated taking into account the absorption maxima
(r2 = 0.531, 0.164, 0.645 and 0.296 to 3a, 3b, 3d and 4a respec-
tively), which indicates that no ICT takes place in the ground
state. The same could be observed in the excited state for
Theoretical results
To provide a better understanding of the excited state of the
studied dihydropyridines, the calculations were performed as-
suming normal and aromatized forms of the structures, as
Figure 3. Solvent effect on the spectral position of the (a) absorption
and (b) fluorescence emission maxima, for the 1,4-dihydropyridines
3a, 3b, 3d and the aromatic 4a. The Lippert–Mataga solvent polarity
function Δf is given in Eqn (2)
Figure 4. Normalized absorption and emission spectra of 3a and 4a in
dichloromethane
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J. Phys. Org. Chem. (2012)

PHOTOPHYSICAL PROPERTIES OF A SERIES OF 4-ARYL SUBSTITUTED 1,4-DIHYDROPYRIDINES
Figure 5. DHP derivative 3b optimized in the excited state S1 for the (a) aromatized and (b) non-aromatized structure at CIS/6-31G* level and (c) in the
ground state S0 at B3LYP/6-31G* level.
already discussed. The most stable calculated conformations for
the ground and excited state (both normal and aromatized
forms) are represented in Fig. 5, for the representative struc-
ture 3b. No significant structural changes were observed for
the rest of the series. The most important alteration with the
electronic transition is the partial planarization of the pyridine
moiety in the excited state. In the aromatized form, the structural
changes are more drastic – both rings are forced in a nearly co-
planar geometry (full coplanarity is not reached possibly because
of the steric hindrance from the carboxyl oxygen). Geometry
optimization of aromatized structure DHP 3h failed to reach a
stable minimum, leading to ring opening during calculations.
In accordance with other studies applying the TDDFT/PBE0
approach on chromophores presenting large Stokes shifts[42–44]
the absorption wavelengths are remarkably well matched by
calculated transitions, where an average absolute deviation of
10.7 nm for the whole series could be obtained. Figure 6 shows
a comparison between experimental and calculated absorption
spectra of a representative structure, convoluted using SWizard
program, revision 5.6.[64,65]
The TDDFT formalism[66–79] has proven to be a useful method
for an accurate prediction of electronic absorption energies,[80–87]
Table 2. B3LYP/6-31*G(d) and PBE0/6-31*G(d) TDDFT-PCM
calculated transition maxima of 1,4-DHPs
Solvent DHP
labs (nm)
lem (nm)
Exp. Calc. Exp. Calc. (normal/aromatized)
CH2Cl2
3a
3b
3d
3e
3f
3g
3h
3a
3b
3d
3e
3f
344 334 417
326 332 477
343 332 415
342 333 416
351 327 418
352 347 413
338 331 402
347 335 420
331 334 516
347 333 419
344 334 422
356 328 421
356 350 429
340 333 410
410/380
333/438
333/402
332/416
341/355
344/353
332/–
410/381
334/438
334/410
333/396
343/355
347/352
334/–
CH3CN
3g
3h
Figure 6. Experimental and calculated spectra of DHP 3d
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
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R. F. AFFELDT ET AL.
but not so accurate concerning emission processes.[42–44] In this
work, the calculated fluorescence emission maxima from normal
or aromatized species do not match with the experimental data
(Table 2). It has to be remarked that the geometry optimization
of the structures in the ground and excited states were per-
formed with different formalisms (HF and CIS), to which the
large differences in average errors between experimental
and calculated wavelengths for absorption and emission can
be attributed, and have been observed in other similar
studies.[43] For this series, the calculated normal fluorescence
wavelengths are practically not redshifted with respect to
the absorption values (Table 2), excepting the DHP 3a,
indicating that the applied methodology works well to non-
substituted 4-aryl-DHPs. The obtained calculated fluorescence
spectra indicates that the theoretical model does not fit very
well the emission maxima location when energy or electron
transfer processes take place in the excited state, ruling out
a normal fluorescence relaxation process.[88] The theoretical
method also seems to be unable to predict the emission
maxima for structures with intramolecular charge transfer in
the excited state, as observed to the DHP 3b.
Although the calculated transitions for most of the aromatized
structures fall closer to the experimental values than the normal
structure one (Table 2), a comparison between normal and aro-
matized excited state energy minima shows that the
aromatized form is less stable in all the compounds studied
(see Table S1). Further investigations are currently being carried
out employing TDDFT to optimized the excited state as well,
which is known to provide better results for the fluorescence
emission.[41,42]
Acknowledgements
We are grateful for the financial support and scholarships from
Conselho Nacional para o Desenvolvimento Científico e Tecnoló-
gico (CNPq). The authors also thank the Theoretical Chemistry
Group at the Instituto de Química/UFRGS for providing computa-
tional resources and Fabiano da Silveira Santos for the Lippert-
Mataga experiments.
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