Drastic solid-state fluorescence enhancement behaviour of phenanthro[9,10-d]imidazole-type fluorescent hosts upon inclusion of carboxylic acids

Article abstract of DOI:10.1002/ejoc.200900823

The crystals of phenanthro[9,10-d]imidazole-type fluorescent host 1 exhibit drastic fluorescence enhancement behaviour with a redshift in the emission maximum upon enclathration of various carboxylic acids such as formic acid, acetic acid and propionic ac

Full text of DOI:10.1002/ejoc.200900823

FULL PAPER
DOI: 10.1002/ejoc.200900823
Drastic Solid-State Fluorescence Enhancement Behaviour of
Phenanthro[9,10-d]imidazole-Type Fluorescent Hosts upon Inclusion of
Carboxylic Acids
Yousuke Ooyama,*[a] Kazuki Uwada,[a] Hironori Kumaoka,[a] and Katsuhira Yoshida*[a]
Keywords: Dyes/Pigments / Clathrates / Carboxylic acids / Structure elucidation / Fluorescence
The crystals of phenanthro[9,10-d]imidazole-type fluorescent
host 1 exhibit drastic fluorescence enhancement behaviour
with a redshift in the emission maximum upon enclathration
of various carboxylic acids such as formic acid, acetic acid
and propionic acid. The optical changes are greatly depend-
ent on the identity of the enclathrated carboxylic acids. The
fluorescent clathrate compounds are formed not only by
cocrystallization from carboxylic acid solutions but also by
solid (fluorescent host)–gas (carboxylic acid vapour) contact.
Furthermore, when the acetic acid inclusion crystals are ex-
posed to propionic acid vapour, acetic acid is gradually re-
placed by propionic acid. The guest exchange of the in-
clusion crystals was accompanied with colour and fluores-
cent intensity changes. The X-ray structural analyses of the
guest-free and carboxylic acid inclusion compounds demon-
strated that the destructions of the π–π interactions, and the
intermolecular hydrogen bonds binding fluorophores were
induced by the enclathrated carboxylic acid molecules.
Moreover, the imidazole ring of the host is protonated by the
enclathrated carboxylic acid proton. On the basis of the spec-
troscopic data and the crystal structures, the effects of the
enclathrated carboxylic acid on the solid-state photophysical
properties of the clathrate compounds are discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
tals exhibit guest-dependent fluorescence enhancement with
a redshift in the emission maximum upon contact with gas-
eous carboxylic acids such as formic acid, acetic acid and
propionic acid.[11] Recently, we found that when the acetic
acid inclusion crystals are exposed to propionic acid vap-
our, acetic acid is gradually replaced by propionic acid. The
guest exchange of the inclusion crystals were accompanied
by colour and fluorescent intensity changes. Furthermore,
the fluorescent clathrate compounds are formed by
cocrystallization from various carboxylic acid solutions,
and the degree of fluorescence enhancement and the red-
shift in the emission maximum are greatly dependent on the
identity of the enclathrated carboxylic acids. In this paper,
the effects of enclathrated carboxylic acids on the solid-state
photophysical properties and crystal-packing structures of
1 are discussed on the basis of the X-ray crystal structures
of the guest-free and carboxylic acid inclusion compounds.
Introduction
Solid-state fluorescent organic hosts are attractive mate-
rials in the fundamental research field of solid-state photo-
chemistry and in the applied field of optoelectronic devices,
because the organic fluorescent host can be the most prom-
ising material for the construction of desirable solid-state
fluorescent systems.[1–4] Several types of fluorescent hosts
exhibit fluorescence enhancement behaviour with a blue-
shift in the fluorescence wavelength maximum upon in-
clusion of various organic solvent molecules.[5–11] From
comparison of the X-ray crystal structures of the guest-free
and several clathrate compounds, it was concluded that the
changes in molecular arrangement by the formation of CH–
π interactions and the destructions of π–π interactions and
the intermolecular hydrogen bonds binding fluorophores by
the enclathrated guest molecules are the main reason for
the guest-dependent fluorescence enhancement and the
blueshift in the fluorescence maximum of the crystals. In
contrast, we reported that a new phenanthro[9,10-d]imid-
azole-type fluorescent host,[12] 2-[4-(diethylamino)phenyl]-
1H-phenanthro[9,10-d]imidazole (1; Scheme 1), whose crys-
[a] Department of Material Science, Faculty of Science, Kochi Uni-
versity
Akebono-cho, Kochi 780-8520, Japan
Fax: +81-88-844-8359
E-mail: yooyama@hiroshima-u.ac.jp
kyoshida@cc.kochi-u.ac.jp
Scheme 1. Chemical structure of phenanthro[9,10-d]imidazole-type
fluorescent clathrate host 1.
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fluorescence maxima of 2-[4Ј-(N,N-dimethylamino)phenyl]-
benzimidazole in acidic solution are redshifted relative to
those in neutral solution.[13] They demonstrated that the
redshifts are induced by protonation of an imino nitrogen
atom of the imidazole ring. Therefore, it is suggested that
the observed redshifts of 1 in acetic acid are caused by pro-
tonation of an imino nitrogen atom of the imidazole ring,
as shown in Scheme 2. The fluorescence quantum yield (Φ)
of 1 in 1,4-dioxane, ethanol and acetic acid are 0.56, 0.52
and 0.68, respectively.
Results and Discussion
Spectroscopic Properties of 1 in Solution
The absorption and fluorescence spectroscopic data of 1
in 1,4-dioxane, ethanol and acetic acid summarized in
Table 1 and the spectra of 1 are shown in Figure 1. Fluoro-
phore 1 exhibits an absorption band at around 347 nm and
a fluorescence band at around 408 nm in 1,4-dioxane. In
contrast, in ethanol, fluorophore 1 exhibits an absorption
band at around 341 nm with a shoulder at 371 nm and a
fluorescence band at around 419 nm. Interestingly, in acetic
acid, the absorption and fluorescence bands of 1 appear at
around 376 and 423 nm, respectively, which are redshifted
by 29 and 15 nm, respectively, relative to those observed in
1,4-dioxane. Dogra et al. reported that the absorption and
Table 1. Spectroscopic properties of 1 in solution.
Solvent
λmaxabs [nm][a]
λmaxfl [nm][b] Φ[c]
SS[d]
∆λmax
(εmax [dm3mol–1cm–1])
1,4-Dioxane
Ethanol
Acetic acid
347 (36700)
341 (35100), 372 (22200)
376 (40600)
408
419
423
0.56
0.52
0.68
61
78
47
[a] 1.0ϫ10–5 . [b] 1.0ϫ10–6 . [c] The Φ values were determined
by using a calibrated integrating sphere system (λex = 363 nm).
[d] Stokes shift value.
Scheme 2.
Semiempirical MO Calculations (AM1, CNDO/S)
To understand the photophysical properties of 2-[4-(di-
ethylamino)phenyl]-1H-phenanthro[9,10-d]imidazole
(1),
we carried out semiempirical molecular orbital (MO) calcu-
lations of 1 by the CNDO/S method[14] after geometrical
optimization by the MOPAC/AM1 method.[15] Further-
more, we estimated the photophysical properties for 1H+
with the protonated imidazole ring, because it was assumed
that the imino nitrogen atom of the imidazole ring is pro-
tonated in acidic solution. As shown in Figure 2, the opti-
mized geometry of 1 shows that he bond lengths between
the N-1 and N-2 nitrogen atoms and C-2 in the imidazole
ring are 1.41 and 1.36 Å, respectively. This result shows that
the differences between the two lengths for 1 are attributed
to the C–N and C=N bonds of the imidazole ring. In con-
trast, the two bond lengths for 1H+ with the protonated
imidazole ring are both 1.39 Å, which reveals the contri-
bution of the resonance structure 1H+(I), as shown in
Scheme 2. In contrast, the bond lengths between C-3 of the
phenyl group and C-2 in the imidazole ring are 1.46 Å for
1 and 1.44 Å for 1H+, and the bond lengths between C-1
of the phenyl group and N3 of the diethylamino group are
1.40 Å for 1 and 1.37 Å for 1H+. The two bond lengths of
1H+ are shorter than those of 1, which indicates the contri-
bution of the resonance structure 1H+(II), as shown in
Scheme 2. The calculated absorption wavelengths and the
Figure 1. (a) Absorption and (b) fluorescence spectra of 1 in 1,4-
dioxane, ethanol and acetic acid.
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Eur. J. Org. Chem. 2009, 5979–5990

Fluorescence Enhancement of Phenanthro[9,10-d]imidazole Hosts
nature of the transition of the first absorption bands are Fluorescence Sensing Behaviour of Crystals of 1 upon
collected in Table 2. The calculated absorption wavelengths Contact with Gaseous Carboxylic Acids
and the oscillator strength values of 1 and 1H+ are compar-
able with the observed spectra in 1,4-dioxane and acetic
Recrystallization of 1 from acetonitrile gave needle-
acid; the calculated absorption wavelength of 1H+ is shifted shaped crystals that contain no solvent molecules. Interest-
to longer wavelength relative to that of 1, which is in good ingly, when the guest-free crystals were placed in a vessel
agreement with the experimental data in 1,4-dioxane and saturated with carboxylic acid vapour at 30 °C, the colour
acetic acid. Therefore, the redshift of 1 in acetic acid was of the crystals turned from colourless to yellow. By solid-
explained by the resonance interaction of the diethylamino state fluorescence measurement, we found that a redshift in
group with the imidazole ring, leading to a resonance hy- the excitation and emission maxima and a guest-dependent
brid of 1H+(I) and 1H+(II).
fluorescence enhancement are induced upon inclusion of
the guest molecules. As an example, the fluorescence exci-
tation and emission spectral changes in the guest-free crys-
tals of 1 upon exposure to acetic acid vapour are shown in
Figure 3, and Table 3 summarizes the time-dependent
changes in the excitation and emission spectra of the guest-
free crystals upon exposure to gaseous formic acid, acetic
acid and propionic acid, respectively. At the time intervals,
a small portion of the exposed crystals was withdrawn and
checked by 1H NMR spectroscopy, which demonstrated
that carboxylic acid molecules are gradually included into
the crystals, and clathrate formation by solid–gas contact is
suggested. The maximum fluorescence was obtained when
the host-to-guest stoichiometric ratio attained was near 1:2
in every case. The initial excitation band at 378 nm of the
guest-free crystals was shifted to 447 nm for 1·formic acid
(1:2), 442 nm for 1·acetic acid (1:2) and 443 nm for
1·propionic acid (1:2). The corresponding fluorescence
spectra showed an increase in the fluorescence intensity
with a redshift in the emission maximum from 468 to
474 nm for 1·formic acid (1:2), 473 nm for 1·acetic acid
(1:2), and 511 nm for 1·propionic acid (1:2). The maximum
fluorescence intensities of the guest-inclusion crystals after
exposure to gaseous formic acid, acetic acid and propionic
acid were about 5-, 33- and 50-fold stronger than the fluo-
rescence intensity of the starting guest-free crystals, respec-
tively. Moreover, continuous exposure of the crystals re-
sulted in further inclusion of carboxylic acid molecules until
the host-to-guest stoichiometric ratio was 1:3, and the fluo-
rescence intensity decreased slightly in every case. It took
about 150 min for 1·formic acid (1:3), 900 min for 1·acetic
acid (1:3), and 2400 min for 1·propionic acid (1:3). These
results suggest that increasing the molecular size of the en-
clathrated carboxylic acid tends to induce a larger fluores-
cence enhancement, but it also slows down the formation
of the clathrate.
Figure 2. Calculated electron density changes accompanying the
first electronic excitation of 1 and 1H+. The black and white lobes
signify decreases and increase in electron density accompanying the
electronic transition. Their areas indicate the magnitude of the elec-
tron density change.
More interestingly, when the above-mentioned powders
of 1·acetic acid (1:3) and 1·propionic acid (1:3) were ex-
posed to propionic acid and acetic acid vapour, respectively,
exchange of the guest molecules was observed. Guest mole-
cules of acetic acid and propionic acid were gradually re-
placed by propionic acid and acetic acid, respectively. In
the above case, the former guest molecules were completely
removed when the ratio of the host to the latter guest
reached 1:2 after about 10–20 min. Continuous exposure of
the crystals resulted in further inclusion of carboxylic acid
molecules until the host to the latter guest stoichiometric
ratio became 1:3. The guest exchange of the inclusion crys-
Table 2. Calculated absorption spectra for 1 and 1H+.
Species
µ [D][a]
Absorption (calcd.)
λmax [nm]
CI component[c] ∆µ [D][d]
f[b]
1
4.55
2.12
371
0.67
HOMOǞLUMO
(75%)
HOMOǞLUMO
(90%)
2.66
8.43
1H+
446
0.70
[a] Values of the dipole moment in the ground state. [b] Oscillator
strength. [c] The transition is shown by an arrow from one orbital
to another, followed by its percentage CI (configuration interac-
tion) component. [d] The values of the difference in the dipole mo-
ment between the excited and ground states.
Eur. J. Org. Chem. 2009, 5979–5990
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Y. Ooyama, K. Uwada, H. Kumaoka, K. Yoshida
FULL PAPER
tals was accompanied by colour and fluorescent intensity
changes. When the powders of 1·acetic acid (1:3) were ex-
posed to propionic acid vapour, a redshift in the excitation
and emission maxima and a fluorescence enhancement were
induced upon exchange of acetic acid with propionic acid
(Figure 4a). In contrast, when the powders of 1·propionic
acid (1:3) were exposed to acetic acid, a blueshift in the
excitation and emission maxima and a slight change in fluo-
rescence intensity were induced upon exchange of propionic
acid with acetic acid (Figure 4b). Table 4 summarizes the
time-dependent changes in the excitation and emission
spectra upon exchange of acetic acid with propionic acid
and upon exchange of propionic acid with acetic acid,
respectively. It is worth noting that the excitation and fluo-
rescence wavelength maxima of 1·acetic acid (1:2) and
1·propionic acid (1:2), which were prepared by exchange of
guest molecules, are slightly different from those of 1·acetic
acid (1:2) and 1·propionic acid (1:2) prepared by guest-free
crystals/carboxylic acid vapour contact (Table 3). These re-
sults indicate that the photophysical spectral changes upon
solid–gas contact reflect strictly the changes in molecular
packing induced by the enclathrated guest molecules in the
crystals, and there are slight differences in the degree of
changes in molecular packing between the guest-free crys-
tals/carboxylic acid vapour contact and the exchange of
guest molecules.
Figure 3. Time-dependent spectral changes of the guest-free crys-
tals of 1 upon exposure to acetic acid vapour at 30 °C; (a) the
excitation and (b) emission spectra were recorded at their corre-
sponding emission and excitation maxima.
Table 3. Time-dependent changes in the excitation and emission
wavelength, fluorescence intensity and host-to-guest ratios of the
guest-free crystals of 1 upon contact with gaseous carboxylic acids
at 30 °C.
Guest
acid
Time
[min]
Excitation
λex [nm] λem [nm]
Emission
Ratio[b]
host/guest
RFI[a]
Formic
acid
0
378
445
446
447
448
468
473
474
474
474
1.00
4.61
5.05
5.14
4.42
1:0
10
20
50
150
1:0.6
1:1.2
1:2.2
1:2.9
Acetic
acid
0
50
378
436
440
442
442
442
442
468
468
471
474
473
468
464
1.00
17.9
26.9
32.8
33.4
31.1
28.1
1:0
1:0.5
1:1.1
1:1.9
1:2.3
1:2.8
1:3.0
100
200
300
500
900
Propionic
acid
0
378
441
443
443
443
443
468
511
511
511
513
513
1.00
35.2
43.4
50.8
50.4
49.6
1:0
120
300
420
1200
2400
1:0.3
1:1.1
1:2.0
1:2.9
1:3.0
[a] Relative fluorescence intensity was determined by considering
Figure 4. Time-dependent changes in the fluorescence intensity
upon guest exchange by gas–solid contact at 30 °C; (a) 1·acetic acid
(1:3) exposed to propionic vapour and (b) 1·propionic acid (1:3)
exposed to acetic acid vapour.
the fluorescence intensity of the guest-free crystal of 1 as 1.0.
1
[b] Determined by integration of the signals in the H NMR spec-
tra.
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Eur. J. Org. Chem. 2009, 5979–5990

Fluorescence Enhancement of Phenanthro[9,10-d]imidazole Hosts
Table 4. Time-dependent changes in the excitation and emission zation of 1 from carboxylic acid solutions are summarized
wavelength, fluorescence intensity and host-to-guest ratios upon
in Table 5. Interestingly, for formic acid, acetic acid, propi-
guest exchange by solid–gas contact at 30 °C; 1·acetic acid (1:3)
onic acid, 2,2-dimethyl-n-butyric acid and 3,3-dimethyl-n-
was exposed to propionic vapour and 1·propionic acid (1:3) was
butyric acid, the host/guest ratio varies depending on the
exposed to acetic acid vapour.
carboxylic acid solutions used as recrystallization solvents.
Guest
exchange
Time
[h]
Excitation
λex [nm]
Emission
λem [nm]
Ratio[b]
host/acetic acid/
propionic acid
When a mixture of carboxylic acid and acetonitrile was
used, the host/guest ratio became 1:2. In contrast, when
only carboxylic acid was used, the host/guest ratio became
1:3, except for formic acid. The solution jelled when only
formic acid was used. In comparison to the guest-free crys-
tals, the colour of the carboxylic acid inclusion crystals var-
ied from colourless to yellow and a dramatic fluorescence
enhancement was observed.
In order to investigate the effect of clathrate formation
on the solid-state photophysical properties, the fluorescence
excitation and emission spectra of the guest-free and the
carboxylic acid inclusion crystals were measured (Figure 5).
The solid-state photophysical changes are greatly depend-
ent on the identity of the enclathrated carboxylic acids. In
comparison to the guest-free crystal, the excitation and
emission maxima of the guest-inclusion crystals exhibited a
redshift and the fluorescence intensity was enhanced to
various degrees depending on the identity of the enclath-
rated carboxylic acid molecules. The excitation and fluores-
cence spectroscopic data of the guest-free and carboxylic
acid inclusion crystals are summarized in Table 6. The 428-
nm band observed in the excitation spectrum of the guest-
free crystals shifted to around 430–451 nm in those of the
carboxylic acid inclusion crystals. The guest-free host crys-
tal exhibits relatively weak fluorescence with emission maxi-
mum at 468 nm, whereas the carboxylic acid inclusion crys-
RFI[a]
Acetic
acid
Ǟ
Propionic
acid
0
0.5
1
1.5
6
9
20
44
442
438
438
438
437
437
437
438
463
489
491
491
492
492
493
507
30.3
37.4
39.9
41.4
44.6
43.0
41.0
40.7
1:3:0
1:1.71:0.89
1:0.75:1.30
1:0.48:1.55
1:0.06:1.94
1:0.05:1.95
1:0.03:2.00
1:0:2.94
Propionic
acid
Ǟ
Acetic
acid
0
0.5
1
3
4
6
11
20
443
443
442
438
436
436
436
438
513
513
508
483
478
477
473
462
50.6
50.6
41.0
51.0
51.3
51.3
48.5
30.6
1:0:3
1:0.56:2.17
1:0.86:1.78
1:1.36:0.63
1:1.59:0.35
1:1.77:0.21
1:1.89:0.11
1:3:0
[a] Relative fluorescence intensity was determined by considering
the fluorescence intensity of the guest-free crystals of 1 as 1.0.
[b] Determined by integration of the signals in the H NMR spec-
1
tra.
Solid-State Fluorescence Properties of the Clathrate
Compounds Prepared by Cocrystallization of 1 from
Carboxylic Acid Solutions
In order to investigate the carboxylic acid inclusion abil- tals exhibit much stronger fluorescence intensity with an
ity of 1, we recrystallized fluorophore 1 from various car- emission maximum redshifted to around 499–512 nm. In
boxylic acid solutions. We found that fluorophore 1 yields comparison with the guest-free crystal, the fluorescence in-
various host–guest inclusion compounds in stoichiometric tensities of the guest-inclusion crystals were ca. 26-fold in
ratios with carboxylic acids such as formic acid, acetic acid, 1·(CH3COOH)2, ca. 40-fold in 1·(CH3CH2COOH)2, ca. 56-
propionic acid, benzoic acid and sebacic acid in the crystal- fold in 1·[CH3CH2C(CH2)2COOH]2 and ca. 51-fold in
line state. The characteristics of the guest-free and various 1·[HOOC(CH2)8COOH]2. It is noteworthy that the exci-
carboxylic acid inclusion crystals obtained by cocrystalli- tation and fluorescence wavelength maxima and solid-state
Table 5. Host–guest molar ratio, crystal form and crystal colour of the guest-free and the guest-inclusion crystals of 1.
Clathrate compound
Host/ Guest
guest
Crystal
colour
Crystal Recrystallization solvent
form
Guest-free
1·(HCOOH)2
1·(CH3COOH)2
1·(CH3COOH)3
1·(CH3CH2COOH)2
1·(CH3CH2COOH)3
1·[CH3(CH2)4COOH]2
1·[CH3(CH2)4COOH]3
1·[(CH3)3CCH2COOH]2
1·[(CH3)3CCH2COOH]3
1·[CH3CH2C(CH2)2COOH]2
1·[CH3CH2C(CH2)2COOH]3
1·benzoic acid
1:0
1:2
1:2
1:3
1:2
1:3
1:2
1:3
1:2
1:3
1:2
1:3
1:1
1:1
none
formic acid
acetic acid
acetic acid
propionic acid
propionic acid
hexanoic acid
hexanoic acid
2,2-dimethy-n-butyric acid
2,2-dimethy-n-butyric acid
3,3-dimethy-n-butyric acid
3,3-dimethy-n-butyric acid
benzoic acid
milk-white
yellow
yellow
yellow
yellow
yellow
yellow
yellow
yellow
yellow
yellow
yellow
white
needle
needle
prism
prism
prism
prism
prism
prism
needle
needle
prism
prism
needle
needle
acetonitrile
formic acid/acetonitrile (1:6)
acetic acid/acetonitrile (1:3)
acetic acid
propionic acid/acetonitrile (1:1)
propionic acid
hexanoic acid/acetonitrile (1:3)
hexanoic acid
2,2-dimethy-n-butyric acid/acetonitrile (1:1)
2,2-dimethy-n-butyric acid
3,3-dimethy-n-butyric acid/acetonitrile (1:3)
3,3-dimethy-n-butyric acid
acetonitrile solution containing benzoic acid
acetonitrile solution containing m-dimethylamino-
benzoic acid
1·m-dimethylaminobenzoic acid
m-dimethylaminobenzoic acid
yellow
1·[HOOC(CH2)2COOH]2
1·[HOOC(CH2)8COOH]2
2:1
2:1
succinic acid
sebacic acid
yellow
yellow
prism
prism
acetonitrile solution containing succinic acid
acetonitrile solution containing sebacic acid
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fluorescence intensity of the carboxylic acid inclusion crys- These results indicate that there are some differences in the
tals prepared by recrystallization are slightly different from molecular packings between the clathrate compounds pre-
those of carboxylic acid inclusion solids prepared by guest- pared by cocrystallization and by solid–gas contact.
free crystals/ carboxylic acid vapour contact (Table 3).
Relation between Solid-State Fluorescence Properties and
X-ray Crystal Structures of Carboxylic Acid Inclusion
Compounds
To investigate the relationship between the observed so-
lid-state fluorescence properties and the changes in molecu-
lar packing structure upon inclusion of the guest molecules,
the crystal structures of the guest-free crystal of 1 and the
carboxylic acid inclusion compounds 1·(CH3COOH)2,
1·(CH3CH2COOH)2, 1·[CH3CH2C(CH2)2COOH]2 and
1·[HOOC(CH2)8COOH]2 were determined by X-ray dif-
fraction analysis.
In the guest-free crystal, host molecules are linked by
intermolecular NH···N hydrogen bonds between the imid-
azole nitrogen atoms to form a linear molecular chain along
the c axis (Figure 6a). The host molecules along the chain
are related by 41 (fourfold screw axis) symmetry, as shown
in Figure 6b. The bond lengths between the N-1 and N-2
nitrogen atoms and C-2 in the imidazole ring are 1.39 and
1.30 Å, respectively. The differences between the two
lengths for 1 are attributed to the C–N and C=N bonds of
the imidazole ring. Between the adjacent chains there are
two short molecular contacts between host molecules; one
results from the overlapping of the p-(diethyamino)phenyl
parts (Figure 6c: stacking I) and the other is the π–π over-
lapping of the phenanthrene parts, where there are 11 short
interatomic π–π contacts of less than 3.6 Å (Figure 6d:
stacking II). These long-range host–host interactions based
on continuous intermolecular hydrogen bonds and π–π in-
teractions would cause a strong fluorescence quenching in
Figure 5. (a) Solid-state excitation and (b) fluorescence spectra of
the guest-free and the carboxylic acid inclusion crystals of 1; (A) 1
(guest-free), (B) 1·[HOOC(CH2)2COOH]2, (C) 1·(CH3COOH)2,
(D) 1·(CH3CH2COOH)2, (E) 1·[HOOC(CH2)8COOH]2, (F)
1·[CH3CH2C(CH2)2COOH]2 and (G) 1·(HCOOH)2.
the guest-free crystal.[5–8,11]
In contrast, in the case of the carboxylic acid inclusion
crystals, such direct intermolecular hydrogen bonding be-
tween host molecules is not found. Instead, the carboxylic
acid inclusion crystals, except those of 1·[HOOC(CH2)8-
COOH]2, are made up of a hydrogen-bonded cluster unit
composed of two imidazole hosts and four carboxylic acid
guests (Figures 7, 8 and 9). The two hosts and two of the
four guests are alternately linked by four NH···O hydrogen
bonds to form a centrosymmetric ring, in which the imid-
azole ring is protonated by the carboxylic acid proton (Fig-
ures 7b, 8b and 9b). Actually, the two bond lengths between
the N(1) and N(2) nitrogen atoms and C(15) in the imid-
azole ring are ca. 1.34 and ca. 1.35 Å, respectively, which
are nearly the same. The other two guests are linked by a
hydrogen bond to each of the two ring-forming guests to
form a hexamer cluster unit. These results suggest that the
imidazole ring effectively fixes carboxylic acid molecules in
the crystalline state. There are two styles of π–π stacking
between host molecules. One is the π–π overlapping be-
tween the phenanthroimidazole and p-(diethylamino)phenyl
parts in the cluster unit (Figures 7c, 8c and 9c: stacking I).
For 1·(CH3COOH)2, 1·(CH3CH2COOH)2 and 1·[CH3-
Table 6. Excitation and fluorescence spectroscopic data of the guest
free and various carboxylic acid inclusion crystals of 1.
Clathrate compound
Excitation
Emission
RFI[a,b]
λex [nm]
λem [nm]
Guest-free
1·(HCOOH)2
1·(CH3COOH)2
378
446
435
439
446
437
447
451
420
447
444
468
519
484
502
506
499
510
497
500
513
508
1.0
56.3
26.0
40.4
44.5
56.2
46.4
4.5
28.0
13.4
51.2
1·(CH3CH2COOH)2
1·[CH3(CH2)4COOH]2
1·[CH3CH2C(CH2)2COOH]2
1·[(CH3)3CCH2COOH]2
1·benzoic acid
1·m-dimethylaminobenzoic acid
1·[HOOC(CH2)2COOH]2
1·[HOOC(CH2)8COOH]2
[a] Relative fluorescence intensity was determined by considering
the fluorescence intensity of the guest-free crystal of 1 as 1.0. [b] As
the standard sample, the RFI of anthracene crystal (λex = 370 nm,
λem = 445 nm) is 42.[9]
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Fluorescence Enhancement of Phenanthro[9,10-d]imidazole Hosts
are alternately linked by four NH···O hydrogen bonds to
form a centrosymmetric ring, in which the imidazole ring is
protonated by the sebacic acid proton [N(1)–C(15) ca.
1.34 Å and N(2)–C(15) ca. 1.35 Å], interestingly the neigh-
bouring cluster units are connected through two carboxyl
Figure 6. Crystal structure of 1: (a) a stereoview of the molecular
packing structure, (b) intermolecular hydrogen bonds between the
fluorophores, (c) top view of a cluster unit (stacking I) and (d) a
top view between two cluster units (stacking II). The dotted lines
in (b) indicate hydrogen bonds. The circles represented in (d) show
short interatomic π–π contacts of less than 3.6 Å.
CH2C(CH2)2COOH]2, there are 9, 23 and 23 short in-
teratomic π–π contacts of less than 3.6 Å, and the average
distances of the interatomic π–π contacts are ca. 3.39, ca.
3.49 and ca. 3.47 Å, respectively. The other is observed be-
tween the neighbouring cluster units, where the phenan-
threne ring parts are overlapping (Figures 7d, 8d and 9d:
stacking II). There are 32, 22 and 7 short interatomic π–π
contacts of less than 3.6 Å, and the average distances of the
interatomic π–π contacts are ca. 3.50, ca. 3.51 and ca.
3.53 Å, respectively. In contrast, the crystal of
1·[HOOC(CH2)8COOH]2 is made up of a hydrogen-bonded
cluster unit composed of two hosts and two sebacic acid
Figure 7. Crystal structure of 1·(CH3COOH)2: (a) a stereoview of
the molecular packing structure, (b) intermolecular hydrogen
bonds, (c) top view of a cluster unit (stacking I) and (d) a top
view between two cluster units (stacking II). The dotted lines in (b)
indicate hydrogen bonds. The circles represented in (c) and (d) show
guests (Figure 10). Although the two hosts and two guests short interatomic π–π contacts of less than 3.6 Å.
Eur. J. Org. Chem. 2009, 5979–5990
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5985

Y. Ooyama, K. Uwada, H. Kumaoka, K. Yoshida
FULL PAPER
groups on both sides of the sebacic acid. Therefore, a con- throimidazole and p-(diethylamino)phenyl parts in the clus-
tinuous intermolecular hydrogen-bonding chain of ter unit was observed, and there are 23 short interatomic
(···H···G···H···) is formed through the above intermolecular π–π contacts of less than 3.6 Å, and the average distance of
hydrogen bonding (NH···O) between hosts and sebacic acid
molecules. The π–π overlapping between the phenan-
Figure 8. Crystal structure of 1·(CH3CH2COOH)2: (a) a stereoview
of the molecular packing structure, (b) intermolecular hydrogen
bonds, (c) top view of a cluster unit (stacking I) and (d) a top
view between two cluster units (stacking II). The dotted lines in (b)
indicate hydrogen bonds. The circles represented in (c) and (d) show
short interatomic π–π contacts of less than 3.6 Å.
Figure 9. Crystal structure of 1·[CH3CH2C(CH2)2COOH]2: (a) a
stereoview of the molecular packing structure, (b) intermolecular
hydrogen bonds, (c) top view of a cluster unit (stacking I) and (d)
a top view between two cluster units (stacking II). The dotted lines
in (b) indicate hydrogen bonds. The circles represented in (c) and
(d) show short interatomic π–π contacts of less than 3.6 Å.
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Fluorescence Enhancement of Phenanthro[9,10-d]imidazole Hosts
the interatomic π–π contacts is ca. 3.45 Å. However, the π– four carboxylic acid inclusion compounds clarified that the
π overlapping of two hosts between the neighbouring clus- π–π interactions between the host molecules decreased with
ter units was not observed.
an increase in the size of the enclathrated guest molecules:
1·(CH3COOH)2 Ͼ 1·(CH3CH2COOH)2 Ͼ 1·[CH3CH2C-
(CH2)2COOH]2 Ͼ 1·[HOOC(CH2)8COOH]2. These results
confirm that the differences in the destruction of the host–
host π–π interactions by enclathration of the carboxylic
acid molecules are reflected on the guest size-dependent
fluorescence enhancement. However, the solid-state fluores-
cence intensity of 1·[HOOC(CH2)8COOH]2 is weaker than
that of 1·[CH3CH2C(CH2)2COOH]2. It is considered that
the continuous intermolecular hydrogen bond ranging alter-
nately host and guest (···H···G···H···) was observed only in
1·[HOOC(CH2)8COOH]2, which is a principal factor lead-
ing to fluorescence quenching in the solid state.[8] In con-
trast, on the basis of the absorption and fluorescence spec-
tra of 1 in acetic acid, the semiempirical MO calculations
and the X-ray crystal structure of the carboxylic acid in-
clusion compounds, the protonation of the imidazole ring
of host 1 in the carboxylic acid inclusion crystals would
contribute to a redshift in the absorption and emission
spectra of the crystals because of an increase in intramolec-
ular charge-transfer character of the host fluorophore by
strong resonance interaction of the diethylamino group
with the protonated imidazole ring.
Conclusions
We have designed and synthesized a phenanthro[9,10-d]-
imidazole-type fluorescent clathrate host, 2-[4-(dieth-
ylamino)phenyl]-1H-phenanthro[9,10-d]imidazole (1), con-
taining an imidazole ring and a diethylamino group. The
crystals of 1 exhibit a drastic fluorescence enhancement
with a redshift in the emission maximum upon enclathra-
tion of various carboxylic acids. The X-ray crystal struc-
tures of the guest-free and carboxylic acid inclusion com-
pounds demonstrated that the differences in the destruction
of the host–host π–π interactions by enclathration of the
carboxylic acid molecules are reflected in the guest size-de-
pendent fluorescence enhancement. Furthermore, on the
basis of the absorption and fluorescence spectra of 1 in ace-
tic acid, semiempirical MO calculations and X-ray crystal
structure of the carboxylic acid inclusion compounds, the
protonation of the imidazole ring of host 1 in the carboxylic
acid inclusion crystals would contribute to a redshift in the
absorption and emission spectra of the crystals because of
an increase in the intramolecular charge-transfer character
of the host fluorophore by strong resonance interaction of
the diethylamino group with the protonated imidazole ring.
Our results indicate that fluorescent clathrand 1 can be uti-
lized as a chemical solid sensor for recognition of gaseous
carboxylic acids.
Figure 10. Crystal structure of 1·[HOOC(CH2)8COOH]2: (a) a
stereoview of the molecular packing structure, (b) intermolecular
hydrogen bonds and (c) top view of a cluster unit (stacking I) The
dotted lines in (b) indicate hydrogen bonds. The circles represented
in (c) show short interatomic π–π contacts of less than 3.6 Å.
On the basis of the solid-state photophysical data and
the crystal structures of the guest-free and carboxylic acid
inclusion compounds, we discuss the effects of the enclath-
rated carboxylic acid on the solid-state photophysical prop-
erties of the clathrate compounds. The styles of π–π stack-
ing between host molecules of the four carboxylic acid in-
clusion compounds were quite similar to each other; how-
Experimental Section
General: Elemental analyses were measured with a Perkin–Elmer
ever, a detailed comparison of the crystal structures of the 2400 II CHN analyzer. IR spectra were recorded with a JASCO
Eur. J. Org. Chem. 2009, 5979–5990
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5987

Y. Ooyama, K. Uwada, H. Kumaoka, K. Yoshida
FULL PAPER
FT/IR-5300 spectrophotometer for samples in KBr pellet form.
Single-crystal X-ray diffraction was performed with a Rigaku
AFC7S diffractometer. Absorption spectra were observed with a
JASCO U-best30 spectrophotometer, and fluorescence spectra were
measured with a JASCO FP-777 spectrophotometer. The fluores-
cence quantum yields (Φ) were determined with a Hamamatsu
C9920-01 equipped with CCD by using a calibrated integrating
sphere system (λex = 363 nm). For measurement of the solid-state
fluorescence excitation and emission spectra of the crystals, a Jasco
FP-1060 attachment was used. 1H NMR spectra were recorded
with a JNM-LA-400 (400 MHz) FTNMR spectrometer with tet-
ramethylsilane (TMS) as an internal standard.
1·2,2-Dimethyl-n-Butyric Acid (H/G, 1:2): Host 1 (300 mg) was dis-
solved by warming in a mixture of 2,2-dimethyl-n-butyric acid and
acetonitrile (volume ratio of 1:1, 30 mL), and the resulting solution
was allowed to stand at room temperature. The crystals (yellow,
prism, 411 mg) were collected and dried on the filter paper.
1·3,3-Dimethyl-n-butyric Acid (H/G, 1:2): Host 1 (300 mg) was dis-
solved by warming in a mixture of 2,2-dimethyl-n-butyric acid and
acetonitrile (volume ratio of 1:3, 30 mL), and the resulting solution
was allowed to stand at room temperature. The crystals (yellow,
prism, 477 mg) were collected and dried on the filter paper.
1·Benzoic Acid (H/G, 1:1): Host 1 (500 mg) and benzoic acid (1 g)
were dissolved by warming in acetonitrile (600 mL), and the re-
sulting solution was allowed to stand at room temperature. The
crystals (white, needle, 608 mg) were collected and dried on the
filter paper. C32H29N3O2 (487.59): calcd. C 78.82, H 5.99, N 8.62;
found C 79.01, H 6.06, N 8.75.
2-[4-(Diethylamino)phenyl]-1H-phenanthro[9,10-d]imidazole (1): A
solution of 9,10-phenanthrenequinone (5.00 g, 24.0 mmol), p-dieth-
ylaminobenzaldehyde (4.26 g, 24.0 mmol) and ammonium acetate
(29.6 g, 0.38 mol) in acetic acid (90 mL) was stirred at 110 °C for
3 h. The reaction mixture was poured into ice-cold water. The re-
sulting precipitate was filtered, washed with water and dried. The
residue was purified by chromatography on silica gel (CH2Cl2/ethyl
acetate, 3:1) to give 1 (7.09 g, 81%) as a milk-white powder. M.p.
1·m-Dimethylaminobenzoic Acid (H/G, 1:1): Host 1 (460 mg) and
m-dimethylaminobenzoic acid (1.25 g) were dissolved by warming
in acetonitrile (600 mL), and the resulting solution was allowed to
stand at room temperature. The crystals (yellow, needle, 594 mg)
were collected and dried on the filter paper. C34H34N4O2 (530.66):
calcd. C 76.95, H 6.46, N 10.56; found C 76.83, H 6.57, N 10.56.
1
319–320 °C. H NMR (400 MHz, [D6]DMSO, TMS): δ = 1.14 (t,
6 H), 3.45 (m, 4 H), 6.83 (d, J = 7.81 Hz, 2 H), 7.57–7.72 (m, 4
H), 8.09 (d, J = 7.56 Hz, 2 H), 8.53 (d, J = 7.56 Hz, 2 H), 8.82 (d,
J = 7.81 Hz, 2 H) ppm. IR (KBr): ν = 3069, 1611 cm–1. C H N
˜
25 23
3
1·Succinic Acid (H/G, 2:1): Host 1 (400 mg) and succinic acid
(188 mg) were dissolved by warming in acetonitrile (400 mL), and
the resulting solution was allowed to stand at room temperature.
The crystals (yellow, prism, 295 mg) were collected and dried on
the filter paper.
(365.47): calcd. C 82.16, H 6.34, N 11.50; found C 82.45, H 6.09,
N 11.62.
Preparation of Guest-Inclusion Crystals for Measurement of Solid-
State Fluorescence Excitation and Emission Spectra: Host com-
pound 1 was dissolved with heating in the respective guest solvent.
The solution was filtered and kept for a few days at room tempera-
ture. The crystals that formed were collected by filtration. The host
(H)/guest (G) stoichiometric ratio of the inclusion compounds was
1·Sebacic Acid (H/G, 2:1): Host 1 (400 mg) and sebacic acid
(221 mg) were dissolved by warming in acetonitrile (300 mL), and
the resulting solution was allowed to stand at room temperature.
The crystals (yellow, prism, 346 mg) were collected and dried on
the filter paper. C60H64N6O4 (933.19): calcd. C 77.22, H 6.91, N
9.01; found C 76.92, H 6.97, N 8.82.
1
determined by integration of the signals in the H NMR spectrum
and CHN analysis.
1 (Guest-free): Host 1 (500 mg) was dissolved by warming in aceto-
nitrile (600 mL), and the resulting solution was allowed to stand at
room temperature. The crystals (milk-white, needle, 420 mg) were
collected and dried on the filter paper.
X-ray Crystallographic Studies: The reflection data were collected
at 23 Ϯ1 °C with a Rigaku AFC7S four-circle diffractometer by
2θ–ω scan technique and by using graphite-monochromated Mo-
Kα (λ = 0.71069 Å) radiation at 50 kV and 30 mA. In all case, the
data were corrected for Lorentz and polarization effects. A correc-
tion for secondary extinction was applied. The reflection intensities
were monitored by three standard reflections for every 150 reflec-
tions. An empirical absorption correction based on azimuthal scans
of several reflections was applied. All calculations were performed
by using the teXsan[16] crystallographic software package of Molec-
ular Structure Corporation. CCDC-740518 (for 1), -740519 [for
1·(CH3COOH)2], -740520 [for 1·(CH3CH2COOH)2], -740521 {for
1·[CH3CH2C(CH2)2COOH]2} and -740522 {for 1·[HOOC(CH2)8-
COOH]2} contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.au.uk/
data_request/cif.
1·Formic Acid (H/G, 1:2): Host 1 (300 mg) was dissolved by warm-
ing in a mixture of formic acid and acetonitrile (volume ratio of
1:6, 21 mL), and the resulting solution was allowed to stand at
room temperature. The crystals (yellow, needle, 382 mg) were col-
lected and dried on the filter paper.
1·Acetic Acid (H/G, 1:2): Host 1 (500 mg) was dissolved by warm-
ing in a mixture of acetic acid and acetonitrile (volume ratio of 1:3,
10 mL), and the resulting solution was allowed to stand at room
temperature. The crystals (yellow, prism, 297 mg) were collected
and dried on the filter paper. C29H31N3O4 (485.57): calcd. C 71.73,
H 6.43, N 8.65; found C 71.94, H 6.57, N 8.76.
1·Propionic Acid (H/G, 1:2): Host 1 (565 mg) was dissolved by
warming in a mixture of propionic acid and acetonitrile (volume
ratio of 1:1, 20 mL), and the resulting solution was allowed to
stand at room temperature. The crystals (yellow, prism, 661 mg)
were collected and dried on the filter paper. C31H35N3O4 (513.63):
calcd. C 72.49, H 6.87, N 8.18; found C 72.69, H 7.03, N 8.28.
Crystal of 1: The transmission factors ranged from 0.96 to 1.00.
The crystal structure was solved by direct methods by using
SIR92.[17] The structures were expanded by using Fourier tech-
niques.[18] The non-hydrogen atoms were refined anisotropically.
Some hydrogen atoms were refined isotropically; the rest were fixed
geometrically and not refined. Crystallographic data: C25H23N3, M
= 365.48, tetragonal, a = 20.675(2) Å, b = 20.675(2) Å, c =
19.151(6) Å, U = 8186(2) Å3, ρcalcd. = 1.186 gcm–3, T = 296.2 K,
space group I41/a (no.88), Z = 16, µ(Mo-Kα) = 0.70 cm–1, 3834
reflections measured, 3616 unique (Rint = 0.073), which were used
1·Hexanoic Acid (H/G, 1:2): Host 1 (300 mg) was dissolved by
warming in a mixture of hexanoic acid and acetonitrile (volume
ratio of 1:3, 14 mL), and the resulting solution was allowed to
stand at room temperature. The crystals (yellow, prism, 412 mg)
were collected and dried on the filter paper.
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Eur. J. Org. Chem. 2009, 5979–5990

Fluorescence Enhancement of Phenanthro[9,10-d]imidazole Hosts
in all calculations. The final R indices [IϾ2σ(I)], R1 = 0.121,
wR(F2) = 0.273.
tion interaction), which includes the configuration with one elec-
tron excited from any occupied orbital to any unoccupied orbital,
where 225 configurations were considered [keyword CI (15 15)].
Crystal of 1·(CH3COOH)2: The transmission factors ranged from
0.97 to 1.00. The crystal structure was solved by direct methods by
using SAPI91.[19] The structures were expanded by using Fourier
techniques.[18] The non-hydrogen atoms were refined anisotropi-
cally. Some hydrogen atoms were refined isotropically; the rest were
fixed geometrically and not refined. Crystallographic data:
Acknowledgments
This work was partially supported by a Grant-in-Aid for Science
and Research from the Ministry of Education, Science, Sport and
Culture of Japan (Grant 21550181) and by a Special Research
Grant for Green Science from Kochi University.
C29H31N3O4,
M = 485.58, triclinic, a = 11.423(1) Å, b =
14.126(2) Å, c = 8.770(2) Å, α = 105.92(1)°, β = 105.44(1)°, γ =
83.68(1)°, U = 1310.7(4) Å3, ρcalcd. = 1.230 gcm–3, T = 296.2 K,
–1
¯
space group P1 (no.2), Z = 2, µ(Mo-Kα) = 0.83 cm , 4880 reflec-
tions measured, 4627 unique (Rint = 0.030), which were used in all
calculations. The final R indices [IϾ2σ(I)], R1 = 0.085, wR(F2) =
0.207.
[1] a) E. Weber, M. Czugler in Topics in Current Chemistry Vol.
149: Molecular and Molecular Recognition – Clathrates II (Ed.:
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1991, 1707–1713; b) I. Csöregh, E. Weber, L. R. Nassimbeni,
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Crystal of 1·(CH3CH2COOH)2: The transmission factors ranged
from 0.99 to 1.00. The crystal structure was solved by direct meth-
ods by using SAPI91.[19] The structures were expanded by using
Fourier techniques.[18] The non-hydrogen atoms were refined aniso-
tropically. Some hydrogen atoms were refined isotropically; the rest
were fixed geometrically and not refined. Crystallographic data:
C31H35N3O4,
M = 513.64, triclinic, a = 11.712(2) Å, b =
13.955(2) Å, c = 9.611(2) Å, α = 107.03(1)°, β = 108.59(2)°, γ =
84.76(1)°, U = 1423.6(5) Å3, ρcalcd. = 1.198 gcm–3, T = 296.2 K,
–1
¯
space group P1 (no.2), Z = 2, µ(Mo-Kα) = 0.80 cm , 5272 reflec-
tions measured, 5007 unique (Rint = 0.012), which were used in all
calculations. The final R indices [IϾ2σ(I)], R1 = 0.073, wR(F2) =
0.193.
Crystal of 1·[CH3CH2C(CH2)2COOH]2: The transmission factors
ranged from 0.97 to 1.00. The crystal structure was solved by direct
methods by using SIR92.[17] The structures were expanded by using
Fourier techniques.[18] The non-hydrogen atoms were refined aniso-
tropically. Some hydrogen atoms were refined isotropically; the rest
were fixed geometrically and not refined. Crystallographic data:
C37H47N3O4, M = 597.80, orthorhombic, a = 22.296(5) Å, b =
27.49(1) Å, c = 11.342(4) Å, U = 6951(2) Å3, ρcalcd. = 1.141 gcm–3,
T = 296.2 K, space group Pbca (no.61), Z = 8, µ(Mo-Kα) =
0.74 cm–1, 6125 reflections measured, 6123 unique (Rint = 0.083),
which were used in all calculations. The final R indices [IϾ2σ(I)],
R1 = 0.097, wR(F2) = 0.252.
Crystal of 1·[HOOC(CH2)8COOH]2: The transmission factors
ranged from 0.96 to 1.00. The crystal structure was solved by direct
methods by using SIR92.[17] The structures were expanded by using
Fourier techniques.[18] The non-hydrogen atoms were refined aniso-
tropically. Some hydrogen atoms were refined isotropically; the rest
were fixed geometrically and not refined. Crystallographic data:
C30H32N3O2, M = 466.6, monoclinic, a = 10.819(4) Å, b =
18.650(3) Å, c = 13.063(3) Å, β = 102.89(2)°, U = 2569(1) Å3, ρcalcd.
= 1.206 gcm–3, T = 296.2 K, space group P21/c (no.14), Z = 4,
µ(Mo-Kα) = 0.76 cm–1, 4765 reflections measured, 4514 unique
(Rint = 0.049), which were used in all calculations. The final R in-
dices [IϾ2σ(I)], R1 = 0.075, wR(F2) = 0.219.
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www.eurjoc.org
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Received: July 23, 2009
Published Online: October 20, 2009
5990
www.eurjoc.org
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 5979–5990