Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer

Article abstract of DOI:10.1021/ja047815i

In this work, we report a peculiar positive solvatochromism in the keto emission of the acceptor-substituted 2-(2′-hydroxyphenyl)benzoxazoles (HBO), which originates from the excited-state intramolecular proton transfer (ESIPT) followed by the intramolecu

Full text of DOI:10.1021/ja047815i

Published on Web 08/19/2004
Strong Solvatochromic Fluorescence from the Intramolecular Charge-Transfer
State Created by Excited-State Intramolecular Proton Transfer
Jangwon Seo, Sehoon Kim, and Soo Young Park*
School of Materials Science and Engineering, Seoul National UniVersity, Seoul, 151-744, Korea
Received April 15, 2004; E-mail: parksy@plaza.snu.ac.kr
Significant interest has existed in the fundamental investigation
and the application of organic molecules exhibiting excited-state
intramolecular proton transfer (ESIPT) because of their four-level
photophysical scheme, spectral sensitivity to the surrounding
medium and a large Stokes’-shifted fluorescence.1 ESIPT is a
phototautomerization, that is, enol (E) to keto (K) transformation
in the excited state via an intramolecular hydrogen bond involving
the transfer of hydroxyl proton to the electronegative atom, which
occurs extremely fast in the subpicosecond time scale.2 After the
relaxation of the keto form to the ground state, the energetically
favored enol form is recovered spontaneously by reverse proton
transfer, to complete the cyclic four-level scheme. Consequently,
this process gives rise to the transient chemical change from enol
to keto tautomer, leading to the transient alternation of the electronic
properties such as electron density distribution, energies of elec-
tronic states, and dipole moments, etc. Recently, it has been
Figure 1. Absorption and fluorescence spectra of HBOCE and HBODC
excited at the wavelength of absorption maxima in (a) cyclohexane, (b)
chloroform, (c) methanol, and (d) PMMA film doped with 2 wt % of each
dye.
demonstrated that the proper structural design of the ESIPT
molecule leads to the peculiar photoexcitation-gated properties,
including electrochromic modulation,3 perturbation of electronic
state by variation of solvent polarity,4 and anion emission.5
In this communication, we report an unusually large positive
solvatochromic shift in ESIPT keto fluorescence originating from
the creation of an intramolecular charge-transfer (ICT) state after
the ESIPT process. Molecules investigated in this work are 2-(2′-
hydroxyphenyl)benzoxazole (HBO) derivatives with conjugative
electron acceptors (HBOCE and HBODC; Figure 1), which are
capable of rearranging themselves into the dipolar push-pull
structure after ESIPT.
that HBODC and HBOCE constitute a new class of second-order
nonlinear optical materials whose hyperpolarizability can be
modulated by light absorption. Another consequence of the ESIPT-
induced transient push-pull structure is a unique spectral change
depending on the solvent polarity to be discussed below.
HBOCE and HBODC were synthesized using a Knoevenagel
condensation of the aldehyde-substituted HBO with cyano-acetic
acid 2-ethyl-hexyl ester and malononitrile, respectively (Scheme 1
of Supporting Information).
It was peculiarly observed that the keto emission of HBODC
and HBOCE showed large bathochromic shift with increasing
solvent polarity in contrast to the negligible shift of enol emission,
which is, in addition, totally different from the small negative
solvatochromism of keto emission in nonsubstituted HBO.2,6 Most
probably, it is speculated that the large difference in electronic
properties between the enol and keto forms is associated with the
ESIPT-gated transient evolution of amine in keto tautomer from
imine in enol tautomer, resulting in the significantly enhanced
charge-transfer interaction with conjugative electron acceptor in the
keto tautomer. To identify the transient alternation of electronic
properties between the enol and keto forms, their first hyperpolar-
izabilities (â) were evaluated using the MOPAC 97 program with
PM3 method (Supporting Information). For HBOCE, the calculated
â value was 22.99 × 10-30 esu for enol and 28.56 × 10-30 esu for
the keto tautomer. Similar enhancement (25%) was also obtained
in the relative â values of HBODC, providing a generality of â
enhancement for this class of molecules. Enhanced â value of keto
tautomer is attributed to the transient push-pull structure reinforced
and modulated by the ESIPT process. This structure is similar to
those of 2-(4-diethylamino-benzylidene)-malononitrile and its ana-
logues, which have been intensively investigated as the second-
order nonlinear optical chromophores.7 Therefore, it is considered
Figure 1 shows representative absorption and emission spectra
of HBOCE and HBODC in the aprotic nonpolar (cyclohexane, a),
polar (chloroform, b), and protic (methanol, c) solvents as well as
in the solid state (PMMA film, d). Ground-state enol absorptions
in three solvents indicate no significant spectral dependency on the
solvent nature. Dual emissions (E and K) from the enol and keto
forms are observed in aprotic solvents (Figure 1a,b), where the keto
emission is much stronger and far separated from the absorption
band with a Stokes’ shift more than 150 nm. In contrast, only a
single enol emission with normal Stokes’ shift (<75 nm) is observed
in protic solvent (Figure 1c) because the intramolecular hydrogen-
bonding (-OH- - -N-) needed for the ESIPT process is
interrupted in this solvent. Characteristic dual emissions from
HBOCE and HBODC in different aprotic solvents are contrasted
with the virtual single keto emission from HBO,2,6 which is most
likely attributed to the increased energy barrier between the tautomer
states due to the incorporation of conjugated polar substituents.8
In the solid solution in PMMA, fluorescence emission spectra of
enol and keto forms are similar to those in nonpolar solvent except
for their slightly broader shape. Irrespective of the solvents, all the
enol fluorescences are located in the limited spectral range of blue
emission compared to the keto emission. However, it is peculiarly
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10.1021/ja047815i CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
the solvent relaxation clearly explains the large positive solvato-
chromism of keto emission as well as the insensitivity of enol
emission with increasing solvent polarity as shown in Figure 2b. It
must be noted that keto tautomers of common ESIPT materials
including the unsubstituted HBO were more influenced by solvent
stabilization for ground state than for excited state, showing a
negative solvatochromic keto emission.2,6 Additional evidence for
this consecutive photophysical process was obtained by the tem-
perature-dependent photoluminescence study (Figure 3a,b of Sup-
porting Information). HBOCE in chloroform solution shows bright
orange emission (λem ) 585 nm) at room temperature, which
changes to yellowish white emission (λem ) 540 nm) in the frozen
state at 205 K. It should be noted that the latter one is identical to
the keto emission observed in the nonpolar solvent (compare the
spectra with those in Figure 1a). Such a unique evolution of keto
emission toward higher energy at low temperature is mainly due
to the restricted solvent relaxation. Consistent with this point of
view, keto emission in the solid film state was also observed to be
almost identical to that in nonpolar solvent due to the hampered
molecular relaxation in the solid media (Figure 1d).
Figure 2. (a) Plot of absorption (b) and emission maxima in enol (2) and
keto (9) forms of HBOCE as a function of solvent polarity parameter. n
and ꢀ are the refractive index and dielectric constant of solvent, respectively.
(b) Proposed consecutive photophysical process.
noted that the spectral positions of keto emission are largely varied
from the green to the orange range with increasing polarity in aprotic
solvents. This positive solvatochromism of keto emission is just
opposite to the spectral trend in nonsubstituted HBO.2,6 In view of
the positive solvatochromic shift, keto emission of HBOCE seems
to be unusual and unique in comparison to that of HBO. For
HBODC, the same effect is observed, but the stronger electron-
accepting dicyanovinyl group than the 2-cyano-acrylate group in
HBOCE resulted in the larger red shifts in the absorption and
emission bands. Such an unusual spectral behavior of HBODC and
HBOCE suggests that their keto emission is likely to be related to
a more complicated process rather than the common ESIPT
phenomena.
In summary, the introduction of a conjugative electron acceptor
in HBO caused a strong positive solvatochromism in ESIPT keto
emission. This unique spectral change was attributed to the
consecutive ESIPT/ICT process in the acceptor-substituted HBO
compounds. Potential application of HBOCE and HBODC as the
fast hyperpolarizability modulators was demonstrated by the
semiempirical calculation.
Acknowledgment. This work was supported in part by CRM-
To get an insight into the different solvatochromic behaviors of
enol and keto forms, the spectral dependency of HBOCE on solvent
polarity was studied on the basis of the Lippert-Mataga model,9
as shown in Figure 2a. The solvents used and the corresponding
polarity parameters9 are: cyclohexane (∼0), chloroform (0.146),
diethyl ether (0.162), ethyl acetate (0.199), tetrahydrofuran (0.210),
and dichloromethane (0.218). It is seen that the spectral positions
of the absorption as well as the enol emission do not vary with
solvent polarity and the spectral change is within 10 nm over the
entire range of solvents examined. On the other hand, keto emission
shows a significant bathochromic shift in the broad range of 526-
614 nm with increasing solvent polarity. A good linear correlation
with large slope (12 000 cm-1) between the emission frequency
and solvent polarity strongly suggests that the spectral behavior of
keto emission is related to ICT characteristic.9,10 Comparing the
spectral behaviors of enol and keto emission, it is concluded that
the ICT must be developed after the ESIPT process.11
One acceptable explanation for this observation is the creation
of specific molecular structure via ESIPT, which is capable of
effective charge transfer into the electron acceptor unit. Practically
in HBOCE, imino nitrogen in the para position of the electron
acceptor is converted into an amine group by the process of ESIPT
(see the chemical structure in Figure 2a), which triggers the strong
push-pull ICT interaction. Additionally, the aromatic delocalization
energy for ICT is no longer needed as much as that in the enol
tautomer due to the breakage of heteroaromatic oxazole in the keto
tautomer. Therefore, the excited ICT state (KICT*) of HBOCE is
most probably and easily generated from ICT immediately after
KOSEF.
Supporting Information Available: Synthetic and experimental
details, temperature-dependent photoluminescence spectra of HBOCE,
and calculation details for hyperpolarizability. This material is available
free of charge via the Internet at http://pubs.acs.org.
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ESIPT process (Figure 2b). According to this model, KICT
*
stabilized by solvent relaxation dependent on its polarity is then
deactivated to K by radiative or nonradiative decay, and finally, E
is recovered from K through reverse proton transfer. Such a
consecutive photophysical process (ICT after ESIPT, depicted in
Figure 2b) is clearly distinguished from the general photophysical
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low compared to that of HBOCE.
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