Bright, emission tunable fluorescent dyes based on imidazole and π-expanded imidazole

Article abstract of DOI:10.1039/c2jm33891b

A diverse set of imidazole- and π-expanded imidazole derivatives displaying excited state intramolecular proton transfer (ESIPT) was designed and synthesized. The effect of structural variation on photophysical properties was studied in detail for nine dyes. The relationship between the structure and photophysical properties was thoroughly elucidated also by comparing with analogues with blocked ESIPT functionality. All but one of the obtained compounds exhibit ESIPT, as demonstrated by large Stokes shifts (6500-15 600 cm^-1). The type of π-expansion strongly influences the overall optical phenomena: while typical π-expansion preserves ESIPT activity, the direct fusion of imidazole with a naphthalene unit at positions 4 and 5 results in dyes which do not exhibit ESIPT. The compound possessing an acidic NH group as part of an intramolecular hydrogen bond system has a much higher fluorescence quantum yield and Stokes shift than its analogue bearing an OH group. The occurrence of ESIPT for tosylamide analogues is less affected by the hydrogen-bonding ability of the solvents compared to the unprotected amines. Two-photon absorption cross-sections of the selected derivatives are in the range of 5-100 GM.

Full text of DOI:10.1039/c2jm33891b

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PAPER
Bright, emission tunable fluorescent dyes based on imidazole and p-expanded
imidazole†
a
Kamil Skonieczny, Adina I. Ciuciu, Eva M. Nichols,‡ Vincent Hugues, Mireille Blanchard-Desce,
b
a
c
c
*
b
Lucia Flamigni and Daniel T. Gryko
ad
*
*
Received 16th June 2012, Accepted 10th August 2012
DOI: 10.1039/c2jm33891b
A diverse set of imidazole- and p-expanded imidazole derivatives displaying excited state intramolecular
proton transfer(ESIPT) wasdesignedandsynthesized. Theeffect of structural variation onphotophysical
properties was studied in detail for nine dyes. The relationship between the structure and photophysical
properties was thoroughly elucidated also by comparing with analogues with blocked ESIPT
functionality. All but one of the obtained compounds exhibit ESIPT, as demonstrated by large Stokes
shifts (6500–15 600 cmꢀ1). The type of p-expansion strongly influences the overall optical phenomena:
while typical p-expansion preserves ESIPT activity, the direct fusion of imidazole with a naphthalene unit
at positions 4 and 5 results in dyes which do not exhibit ESIPT. The compound possessing an acidic NH
group as part of an intramolecular hydrogen bond system has a much higher fluorescence quantum yield
and Stokes shift than its analogue bearing an OH group. The occurrence of ESIPT for tosylamide
analogues is less affected by the hydrogen-bonding ability of the solvents compared to the unprotected
amines. Two-photon absorption cross-sections of the selected derivatives are in the range of 5–100 GM.
pyridines,12 possess a large Stokes shift and hence have found
applications in laser dyes,13 fluorescence recording,14 ultraviolet
stabilisers,15 probes for solvation dynamics,16 probes for bio-
logical environments17 and recently organic light emitting devi-
ces.6b,18 Excited-state proton transfer is an increasingly important
photophysical process in the design of fluorescent probes.19
Although many functional dyes display ESIPT, only a handful of
them are known to simultaneously exhibit a high fluorescence
quantum yield. Among those studied extensively is a variety of 2-
hydroxyphenylbenzoxazole9 and 2-hydroxyphenylbenzothiazole
derivatives.20 Very few derivatives and/or analogues of 2-
(2-hydroxyphenyl)imidazole have been photophysically studied.
Given the promising optical properties of a handful of imidazole
derivatives synthesized so far, it is desirable to gain an in-depth
knowledge on the structure–property relationship for this group
of compounds. In particular, one can envision that p-expansion
of multisubstituted imidazoles will significantly alter their optical
properties, as is the case for many homoaromatic systems.21
While phenanthro[9,10-d]imidazoles have been previously
described,22a derivatives possessing a 2-hydroxyphenyl group at
position 2 of the imidazole core were studied only recently.22b In
fluorescence imaging it is desirable to use functional dyes which
can be excited using red or NIR light. Given that the majority of
ESIPT-active compounds efficiently absorb UV or violet light
(hence tissue penetration is rather low and applications in
biological imaging are very problematic), we were also interested
in studying the two-photon absorption (2PA) cross-section of the
selected molecules (which in turn allows for excitation in a bio-
logical window) The aim of this study is to explore strategies to
Introduction
Imidazole1 derivatives, pioneered by Debus2 and Radziszewski,3
are continuing to attract attention due to their intriguing photo-
physical properties.4 While triphenylimidazoles played an
important role in the discovery of chemiluminescence,5 tetraphe-
nylimidazoles have recently attracted significant interest. Inge-
nious work by Park and co-workers6 showed that this scaffold,
with the proper choice of substituents, can lead to compounds that
emit white light through a combination of excited-state intra-
molecular proton transfer and restricted energy transfer.
Excited-state intramolecular proton transfer (ESIPT)7 has
emerged in recent years as a very interesting phenomenon which
can be applied to the design of fluorescent sensors.8 Typical
compounds that display ESIPT, such as benzoxazoles,9
flavones,10 10-hydroxybenzo[h]quinoline11 or imidazo[1,2-a]
aInstitute of Organic Chemistry Polish Academy of Sciences, Kasprzaka
44/52 01-224, Warsaw, Poland. E-mail: dtgryko@icho.edu.pl; Fax: +48
22 632 66 81; Tel: +48 22 343 30 63
bIstituto per la Sintesi Organica e Fotoreattivita (ISOF), CNR, Via P.
Gobetti 101, 40129 Bologna, Italy. E-mail: flamigni@isof.cnr.it; Fax:
+39 (0)51 639 98 44; Tel: +39 (0)51 639 98 12
cUniversiteꢀ de Bordeaux, ISM (UMR5255 CNRS), F 33400, Bordeaux,
France. E-mail: m.blanchard-desce@ism.u-bordeaux1.fr; Fax: +33 (0)5
40 00 69 94; Tel: +33 (0)5 40 00 67 32
dWarsaw University of Technology, Noakowskiego 3, 00-664, Warsaw,
Poland. Fax: +48 22 628 27 41; Tel: +48 22 234 58 01
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm33891b
‡ On leave from California Institute of Technology
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alter the basic imidazole-based ESIPT system, to obtain a range
of derivatives and investigate their fundamental optical proper-
ties. This would allow us to address one of the most important
challenges regarding ESIPT systems, i.e. the wide tunability of
chromophore absorption as well as proton transfer emission.
Optimization of fluorescence from the keto tautomer remains a
challenge in molecular design, with success depending on a more
comprehensive understanding of the ESIPT process. The aim of
this work is to correlate various structural features with the
properties associated with ESIPT and identify promising designs.
Herein we present the synthesis and optical properties of a
diverse set of imidazole derivatives and their analogues,
combining for the first time a study of ESIPT and 2PA activity.
moderate yield of the Debus–Radziszewski condensation using
ketone 13 is expected given previous literature data.27 As previ-
ously reported the main product of this reaction is acenaphtho
Results and discussion
Design and synthesis
The occurrence of ESIPT usually leads to a significant decrease
in fluorescent quantum yields (ffl) in comparison to analogous
non-ESIPT systems.23 This is mainly due to the competitive
internal conversion followed by vibrational relaxation. Park and
co-workers showed that tetrasubstituted imidazole derivatives
bearing a 2-hydroxyphenyl substituent at position 2 display
ESIPT and at the same time possess a fairly high fluorescence
quantum yield (0.18).6
1,4,5-Triphenyl-2-(2-hydroxyphenyl)imidazole
and
its
analogues absorb around 320 nm and emit light in the range of
460–570 nm. While designing structural variations of this basic
chromophore, we kept in mind the two principles: (a) p-expansion
of the chromophore should lead to larger 3 and may result in
higher s2; (b) both linear and 2PA properties will depend on the
type of p-expansion, thus suggesting that various types of 1,2-
diketones should be utilized. We also would like to include NH
hydrogen bond donors in addition to commonly used phenolic
OH groups. Keeping in mind the above considerations, a small
library of nine derivatives was designed. We envisioned that all
ESIPT-compounds, as well as analogs with blocked ability to
form hydrogen bonds, could be synthesized via the Debus–Rad-
ziszewski method. Among various possibilities to expand the
imidazole chromophore, conceptually the easiest is to start from
1,10-phenantroquinone (1) (Scheme 1). Condensation of diketone
quinone 1 with aniline (2) or p-toluidine (3) and salicylaldehyde
gave a mixture of p-expanded imidazole 7 and known24 p-
expanded oxazole 8 in moderate yields. The concomitant forma-
tion of oxazole derivatives is in agreement with the well-known
Japp reaction for benzoxazole synthesis.25 The model compounds
6 and 9 were synthesized either via direct condensation starting
from o-anisaldehyde or via methylation of hydroxyl-derivative 8
(Scheme 1). Two analogous compounds 11 and 12 were synthe-
sized from quinone 10 (Scheme 2). They differ from 6 and 7 in that
they possess two additional pyridine-type nitrogens which
decrease electron density in the heterocyclic system, allowing at
the same time to build complexes of Ru(phen)3 type.
It is known that rylene type p-expansion gives a significantly
larger bathochromic shift in absorption compared to the addi-
tion of a naphthalene unit in an anthracene-like fashion.26 Along
these lines, we designed and synthesized expanded imidazoles 14
and 15 starting from acenaphthoquinone (13) (Scheme 3). A
Scheme 1
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[10,20:4,5]imidazo[2,1-a]benzo[de]isoquinolin-14-one.28 Due to
the significant difference in Rf it was easily separated from the
desired compound 14.
A second group of compounds was designed to study the N–
H–N hydrogen bond system. We were curious to investigate the
Scheme 2
Scheme 3
Scheme 4
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Compound 8, the only benzoxazole of the series, could in
principle exist, in analogy to similar benzoxazoles, in the two
different planar conformers syn and anti enols in equilibrium in
the ground state. In addition, in H-bonding solvents a third non
planar rotamer, referred to as open or solvated, where the phe-
noxy group is H-bonded to a solvent molecule, can be present
Scheme 5
difference in optical properties of these compounds as a function
of acidity of the N–H bond. Five compounds (19, 20, 24, 25 and
27) were designed and synthesized. The key compounds 20 and
25 possess an N-tosylamino group at the ortho position of the
phenyl ring. Since the corresponding aldehyde is not commer-
cially available, our syntheses started from 2-nitrobenzaldehyde
17 (Schemes 4 and 6).
The synthesized imidazole 18 was easily reduced to compound
19 and tosylated under classical conditions to obtain the target
derivative 20 in a 92% overall yield. In order to compare optical
properties of imidazole 20 with amine 19 and the model
compound lacking ESIPT activity, we synthesized derivative 22
from the corresponding aldehyde 21 29 (Scheme 5).
Synthesis of the fused analogue of compound 20 was realized
using the same strategy starting from 2-nitrobenzaldehyde (17)
and 1,10-phenantroquinone (1) in an overall yield of 84%
(Scheme 6). Imidazole 27 was synthesized starting from aldehyde
26 (Scheme 7). Given the moderate acidity of the N–H bond, it
was expected that this arrangement would lead to a weak
hydrogen bond. This type of structural variation is interesting
since it results in a different geometry of the intramolecular
hydrogen bond.
Linear optical properties
The ESIPT process is very complex and its efficiency is affected
by internal (e.g. substitution pattern) or external parameters (e.g.
solvent). In order to ascertain the ESIPT properties of the new
compounds, we determined their basic spectroscopic properties
and compared them to those of non-ESIPT models in solvents of
different dielectric constants and proticities, namely toluene
(Tol), dichloromethane (DCM) and methanol (MeOH). Molar
absorption coefficients and related absorption maxima, emission
maxima, Stokes shifts, and luminescence quantum yields are
reported in Table 1. Where appropriate, two separate lines for
the enol and keto forms are reported.
Scheme 6
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pattern, and a different distribution of the emission of the keto
and enol forms has been found in hydroxynaphthyl derivatives,
for example.31 The main reason for the different behavior
depends on the energy barrier between the syn and anti forms in
the enol ground state whose height is strongly affected by
substitution.
Scheme 7
(Fig. 1).30 The syn form, due to favorable electronic and struc-
tural properties, exhibit ESIPT whereas the anti form does not
lead to ESIPT because of unfavorable thermodynamic parame-
ters. In apolar or low polarity solvents the syn form prevails due
to the relatively stronger H-bond with the benzoxazole N rather
than with O. In polar solvents, on the contrary, the perpendicular
open form prevails but still some anti and syn forms are present in
low concentration. Therefore, it has been shown that in simple 2-
hydroxyphenylbenzoxazole (HBO) in apolar solvent only the
keto form emission prevails, that in polar solvents the enol form
appears, whereas in protic methanol both forms are present.30
This phenomenology is actually affected by the substitution
Fig. 2 Fluorescence from optically matched solutions of 9 (line) and 8
(scattered) in Tol, DCM and MeOH. The signal from 8 is multiplied by
10. The excitation wavelength is 325 nm in Tol, 327 nm in DCM and 315
nm in MeOH.
Fig. 1 Possible conformers of compound 8.
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Going back to the case of 8 in Tol, this is characterized by an
enol form with a structured emission band with maxima at 367,
389, and 413 nm, well reproducing the non-ESIPT model 9 with
maxima at 368, 389 and 410 nm (Fig. 2).
yield with respect to the former less polar, aprotic solvents. This
seems to indicate that in the non-ESIPT model 12 in protic polar
solvents, the excited state is different than in less polar solvents.
Accordingly, also in the ESIPT compound 11 the luminescence
in the higher energy side is bathochromically shifted whereas the
keto form emission disappears. We ascribe this behavior in the
protic polar solvent either to the stabilization of a CT state,
which becomes the emitting one or, less likely, to the emission
from a differently protonated form of phenanthroline derivative.
When compared to compound 7, dye 11 displays a significantly
The luminescence however is greatly quenched, to less than 1%
that of the model (Table 1). The keto form with a maximum at
495 nm has a yield of 0.111. In DCM, the luminescence prop-
erties are very similar to Tol – a very low emission yield of the
enol form, and emission from the keto form with ffl ¼ 0.073. In
MeOH, however, the ratio between enol and keto form changes,
and the enol emission yield grows above 1% of the model emis-
sion whereas the luminescence of the keto form drops to an
emission quantum yield of 0.013.
higher Stokes shift in non-polar solvents (9300 vs. 6500 cmꢀ1
)
and lower emission quantum yield (Table 1). For both dyes 6 and
12 absorption maximum is basically solvent-independent and the
presence of two nitrogen atoms has a negligible effect on the
spectrum. As for dye 12 this is in contrast with results published
for 1,2-diaryl-phenantrolinoimidazoles with 4-substituted aryl
groups, which display a slight but noticeable bathochromic shift
of absorption while moving to polar solvents.36a,b The situation is
entirely different for emission. While emission maximum of
model compound 6 is almost solvent-independent, for dye 12 it
does bathochromically shift with increasing solvent polarity. The
fluorescence quantum yield of dye 12 is also decreased when
compared to analogue 6 (only 0.04–0.075 depending on the
solvent). This cannot however be simply explained by the pres-
ence of two pyridine-type nitrogen atoms, since fluorescence
quantum yields reported for analogous compounds lacking steric
hindrance imparted by the 2-methoxy group are around 0.4.36a,b
Crystallographic studies show that the mean plane angle between
the imidazole ring makes a dihedral angle of 56ꢂ with a phenyl
ring attached to the C(2) carbon.36a,b The presence of an addi-
tional substituent at position 2 imparts steric hindrance which in
turn can probably increase this dihedral angle, decreasing
effective conjugation in the ground state. In spite of that we did
not observe a notable hypsochromic shift of absorption of
compounds 6 and 12 compared to non-hindered analogues.36c
Altering the mode of p-expansion resulted in a complete change
in the optical properties of the resulting dyes 14 and 15.
Compound 14 has luminescence properties which are hardly
distinguishable from those of the non-ESIPT model 15 (Fig. 5).
The only difference is the presence in model 15 of a weak emission
around 385 nm, absent in 14. Both model and ESIPT display a
strong Stokes-shifted broad band, bathochromically shifted in 14
with respect to the model by 5 nm in non-polar Tol and by ca. 15
nm in DCM and MeOH. The luminescence yield of the Stokes-
shifted band in 14 (ffl # 10ꢀ2) is lower with respect to model 15 by
10% in Tol and by almost 30% in DCM and MeOH. The same
behavior is obtained with excitation on the visible band at 580 nm.
The plausible rationale behind these data is that the energy
difference between S1 levels of the keto and enol tautomers is
small and this precludes ESIPT to occur for thermodynamic
reasons (in analogy to a 10-hydroxy-1-azaperylene scaffold).37
Rather large Stokes shifts for both 14 and 15 suggest an
important geometry and/or electron-distribution rearrangement
in the S1 excited state. The comparison of two types of p-
expansion (14 vs. 7) reveals a significant difference. Dye 14 has
bathochromically shifted (although weak) absorption toward the
visible region, relatively high Stokes shift and ffl is only 1%. It is
noteworthy to mention that this is the first photophysical
investigation of p-expanded imidazoles of this type.
This is in agreement with the fact that the anti form is more
stable in polar solvents, as formerly observed.24 The present case
seems to have more similarities with that of similar 2-hydroxy-
naphtyl derivatives, where the enol form could be observed also
in benzene, than with that of HBO. Very likely, also in this case
the barrier between syn and anti conformers is rather low. The
replacement of the oxygen atom with the NC6H5 group leads to
p-expanded imidazole 7. This alteration changes the central unit
to the one which is less electron deficient.
A single isomer is possible; in Tol the enolic form has bands at
369 and 383, very similar to the spectrum of model 6. The
luminescence of this form is however almost completely
quenched in favor of the appearance of the keto form emission at
478 nm, with a ffl of almost 0.2. The results in DCM are rather
similar, with a slightly lower emission quantum yield of the keto
form of 0.144, and a 5 nm hypsochromically shifted maximum.
In the protic solvent (MeOH) the luminescence of the keto form
diminishes by a factor of almost 10 compared to Tol whereas the
emission of the enol form appears at 365 and 379 nm, in good
agreement with that of model 6 (Fig. 3). Whereas in non-protic
solvents the ground state configuration with a pre-formed H
bond between hydroxyl H and the N3 of the imidazole favors
ESIPT, in MeOH intermolecular H-bonding with the solvent
competes with the intramolecular one, limiting the ESIPT
process only to the intramolecularly bound structures.
Because the effective conjugation length is affected by the
planarity of the molecule, phenanthro[9,10-d]imidazole 7 when
compared to 1,4,5-triphenyl-2-(2-hydroxyphenyl)imidazole6a
reveals a significant bathochromic shift of absorption (ꢁ45 nm).
The extended conjugation length however has only an indirect
and limited effect on the stabilization of the keto excited state,
resulting in a small change in the emission (ꢁ10 nm). Needless to
say the Stokes shift (Dnss) decreases from 11 000 cmꢀ1 to ꢁ6500
cmꢀ1. At the same time ffl remains on the same level. The direct
comparison of optical properties of compound 7 with analogues
studied by Park et al. as well as with other tri- and tetrasub-
stituted imidazole derivatives32–35 reveals similar trends.
The phenanthroline derivative 11 displays in Tol a lumines-
cence mainly of keto type, with a maximum at 484 nm and an
emission quantum yield of 0.125, twice the one of the enol
luminescence of the reference model 12 (Fig. 4). In Tol the enol
luminescence is reduced to an extremely low value, which
increases slightly in DCM. Conversely in DCM the luminescence
of the keto form decreases to about half with respect to that in
Tol. In MeOH model 12 displays a different spectrum, bath-
ochromically shifted and with a reduced luminescence quantum
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Fig. 3 Fluorescence from optically matched solutions of 6 (line) and 7
(scattered) in Tol, DCM and MeOH. The signal from 7 in MeOH is
multiplied by 10. The excitation wavelength is 343 nm in Tol, 324 nm in
DCM and 315 nm in MeOH.
Fig. 4 Fluorescence from optically matched solutions of 12 (line) and 11
(scattered) in Tol, DCM and MeOH. The excitation wavelength is 325
nm in Tol, 324 nm in DCM and 325 nm in MeOH.
of such a low energy state is compatible with the presence of the
electron rich amino group in the model which might act as an
electron donor toward the imidazole moiety. Interestingly the
effect of the NMe2 group at position 2 of the phenyl ring is
different than for an analogous tetraphenylimidazole possessing a
4-N,N-dimethylaminophenyl substituent ffl ¼ 0.31.38 Although
ffl of compound 22 is almost the same (0.28) in comparison with
In the second part of photophysical studies we focused on
compounds bearing NH group as a donor in a hydrogen bond.
The properties of 19 and 20 will be discussed relative to the non-
ESIPT model 22. The luminescence of model 22 is rather atypical
for an enol luminescence, generally structured, and points to a
mixing with a low energy charge transfer (CT) state. The presence
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Fig. 6 Fluorescence from optically matched solutions of 22 (line) and 19
(scattered) in Tol, DCM and MeOH. The signal from 19 in Tol and DCM
is multiplied by 5. The excitation wavelength is 325 nm in Tol, in DCM
and in MeOH.
Fig. 5 Fluorescence from optically matched solutions of 15 (line) and 14
(scattered) in Tol, DCM and MeOH. The excitation wavelength is 342
nm in Tol, 326 nm in DCM and 326 nm in MeOH.
imidazole bearing 4-N,N-dimethylaminophenyl group, the fluo-
rescence maximum is strongly red shifted and the absorption
maximum is strongly blue shifted. As a result the Stokes shift is
significantly higher (ꢁ16 000 cmꢀ1 vs. 8500 cmꢀ1). The relative
blue shift of absorption in compound 22 originates from steric
hindrance induced by the presence of the NMe2 group at position
two of the phenyl ring decreasing the effective interaction between
this ring and the imidazole core in the ground state.
In 19 the luminescence is shifted to higher energies with respect
to model 22, which is compatible with a less electron rich NH
unit which destabilizes the CT state. However, the ‘‘enol’’ lumi-
nescence is generated also in this case from a state with a sizeable
CT component. Accordingly, the fluorescence band shifts to a
lower energy in the more polar solvent (MeOH) which stabilizes
CT. The luminescence maximum shifts in fact from 398 nm in
TOL to 407 nm in MeOH (Fig. 6). Whereas in Tol and DCM
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Fig. 8 Fluorescence from optically matched solutions of 24 (line) and 25
(scattered) in Tol, DCM and MeOH. The excitation wavelength is
343 nm in Tol, DCM and MeOH.
Fig. 7 Fluorescence from optically matched solutions of 22 (line) and 20
(scattered) in Tol, DCM and MeOH. The excitation wavelength is 325
nm in Tol, 325 nm in DCM and 330 nm in MeOH.
substituent, therefore a low energy CT excited state involving the
imidazole unit is no longer possible. The difference in peak
wavelength between absorption and emission as large as 15 000
cmꢀ1 unambiguously supports the occurrence of ESIPT, forming
a proton-transfer tautomer adiabatically in the excited state.
In Tol a strongly quenched enol fluorescence at 375 nm with
ffl ¼ 0.004 is observed, but an intense keto luminescence with ffl
of almost 0.4 is registered at 499 nm. In DCM the behavior is
similar, with a similarly low enol fluorescence at 378 nm and a
slightly hypsochromically shifted keto luminescence at 484 nm
some keto forms can be detected (though to an extremely low
extent, ffl < 10ꢀ2) it disappears completely in MeOH, where the
emission is only from the enol/CT state. Proton coupled electron
transfer (PCET) reactions associated with electronically excited
states is an interesting class of reactions39 and the present case
seems to belong to this class.
The introduction of the electron-withdrawing tosyl group in 20
leads to a completely different behavior (Fig. 7). The NH group
is now less electron rich due to the inductive effect of the
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Dyes 24 and 25 are the equivalents of compounds 19 and 20
respectively, but with the more extended aromatic unit (phenan-
threne). Compound 24 shows a moderately intense, enol like
emission in Tol and DCM (ffl ca. 0.15) but with a slightly higher
Stokes-shifted emission than expected (Fig. 8 and Table 1). This
indicates some charge transfer characters in the excited state,
though much lower than the one detected in 19 (where it is visible
both from the broad spectral shape and from the larger Stokes
shifted emission). Only in MeOH the structureless spectral shape
and the higher Stokes shift of 24 are more clearly indicative of a
charge transfer emitting excited state. In this solvent, the lumi-
nescence yield is rather intense (ꢁ0.3). In comparison with 19, 24
has a much higher fluorescence intensity and does not display the
minimal formation of a keto form, which was on the contrary
detectable in 19. Obviously, in the present case there is no suffi-
cient driving force for proton transfer to occur in the excited state.
Upon introduction of the electron-withdrawing tosyl group in
25, which renders the NH group less electron rich, the enolic
excited state loses its charge transfer character and displays a
structured emission. As in the case of 20, the keto form is effi-
ciently formed, displaying ffl on the order of 0.5 in non-polar and
moderately polar solvents. Even in MeOH, where the competi-
tion with the solvent generally precludes or disfavors ESIPT, the
keto form has a high yield of formation, as testified by the still
moderately high emission yield of the keto form, on the order of
0.2. The phenomenology in both compounds 24 and 25 can be
interpreted along the same lines as 19 and 20.
As described by Fahrni and co-workers for 2-(20-tosylamino-
phenyl)benzimidazoles, the cis-rotamer is thermodynamically
substantially favored over the trans-rotamer.40 As a result the
ground-state equilibrium of that molecule is dominated by a
single species, and the ESIPT process is essentially unaffected by
the nature of the solvent.40 In the case of tosylamides 20 and 25
ESIPT is more affected by solvent properties than in the corre-
sponding benzimidazole case40 but much less than for phenols 7
and 11. Notably, both derivatives 20 and 25 exhibit mostly keto-
emission, indicating that formation of ESIPT tautomer is very
efficient and neither compromised through solvent–solute
hydrogen-bonding interactions,41 solvent-polarization-induced
barriers,42 or ground-state stabilization of tautomer that cannot
undergo ESIPT.43
Compound 27 displays a spectrum clearly ascribable to the
enolic form, with a structured emission with maxima at 377, 398
and 421 nm in Tol and an emission quantum yield of 0.17. A very
weak emission of a keto form with maximum at 566 nm and ffl ¼
0.015 can be detected in Tol (Fig. 9). The emission quantum yield
of the enolic form increases in polar solvents whereas that of the
keto form decreases. In this molecule ESIPT appears to be
ineffective very likely for the low acidity of the indolic proton.
Fig. 9 Fluorescence from solutions of 27 in Tol, DCM and MeOH. The
excitation wavelength is 325 nm in Tol, 327 nm in DCM and 315 nm in
MeOH.
Non-linear optical properties
Two-photon absorption measurements of selected ESIPT chro-
mophores (7, 8, 11, 19 and 20) were performed using the
two-photon excited fluorescence (TPEF) method (Table 2).
Unfortunately, measurements of compounds 11 and 20 were not
possible as they did not give a reliable TPEF signal (the fluo-
rescence quantum yield of 11 is too low and the 2PA of 20 in the
700–900 nm region is too low due to its blue-shifted absorption).
with an almost identical emission quantum yield to that of Tol.
In MeOH the protic solvent competes with the intramolecular H-
bond formation decreasing the ESIPT efficiency. As a conse-
quence the keto fluorescence at 482 nm is decreased to ffl ¼ 0.201
in MeOH whereas the enol luminescence is almost constant with
respect to Tol and DCM.
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Table 2 Two-photon absorption (2PA) cross-section of compounds 7, 8,
11, 19 and 20
via EI-MS. Aldehyde 21 (ref. 29) and diketone 10 (ref. 44) were
prepared according to the literature procedures.
max1
2PA
Compound
Solvent
l
(nm)
sm2 ax1 (GM)
General procedure for 1. To a solution of aromatic aldehyde
(1 eq.) and aromatic amine (1.5 eq.) in glacial acetic acid, a-
diketone (1 eq.) and ammonium acetate (5 eq.) were added. The
mixture was stirred at 110 ꢂC for 12 h. After cooling, the solution
was poured into a copious amount of water. The dark precipitate
was filtered. The details of purification were described for each
case as follows:
7
8
11
19
20
CH3CN
CHCl3
CH3CN
CHCl3
CHCl3
<700
710
—
<700
—
>18
5
—
>95
—
Among all compounds studied, dye 19 gave the highest 2PA
cross-section value (about 100 GM), while compounds 7 and 8
possess lower values (ꢁ5 to 20 GM). Interestingly, we found a
five-fold increase of 2PA at 700 nm on going from p-expanded
oxazole 8 to p-expanded imidazole 7. However, since the oxazole
derivative has a lower quantum yield, this results in similar two-
photon brightness. Furthermore, replacing OH with NH2 (a
stronger donor) leads to much larger 2PA in the 700–800 nm
region (although it is hypsochromically shifted and fluorescence
blue-shifted).
2-(2-Methoxyphenyl)-1-(p-tolyl)-1H-phenanthro[9,10-d]imid-
azole (6). The product was purified by column chromatography
on silica using hexanes–ethyl acetate (3/1) solution as eluent.
Crystallization from chloroform–hexanes afforded a white
ꢂ
crystalline solid (2.87 g, 86%); m.p. ¼ 210–212 C (chloroform–
1
hexanes); H NMR (500 MHz, CDCl3) d 2.41 (s, 3H), 3.57 (s,
3H), 6.75 (dd, 1H, J ¼ 8.3 Hz, J ¼ 0.8 Hz), 6.95–6.98 (m, 1H),
7.18–7.21 (m, 2H), 7.25–7.33 (m, 5H), 7.48–7.51 (m, 2H), 7.59–
7.64 (m, 1H), 7.68–7.73 (m, 1H), 8.69–8.71 (m, 1H), 8.75–8.77
(m, 1H), 8.83–8.86 (m, 1H); 13C NMR (125 MHz, CDCl3) d 21.3,
54.9, 110.5, 120.3, 120.3, 121.0, 122.8, 123.0, 123.1, 124.0, 124.7,
125.3, 126.1, 127.1, 127.4, 127.5, 128.1, 128.2, 129.1, 129.6, 131.0,
132.5, 135.6, 137.3, 138.9, 150.0, 157.7; HRMS (EI) calcd for
C29H22N2O 414.1732, found 414.1752 [M+_].
Conclusions
In conclusion, we report new functional dyes combining record
Stokes shifts (15 000 cmꢀ1) with good fluorescence quantum
yields and reasonable 3 values. We also discovered that specific
p-expansion of imidazole derivatives leads to an ESIPT-silent
system that still possesses a large Stokes shift. Another non-
2-(2-Hydroxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imid-
azole (7) and 2-(2-hydroxyphenyl)phenanthro[9,10-d]oxazole (8).
Two products were purified by column chromatography on silica
using hexanes–ethyl acetate (4/1) solution as eluent. 2-(2-
Hydroxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (7):
crystallization from chloroform–hexanes afforded a white crys-
talline solid (0.66 g, 42%); m.p. ¼ 184–186 ꢂC (CHCl3–hexanes);
1H NMR (500 MHz, CDCl3) d 6.49–6.53 (m, 1H), 6.75 (dd, 1H, J
¼ 8.1 Hz, J ¼ 1.4 Hz), 7.05 (d, 1H, J ¼ 7.9 Hz), 7.13 (dd, 1H, J ¼
8.2 Hz, J ¼ 1.1 Hz), 7.19–7.23 (m, 1H), 7.23–7.27 (m, 1H), 7.50–
7.54 (m, 1H), 7.61–7.64 (m, 2H), 7.67–7.70 (m, 1H), 7.70–7.80
(m, 4H), 8.68–8.78 (m, 3H), 13.77 (s, 1H); 13C NMR (125 MHz,
CDCl3) d 113.0, 118.1, 120.9, 122.5, 122.6, 123.2, 124.2, 125.3,
125.6, 126.1, 126.2, 126.5, 127.0, 127.5, 128.5, 129.1, 129.5, 130.6,
130.8, 130.8, 134.2, 138.9, 148.4, 159.1; HRMS (EI) calcd for
C27H18N2O 386.1419, found 386.1413 [M+_]; labs [nm] (CH3CN, 3
ꢃ 10ꢀ3) 288 (47.6); lem [nm] (CH3CN) 468, (MeOH) 366,
(toluene) 474, (hexane) 476, (AcOH) 401.
ESIPT architecture which leads to
a similar effect is
1,4,5-triphenyl-2-(20-dimethylaminophenyl)imidazole. The other
notable findings are as follows: (a) in most cases, p-expansion of
the chromophore leads to compounds that display an excited
state intramolecular proton transfer, as evidenced by a large
Stokes shift. A detailed study of the optical properties of the
products and their ESIPT-blocked models allowed us to observe
that substitution of OH with H–N–Tos led to an increase in the
Stokes shift while hypsochromically shifting the absorption
band. We found that chemical modifications resulted in
significantly altered spectroscopic properties (i.e., red-shifted
absorption and emission spectra) relative to the parent 1,4,5-
triphenyl-2-(2-hydroxyphenyl)imidazole. The key difference
between dyes possessing the OH group versus the NHT group is
that for the latter ones the keto state is achievable even in MeOH.
These results are not only of theoretical significance in that they
provide new insight into factors influencing the ESIPT
phenomenon, but they may also open doors to practical appli-
cations in biological imaging.
2-(2-Hydroxyphenyl)phenanthro[9,10-d]oxazole (8). Crystalli-
zation from chloroform–hexanes afforded a white crystalline
solid (0.1 g, 8%); m.p. ¼ 238–240 ꢂC (CHCl3–hexanes); 1H NMR
(500 MHz, CDCl3) d 7.04–7.08 (m, 1H), 7.15–7.19 (m, 1H), 7.42–
7.47 (m, 1H), 7.65–7.75 (m, 4H), 8.13 (dd, 1H, J ¼ 7.8 Hz, J ¼ 1.6
Hz), 8.26–8.30 (m, 1H), 8.43–8.47 (m, 1H), 8.66–8.72 (m, 2H),
11.43 (bs, 1H); 13C NMR (125 MHz, CDCl3) d 117.4, 119.6,
120.6, 120.8, 122.8, 123.4, 123.7, 125.0, 126.4, 126.6, 126.7, 127.4,
127.5, 129.0, 129.3, 133.0, 133.3; HRMS (ESI) calcd for
C21H14NO2 312.1019, found 312.1015 [M + H]+.
Experimental part
Synthesis
All chemicals were used as received unless otherwise noted.
Reagent grade solvents (CH2Cl2, hexanes) were distilled prior to
use. All reported NMR spectra were recorded on 500 MHz and
600 MHz spectrometers. UV-vis absorption and fluorescent
spectra were recorded in acetonitrile. Chromatography was
performed on silica (200–400 mesh). Mass spectra were obtained
2-(2-Hydroxyphenyl)-1-phenyl-1H-imidazo[4,5-f][1,10]phenan-
throline (11). The product was purified by column chromatog-
raphy on silica using CH2Cl2 / CH2Cl2–MeOH (40/1) solution
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as eluent. Crystallization from ethyl acetate–pentane afforded a
white crystalline solid (0.71 g, 61%); m.p. ¼ 336 ꢂC (ethyl
acetate–pentane); 1H NMR (500 MHz, CDCl3) d 6.52 (t, 1H, J ¼
7.6 Hz), 6.75 (d, 1H, J ¼ 8.2 Hz), 7.12 (d, 1H, J ¼ 8.2 Hz), 7.21–
7.27 (m, 3H), 7.66 (d, 2H, J ¼ 7.4 Hz), 7.72–7.84 (m, 4H), 8.92 (d,
1H, J ¼ 8.0 Hz), 9.03 (s, 1H), 9.17 (d, 1H, J ¼ 3.7 Hz), 13.32 (s,
1H); 13C NMR (125 MHz, CDCl3) d 112.6, 118.2, 118.3, 119.5,
122.3, 122.6, 123.7, 125.8, 126.2, 128.1, 128.8, 130.4, 131.1, 131.2,
131.3, 133.1, 138.3, 144.2, 144.8, 148.2, 149.3, 149.8, 159.1;
HRMS (EI) calcd for C25H16N4O 388.1324, found 388.1338
[M+_]; labs [nm] (CH3CN, 3 ꢃ 10ꢀ3) 277 (35.1), 332 (21.8); lem
[nm] (CH3CN) 470, (CH2Cl2) 475.
Crystallization from ethyl acetate–hexanes afforded a white
crystalline solid (2.12 g, 82%); m.p. ¼ 168–169 ꢂC (ethyl acetate–
1
hexanes); H NMR (500 MHz, CDCl3) d 2.19 (s, 3H), 2.31 (s,
6H), 6.65 (dd, 1H, J ¼ 8.2 Hz, J ¼ 1.0 Hz), 6.68–6.69 (m, 2H),
6.81–6.85 (m, 2H), 6.93–6.97 (m, 1H), 7.13–7.26 (m, 9H), 7.58–
7.63 (m, 3H); 13C NMR (125 MHz, CDCl3) d 21.0, 42.1, 117.1,
120.6, 123.6, 126.3, 126.5, 127.3, 127.7, 128.0, 128.3, 128.3, 129.3,
129.8, 131.1, 131.2, 132.8, 134.6, 134.8, 136.7, 138.0, 147.6, 151.7;
HRMS (EI) calcd for C30H27N3 429.2205, found 429.2216 [M+_].
2-(2-Nitrophenyl)-1-(p-tolyl)-1H-phenanthro[9,10-d]imidazole
(23). The product was purified by column chromatography on
silica using hexanes–ethyl acetate (3/2 to 1/1) solution as eluent.
Crystallization from ethyl acetate afforded an orange crystal-
2-(2-Methoxyphenyl)-1-phenyl-1H-imidazo[4,5-f][1,10]phenan-
throline (12). The product was purified by column chromatog-
raphy on silica using CH2Cl2–MeOH (40/1) solution as eluent.
Crystallization from ethyl acetate–pentane afforded a white
crystalline solid (1.01 g, 70%); m.p. ¼ 275–276 ꢂC (ethyl acetate–
1
ꢂ
line solid (1.56 g, 91%); m.p. ¼ 262–263 C (ethyl acetate); H
NMR (500 MHz, CDCl3) d 2.42 (s, 3H), 7.21–7.32 (m, 6H),
7.50–7.55 (m, 2H), 7.60–7.66 (m, 3H), 7.68–7.73 (m, 1H), 8.01
(d, 1H, J ¼ 8.0 Hz), 8.71 (d, 1H, J ¼ 8.4 Hz), 8.74–8.79 (m,
2H); 13C NMR (125 MHz, CDCl3) d 21.3, 120.9, 122.7, 122.9,
123.1, 124.1, 124.6, 125.1, 125.6, 126.3, 126.7, 127.2, 127.3,
127.7, 128.3, 128.3, 129.3, 130.3, 130.4, 132.9, 133.5, 134.5,
137.5, 139.7, 147.7, 148.9; HRMS (EI) calcd for C28H19N3O2
429.1477, found 429.1488 [M+_].
1
pentane); H NMR (500 MHz, CDCl3) d 3.56 (s, 3H), 6.76 (dd,
1H, J ¼ 8.5 Hz, J ¼ 0.5 Hz), 6.97–7.03 (m, 1H), 7.28–7.57 (m,
9H), 7.73 (q, 1H, J ¼ 4.3 Hz), 9.06 (dd, 1H, J ¼ 4.3 Hz, J ¼ 1.6
Hz), 9.14 (dd, 1H, J ¼ 8.1 Hz, J ¼ 1.9 Hz), 9.18 (dd, 1H, J ¼ 4.3
Hz, J ¼ 1.8 Hz); 13C NMR (125 MHz, CDCl3) d 54.9, 110.5,
119.4, 119.8, 120.5, 122.0, 123.4, 124.1, 126.2, 128.1, 128.2, 129.3,
129.5, 130.5, 131.5, 132.3, 136.2, 137.5, 144.3, 144.8, 147.9, 148.8,
151.3, 157.4; HRMS (EI) calcd for C26H18N4O 402.1481, found
402.1475 [M+_].
2-(1H-Indol-7-yl)-1-(p-tolyl)-1H-phenanthro[9,10-d]imidazole
(27). Following the general procedure for 1, the product was
purified by column chromatography on silica using hexanes–
ethyl acetate (7/1) solution as eluent. Crystallization from ethyl
acetate–hexanes afforded a white crystalline solid (40 mg, 10%)
m.p. ¼ 105–106 ꢂC (ethyl acetate–hexanes); 1H NMR (500 MHz,
CDCl3) d 2.60 (s, 3H), 6.61–6.64 (m, 1H), 6.73 (dd, 1H, J ¼ 7.6
Hz, J ¼ 0.6 Hz), 6.81 (t, 1H, J ¼ 7.7 Hz), 7.18 (dd, 1H, J ¼ 8.4
Hz, J ¼ 1.0 Hz), 7.25–7.30 (m, 1H), 7.45–7.53 (m, 6H), 7.62 (d,
1H, J ¼ 7.8 Hz), 7.64–7.69 (m, 1H), 7.75–7.80 (m, 1H), 8.71 (d,
1H, J ¼ 8.4 Hz), 8.77 (d, 1H, J ¼ 8.3 Hz), 8.89 (dd, 1H, J ¼ 7.9
Hz, J ¼ 1.1 Hz), 11.50 (bs, 1H); 13C NMR (125 MHz, CDCl3) d
21.6, 102.4, 118.6, 120.2, 121.0, 122.0, 122.5, 123.1, 123.2, 124.1,
124.9, 125.6, 126.4, 127.2, 127.7, 128.3, 128.8, 128.8, 129.3, 131.2,
135.0, 136.7, 140.4, 148.8; HRMS (EI) calcd for C30H21N3
423.1735, found 423.1746 [M+_]; lem [nm] (CH3CN) 379, 397.
8-(2-Hydroxyphenyl)-7-phenyl-7H-acenaphtho[1,2-d]imidazole
(14). The product was purified by column chromatography on
silica using hexanes–ethyl acetate (3/1 to 1/1) solution as eluent.
Crystallization from CHCl3–hexanes afforded an orange crys-
talline solid (0.76 g, 46%); m.p. ¼ 192–194 ꢂC (CHCl3–hexanes);
1H NMR (500 MHz, CDCl3) d 6.51–6.56 (m, 1H), 6.81 (dd, 1H, J
¼ 8.1 Hz, J ¼ 1.4 Hz), 7.03 (dd, 1H, J ¼ 6.9 Hz, J ¼ 0.6 Hz),
7.09–7.12 (m, 1H), 7.15–7.20 (m, 1H), 7.31–7.36 (m, 1H), 7.55–
7.60 (m, 3H), 7.62–7.66 (m, 3H), 7.69–7.74 (m, 2H), 7.89–7.92
(m, 1H), 12.85 (bs, 1H); 13C NMR (125 MHz, CDCl3) d 113.6,
117.8, 118.1, 119.1, 121.2, 125.9, 126.4, 126.6, 126.9, 127.0, 127.3,
127.8, 129.4, 129.5, 129.6, 129.9, 130.3, 131.6, 138.1, 138.2, 144.5,
148.3, 157.8; HRMS (EI) calcd for C25H16N2O 360.1263, found
360.1258 [M+_]; labs [nm] (CHCl3, 3 ꢃ 10ꢀ3) 327 (35.0), 429 (1.4);
lem [nm] (CHCl3) 558.
2-(2-Methoxyphenyl)phenanthro[9,10-d]oxazole (9). CH3I (46
mg, 0.32 mmol) was added dropwise to a suspension of 8 (50 mg,
0.16 mmol) and K2CO3 (45 mg, 0.32 mmol) in acetone (40 ml).
The reaction was refluxed overnight. After cooling, the organic
layer was distilled off and the product was purified by column
chromatography using hexanes–ethyl acetate (2/1) solution as
eluent. Crystallization from chloroform–hexanes afforded a
white crystalline solid (46 mg, 88%); 1H NMR (500 MHz,
CDCl3) d 4.06 (s, 3H), 7.08–7.17 (m, 2H), 7.48–7.53 (m, 1H),
7.64–7.77 (m, 4H), 8.26 (ddd, 1H, J ¼ 7.8 Hz, J ¼ 1.8 Hz, J ¼ 0.3
Hz), 8.31–8.35 (m, 1H), 8.66–8.76 (m, 3H); 13C NMR (125 MHz,
CDCl3) d 56.2, 112.2, 116.7, 120.8, 121.0, 121.1, 123.1, 123.3,
123.7, 126.0, 126.3, 127.1, 127.3, 128.8, 129.2, 131.1, 132.3, 135.3,
158.2, 160.8; HRMS (EI) calcd for C22H15NO2 325.1103, found
325.1095 [M+_].
2-(2-Nitrophenyl)-4,5-diphenyl-1-(p-tolyl)-1H-imidazole (18).
Recrystallization from ethyl acetate solution afforded a yellow
crystalline solid (3.41 g, 98%); m.p. ¼ 184 ꢂC (ethyl acetate); 1H
NMR (400 MHz, CDCl3) d 2.23 (s, 3H), 6.82 (d, 2H, J ¼ 8.4
Hz), 6.93 (d, 2H, J ¼ 8.0 Hz), 7.16–7.30 (m, 8H), 7.47–7.52 (m,
1H), 7.55–7.69 (m, 4H), 7.89–7.93 (m, 1H); 13C NMR (100
MHz, CDCl3) d 21.3, 124.7, 126.9, 127.6, 127.9, 128.3, 128.4,
128.7, 129.8, 130.2, 130.5, 130.9, 131.2, 133.1, 133.2, 133.6,
134.5, 138.3, 138.8, 143.5, 149.1, 182.4; HRMS (EI) calcd for
C28H21N3O2 431.1634, found 431.1619 [M+_]; labs [nm]
(CH3CN, 3 ꢃ 10ꢀ3) 268 (21).
2-(2-N,N-Dimethylaniline)-4,5-diphenyl-1-(p-tolyl)-1H-imid-
azole (22). The product was purified by column chromatography
on silica using hexanes–ethyl acetate (3/1) solution as eluent.
8-(2-Methoxyphenyl)-7-phenyl-7H-acenaphtho[1,2-d]imidazole
(15). CH3I (46 mg, 0.32 mmol) was added dropwise to a
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suspension of 14 (59 mg, 0.16 mmol) and K2CO3 (45 mg, 0.32
mmol) in acetone (40 ml). The reaction was refluxed overnight.
After cooling, the organic layer was distilled off and the
product was purified by crystallization from chloroform–
hexanes, affording an orange crystalline solid (61 mg, 99%);
m.p. ¼ 154–156 ꢂC (CHCl3–hexanes); 1H NMR (500 MHz,
CDCl3) d 3.31 (s, 3H), 6.73 (d, 1H, J ¼ 8.0 Hz), 7.02–7.04 (m,
1H), 7.32–7.44 (m, 8H), 7.52–7.56 (m, 1H), 7.66–7.72 (m, 3H),
7.92 (d, 1H, J ¼ 6.6 Hz); 13C NMR (125 MHz, CDCl3) d 54.6,
110.8, 118.8, 120.4, 120.5, 120.8, 124.4, 126.3, 126.8, 126.9,
127.3, 127.7, 127.8, 129.0, 129.5, 130.8, 130.9, 131.5, 132.1,
137.1, 138.4, 148.0, 148.6, 156.8; HRMS (EI) calcd for
C26H18N2O 374.1419, found 374.1411 [M+_].
filtrate was evaporated under reduced pressure. Crystallization
from ethyl acetate solution afforded a white crystalline solid
(0.50 g, 99%); m.p. ¼ 202–203 ꢂC (ethyl acetate); 1H NMR (500
MHz, CDCl3) d 2.49 (s, 3H), 5.26 (s, 2H), 6.41–6.46 (m, 1H),
6.76 (dd, 1H, J ¼ 8.0 Hz, J ¼ 0.7 Hz), 6.87 (dd, 1H, J ¼ 7.9
Hz, J ¼ 1.3 Hz), 7.03–7.08 (m, 1H), 7.23–7.38 (m, 6H), 7.47–
7.52 (m, 1H), 7.61–7.66 (m, 1H), 7.69–7.74 (m, 1H), 8.70 (d,
1H, J ¼ 8.4 Hz), 8.76 (d, 1H, J ¼ 8.4 Hz), 8.80 (dd, 1H, J ¼ 7.9
Hz, J ¼ 1.0 Hz); 13C NMR (125 MHz, CDCl3) d 21.4, 113.9,
116.3, 116.7, 121.1, 122.6, 123.1(2), 124.0, 124.8, 125.5, 126.3,
127.0, 127.2, 127.2, 128.2, 128.7, 129.1, 129.8, 130.3, 130.6,
136.0, 136.6, 139.6, 147.3, 149.5; HRMS (EI) calcd for
C28H21N3 399.1735, found 399.1727 [M+_].
2-(2-Aniline)-4,5-diphenyl-1-(p-tolyl)-1H-imidazole
(19).
4-Methyl-N-(2-(1-(p-tolyl)-1H-phenanthro[9,10-d]imidazol-2-
yl)phenyl)benzenesulfonamide (25). p-Toluenesulfonyl chloride
(72 mg, 0.38 mmol) was added to a solution of 24 (120 mg, 0.25
mmol) in pyridine (5 ml), and the mixture was heated at reflux
for 2 h. After cooling, the solution was poured into a copious
amount of water. The white precipitate was filtered and washed
with water. The product was purified by column chromatog-
raphy on silica using dichloromethane as eluent. Recrystalli-
zation from dichloromethane–pentane solution afforded a
white crystalline solid (154 mg, 93%); m.p. ¼ 195–196 ꢂC
(dichloromethane–pentane); 1H NMR (500 MHz, CDCl3) d
2.03 (s, 3H), 2.51 (s, 3H), 6.70–6.81 (m, 6H), 7.08 (dd, 1H, J ¼
8.2 Hz, J ¼ 0.7 Hz), 7.23–7.31 (m, 4H), 7.36 (d, 2H, J ¼ 8.4
Hz), 7.53–7.57 (m, 1H), 7.69–7.74 (m, 1H), 7.79–7.83 (m, 2H),
8.73 (d, 1H, J ¼ 8.4 Hz), 8.80 (d, 1H, J ¼ 8.4 Hz), 8.84 (dd,
1H, J ¼ 7.9 Hz, J ¼ 0.8 Hz), 11.66 (s, 1H); 13C NMR (125
MHz, CDCl3) d 21.1, 21.4, 120.2, 121.0, 122.6, 122.7, 123.1,
123.9, 124.2, 124.7, 125.4, 126.2, 126.4(2), 127.0, 127.2, 127.9,
128.2, 128.4, 128.8, 129.0, 129.6, 129.7, 130.8, 135.3, 136.1,
136.9(2), 140.1, 143.0, 146.8; HRMS (EI) calcd for
C35H27N3O2S 553.1824, found 553.1835 [M+_].
Compound 18 (1.5 g, 3.48 mmol) was dissolved in 150 ml of
ethanol–ethyl acetate (2/1) and reduced with hydrogen (1 atm)
on 10% Pd–C (0.22 g) as a catalyst. The catalyst was filtered off
and the filtrate was evaporated under reduced pressure.
Recrystallization from ethyl acetate solution afforded a yellow
crystalline solid (1.33 g, 95%); m.p. ¼ 202–203 ꢂC (ethyl
1
acetate); H NMR (500 MHz, DMSO) d 2.20 (s, 3H), 6.06 (s,
2H), 6.21–6.25 (m, 1H), 6.63 (dd, 1H, J ¼ 7.8 Hz, J ¼ 1.5 Hz),
6.73 (dd, 1H, J ¼ 8.2 Hz, J ¼ 1.1 Hz), 6.91–6.95 (m, 1H), 7.01–
7.06 (m, 4H), 7.12–7.17 (m, 1H), 7.19–7.24 (m, 4H), 7.26–7.30
(m, 3H), 7.42–7.46 (m, 2H); 13C NMR (125 MHz, DMSO) d
21.0, 113.1, 115.2, 116.0, 126.7, 126.8, 128.6, 128.7, 128.8,
128.9, 129.5, 129.9, 130.0, 130.5, 131.1, 131.6, 134.7, 134.8,
136.2, 138.1, 145.7, 148.2; HRMS (EI) calcd for C28H23N3
401.1892, found 401.1896 [M+_]; labs [nm] (CHCl3, 3 ꢃ 10ꢀ3
282 (17); lem [nm] (CH2Cl2) 393, (CH3CN) 401.
)
N-{2-[4,5-Diphenyl-1-(p-tolyl)-1H-imidazol-2-yl]phenyl}-4-meth-
ylbenzenesulfonamide (20). p-Toluenesulfonyl chloride (129 mg,
0.675 mmol) was added to a solution of 19 (201 mg, 0.5 mmol)
in pyridine (3 ml), and the mixture was heated at reflux for 16
h. After cooling, the solution was poured into a copious
amount of water. The light brown precipitate was filtered and
washed with water. The product was purified by column
chromatography on silica using hexanes–ethyl acetate (4/1)
solution as eluent. Recrystallization from ethyl acetate–pentane
solution afforded a white crystalline solid (272 mg, 98%); m.p.
Linear optical properties
The solvents used were spectroscopic grade toluene, dichloro-
methane and methanol (C. Erba). Absorption spectra were
recorded with a Perkin-Elmer Lambda 650 spectrophotometer
and the emission spectra, corrected for the photomultiplier
response if not otherwise stated, were detected by an Edinburgh
FSP920 fluorometer equipped with a R928P Hamamatsu pho-
tomultiplier. Luminescence quantum yields of the samples, fs,
were evaluated against a standard with known emission quantum
yield, fr, by comparing areas under the corrected luminescence
spectra using the equation: fs/fr ¼ Arn2 (area)s/Asn2 (area)r,
1
¼ 258 ꢂC (ethyl acetate–pentane); H NMR (500 MHz, CDCl3)
d 2.28 (s, 3H), 2.34 (s, 3H), 6.27 (bs, 2H), 6.53 (d, 1H, J ¼ 7.4
Hz), 6.73 (t, 1H, J ¼ 7.4 Hz), 6.93 (d, 2H, J ¼ 8.2 Hz), 7.05 (d,
2H, J ¼ 6.8 Hz), 7.14 (d, 2H, J ¼ 8.2 Hz), 7.18–7.23 (m, 1H),
7.23–7.33 (m, 6H), 7.53 (d, 2H, J ¼ 8.0 Hz), 7.61 (d, 2H, J ¼
7.4 Hz), 7.76 (bs, 1H), 11.76 (bs, 1H); 13C NMR (125 MHz,
CDCl3) d 21.1, 21.4, 120.1, 123.9, 124.6, 126.9, 127.1, 127.4,
127.7, 128.5, 128.8, 129.1, 129.7, 130.1, 130.5, 131.1, 133.4,
133.7, 136.4, 137.2, 138.5, 143.0, 143.1; HRMS (EI) calcd for
C35H29N3O2S 555.1980, found 555.1959 [M+_]; labs [nm]
(CHCl3, 3 ꢃ 10ꢀ3) 289 (20.4), 276 (20); lem [nm] (CH2Cl2) 475,
(CH3CN) 429.
s
r
where A is the absorbance, n is the refractive index of the solvent
employed and s and r stand for sample and reference, respec-
tively. The standard used was air-equilibrated quinine sulphate in
1 N H2SO4 with an emission quantum yield ffl ¼ 0.546.45 Keto
and enol quantum yields were obtained by spectral deconvolu-
tion by Microcalꢀ Originꢁ 6.1.
2-(1-(p-Tolyl)-1H-phenanthro[9,10-d]imidazol-2-yl)aniline (24).
Compound 23 (0.54 g, 1.36 mmol) was dissolved in 200 ml
ethyl acetate and reduced with hydrogen (1 atm) on 10% Pd–C
(0.08 g) as a catalyst. The catalyst was filtered off and the
Estimated errors are 10% on molar extinction coefficients
and quantum yields and 2 nm on emission and absorption
maxima. The working temperature, if not otherwise specified, is
295 ꢄ 2 K.
20662 | J. Mater. Chem., 2012, 22, 20649–20664
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D.-J. Jang, B. M. Medina, J. Gierschner and S.-Y. Park, J. Am.
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Two-photon absorption (TPA) measurements were conducted
by investigating the two-photon excited fluorescence (TPEF) of
the fluorophores in solution at room temperature on air-
equilibrated solutions (10ꢀ4 M), according to the experimental
protocol established by Xu and Webb.46 This protocol avoids
contributions from excited-state absorption that are known to
result in overestimated TPA cross-sections. To span the 700–
980 nm range, a Nd:YLF-pumped Ti:sapphire oscillator was
used generating 150 fs pulses at a 76 MHz rate. The excitation
was focused onto the cuvette through a microscope objective
(10ꢃ, NA 0.25). The fluorescence was detected in epifluor-
escence mode via a dichroic mirror (Chroma 675dcxru) and a
barrier filter (Chroma e650sp-2p) by a compact CCD spec-
trometer module BWTek BTC112E. Total fluorescence inten-
sities were obtained by integrating the corrected emission
spectra measured by this spectrometer. TPA cross-sections (s2)
were determined from the two-photon excited fluorescence
(TPEF) cross-sections (s2F) and the fluorescence emission
quantum yield (F). TPEF cross-sections were measured relative
to fluorescein in 0.01 M aqueous NaOH for 715–980 nm,47 and
the appropriate solvent-related refractive index corrections.48
Data points between 700 and 715 nm were corrected according
to ref. 49. The quadratic dependence of the fluorescence
intensity on the excitation power was checked for each sample
and all wavelengths, indicating that the measurements were
carried out in intensity regimes where saturation or photo-
degradation did not occur.
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Financial support for our work from the Foundation for Polish
Science (Grant number TEAM/2009-4/3) is gratefully acknowl-
edged. The research leading to these results has received partial
funding from the European Community’s Seventh Framework
Programme under the TOPBIO project-grant agreement no.
264362. E. N. thanks Caltech SURF Program and Hugh F. and
Andy Lou Colvin.
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