Synthesis, spectral and photochemical properties of the styrylquinoline-naphthol dyad with a dioxytetramethylene bridge

Article abstract of DOI:10.1016/j.mencom.2011.04.013

The luminescence of 2-naphthol and 2-styrylquinoline fragments, intramolecular energy transfer from the former to the latter, and photoisomerization of the latter are observed in the newly synthesized bichromophoric dyad.

Full text of DOI:10.1016/j.mencom.2011.04.013

Mendeleev
Communications
Mendeleev Commun., 2011, 21, 151–152
Synthesis, spectral and photochemical properties of the
styrylquinoline–naphthol dyad with a dioxytetramethylene bridge
Mikhail F. Budyka,*a Kristina F. Sadykovaa,b and Tatyana N. Gavrishovaa
a Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka,
Moscow Region, Russian Federation. Fax: +7 496 514 3244; e-mail: budyka@icp.ac.ru
b Department of Chemistry, Bashkir State University, 450074 Ufa, Russian Federation
DOI: 10.1016/j.mencom.2011.04.013
The luminescence of 2-naphthol and 2-styrylquinoline fragments, intramolecular energy transfer from the former to the latter, and
photoisomerization of the latter are observed in the newly synthesized bichromophoric dyad.
Supramolecular bichromophoric molecular systems are of interest
O
from the viewpoint of investigating photophysical and photo-
chemical properties such as exciplex and excimer formation1,2
OH
and electron and energy transfer3–7 with possible applications
Microwave
irradiation
N
N
in light harvesting and artificial photosynthesis,8–10 molecular
devices,11 switches12 and logic gates.13
OH
,
1
The molecular device is a molecular system that can be switched
from one state to another by an external stimulus. For example,
2-styrylquinoline (2SQ) undergoes reversible trans–cis photo-
isomerization under UV irradiation,14,15 which has been used for
designing molecular logic gates.16 Additionally, the pKa of 2SQ
increases from 4.8 to pKa* 12.0 upon excitation from the ground
(S0) to the lowest excited singlet (S1) state; therefore, 2SQ is a
photobase.14 It was interesting to bind 2SQ in one supramole-
cular system with a photoacid, namely, 2-naphthol; it is known
that the acidity of the latter increases from pKa 9.5 to pKa* 2.8 on
going from the S0 to S1 state.17 For this purpose, we synthesized
the styrylquinoline–naphthol dyad 2-(E)-{4-[4-(3-hydroxynaph-
thalen-2-yloxy)butoxy]styryl}quinoline (SQ4Np).
HO
HO
Br(CH2)4Br,
K2CO3
K2CO3
N
O(CH2)4Br
2
HO
N
O(CH2)4O
3 (SQ4Np)
Scheme 1
The dyad SQ4Np was synthesized in three steps in an overall
yield of 67%.† The synthesis started with reaction between
quinaldine and 4-hydroxybenzaldehyde followed by alkylation
with 1,4-dibromobutane and final treatment with 2,3-dihydroxy-
naphthalene (Scheme 1).
The absorption spectrum of the dyad SQ4Np (Figure 1, spec-
trum 1) is a superposition of the spectra of model compounds,
2-(4-ethoxystyryl)quinoline EtOSQ (spectrum 2) and 3-methoxy-
2-naphthol MeONp (spectrum 3). This implies an absence of
considerable interaction between two constituting chromophoric
groups of SQ4Np in the ground state.
†
2-(E)-(4-Hydroxystyryl)quinoline 1. An open glass test-tube containing
quinaldine (2 mmol) and 4-hydroxybenzaldehyde (4 mmol) was put in a
bulb (V = 100 ml) containing 15–20 ml of water and subjected to micro-
wave irradiation (P = 600 W) in a three step mode 3×3 min with 30 s
intervals. After cooling, the reaction mixture was treated with aqueous
ethanol. The precipitate was filtered off and washed with hot ethanol
(2×20 ml). Pale yellow solid of compound 1 was obtained (0.44 g, 90%);
mp 268°C (EtOH).17
(dCH=CH), 826, 757, 511 (nC–Br). Found (%): C, 65.62; H, 5.23; N, 3.47.
Calc. for C21H20NOBr (%): C, 65.98; H, 5.27; N, 3.66.
2-(E)-{4-[4-(3-hydroxynaphthalen-2-yloxy)butoxy]styryl}quinoline
SQ4Np 3. A mixture of 2 (0.5 mmol), 2,3-dihydroxynaphthalene (2.5 mmol)
and K2CO3 (1.3 mmol) in 40 ml of butan-2-one was refluxed for 6 h. The
solvent was partially removed on a rotary evaporator. The resulting mix-
ture was acidified to pH 6 with dilute HCl. The precipitate formed was
filtered off, washed with Et2O, then repeatedly washed with acetone–water
solution to afford white precipitate of product 3 (0.19 g, 82%); mp 191–
2-(E)-[4-(4-Bromobut-1-oxy)styryl]quinoline 2. A mixture of 1 (1 mmol),
1,4-dibromobutane (10 mmol) and K2CO3 (1 mmol) in 20 ml of acetone
was refluxed for 6 h. NaOH solution was added to the mixture and the
organic layer was separated and acidified with HCl. The resulting orange
precipitate was filtered off, washed with hexane (5×3 ml) and neutralized
with NaHCO3 in acetone–water solution. The precipitate formed was filtered
off, dried and recrystallized from hot hexane to afford white crystals of
192°C. H NMR ([2H6]DMSO) d: 1.94–2.04 (m, 4H, CH2), 4.09–4.23
1
(m, 4H, CH2), 7.03 (d, 2H, o-C6H4, J 8.4 Hz), 7.15 (s, 1H, naphthalene),
7.20–7.26 (m, 2H, naphthalene), 7.28 (s, 1H, naphthalene), 7.34 (d, 1H, =CH,
J 16.3 Hz), 7.54 (t, 1H, quinoline, J 7.6 Hz), 7.61 (d, 1H, naphthalene,
J 6.7 Hz), 7.65–7.71 (m, 3H, o-C6H4, naphthalene); 7.74 (t, 1H, quinoline,
J 7.6 Hz), 7.79 (d, 1H, =CH, J 16.3 Hz), 7.84 (d, 1H, quinoline, J 8.6 Hz),
7.93 (d, 1H, quinoline, J 8.1 Hz), 7.97 (d, 1H, quinoline, J 8.3 Hz), 8.32
(d, 1H, quinoline, J 8.6 Hz), 9.42 (s, 1H, OH). IR (n/cm–1) d: 3600–2400
1
compound 2 (0.35 g, 91%); mp 92–93°C (hexane). H NMR (CDCl3)
d: 1.94–2.01 (m, 2H, CH2), 2.05–2.13 (m, 2H, CH2), 3.51 (t, 2H, CH2,
J 6.6 Hz), 4.04 (t, 2H, CH2, J 6.0 Hz), 6.92 (d, 2H, o-C6H4, J 8.6 Hz),
7.28 (d, 1H, =CH, J 16.4 Hz), 7.48 (t, 1H, quinoline, J 7.6 Hz), 7.57 (d,
2H, m-C6H4, J 8.6 Hz), 7.60–7.66 (m, 2H, quinoline, =CH), 7.69 (t, 1H,
quinoline, J 7.6 Hz), 7.77 (d, 1H, quinoline, J 8.0 Hz), 8.06 (d, 1H, quinoline,
J 8.4 Hz), 8.10 (d, 1H, quinoline, J 8.6 Hz). IR (n/cm–1): 3033, 2953,
(nOH), 3057, 3040, 2940, 2857 (nCH ), 1635 (nC=C), 1597, 1511, 1262, 1236
(nCOC), 1176, 976 (nCH=CH). Found (%): C, 80.44; H, 5.66; N, 2.84. Calc.
2
2871 (nCH ), 1633 (nC=C), 1598, 1515, 1249 (dCOC), 1231, 1184, 958
for C31H27NO3 (%): C, 80.73; H, 5.90; N, 3.03.
2
© 2011 Mendeleev Communications. All rights reserved.
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Mendeleev Commun., 2011, 21, 151–152
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
80000
60000
40000
20000
0
1
5
4
1
7
2
8
3
250
300
350
400
250
300
350
l/nm
400
450
500
l/nm
Figure 2 Spectral variations during the irradiation of air-saturated solution
of trans-SQ4Np (1.66×10–5 m) in ethanol with light at 365 nm, intensity,
1.25×10–9 Einstein cm–2 s–1, irradiation time, (1) 0, (2) 5, (3) 15, (4) 30, (5)
50, (6) 80 and (7) 580 s, the last spectrum corresponds to photostationary
state PS365; (8) spectrum of cis-SQ4Np calculated by Fischer’s method.
Figure 1 Absorption spectra in ethanol: (1) trans-SQ4Np, (2) trans-
2-(4-ethoxystyryl)quinoline, (3) 3-methoxy-2-naphthol; normalized (4) lumi-
nescence (excited at 280 nm) and (5) excitation (monitored at 430 nm) spectra
of trans-SQ4Np.
From a comparison with the model spectra, it follows that, at
wavelengths l < 330 nm, both the naphthol (Np) and styrylquinoline
(SQ) fragments of SQ4Np absorb light, whereas the long-wave-
length absorption band (LWAB) in the region of 340–390 nm
belongs to the SQ fragment only.
In accordance with this fact, upon excitation of SQ4Np in the
region of l < 330 nm, emission from both chromophores was
observed: from the Np fragment at 350 nm, and from the SQ
fragment at 430 nm (Figure 1, spectrum 4). Irradiation of SQ4Np
within LWAB resulted in excitation and subsequent emission of
the SQ fragment only. Based on absorption maxima, the energies
of the lowest excited (Franck–Condon) singlet states localized on
Np (S1_Np) and SQ (S1_SQ) fragments were calculated to be 3.80
and 3.46 eV.
wavelength (l). The observation of an isosbestic point at 259 nm
indicated the absence of secondary reactions. Based on the spectra
of trans-isomer and two photostationary states, PS313 and PS365
,
the spectrum of cis-isomer was calculated by Fischer’s method23
(Figure 2, spectrum 8). The quantum yields of trans–cis (jtc) and
cis–trans (jct) photoisomerization found by the numerical solution
of differential kinetic equations are jtc = 0.58 and jct = 0.52.
These values coincide with the data for EtOSQ15 and corroborate
that the SQ fragment in SQ4Np maintains photochemical activity.
Thus, the bichromophoric dyad SQ4Np was synthesized, and
its properties were investigated. Due to a variety of photopro-
cesses observed in the dyad, it possesses high potential as a
controllable molecular photoswitch.
The important feature of the excitation spectrum of SQ4Np
(Figure 1, spectrum 5) is maxima at ~230 and 320 nm, which
correspond to the absorption of the Np fragment, whereas moni-
toring the luminescence of the SQ fragment at 430 nm. Naphthol
contribution to the styrylquinoline fluorescence suggests the
Forster resonance energy transfer from the excited Np fragment
to the SQ fragment with the subsequent emission of the latter.
According to quantum-chemical calculations,19 the most elong-
ated conformer of SQ4Np with the trans-configuration of all
single C–C and C–O bonds is 2.94 nm in length, with a center-
to-center distance of 1.67 nm between two fragments. This is less
than the value of the Forster radius R0 = 3.6 nm, as calculated for
the naphthol–styrylquinoline donor–acceptor pair according to
ref. 20 and explains effective energy transfer in the covalently
bound dyad.
The acidity of the two fragments of SQ4Np changes in the
opposite direction upon excitation. The measured ground state
pKa of SQ4Np are 5.0 (quinoline ring) and 10.4 (hydroxyl group),
and its acidities in the excited state (pKa*) calculated by the Forster
cycle21 are 12.9 (quinoline ring) and 6.6 (hydroxyl group), re-
spectively. Compared to unsubstituted 2-naphthol, the acidity of
hydroxyl group in the Np fragment decreased, especially in the
excited state (pKa* changed from 2.8 to 6.6).
Obviously, this is the effect of the oxygen atom in the neigh-
bouring alkoxy group. According to B3LYP/6-31G* calculation,
there is an intramolecular hydrogen bond between hydroxyl and
alkoxy groups. The conformer with hydroxyl hydrogen turned to
the neighbouring oxygen is 4.3 kcal mol–1 more stable than the
conformer with the opposite disposition of hydroxyl hydrogen.
In the former conformer, the calculated distance between hydroxyl
hydrogen and alkoxy oxygen is 2.05 Å, which is less than the sum
of van der Waals radii (1.2 Å for H and 1.52 Å for O).22
Irradiation of SQ4Np solution with UV light resulted in spectral
changes characteristic of photoisomerization: reduction and hypso-
chromic shift of LWAB (Figure 2). The system achieved a photo-
stationary state (PSl), whose composition depended on irradiation
This work was supported by the Russian Foundation for Basic
Research (grant no. 10-03-00751).
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Received: 20th December 2010; Com. 10/3647
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