Journal of the American Chemical Society
Article
reversible process.31−36 On the other hand, the diversification
in the ESIPT mechanism renders different emission properties
in terms of single, dual, and even multiple emissions that can
be fine-tuned by varying the functionality, offering the
versatility in applications such as in sensing,37−40 bioimag-
ing,41−43 organic lasing,44−46 and light-emitting di-
odes.34,44,47,48
The above OH (NH)···N (or O) type H-bond, in
theory, is formed via a static dipole−dipole interaction where
both the proton-donating/-accepting elements possess large
electronegativities (>3.0 in Pauling scale). Driven by the
changes of electronic configuration, ESIPT along the H-bond
then takes place. In comparison, the H-bonds involving thiol
SH (proton donor) or thioneS (proton acceptor) are a
result of their inherent dispersity of the electron cloud.
Therefore, whether ESIPT takes place via the sulfur H-bond is
of prime fundamental importance. In this regard, in the
cryogenic temperature, the matrix-isolated thiotropolone
seems to be capable of undergoing proton transfer via
vibrational excitation in the electronic ground state.49 Also,
in the low-temperature matrixes, Waluk and co-workers have
reported the occurrence of ESIPT in β-thioxoketones via
vibration analyses.9,50−52 One report proposed the occurrence
of ESIPT in a four-membered ring H-bonding system with a
thiol group as a proton donor; however, the lack of direct
evidence and the calculated rather high barrier make the
conclusion elusive.53 We recently made attempts by synthesiz-
ing the thione (S)···HO H-bonded molecule 7-hydroxy-
2,2-dimethyl-2,3-dihydro-1H-indene-1-thione (DM-7HIT)54
and examining its possibility of spawning ESIPT. The results,
on the one hand, were disappointing when we found no
ESIPT. We tentatively rationalized the results by the fact that
the lowest lying excited singlet state is dominated by the nπ*
character, i.e., an S1(nπ*) state, where n is ascribed to the
higher lying lone-pair electrons of thione (S). This results in
the drastic decrease of the thione electron-accepting strength
and hence the prohibition of ESIPT after photoexcitation. On
the other hand, the decrease of thione basicity ruptures the (
S)···HO intramolecular H-bond such that the H-bond on/
off reaction takes place with respect to light on/off,
demonstrating a prototype of photoinduced ultrafast molecular
switch.54
Therefore, up to this stage, on the basis of the absorption
and emission spectroscopies, unambiguous experimental
evidence clearly addressing the ESIPT of sulfur-containing
H-bonded molecules at room temperature, which is practical in
applications, is still pending. Amid the quest of ESIPT
associated with sulfur H-bond, in light of the aforementioned
higher lying energy of the thione lone-pair electrons, we then
strategically switched the sulfur H-bond from the OH
(donor)/thione (acceptor) pair to the configuration of SH
(thiol, donor)/CO (acceptor) pair. In comparison to thione,
the lower lying lone-pair electrons of thiol, in theory, should
make the S2(nπ*) state appreciably higher in energy than that
of the S1(ππ*) state. Accordingly, in this study, a series of
sulfur intramolecular H-bonded molecules, 3-thiolflavone
(3TF) and its derivatives 3FTF and 3NTF, were strategically
designed and synthesized (see Figure 1). Also, the methylated
(SCH3) derivative for 3NTF, namely, 3MeNTF (see Figure
1), was synthesized for comparison. 3MeNTF lacks a labile
proton to ensure no occurrence of ESIPT. Note that the
replacement of thiol by the hydroxyl group of 3TF forms 3-
hydroxyflavone (3HF) possessing the OH···OC intra-
Figure 1. Molecular structures of 3HF and its derivatives of 3TF and
3MeNTF.
molecular H-bond. 3-HF and its derivatives undergo
prominent OH-type ESIPT, which have been widely
studied.13,55−58
We then report herein, for the first time, the observation of
ESIPT in 3NTF incorporating thiol proton, forming a proton-
transferred thione tautomer, which gives rise to a prominent
red emission maximized at 710 nm in solution and solid state
at room temperature. We also infer that ESIPT may not be
uncommon in the thiol H-bonded polyaromatic molecules,
opening up a new chapter of excited-state intramolecular thiol
proton transfer reaction. Details of the results and discussions
are elaborated below.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. One may be skeptical
about why reports on the photophysics of thiol H-bonded
molecules are so scarce. Chemically, one major reason lies in
its stability. Thiol contained molecules, similar to the natural
amino acid cysteine, are prone to undergo oxidation to form a
disulfide SS bond and thus are difficult to obtain the pure
thiol compounds and perform photophysical studies. It
requires special caution to exclude air during the synthesis
and purification to prevent the occurrence of oxidation.
Syntheses and characterization of 3TF and its derivatives are
elaborated below with pertinent characterization data listed in
Synthesis and Characterization of 3TF and its
Derivatives. Two different synthetic routes (shown in
Scheme 1) were attempted to synthesize 3TF. The first
route, in part, was according to the literature,59 where flavone
(2) molecular framework was constructed first by reacting 2′-
hydroxyacetophenone (1) with either benzoyl chloride60 or
benzaldehyde61 and then followed by LDA-mediated lithiation
and nucleophilic addition with sulfur powder to yield 3TF
along with the 3TF dimer with an SS linkage, namely, SSF
(see Figure S1) as a minor product. It was suspected that the
oxidation of 3TF to SSF occurred during the workup
procedure as well as during the course of purification. Thus,
several freeze−pump−thaw cycles were taken to degas all the
solvents used in step b (see Scheme 1) to prevent the
oxidation. However, these efforts turned out to be in vain.
Therefore, the alternative synthetic route was sought, where
reacting 1 with benzaldehyde gave conjugated ketone (3) via
Claisen−Schmidt condensation, then followed by intra-
molecular Michael addition to yield chromanone (4). The
bromination of 4 led to diastereomers (5), which were used
directly for the subsequent nucleophilic substitution with
potassium thioacetate in the presence of 18-crown-6 ether as a
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX