Molecular fluorescent pH-probes based on 8-hydroxyquinoline

Article abstract of DOI:10.1039/b600912c

Three 5,7-π-extended 8-benzyloxyquinolines, namely 5,7-diphenyl-, 5,7-bis(biphenyl-4-yl)- and 5,7-bis(4-dibenzothiophenyl)-8-benzyloxyquinoline were prepared and investigated as fluorescent pH-probes in nonaqueous solution. Absorption and photoluminescenc

Full text of DOI:10.1039/b600912c

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Molecular fluorescent pH-probes based on 8-hydroxyquinoline†
Stefan Kappaun,a Tanja Sovic´,a Franz Stelzer,a Alexander Pogantsch,b Egbert Zojer*b,c and
Christian Slugovc*a
Received 19th January 2006, Accepted 15th February 2006
First published as an Advance Article on the web 14th March 2006
DOI: 10.1039/b600912c
Three 5,7-p-extended 8-benzyloxyquinolines, namely 5,7-diphenyl-, 5,7-bis(biphenyl-4-yl)- and
5,7-bis(4-dibenzothiophenyl)-8-benzyloxyquinoline were prepared and investigated as fluorescent
pH-probes in nonaqueous solution. Absorption and photoluminescence spectra of the introduced
compounds also including the starting material 8-benzyloxy-5,7-dibromoquinoline as well as their
N-protonated counterparts were recorded and the results were rationalized by quantum-chemical
calculations. A pronounced red shift of the emission occurred upon protonation of the non halogenated
derivatives, while the dibromo-derivative is hardly emissive and is virtually not protonated under the
conditions used. The diphenyl- and the bis(biphenyl)-derivative especially show promising
photoluminescence quantum yields both in the parent and the protonated state making them
candidates for the active component in pH sensing applications.
As a further advance in the development of fluorescent pH-
Introduction
probes, we report the synthesis, and photophysical as well as
quantum-chemical characterization of modified 8-hydroxyquino-
line derivatives starting from 5,7-dibromo-8-hydroxyquinoline. As
it is well known that 8-hydroxyquinoline is weakly fluorescent be-
cause of the excited-state intramolecular proton transfer (ESIPT)
of the proton of the OH-function to the N of the pyridine moiety,3
our design is based on bonding alkyl chains to the OH-function
of 8-hydroxyquinoline inhibiting this ESIPT pathway.6
Moreover, we pay particular attention to the effects of ex-
tending the p-system on emission behaviour. Two of the in-
vestigated molecules proved suitable as ratiometric pH probes
in a non-aqueous environment making use of the protona-
tion/deprotonation of the quinoline nitrogen atom.
The design and synthesis of fluorescent molecules with a sensing
function is an area of intense research activity. A large number
of chemosensors whose emission properties are modulated by
external inputs have been described so far. Due to the high
sensitivity of fluorescence spectroscopy, allowing the detection
of very low concentrations of the analyte, in certain cases even
down to individual molecules, fluorescent sensors and probes
have been utilised in many areas such as medicine, industry and
the environment.1 While in many fluorescent chemosensors the
interaction with the analyte only affects the emission intensity,
a shift in the emission wavelength through the sensing process
is preferable because of the possibility of signal rationing to
increase the dynamic range and provide built-in correction for
environmental effects.2
8-Hydroxyquinoline and its derivatives have found many ap-
plications in the fluorescent sensing of biological and environ-
mentally important metal ions.3 Nevertheless, just one study
has hitherto been done dealing with the influence of acids and
bases on 8-hydroxyquinoline and its derivatives.4 Aside from
this, 8-hydroxyquinolines have been used in electroluminescent
complexes, most prominently tris(8-hydroxyquinoline)aluminium
([Alq3]). [Alq3] was introduced as an emitting material by Tang
and VanSlyke. Since then these and analogous metal complexes
have attracted a great interest in display applications.5
Experimental
Materials and methods
Unless otherwise noted, materials were obtained from commercial
sources (Aldrich, Fluka or Lancaster) and used without further
purification. 8-Benzyloxy-5,7-dibromoquinoline (2), 8-benzyloxy-
5,7-diphenylquinoline (3) and 8-hydroxy-5,7-diphenylquinoline
(6) were prepared as reported in the literature.7 1H-NMR spectra
were recorded on a Varian INOVA 500 MHz spectrometer versus
SiMe4 as standard. 13C{ H}-NMR spectra were recorded at
1
125 MHz. The solvent residual peak of CDCl3 was used for refer-
encing the NMR-spectra to 7.26 ppm and 77.16 ppm, respectively.8
FT-IR spectra were obtained from films on NaCl windows
with a Perkin Elmer Spectrum One and a DTGS-detector.
UV/VIS absorption spectra were recorded on a Cary 50 Bio
UV/Visible spectrophotometer, fluorescence spectra on a Perkin
Elmer Luminescence Spectrometer LS50B. PL Quantum yields
were measured using a Shimadzu RF-5301PC spectrofluorimeter
(detector corrected) and Perkin Elmer Lambda 9 UV/VIS/NIR
spectrophotometer using quinine sulfate dihydrate in 0.05 M
H2SO4 as standard. UV/VIS absorption and photoluminescence
aInstitute of Chemistry and Technology of Organic Materials (ICTOS),
Graz University of Technology, Stremayrgasse 16, A-8010, Graz, Austria.
E-mail: slugovc@tugraz.at; Fax: +43316 873 8951; Tel: +43316 873 8454
bInstitute of Solid State Physics, Graz University of Technology, Petersgasse
16, A-8010, Graz, Austria. E-mail: egbert.zojer@tugraz.at; Fax: +43316
873 8466; Tel: +43316 873 8475
cSchool of Chemistry and Biochemistry, Georgia Institute of Technology,
770 State Street, Atlanta, GA, 30332-0400, USA
† Electronic supplementary information (ESI) available: details of syn-
thesis, detailed absorption and fluorescence spectra of the introduced
materials, details of computational methodology and CI description of
the excited states. See DOI: 10.1039/b600912c
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(PL) spectra were recorded in diluted solutions of CHCl3–
MeOH (1 : 1) at room temperature under ambient conditions.
Fluorescence titrations and the evaluation of PL quantum yields of
the protonated and/or deprotonated species were also performed
in CHCl3–MeOH (1 : 1) by adding diluted solutions of CF3COOH
or KOtBu. The refractive index of the solvent mixture CHCl3–
MeOH (1 : 1) was determined with an Abbe-Refractometer at
General procedure for the preparation of 5,7-disubstituted
8-hydroxyquinoline derivatives (6–8)
Referring to a known procedure,9 a suspension of the benzylated
derivatives (3–5) and KI (15.9–25.8 eq.) in acetonitrile was
carefully degassed. After adding BF3·Et2O (11.5–20.6 eq.) under
an inert argon atmosphere, the reaction mixture was heated for
24 hours at 80 ◦C. Quenching with H2O, adding aqueous Na2S2O3
(10%, 4 mL) and aqueous HCl (10%, 2 mL) gave the crude
products after extraction with CH2Cl2 and drying over Na2SO4.
The products were purified by precipitation from hot methanol.
20 ◦C for the calculation of the PL quantum yield (nD 1.388).
20
Synthesis
8-Benzyloxy-5,7-bis(biphenyl-4-yl)quinoline (4). Similarly to
the procedure of Shoji et al.,7 a suspension of 2 (100 mg,
0.254 mmol), biphenyl boronic acid (125 mg, 0.631 mmol) and
Na2CO3 (300 mg, 2.830 mmol) in toluene (3 mL), EtOH (3 mL)
and H2O (3 mL) was degassed for 30 minutes. After adding
[Pd(PPh3)4] (25 mg, 0.022 mmol) the reaction mixture was heated
for 24 hours at 95 ◦C under an inert argon atmosphere. Products
were extracted with CH2Cl2, and dried over Na2SO4. The band
with Rf = 0.5 (cy : ee = 3 : 1) was sampled in the course of column
chromatography on silica using cy : ee = 10 : 1 as solvent giving 4
(120 mg, 87.4%) as a yellow solid. Anal. Calc. for C40H29NO: C,
89.02; H, 5.42; Found: C, 89.22; H, 5.29%. mmax(film)/cm−1: 3029m,
2925m, 1599m, 1519w, 1486s, 1453s, 1396m, 1370m, 1338s, 1214m,
1186m, 1164w, 1108w, 1075s, 1007s, 912s, 881w, 840s, 793m, 766s,
8-Hydroxy-5,7-diphenylquinoline (6). Compound 6 was ob-
tained using 3 (160 mg, 0.413 mmol), KI (1.090 g, 6.566 mmol)
and BF3·Et2O (0.6 mL, 4.7 mmol) in CH3CN (20 mL) giving 6
(50 mg, 40.7%) as a yellow-brown solid. Compound 6 has been
prepared from 3 before by Shoji and co-workers using Pd/C and
cyclohexa-1,4-diene.7
5,7-Bis(biphen-4-yl)-8-hydroxyquinoline (7). Compound 7 was
prepared starting from 4 (70 mg; 0.130 mmol) by adding KI
(360 mg, 2.169 mmol) and BF3·Et2O (0.2 mL, 1.6 mmol) in CH3CN
(10 mL) yielding 7 (20.8 mg, 35.8%) in a yellow-brown solid form.
Anal. Calc. for C33H23NO: C, 88.17; H, 5.16; Found: C, 88.25;
H, 5.22%. mmax(film)/cm−1: 3327m, 3028m, 1600w, 1574m, 1520w,
1496s, 1487s, 1456s, 1408s, 1372m, 1325m, 1265m, 1180m, 1162m,
1108w, 1075w, 1007m, 941w, 907m, 841s, 792m, 765s, 733s and
696s. dH(500 MHz, CDCl3): 9.44–8.69 (1H, bs, OH), 8.86 (1H, d,
3JHH 3.4, q2), 8.40ꢀ (1H, d, 3JHH 8.3, q4) and 7.97–7.37 (20H, m, q3,
3
735s and 696s. dH(500 MHz, CDCl3): 9.09 (1H, dd, JHH 3.9 and
3
4
4JHH 1.5, q2), 8.40 (1H, dd, JHH 8.3 and JHH 1.5, q4), 7.82–7.25
ꢀ
ꢀ ꢀ ꢀ ꢀ
(25H, m, bn2,3,4,5,6, 2 × bp2,3,5,6,2 ,3 ,4 5 ,6 , q3, q6) and 5.29 (2H, s, CH2);
dC(125 MHz, CDCl3): 151.4 (1C, q8), 149.9 (1C, q2), 143.8 (1C,
q8a), 141.0, 140.7ꢀ, 140.6, 140.3, 138.3, 137.5, 137.2, 135.7, 133.7
(9C, q5,7, 2 × bp1,1 ,4, bn1), 134.7 (1C, q4), 130.7, 130.5, 129.0, 129.0,
128.7, 128.1, 127.8, 127.ꢀ6, 127.5, 127.4, 127.3, 127.2, 127.0 (24C,
ꢀ
ꢀ
ꢀ ꢀ
q6, 2 × bp2,3,5,6,2 ,3 ,4 ,5 ,6 ). dC(125 MHz, CDCl3): 148.5 (1C, q8), 148.1
(1C,ꢀq2), 141.0, 140.8, 140.4, 140.2, 138.8, 138.4, 136.6 (7C, q8a, 2 ×
bp1,1 ,4), 134.8 (1C, q4), 130.6, 129.9, 129.0, 128.9, ꢀ 127.6, 127.4,
ꢀ
ꢀ ꢀ ꢀ
127.4, 127.3, 127.3, 127.3 (19C, q6, 2 × bp2,3,5,6,2 ,3 ,4 ,5 ,6 ), 130.5,
126.0, 121.9 (q,4a,5,7) and 121.9 (1C, q3).
ꢀ
ꢀ
ꢀ
ꢀ
bn2,3,4,5,6, 2 × bp2,3,5,6,2 ,3 ,4 5 ,6 , q4a), 129.7 (1C, q6), 121.3 (1C, q3) and
76.7 (1C, CH2).
5,7-Bis(dibenzothiophen-4-yl)-8-hydroxyquinoline (8). Com-
pound 8 was obtained by the reaction of 5 (70 mg, 0.117 mmol)
with KI (500 mg, 3.012 mmol) and BF3·Et2O (0.3 mL, 2.4 mmol)
in CH3CN (15 mL) giving 8 (30 mg, 50.4%) as a yellow-brown
solid. Anal. Calc. for C33H19NOS2: C, 77.77; H, 3.76; Found: C,
77.83; H, 3.69%. mmax(film)/cm−1: 3350m, 3063m, 1576m, 1499m,
1480m, 1460s, 1441s, 1416m, 1374m, 1334m, 1302m, 1261s,
1195m, 1170m, 1089m, 1045m, 906s, 817w, 793s, 750s, 728s, 691m
and 600w; dH(500 MHz, CDCl3): 9.30–8.60 (1H, bs, OH), 8.88
5,7-Bis(dibenzothiophen-4-yl)-8-benzyloxyquinoline (5). A sus-
pension of 2 (100 mg, 0.254 mmol), dibenzothiophene-4-boronic
acid (139 mg, 0.609 mmol) and Na2CO3 (300 mg, 2.830 mmol)
in toluene (3 mL), EtOH (3 mL) and H2O (3 mL) was degassed
for 30 minutes. After adding [Pd(PPh3)4] (25 mg, 0.022 mmol)
◦
the reaction mixture was heated for 24 hours at 95 C under an
inert argon atmosphere. Products were extracted with CH2Cl2 and
dried over Na2SO4. Sampling the band with Rf = 0.5 (cy : ee = 3 :
1) by column chromatography on silica with the solvent cy : ee =
7 : 1 yielded 5 (130 mg, 85.2%) as a yellow solid. Anal. Calc.
for C40H25NOS2: C, 80.10; H, 4.20; Found: C, 80.26; H, 4,13%.
mmax(film)/cm−1: 3063m, 2925m, 1571m, 1497m, 1441s, 1385m,
1342m, 1322w, 1303w, 1249m, 1216m, 1169m, 1109m, 1076m,
1046m, 908s, 795m, 751s, 728s and 694s. dH(500 MHz, CDCl3):
3
4
(1H, dd, JHH 3.9 and JHH 1.4, q2), 8.25–8.19 (4H, m, 2 × th1,8),
8.08 (1H, dd, JHH 8.3 and JHH 1.4 Hz, q4), 8.00 (1H, s, q6) and
7.82–7.41 (1H, m, q3, 2 × th2,3,5,6,7); dC(125 MHz, CDCl3): 149.3
(1C, q8), 148.4 (1C, q2), 141.1 (1C, q8a), 139.9, 139.8, 136.3, 136.2,
136.0, 135.9, 133.9, 132.5 (12C, q5,7, 2 × thquart.C), 135.1 (1C, q4),
130.1, 128.8, 128.7, 127.1, 126.8, 125.0, 124.8, 124.6, 124.4, 122.9,
3
4
122.8, 122.2, 122.0, 121.8, 121.0, 121.0 (16C, q3,6, 2 × th1,2,3,5,6,7,8
)
3
4
9.08 (1H, dd, JHH 3.9 and JHH 2.0, q2), 8.26–8.19 (4H, m, 2 ×
and 126.3 (1C, q4a).
th1,8), 8.06 (1H, dd, JHH 8.3 and JHH 2.0, q4), 7.93 (1H, s, q6),
3
4
7.79–7.38 (11H, m, q3, 2 × th2,3,5,6,7), 7.07–7.03 (5H, m, bn2,3,4,5,6
)
Quantum-mechanical calculations
and 5.25 (2H, 2 × d, 2JHH 11.1,CH2); dC(125 MHz, CDCl3): 152.6
(1C, q8), 150.1 (1C, q2), 143.8 (1C, q8a), 140.9, 140.1, 139.8, 137.3,
136.2, 136.0, 135.9, 135.8, 134.0, 133.9, 133.1, 132.8 (13C, q5,7,
bn1, 2 × thquart. C), 134.8 (1C, q4), 129.5, 129.1, 128.7, 127.6, 127.1,
126.8, 124.9, 124.6, 124.4, 122.9, 122.8, 122.0, 121.8, 121.2, 120.9
(16C, bn6, q6, 2 × th1,2,3,5,6,7,8), 128.3 (2C, bn3,5), 128.0 (2C, bn2,6),
127.8 (1C, q4a), 121.7 (1C, q3) and 77.0 (1C, CH2).
To provide microscopic insight into the electronic and optical
properties of the investigated materials, we have calculated the
optical properties of the investigated molecules using semi-
empirical methods. Ground-state equilibrium geometries have
been optimized at the AM1 level10 using Ampac 6.55. Excitation
energies and transition dipole moments have been calculated by
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coupling the INDO/S Hamiltonian11 to a single configuration
interaction (SCI) scheme as implemented by Zerner and co-
workers. The size of the SCI active space is scaled with the
number of p-electrons. To optimize excited state geometries
(which is necessary to describe emission properties), the AM1-
SCI technique as implemented in AMPAC has been used.12 Details
regarding the CI-active spaces can be found in the ESI†. In the
studied model systems (2ꢀ, 3ꢀ, 4ꢀ, 5ꢀ) the benzyloxy group of 2, 3, 4,
and 5 is replaced by a methoxy group.
For comparative reasons, we have also performed studies using
time-dependent density functional theory (TD-DFT) on the basis
of DFT optimized geometries at the B3LYP/6-31G(d,p) level
using Gaussian98 Rev. A.11.13 In the case of the non-protonated
molecules, the obtained results are comparable to the semi-
empirical ones although agreement with the experimental results is
slightly worse. For the protonated systems, we, however, observe a
less satisfactory performance of TD-DFT with the calculations in
some cases, for example predicting excited states located more
than 1 eV below the experimentally observed ones. This is in
fact quite a general observation that we have made investigating
a large number of different protonated/deprotonoted molecules
with various types of conjugated backbones. The origin for that
deviation is not yet fully understood but could be related to
self interaction effects modified by the change of nuclear charge,
although in that case the failure for cations appears somewhat
surprising. To gain a full understanding of this rather technical
aspect, further investigations will be necessary. In this context
it also needs to be stressed that solvent related effects and the
influence of counterions for the protonated molecules have been
neglected in the present calculations.
The benzyloxy-group was attached to 5,7-dibromo-8-hydroxy-
quinoline (1) by the reaction with benzyl bromide and KOH
in EtOH–H2O at 100 ◦C for 20 hours. After cooling to room
temperature, the precipitate was filtered off, washed with cold
methanol and purified by column chromatography on silica. To
remove the by-product benzyl alcohol, the solid residue was
washed with hot methanol several times yielding 8-benzyloxy-
5,7-dibromoquinoline (2) as a white solid (43.4%). Following
this pathway, two main goals could be achieved: (i) 2 is prone
to be coupled via a palladium-catalysed Suzuki cross-coupling
reaction with boronic acids, while 1 is not, due to deactivation of
the catalytically active palladium-species by forming a palladium
complex (presumably [Pdq2])4 and (ii) the ESIPT is blocked by
introduction of the protecting group.
8-Benzyloxy-5,7-diphenylquinoline (3), 8-benzyloxy-5,7-bis-
(biphenyl-4-yl)quinoline (4) and 5,7-bis(dibenzothiophen-4-yl)-8-
benzyloxyquinoline (5) were prepared under palladium-catalysed
conditions by the cross-coupling of 2 with the correspond-
ing boronic acids using the catalyst tetrakis(triphenylphospine)-
palladium ([Pd(PPh3)4]) in a solvent mixture of toluene, EtOH
and H2O in the presence of Na2CO3. After refluxing for 24 hours
at 95 ◦C under an inert argon atmosphere, the products could be
isolated in pure form after column chromatography on silica in
30.4%, 87.4% and 85.2% yield. 3–5 were characterized by IR, 1H-
and 13C-NMR spectroscopy as well as by elemental analysis.
The success of the Suzuki cross-coupling reaction prepar-
ing compounds 3–5 was strongly dependent on the catalyst
used. Experiments to reduce the reaction time and to permit
milder reaction conditions using the recently described, air-stable
catalyst trans-bis(dicyclohexylamine)palladium-diacetate ([trans-
Pd(OAc)2(NHCy2)2], “DAPCy”)14 gave rise to varying amounts
of by-products which were identified as mono-substituted 8-
benzyloxyquinoline derivatives (c.f. ESI†). Although these by-
products could be easily identified in 1H-NMR spectra, attempts
to separate them from 3–5 using column chromatography on silica
failed. However, these experiments showed that the 1H-NMR
signal of the benzyloxy-group can be used for the examination of
the purity of the formed 8-benzyloxy-5,7-disubstituted quinoline
derivatives. As depicted in Fig. 1, a mixture of mono- and
disubstituted products results in two signals for the protons of
the methylene group. While the protons of the OCH2-group of 2,
3 and 4 could be assigned to singlets at 5.47 ppm, 5.22 ppm and
5.29 ppm, the signal-form (two broadened doublets) of the now
diasterotopic CH2O-group of 5 at 5.25 ppm indicates a restricted
rotation of the benzyloxy group (see Fig. 1). Another fact obtained
from 1H-NMR spectra is the different shift of the protons 2 and
4 of the quinoline core (q2 and q4) of mono- and disubstituted
products. Even though the signals of q2 and q4 differ less than
those of the methylene moiety, they can be used for a quick control
of product-purity (see ESI†). Attempts to separate the mono-
substituted products by column chromatography on silica for the
preparation of asymmetrically substituted 8-benzyloxyquinoline
derivatives failed because of similar Rf values. It is worth noting
that beside the residual signal for C-Br no significant difference
between mono- and disubstituted products can be noticed by 13C-
NMR spectroscopy.
Results and discussion
Synthesis
The synthetic pathway used to prepare compounds 2–8 is shown
in Scheme 1.
Scheme 1 Synthesis of 5,7-disubstituted-8-hydroxyquinoline derivatives:
(i) benzyl bromide, KOH, EtOH, H2O, 100 ◦C, 20 hours; (ii) boronic acid,
[Pd(PPh3)4], toluene, EtOH, H2O, Ar, 95 ◦C, 24 hours; (iii) BF3·Et2O, KI,
CH3CN, Ar, 80 ◦C, 24 hours. Bn = benzyl.
For the preparation of 8-hydroxy-5,7-diphenylquinoline (6), 5,7-
bis(biphen-4-yl)-8-hydroxyquinoline (7) and 5,7-bis(dibenzothio-
phen-4-yl)-8-hydroxyquinoline (8), the benzyloxy-group was
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Absorption spectra and quantum mechanical calculations
UV/VIS absorption was recorded in diluted solutions of CHCl3–
MeOH (1 : 1) at room temperature under ambient conditions. The
optical data for the absorption of compounds 2–5 are summarised
in Table 1, their absorption spectra are shown in Fig. 2. In the
measured solutions all compounds exhibit a main absorption peak
at 245 nm, 257 nm, 282 nm and 242 nm, respectively. At longer
wavelengths, absorption peaks in the region of 308 nm, 320 nm and
334 nm can be detected for compounds 2–4. For 5 two absorption
peaks at 289 nm and 330 nm are observed.
Fig. 1 1H-NMR signals of the benzyloxy-group of compounds 2–5.
Mono-coupling products can be identified by different shifts of the
protecting group.
removed after activation with BF3·Et2O by the nucleophilic attack
of KI in acetonitrile. The dark coloured solutions (formation
of iodine in the reaction) were treated with aqueous Na2S2O3,
acidified and purified by precipitation from methanol to remove
the formed benzyl iodide. Compounds 6–8 could be isolated in
analytically pure form in moderate yield (40.7% for 6, 35.8% for 7
and 50.4% for 8). The above described cleavage of the protecting
group was chosen because we encountered difficulties when using
conventional catalytic hydrogenolysis protocols.7 In this case, the
need for separation of unreacted 3, 4 or 5 led, in our hands, to
yields below 10%.15
Fig. 2 Absorption spectra of compounds 2–5 in dilute CHCl3–MeOH
(1 : 1) solution.
The INDO-SCI calculated energies, wavelengths and oscillator
strengths16 for the first excited state as well as higher lying states
with significant oscillator strengths are also listed in Table 1
Table 1 Experimentally obtained absorption maxima for 2, 3, 4 and 5 and INDO/SCI calculated excitation energies, wavelengths and oscillator strengths
of the dominant one-photon states in molecules 2ꢀ, 3ꢀ, 4ꢀ, and 5ꢀ. (Details regarding the CI-description of the states can be found in the ESI†)
Molecule
E/k (eV/nm)
e (L mol−1 cm−1
)
Molecule/state
E (eV)
OS
2
2ꢀ
S1
3.96
0.00
3.73/332
4.05/308
5.28/245
2800
4700
36000
S4
S7
3ꢀ
S1
S2
S6
S7
4ꢀ
4.45
5.23
0.16
0.93
3
4
3.74
3.95
4.65
4.84
0.00
0.22
1.37
0.28
3.92/320
4.83/257
7100
38000
S1
S2
S4
S6
S9
5ꢀ
S1
S2
S4
S8
S9
S10
S18
3.72
3.89
4.26
4.30
4.56
0.01
0.38
1.15
0.78
0.53
3.72/334
4.42/282
shoulder
high energy tail
11000
39000
5
3.75
4.00
4.08
4.49
4.54
4.62
5.33
0.00
0.15
0.12
1.43
0.39
0.72
0.50
3.76/330
3.90/317
4.31/289
9600
8500
18000
5.11/242
81000
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for model-molecules 2ꢀ, 3ꢀ, 4ꢀ, and 5ꢀ. A comparison with the
measured data shows that the experimental observations are fully
reproduced by the calculations. All molecules are characterized
by a lowest lying state with vanishing oscillator strength (which
is thus not visible in the experiments), a relatively weakly allowed
state at smaller energies and a higher energy state with significantly
increased oscillator strength. For the weak low-energy peak (which
in the absorption spectrum of 4 is only visible as a shoulder)
one observes a shift to lower energies (0.33 eV/0.38 eV in the
measurements/calculations) with increasing conjugation length
going from 2 to 3 to 4. A much more pronounced shift is obtained
for the strongest maximum at higher energies (between 4.25 eV
and 5.0 eV) both in the calculations and in the measurements. To
understand this behaviour, it is necessary to have a closer look at
the actual electronic structure of the excited states and, as a first
step, at the molecular orbitals involved in the excitation.
From the plots in Fig. 3 it becomes evident that the highest
occupied molecular orbital (HOMO) and to an even greater extent
the HOMO−1 in 4ꢀ (as a representative example for this class
of molecules) are delocalized over the molecular backbone and
therefore strongly affected by increased conjugation due to the
biphenyl substituents in positions 5 and 7. The lowest unoccupied
molecular orbital (LUMO) is largely concentrated on the central
hydroxyquinoline unit, while the LUMO+1 is again strongly
delocalized. In all molecules, the first optically allowed state is
largely described by an excitation from the HOMO to the LUMO
while the HOMO−1 and LUMO+1 also strongly contribute to
the description of the main maximum (details regarding the CI
descriptions of the excited states can be found in the ESI†). This is
fully consistent with the stronger conjugation length dependence
of the latter peak.
Fig. 4 INDO/SCI calculated e–h two-particle wavefunctions for the S1
(top-left), S2 (top-right), and S4 (bottom-left) states of 4ꢀ. The shading gives
the probability of finding the hole on the site represented by the x-axis,
while the electron is on the site given by the y-axis. The numbering of the
sites is shown schematically in the bottom right corner.
of the hydroxyquinoline). This more pronounced delocalisation
explains why the main maximum is most strongly affected by
increasing the conjugation length.
Compound 5ꢀ somewhat deviates from the general trends
observed for the other three materials. There are several low-
lying states, whose energies and oscillator strengths somewhat
differ for different conformations. This can be explained by the
observation that different orientations of the substituents result
in different sterical constraints, thus different twist angles and, in
turn, different degrees of electronic coupling between the quinoline
core and the substituents in the 5 and 7 positions. The results
contained in Table 1 have been obtained for the conformation
shown in the ESI†. Also in the higher energy region the situation
is clearly more complex than in the other materials with three
states with oscillator strengths >0.3 around 4.5 eV and five states
between 5.3 and 5.7 eV. The superposition of the associated peaks
assuming a Gaussian broadening with a FWHM of 0.4 eV then
yields two maxima at 4.52 eV and 5.64 eV, where the relative
intensity of the lower energy maximum is clearly overestimated
compared to the experiment.
Fig. 3 INDO calculated highest occupied and lowest unoccupied molec-
ular orbitals of 4ꢀ (a coloured figure can be found in the ESI†).
Protonation of 2, 3, 4 and 5
The different nature of the various excited states becomes even
more obvious from the electron-hole two particle wavefunctions
in Fig. 4, which describe the correlated motion of the electron-
hole pair. Here, the shading represents the probability of finding
the hole at the site x, while the electron is at site y.17 The S1
state, which is optically forbidden, is strongly localized on the
hydroxyquinoline unit. In the first allowed state S2 especially,
the hole is already somewhat delocalized and the state res-
ponsible for the main peak (S4) is fully delocalized over the
backbone with, in fact, higher probabilities for the electron and
the hole to be at the substituents (especially that at the 5-position
To test the applicability of the presented materials as pH probes,
we further investigated the photophysical properties of 2, 3, 4 and
5 upon addition of 10 lL conc. trifluoracetic acid (TFA). By doing
so, we anticipated a protonation of all molecules at the quinoline
nitrogen atom forming 2-H+, 3-H+, 4-H+ and 5-H+. From the
resulting absorption and emission spectra (vide infra) it turned out
that in the case of 2 almost no protonation occurred. Even upon
addition of another 10 lL TFA conc. no significant change of the
absorption spectrum was detected. In contrast, 3, 4 and 5 were
completely protonated under the conditions used, as indicated by
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the formation of yellow coloured solutions and thus significantly
altered absorption spectra (c.f. Fig. 5). Addition of another 10 lL
of TFA resulted in no further changes. Absorption maxima for
3-H+, 4-H+ and 5-H+ are presented in Table 2. Again INDO/SCI
calculations were performed in order to understand the observed
changes in the absorption spectra.
(appearance of shoulders) and in the calculations. Concerning
the quantitative agreement between theory and experiment, the
INDO/SCI calculations here somewhat underestimate the ener-
gies of the excited states (i.e., overestimate the shift resulting from
the protonation) but otherwise give a realistic picture. We attribute
this overestimation of the shift to the neglect of interactions with
solvent and/or counterions in the calculations. Hydrogen bonding
and the impact of solvents on the proton transfer has, for example,
been described by Lehtonen et al.18 for the methanesulfonic acid–
pyridine complex. In the actual system such interactions should
similarly weaken the bonding between the proton and the nitrogen
in the hydroxyquinoline.
In 3ꢀ the CI descriptions of the two low-lying states are
dominated by excitations from the HOMO−1 and the HOMO
into the LUMO. Here, the two occupied orbitals are delocalized
over the whole molecule, while the LUMO is strongly localized
on the central quinoline unit. The localisation is even more
pronounced in 4ꢀ where the HOMO and the HOMO−1 are each
largely localized on one of the biphenyl substituents, while the
LUMO is again localized on the quinoline (see orbital plots
in the ESI†). Only the HOMO−4, which plays a role in the
description of the S1 state (see ESI), is delocalized over the whole
backbone. This is consistent with the relatively low oscillator
strength of these excited states as well as with the fact that in
the exciton the hole is delocalized over the backbone, while the
electron is strongly localized on the quinoline (see e–h plots in the
ESI).
Fig. 5 Absorption spectra of compounds 2–5 in their protonated forms.
As far as the electronic states in the protonated molecules are
concerned, the most intriguing aspect is the appearance of two
optically allowed excited states below the first state of the non
protonated systems, as consistently observed in the experiments
The UV/VIS spectra of 6, 7 and 8 are similar to the spectra of
3, 4 and 5 and are therefore not discussed here. Data can be found
in the ESI†.
Table 2 Experimentally obtained absorption maxima for 2-H+, 3-H+, 4-H+, and 5-H+ and INDO/SCI calculated excitation energies, wavelengths and
oscillator strengths of the dominant one-photon states in the protonated species 2ꢀ-H+, 3ꢀ-H+, 4ꢀ-H+, and 5ꢀ-H+. (Details regarding the CI-description of
the states can be found in the ESI†)
Molecule
E/k (eV/nm)
e (L mol−1 cm−1
)
Molecule/state
E (eV)
OS
2-H+
2ꢀ-H+
S1
a
a
—
—
—
—
3.42
4.70
0.11
0.82
a
a
S4
3-H+
3ꢀ-H+
S1
3.27/380
3.63/341
4800
6400
2.84
3.35
4.08
4.53
4.66
5.06
0.18
0.26
0.30
0.22
0.37
0.22
S2
S5
S7
S11
S12
4ꢀ-H+
S1
S2
S3
S7
S8
S12
S19
5ꢀ-H+
S1
4.46/278
4.46/278
19000
19000
4-H+
2.64
2.92
3.74
4.12
4.24
4.44
4.89
0.21
0.41
0.26
0.38
0.39
0.49
0.44
3.08/402
3.44/360
3.99/311
5600
9000
15000
4.54/274
42000
5-H+
3.21/386
3.80/326
3.92/316
4.33/286
4000
7300
7400
2.88
3.42
3.86
4.17
0.24
0.17
0.10
0.42
S5
S6
21000
S10
b
a Almost no protonation occurred as indicated by a virtually unchanged absorption spectrum after addition of TFA. b Several states with intermediate
oscillator strengths at higher energies.
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Table 3 Photoluminescence data of compounds 2–5 and 2-H+–5-H+:
Energies (E) and wavelengths (k) of the luminescence maxima as well
as luminescence quantum yields (øPL) of the respective compounds. 2ꢀ–5ꢀ
and 2ꢀ-H+-5ꢀ-H+: INDO/SCI calculated emission energies, wavelengths,
oscillator strengths (OS) and radiative lifetimes (Trad) for the lowest
excited states of the model molecules as obtained for the S1 excited state
equilibrium geometries
Compound
E (eV)
k (nm)
øPL (%)
OS
Trad
2
3.16
3.58
3.16a
2.95
2.91
3.37
2.41
2.4
2.88
3.24
2.28
2.21
2.95
2.12
393
346
393a
420
426
368
515
518
430
383
545
561
421
584
<0.5
—
<0.5
—
14
—
9
—
16
—
12
—
2
—
0.00
—
0.10
—
0.35
—
0.20
—
0.80
—
0.43
—
—
2ꢀ
0.7 ls
2-H+
2ꢀ-H+
3
26.0 ns
5.8 ns
3ꢀ
3-H+
3ꢀ-H+
4
20.6 ns
2.7 ns
4ꢀ
4-H+
4ꢀ-H+
5
11.0 ns
5-H+
1
Fig. 7 Fluorescence titration of 4 with CF3COOH (kEx = 348 nm).
a No other emission was detected upon addition of CF3COOH.
solutions of TFA in CHCl3–MeOH (1 : 1) were added in microlitre
quantities in the course of fluorescence titration experiments. The
original emission was recovered in each case upon addition of
KOtBu to the acidified solutions indicating the reversibility of the
reaction.19
Compounds 1 and 6–8 as well as their protonated or deproto-
nated forms exhibited quantum yields below 1% rendering these
materials unsuitable as fluorescent pH-probes. Corresponding
data are gathered in the ESI†.
Photoluminescence
Photoluminescence (PL) spectra were obtained by excitation at
348 nm for all compounds. The peak energies of the resulting
emission spectra of 2–5 are listed in Table 3 together with the
measured PL quantum yields. In the non-protonated form, all
molecules emitted in the blue spectral region (with maxima at
393 nm, 426 nm, 430 nm and 421 nm). With the exception of 2, all
compounds emitted at longer wavelengths after protonation with
10 lL TFA conc. (about 515 nm, 545 nm and 584 nm). Among
these materials, 3 and 4 exhibited comparably good fluorescence
intensities as indicated by quantum yields well above 10% (øPL = 14
to 16%), while 2 and 5 showed poor fluorescence intensities with
quantum yields below 2%. Similar behaviour was observed for
the protonated species, albeit quantum yields are generally lower
than in the unprotonated state. The results are best described
by the fluorescence titrations depicted in Figs. 6 and 7. Dilute
Again semi-empirical calculations were performed to rationalize
the emission behaviour of 2–5. The emission properties are
determined by the characteristics of the lowest excited states at
their equilibrium geometries. The energies and oscillator strengths
of these states for the non protonated as well as for the protonated
molecules are shown in Table 3. In 3ꢀ and 4ꢀ the order of the
excited states changes compared to the results obtained for the
ground-state equilibrium geometries (c.f. Table 3) rendering the
lowest excited state optically allowed and the molecule emissive.20
The Stokes shift is, however, somewhat underestimated by the
calculations. In 2ꢀ the optically forbidden state remains lowest in
energy, which results in 2 being non fluorescent as observed in
the experiment. This is a consequence of S1 being a p→r* excited
state, where the LUMO is a r* orbital localized at the C–Br bonds.
For 5, no conclusive answers for the emission properties have been
obtained in the calculations, as here we find a large number of local
minima of the S1 geometry. The resulting transition energies differ
by more than 1 eV (yielding in some cases transition energies
clearly below the experimental observations) depending on the
corresponding twist angle of the 2-dibenzothiophene substituents.
For the lowest energy conformers found in our studies, interactions
of the sulfur with hydrogen atoms in either position 4 or 6 of the
quinoline moiety were detected.
In the protonated state, the calculations predict all molecules
to be emissive, although with smaller ‘oscillator strength’ than in
the non protonated state. In this context it has to be kept in mind
that the oscillator strength is proportional to the energy of the
excited state times the square of the transition dipole to the ground
state; this is a direct measure for the ‘absorption strength’ of a
Fig. 6 Fluorescence titration of 3 with CF3COOH (kEx = 348 nm).
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state. In contrast to that, spontaneous emission probabilities are
proportional to the cube of the transition energy times the square
of the transition dipole. Therefore, calculated radiative lifetimes21
have also been included in Table 3.
References and notes
1 (a) J. I. Peterson and G. G. Vurek, Science, 1984, 224, 123; (b) Prin-
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B. L. Feringa, Wiley-VCH, Weinheim, 2001; (d) Z. Wang, G. Zheng
and P. Lu, Org. Lett., 2005, 7, 3669.
2 (a) H. Ihmels, A. Meiswinkel and C. J. Mohrschladt, Org. Lett., 2000,
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Chou, J.-M. Fang, Z. Slanina and T. J. Chow, Org. Lett., 2002, 4, 3107;
(c) J.-S. Yang, C.-S. Lin and C.-Y. Hwang, Org. Lett., 2001, 3, 889;
(d) T. H. Kim and T. M. Swager, Angew. Chem., Int. Ed., 2003, 42,
4803; (e) J. V. Mello and N. S. Finney, Angew. Chem., Int. Ed., 2001,
40, 1536; (f) L. Fabrizzi, M. Licchelli, N. Marcotte, F. Stomeo and
A. Taglietti, Supramol. Chem., 2002, 14, 127; (g) B. Liu and H. Tian,
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3 (a) M. D. Shults and B. Imperiali, J. Am. Chem. Soc., 2003, 125, 14248;
(b) M. D. Shults, D. A. Pearce and B. Imperiali, J. Am. Chem. Soc.,
2003, 125, 10591; (c) D. A. Pearce, N. Jotterand, I. S. Carrico and B.
Imperiali, J. Am. Chem. Soc., 2001, 123, 5160; (d) N. Jotterand, D. A.
Pearce and B. Imperiali, J. Org. Chem., 2001, 66, 3224; (e) R. T. Bronson,
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(g) L. Prodi, M. Montalti, N. Zaccheroni, J. S. Bradshaw, R. M. Izatt
and P. B. Savage, Tetrahedron Lett., 2001, 42, 2941; (h) L. Prodi, C.
Bargossi, M. Montalti, N. Zaccheroni, N. Su, J. S. Bradshaw, R. M.
Izatt and P. M. Savage, J. Am. Chem. Soc., 2000, 122, 6769; (i) R. T.
Bronson, D. J. Michaelis, R. D. Lamb, G. A. Husseini, P. B. Fransworth,
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2005, 7, 1105; (j) F. Launay, V. Alain, E. Destandau, N. Ramos, E.
Bardez, P. Baret and J.-L. Pierre, New J. Chem., 2001, 25, 1269; (k) J. K.
Winkler, C. M. Bowen and V. Michelet, J. Am. Chem. Soc., 1998, 120,
3237; (l) S. Y. Moon, N. R. Cha, Y. H. Kim and S. K. Chang, J. Org.
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The observed luminescence quantum yields in Table 3 as well as
the emission spectra in Figs. 6 and 7 with the strongly shifted
emission spectrum in the protonated species are thus at least
qualitatively consistent with the measurements (for a quantitative
comparison, a rigorous treatment of non-radiative deexcitation
channels together with their pH dependence would be necessary).
Interestingly, we find that for both the protonated and non
protonated forms of 3ꢀ and 4ꢀ, the coupling between the hydroxy-
quinoline core and the substituent at the 5 position is increased
in the S1 equilibrium geometry compared to the ground state.
This can be concluded from both a reduction of the twist angle
between the plane of the 5-substituent and the hydroxyquinoline
(from 60◦ to 33◦ in 4ꢀ and from 58◦ to 33◦ in 4ꢀ-H+) and the
corresponding e–h two-particle wavefunctions. In both molecules
and both protonation states, the lowest lying optically allowed
states are clearly more delocalized between the hydroxyquinoline
and the substituent in the 5 position for the S1 equilibrium
geometry (for the case of molecule 4ꢀ see ESI†). A similar effect was
described in the literature on the basis of absorption measurements
of Ru(COD)(hydroxyquinoline) complexes featuring differently
substituted hydroxyquinolines.22
Conclusions
In summary, a series of 5,7-disubstituted 8-benzyloxyquinoline
derivatives has been prepared by a Suzuki cross-coupling reac-
tion. Photophysical measurements revealed adequate fluorescence
properties due to an inhibited excited-state intramolecular proton
transfer (ESIPT). For these compounds, modulation of the
emission properties upon an external input (pH) could be detected,
which opens up a possible application as ratiometric fluorescent
pH-probes. The photophysical properties were rationalized by
semi-empirical INDO/SCI calculations. Additionally the corre-
sponding series of 5,7-disubstituted-8-hydroxyquinoline deriva-
tives was synthesized. Albeit these materials alter their emission
properties upon protonation and deprotonation, they suffer from
the drawback of very low photoluminescence quantum yields
which impede their use in pH sensing applications. Nevertheless,
the synthesized p-extended 8-hydroxyquinoline derivatives have
found,5,7 and will find, use in electroactive or electroemissive
materials as ligands for, e.g., aluminium, zinc or boron. Efforts
along these lines are currently underway in our laboratories.
4 R. Ballardini, G. Varani, M. T. Indelli and F. Scandola, Inorg. Chem.,
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1998, 171, 161; (c) V. A. Montes, G. Li, R. Pohl, J. Shinar and P.
Anzenbacher, Adv. Mater., 2004, 16, 2001; (d) R. Pohl, V. A. Montes,
J. Shinar and P. Anzenbacher, J. Org. Chem., 2004, 69, 1723; (e) Q.
Wu, M. Esteghamatian, N.-X. Hu, Z. Popovic, G. Enright, Y. Tao,
M. D’Iorio and S. Wang, Chem. Mater., 2000, 12, 79; (f) H. Jang,
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6 E. Bardez, I. Devol, B. Larrey and B. Valeur, J. Phys. Chem. B, 1997,
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7 E. Shoji, K. Miyatake, A. R. Hlil, A. S. Hay, T. Maindron, V.
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8 H. E. Gottlieb, V. Kotylar and A. Nudelman, J. Org. Chem., 1997, 62,
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9 Y. D. Vankar and C. T. Rao, J. Chem. Res. (S), 1985, 232.
10 M. J. S. Dewar, E. G. Zoebisch, E. F. Healy and J. J. P. Stewart, J. Am.
Chem. Soc., 1985, 107, 3902.
11 J. A. Pople, D. L. Beveridge and P. A. Dobosh, J. Chem. Phys., 1967,
47, 2026; J. Ridley and M. Zerner, Theor. Chim. Acta, 1973, 32,
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Acknowledgements
ꢁ
C
12 Ampac 5.0 User’s Manual, , 1994, Semichem, 7128 Summit, Shwanee,
Financial support by the Spezialforschungsbereich (SFB) Elec-
troactive Materials of the Austrian Science Found FWF (project
numbers F903 and F917) and the Austrian Nano Initiative
(Research Project Cluster 0700-Integrated Organic Sensor and
Optoelectronics Technologies-Research Project 0701 and 0702)
is gratefully acknowledged. The work at Georgia Tech has been
partly supported by the National Science Foundation (CRIF 04-
43564). The authors would like to thank Filipp Furche and Jean-
Luc Bre´das for stimulating discussions.
KS 66216.
13 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr., R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M.
Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
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C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. G.
Johnson, W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez, M. Head-
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and WE. J. List, Chem. Mater., 2004, 16, 4667, rather than applying the
symmetrized version we used in previous works.
18 O. Lehtonen, J. Hartikainen, K. Rissanen, O. Ikkala and L.-O. Pietila,
J. Chem. Phys., 2002, 116, 2417.
19 Molecular Fluorescence, B. Valeur, Wiley-VCH, Weinheim, 2001.
20 In 3ꢀ the splitting between the optically allowed and the lowest forbidden
state is actually considerably smaller resulting in a different order for
one of the tested CI spaces- see also ESI†.
14 B. Tao and D. W. Boykin, J. Org. Chem., 2004, 69, 4330.
15 Attempts to purify compounds 6–8 by column chromatography failed
because of presumably complex formation of the products with metal
ions contained in silica.
16 The convention used for the CI coefficients assumes that each
configuration abbreviated, e.g., by H→L is a normalized singlet
wavefunction.
17 Here we follow the definition given in: L. Romaner, G. Heimel, H.
Wiesenhofer, P. Scandiucci de Freitas, U. Scherf, J. L. Bre´das, E. Zojer
21 For details regarding the calculations of the lifetimes see, e.g., U. Rant,
U. Scherf, M. Rehahn, P. Galda, J. L. Bre´das and E. Zojer, Synth. Met.,
2002, 127, 241.
22 C. Slugovc, A. Koppitz, A. Pogantsch and F. Stelzer, Inorg. Chim. Acta,
2005, 358, 2718.
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