Sulfur-Based Intramolecular Hydrogen-Bond: Excited-State Hydrogen-Bond On/Off Switch with Dual Room-Temperature Phosphorescence

Article abstract of DOI:10.1021/jacs.9b02765

We report O-H-S hydrogen-bond (H-bond) formation and its excited-state intramolecular H-bond on/off reaction unveiled by room-temperature phosphorescence (RTP). In this seminal work, this phenomenon is demonstrated with 7-hydroxy-2,2-dimethyl-2,3-dihydro-1H-indene-1-thione (DM-7HIT), which possesses a strong polar (hydroxy)-dispersive (thione) type H-bond. Upon excitation, DM-7HIT exhibits anomalous dual RTP with maxima at 550 and 685 nm. This study found that the lowest lying excited state (S₁) of DM-7HIT is a sulfur nonbonding (n) to π? transition, which undergoes O-H bond flipping from S₁(nπ*) to the non-H-bonded S′₁(nπ*) state, followed by intersystem crossing and internal conversion to populate the T′₁(nπ*) state. Fast H-bond on/off switching then takes place between T′₁(nπ*) and T₁(nπ*), forming a pre-equilibrium that affords both the T′₁(nπ*, 685 nm) and T₁(nπ*, 550 nm) RTP. The generality of the sulfur H-bond on/off switching mechanism, dubbed a molecule wiper, was rigorously evaluated with a variety of other H-bonded thiones, and these results open a new chapter in the chemistry of hydrogen bonds.

Full text of DOI:10.1021/jacs.9b02765

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Article
Sulfur-Based Intramolecular Hydrogen-Bond: Excited-State Hydrogen-
Bond On/Off Switch with Dual Room-Temperature Phosphorescence
Zong-Ying Liu, Jiun-Wei Hu, Chun-Hao Huang, Teng-Hsing Huang, Deng-
Gao Chen, Ssu-Yu Ho, Kew-Yu Chen, Elise Y. Li, and Pi-Tai Chou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02765 • Publication Date (Web): 05 Jun 2019
Downloaded from http://pubs.acs.org on June 5, 2019
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Sulfur-Based Intramolecular Hydrogen-Bond: Excited-State Hydrogen-Bond On/Off Switch with
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†
,¶
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Zong-Ying Liu, Jiun-Wei Hu, Chun-Hao Huang, Teng-Hsing Huang, Deng-Gao Chen, Ssu-Yu
†
‡*
§*
†*
Ho, Kew-Yu Chen, Elise Y. Li, Pi-Tai Chou
†Department of Chemistry, National Taiwan University, Taipei, 10617, Taiwan, R.O.C.
‡Department of Chemical Engineering, Feng Chia University, Taichung, 40724, Taiwan, R.O.C.
§Department of Chemistry, National Taiwan Normal University, Taipei, 11677, Taiwan, R.O.C.
¶These three authors contributed equally
KEYWORDS: Thiones, sulfur hydrogen bonds, hydrogen bond switch, room-temperature phosphorescence
ABSTRACT: We report O-H----S hydrogen-bond (H-bond) formation and its excited-state intramolecular H-bond on/off reaction
unveiled by room-temperature phosphorescence (RTP). In this seminal work, this phenomenon is demonstrated with 7-hydroxy-2,2-
dimethyl-2,3-dihydro-1H-indene-1-thione (DM-7HIT), which possesses a strong polar (hydroxy)-dispersive (thione) type H-bond.
Upon excitation, DM-7HIT exhibits anomalous dual RTP with maxima at 550 and 685 nm. This study found that the lowest lying
excited state (S
non-H-bonded S’
on/off switching then takes place between T’
and T (n*, 550 nm) RTP. The generality of the sulfur H-bond on/off switching mechanism, dubbed a molecule wiper, was rigorously
1
) of DM-7HIT is a sulfur non-bonding (n) to * transition, which undergoes O-H bond flipping from S
(n*) state, followed by intersystem crossing and internal conversion to populate the T’ (n*) state. Fast H-bond
(n*) and T (n*), forming a pre-equilibrium that affords both the T’ (n*, 685 nm)
1
(n*) to the
1
1
1
1
1
1
evaluated with a variety of other H-bonded thiones, and these results open a new chapter in the chemistry of hydrogen bonds.
particular, sulfur is not only a potential H-bond acceptor, but
INTRODUCTION
the S–H group is also a very good H-bond donor and can form
a variety of H-bonds. For example, the S–H···π intermolecular
Hydrogen bonds (H-bonds) typically involve a combination
of an O-H or an N–H proton donor and an N or an O proton
acceptor, and the corresponding energies can be in a range of
several to ten kcal/mol, depending on the H-bond geometry and
distance. There have been interesting but few reports on H-
bonds that involve a sulfur atom as either the proton donor (S-
H) or acceptor (S). The concept of “sulfur participating in an H-
bond” appeared in early reports despite the relatively low
H-bond between H
stronger than O–H···π, O–H···π, and C–H···π H-bonds.
Apart from the intermolecular H-bond, the intramolecular
2
S and indole/benzene was found to be
1
4-16
1
7-20
sulfur H-bonds have also attracted considerable attention.
In
biological systems, the structure as well as the nature of proteins
are often determined by intramolecular sulfur H-bonds. Despite
the ubiquity of intramolecular sulfur H-bonds, explorations of
their photophysical properties, which help understand the
1
-3
electronegativity (2.58) for sulfur on the Pauling scale.
Recent quantum chemical calculations in conjunction with
high-resolution spectroscopic evidence has revealed that sulfur-
2
1
fundamentals of protein folding, conformation and function,
remain rather scarce.
2
2-25
4
-
containing H-bond can be as strong as conventional H-bonds.
One of the most intriguing photophysical properties
associated with H-bonds can be attributed to the proton-transfer
phenomenon, particularly excited-state intramolecular proton
7
In biological systems, methionine-containing dipeptides have
been reported to form amide-N–H···S intermolecular H-bonds
that are stronger than amide-N–H···O=C H-bonds.
Additionally, the strength of the intermolecular H-bonds in
8
transfer (ESIPT), which has been widely studied for more than
26-29
six decades.
In this context, through the existing
thiobase pairs are reported to be of the same magnitude with the
canonical nucleobase pairs. As a result, the intermolecular
sulfur H-bonds play an important role in artificial DNA or RNA
intramolecular H-bonds, ESIPT involves the transfer of a
proton from O-H or N–H proton donors to =O or N proton
acceptors, forming an excited-state proton-transfer tautomer
that does not exist in the ground state. Accordingly, the
anomalously large Stokes shift of the emission of the tautomer
9
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researches and have been applied to RNAi resistant genes
1
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and protein engineering. From the viewpoint of applications,
Aida and co-workers recently constructed flexible
intermolecular N-H···S H-bonds with thiourea units in a
polymer backbone and successfully demonstrated their unique
3
0-33
allows a variety of applications such as in sensing,
bio-
3
4-35
36-38
37, 39-41
imaging,
organic lasing
and light-emitting diodes.
Such an O-H (N-H)----N (or O) ESIPT occurs in a static dipole-
dipole-type H-bond where both the proton-donating and the
1
3
ability to heal ruptured polymers at ambient temperature. In
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Scheme 1. Overview of the synthetic routes to the title thione compounds. Inset: Displacement ellipsoid representation of
HIT with the atoms labelled. The ellipsoids are drawn at the 50% probability level, and the H atoms are drawn as spheres
of arbitrary radii. The blue dashed line denotes the intramolecular O-H---S hydrogen bond.
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proton-accepting atoms have large electronegativities (> 3.5) on
the Pauling scale. In contrast, H-bonds involving S-H (donor)
or S (acceptor) are a result of their inherent dispersivity. If
ESIPT takes place via the sulfur H-bond, fundamentally, the
properties of its potential energy surface and hence its dynamics
should be unlike those of ESIPT involving conventional
Pauling-type H-bonds. On the other hand, if ESIPT is
prohibited, determining the factors controlling the fate of the
excited-state relaxation pathway of molecules containing
intramolecular sulfur H-bonds would be of fundamental
importance. To the best of our knowledge, no theoretical or
experimental approaches have addressed proton transfer and/or
other distinct photophysical properties of sulfur-containing
intramolecular H-bonds. Using 7-hydroxy-2,2-dimethyl-2,3-
dihydro-1H-indene-1-thione (DM-7HIT) as the model system,
herein, we report a study on intramolecular O-H----S H-bond
formation and the remarkable excited-state behaviour of these
bonds, i.e., the photoinduced intramolecular H-bond on/off
switching reaction. The mechanism was confirmed and its
generality was rigorously tested using a series of new
compounds with strategically placed H-bonded and non-H-
bonded thiones (see Scheme 1).
but its photophysical properties are nearly identical to those of
DM-7HIT (vide infra). Unfortunately, attempts towards the O-
methylation of 7HIT to form the non-H-bonded model system
(7MIT) as a reference failed due to decomposition of the
thiocarbonyl group. On the other hand, direct thionation of 7-
methoxy-2,3-dihydro-1H-inden-1-one (7MI, see Figure S3 for
X-ray structure) with Lawesson's reagent underwent an
44
unexpected cyclization reaction at the C2 position (see
Scheme 1 for the numbering of the atoms). This cyclization
reaction is inhibited by dimethyl substitution at the C2 position,
as seen in DM-7HI and DM-7MI (see Figure S4 for X-ray
structure); as a result, DM-7MIT, the non-H-bonded reference
for DM-7HIT, can be synthesized. Therefore, DM-7HIT was
used as the prototype throughout this study. Attempts to
synthesize DMADM-7HIT from DM-7HI followed by a
Mannich reaction and treatment with Lawesson’s reagent failed
due to the reactive Lawesson’s reagent attacking the amino
group. Instead, DMADM-7HIT was synthesized from DM-
7
HI by treatment with Lawesson’s reagent prior to the Mannich
reaction (Scheme 1). As shown in Figure S14, S18, S20, and
1
S22, the H NMR spectra of DM-7HIT and the other hydroxyl-
thiones reported to have sulfur-containing intramolecular H-
bonds (DTDM-7HIT, DMADM-7HIT and 7HIT) all show
dramatically downfield shifted phenolic –OH protons at >9.0
ppm, suggesting a very electron-deficient proton. Interestingly,
despite 7HIT being a solid (melting point 75-76 °C), DM-7HIT
has a melting point of approximately 10 °C and thus is a liquid
at room temperature. Therefore, a single crystal of 7HIT was
obtained and used as a model for structural elucidation. Scheme
RESULTS AND DISCUSSION
Synthesis and characterization
All the thiones were
synthesized via sulfurization of their corresponding carbonyl
4
2-43
compounds using Lawesson's reagent (Scheme 1).
The
details of synthetic route and product characterization data are
presented in the Supporting Information (SI). Notably, 7HIT,
depicted in Scheme 1, is structurally simpler than DM-7HIT,
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shows the labelled ellipsoid representation of 7HIT. The O–
stabilizes the HOMO (nonbonding orbital). Furthermore, the
lower HOMO energy level (-6.19 eV) of DM-7HIT than that
of DM-7MIT (-5.84 eV), calculated by CV measurement, also
supports the above statement.
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HS distance, which is artificially drawn as a blue dashed
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Figure 1. The mid-IR spectra of DM-7HI (red) and DM-7HIT
blue) at room temperature.
(
line, was calculated to be 2.2 Å with an ∠O–HS angle of
52°, making it nearly coplanar with the main chromophore.
Figure 2. a, Absorption (grey filled area) and emission spectra
(blue filled area and solid red line) of DM-7HI in cyclohexane at
room temperature. The green dashed line represents the emission
spectrum of DM-7DIT. b, Absorption (grey filled area) and
emission spectra (blue filled area and solid red line) of DM-7MIT
in cyclohexane at room temperature. Insets: Expansions of the
absorption spectra over specified wavelength regions. ex: 380 and
1
The short O-HS distances, along with the coplanar
conformation, implies the high possibility of formation of
intramolecular O-HS H-bond.
To check the above conjecture, we then carried out the
Fourier transform infrared spectrum measurement for DM-
3
50 nm for DM-7HIT and DM-7MIT, respectively. Note that the
7
HIT and DM-7HI, the latter acts as a comparison (Figure 1).
labelling “aerated” refers to aeration in atmosphere.
The mid-IR spectrum of DM-7HI shows a broad absorption
band peaked at 3342 cm , which can be assigned to the H-
bonded O-H stretching. The absorption band at 1676 cm for
DM-7HI can be ascribed as the C=O stretching. On the other
hand, the mid-IR spectra of DM-7HIT drastically changes upon
substitution of sulfur for carbonyl oxygen. The C=O stretching
observed in DM-7HI vanishes; instead, a sharp absorption band
can be resolved at 1090 cm , which can be assigned to C=S
stretching signal. Another difference is the disappearance of
the broad O-H stretching signal at 3342 cm ; instead, a broad
absorption band can be observed at about 3059 cm , which
overlaps with C-H asymmetric stretching signal (2962 cm ). In
addition, the deuteration of hydroxyl group was conducted,
forming DM-7DIT, for comparison and the result is shown in
Figure S26. Clearly, the broad band at 3059 cm (DM-7HIT)
shifts to 2285 cm (DM-7DIT), reaffirming that the broad 3059
cm signal is attributed to the O-H stretching motion.
Remarkably, the O-H stretching frequency for DM-7HIT is
lower than that for DM-7HI, which proves the formation of a
stronger intramolecular H-bond than DM-7HI.
-1
Spectroscopy and Dynamics The absorption and emission
spectra of DM-7HIT, DM-7DIT and DM-7MIT in
cyclohexane are shown in Figure 2 and the corresponding
photophysical data are summarized in Table 1. DM-7HIT
showed two lower lying absorption bands with maxima at 310
-1
4
-1
-1
3
-1
-1
nm (1.27 x 10 M cm ) and 380 nm (3.52 x 10 M cm ). Upon
excitation under a standard air atmosphere, weak dual emission
bands with maxima at 550 nm and 685 nm were observed. The
excitation spectra acquired at both emission wavelengths are
identical, and they also resemble the absorption spectra (Figure
S28), indicating that both emission bands originate from the
same ground-state species. Remarkably, the quantum yields,
em values, of both emission bands drastically increases from
0.02% to ~1.0% (more than 50-fold) upon degassing,
confirming their natural triplet character that is quenched by O2.
Importantly, both the 550 nm and 685 nm emission bands
undergo identical population decays in 1.1 s in degassed
cyclohexane (see Figure 3a and Table 2). Note that the rather
low quantum yields for both phosphorescence bands can be
attributed to the operation of energy gap law and the spin-
-1
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-1
-1
-1
-1
-1
To provide further support for H-bond formation, the cyclic
voltammograms (CV) for DM-7HIT and DM-7MIT are also
carried out (Figure S27). Both DM-7HIT and DM-7MIT
undergo irreversible one-electron oxidation and reduction, the
oxidative and reductive potentials are recorded at 1.67 and -1.81
5
0
1 0
forbidden nature of the T to S transition. The former causes
a significant decrease in quantum yield with decreasing
emission energy gap due to the increase of the vibrational
overlap between emitting and ground states, enhancing the
radiationless deactivation. The latter causes an intrinsic long
radiative lifetime. As a result, other non-radiative pathways,
e.g. vibrational relaxation or intersystem crossing, become
competitive with the phosphorescence irradiation and
suppresses the phosphorescence quantum efficiency.
V (versus Ag/AgNO
3
electrode) for DM-7HIT, and 1.32 and -
electrode) for DM-7MIT,
1
.98 (versus Ag/AgNO
V
3
respectively. Remarkably, both the first oxidative and the first
reductive potentials of DM-7MIT are shifted toward more
negative values compared with DM-7HIT, which indicate that
the removal of electrons from DM-7HIT is more arduous than
that of DM-7MIT. According to the previous reports, the
HOMO of these thione compounds are the nonbonding orbital
of the sulfur atom (vide infra).46-49 Hence, the results reaffirm
intramolecular H-bonding formation in DM-7HIT that
To elucidate the emission and hence the photophysical
properties, the -OH group of DM-7HIT was methylated,
forming DM-7MIT (Scheme 1), which serves as a reference
compound as it lacks the intramolecular H-bond. As shown in
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Table 1. The photophysical data of the title compounds in cyclohexane at 298 K.
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Absorption
Emission
Q.Y.a
in %, aer./deg.)b
(
cal (nm)
exp (nm)
cal (nm)
exp (nm)
DM-7HIT
DM-7MIT
469,371
480, 378
607, 764
369, 780
550, 685
380, 690
0.02/1.0
1
.0/1.0 for 380
580, 351
579, 353
0
.02/0.43 for 690
0.04/0.24
DTDM-7HIT
464, 399
468, 434
481, 403
478,390
633, 789
777
586, 703
681
DMADM-7HIT
0.18/1.2
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a
Q.Y. denotes quantum yield. aer. and deg. stand for aeration in atmosphere and degas, respectively.
fluorescence,46-49 which is mainly due to the rather small
S
0
→
ionization energy of the thione non-bonding orbital (n). As a
result, the lowest lying excited state is attributed to an n → *
transition, which is optically forbidden because of the
orthogonality of the n and * orbitals. For DM-7MIT, this is
evidenced by its very weak S
0 1
→ S (n*) absorption maxima at
~580 nm with an absorption extinction coefficient as low as 580
-1
-1
~
10 M cm (see inset of Figure 2b). Conversely, the higher
lying S state mainly possesses * character at ~353 nm (350
2
3
-1
-1
~4×10 M cm ). The energy gap between S
2
(*) and S (n*)
-1
1
was calculated to be as large as ~11,057 cm . This, together
with their distinct orbital configurations, leads to anomalously
slow S
(*) → S
Table 2. The time-resolved data of DM-7HIT and DM-
MIT in cyclohexane at 298 K.
2
(*) → S (n*) internal conversion, allowing the
1
S
2
0
7

exp/ns

exp/ps (pre-factor)
recorded by up-
conversion

mon/n
m
recorded by
TCSPC
0
.3 (0.45), 3.4
0.55)
5
80
1100
(
DM-7HIT
DM-7MIT
a
0
1
.02 (0.91),
100 (0.09)
720
380
0.3 (-0.5), 3.4 (0.5)
Figure 3. a, Microsecond time-resolved phosphorescence decays
recorded for DM-7HIT (in degassed cyclohexane) monitored at
0.085
5
80 nm (black line) and 720 nm (grey line). The Instrument
6
90
1460
response time (IRF) is given by the red line. b, Pico-nanosecond
time-resolved decays recorded for DM-7MIT monitored at 380 nm
(black line). The IRF is given by the red line. Inset of b: The
microsecond time-resolved emission decays recorded for DM-
a
lower than the instrument limit
radiative transition to be competitive. It is thus reasonable to
assign the 380 nm band with a lifetime of 85 ps to the S (*)
fluorescence. This S → S emission is deemed an
2
7
MIT (in degassed cyclohexane) monitored at 700 nm.
→
S
0
2
0
Figure 2b, despite its absorption spectrum being similar to that
of DM-7HIT, the emission spectrum of DM-7MIT is
substantially different, and it shows dual emission bands with
maxima at 380 nm and 690 nm. Upon degassing, also shown in
Figure 2b, the quantum yield of the 690 nm emission band
increases by a factor of approximately 28, whereas the intensity
of the 380 nm emission band remains unchanged, implying that
the 380 nm and 690 nm emission bands can be attributed to
fluorescence and phosphorescence, respectively. This
assignment is also supported by the time-resolved
measurement, which showed that the lifetimes of the 380 nm
and of 690 nm emission bands were 85 ps and as long as 1.5 s,
respectively, in degassed cyclohexane (see Figure 3b and
Table 2).
exception to Kasha’s rule, suggesting that the emission of the
polyatomic molecules in the condensed phase is mostly from
the lowest lying excited state. Similar assignments have been
reported for a congener of DM-7MIT, 2,2-dimethyl-2,3-
dihydro-1H-indene-1-thione (DMIT, Figure S29), which
exhibited dual emission bands with maxima at 401 nm and 630
5
1,
nm that were assigned to S
2
(*) → S
0
fluorescence and
46
T
1
(n*) → S phosphorescence, respectively. Due to the lack
0
of H-bond, the origin of the emission of DM-7MIT is expected
to be the same as that of DMIT, which allows the 690 nm band
of DM-7MIT to be reasonably assigned to the T
phosphorescence.
1
(n*) → S
0
In comparison, one of the obvious differences between DM-
HIT and DM-7MIT lies in the lack of observable S (*) →
fluorescence for DM-7HIT. Shown in the inset of Figure 2a,
7
S
2
The photophysics of thione-containing compounds have
0
received considerable attention owing to the observation of S
2
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the weak S
and has an onset at 530 nm, which is substantially blue shifted
relative to S (n*) of DM-7MIT (onset at ~650 nm). The
results confirm the formation of the sulfur H-bond in DM-
7HIT, and the thione lone-pair electrons are stabilized by the
H-bond. Because the intramolecular H-bond has a slight
influence on the * orbital, a significant increase in the n → *
energy gap for DM-7HIT is observed (cf. DM-7MIT). Based
1
(n*) absorption of DM-7HIT is located at 480 nm
level for DM-7HIT in cyclohexane estimates that the energy of
the tautomer S (*) state is higher than that of the normal
species (S (n*)) by as much as 0.74 eV (~ 17 kcal/mol,
Scheme S1 and Table S6). In addition, the lowest triplet state
of the proton-transfer tautomer, which was calculated to be T
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
1
1
1
(*), is higher in energy than the lowest normal (T (n*)) state
1
by at least 2.3 kcal/mol (see Figure S31 and Table S 7-9 for
details of the state assignments). Although the calculated values
are subject to uncertainty and the data should be varied by using
different functionals and basis sets (Table S 7-9), we found that
the trend in the thermally disfavoured ESIPT holds in both the
singlet and triplet manifolds.
on the S
0
→ S
2
(*) (~380 nm) and S
0
→ S
-S gap was calculated
to be ~5,482 cm , which is half the energy gap (~11,057 cm )
of DM-7MIT. The small energy gap leads to an ultrafast S
internal conversion, and the high rate of this internal
conversion explains the lack of S → S emission in DM-7HIT.
1
(n*) (~480 nm)
absorption maxima for DM-7HIT, the S
2
1
-1
-1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
→
S
1
2
0
Another key difference lies in the observation of dual RT
phosphorescence bands in DM-7HIT instead of the single
phosphorescence band observed in DM-7MIT. We have made
many efforts to explain this difference. One possibility is to
assign the 550 and 685 nm phosphorescence bands to the T
1
and
T
2
phosphorescence emissions, respectively. However,
fundamentally, the ~10 kcal/mol separation in the energy gap
between the 550 and 685 nm emission bands should lead to an
ultrafast T
2 1
→ T internal conversion via a vibrational
relaxation, i.e., an internal conversion. Moreover, this
mechanism cannot explain the identical phosphorescence decay
time constants (1.1 s) for the 550 and 685 nm emission bands
and can thus be discarded. Another possible mechanism is the
assignment of the 550 and 685 nm emission bands to
Figure 4. The phosphorescence spectra of DM-7HIT in a
polyethylene film (black) at 298K and in the solid state (red) at
fluorescence S
this case, the T
be thermally accessible, so the S
1
(n*) and phosphorescence (T
→ S (n*) reverse intersystem crossing has to
(n*) 550 nm emission
1
(n* or *)). In
2
3
73K and DM-7MIT in solid state (blue) at 298K. ex=405 nm and
60 nm for DM-7HIT and DM-7MIT, respectively.
1
1
1
essentially exhibits a thermally activated delayed fluorescence
Moreover, various experimental results cannot be explained
to account for its population decay time constant (1.1 s) being
based on ESIPT. First, in the pure solid at 0 °C (melting point
10 °C for DM-7HIT), DM-7HIT showed only one
phosphorescence band with a maximum at ~590 nm (obs ~4.2
s, Figure 4). Similarly, rigid polyethylene (PE) doped with
DM-7HIT revealed only one phosphorescence band at ~595
nm (obs ~ 1.3 s). Because the occurrence of ESIPT requires
only a slight shift in the proton along the existing H-bond, the
absence of the purported proton-transfer tautomer emission,
i.e., the 685 nm band, cannot be rationalized by the inhibition
of ESIPT in the solid state or in a solid matrix unless DM-7HIT
lacks the intramolecular H-bond in solid environment.
However, the intramolecular H-bond formation in the solid
crystal is supported by the X-ray structure of 7HIT.
Additionally, the emission spectrum of the O-H deuterated
DM-7DIT in cyclohexane is identical with that of the non-
deuterated DM-7HIT (the green dashed line in Figure 2),
affirming that the lack of O-D deuterium isotope effect in its
photophycial behavior. This observation, in an indirect manner,
also devaluates the possibility of ESIPT. Therefore, both the
theoretical and experimental approaches disapprove the ESIPT
mechanism.
identical to that of the 685 nm T
1
phosphorescence. However,
1
energy gap being as large as 10 kcal/mol makes the
the T -S
1
reverse intersystem crossing strictly inaccessible at room
temperature. This proposed mechanism can therefore be
eliminated as well. We also considered the possibility that the
5
50 nm band is delayed fluorescence from T-T annihilation. If
so, the ratio of the intensities of the 550 nm and 685 nm
emission bands should be dependent on the excitation intensity.
Conversely, the results show excitation-intensity independent
ratiometric emissions (Figure S30). In addition, if the 550 nm
delayed fluorescence originated from T-T annihilation,
according to the kinetic expression, the relaxation decay time
5
2
constant should be half of that of the 685 nm emission decay.
The equal decay time constants of the 550 nm and 680 nm
emission bands also exclude the T-T annihilation mechanism.
The existence of an intramolecular H-bond in DM-7HIT is
also reminiscent of the possibility of incorporating ESIPT via –
OH-----S=C hydrogen bond, forming a C=O------H-S-C proton
transfer tautomer (Scheme S1). Accordingly, the 550 nm and
6
80 nm bands of DM-7HIT can be assigned to the normal and
proton-transfer tautomer phosphorescence, respectively,
originating from ESIPT. In this case, we also have to assume
that both the normal and proton-transfer tautomeric triplet states
have similar energies such that a fast equilibrium takes place
between the normal and tautomer triplet states to rationalize the
identical decay rate constants. However, in-depth theoretical
and experimental approaches raise serious concerns about this
proposal. Theoretically, the time-dependent density functional
theory (TD-DFT) approach at the B3LYP/6-311++G(3df,3pd)
The proposed H-bond on/off mechanism
We also
estimated the O-H----S hydrogen bond energy by calculating
the energy difference between DM-7HIT with a H-bond and
DM-7HIT without a H-bond. The latter was done by initially
locating the H atom at the site opposite the H-bond and
optimizing the system to find a local minimum. As a result, an
intramolecular H-bond stabilizes DM-7HIT by 9.8 kcal/mol
(B3LYP/6-311++G(3df,3pd), Figure 5a and Scheme S1), and
this should be the dominant species in the ground state, which
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1
would be consistent with the H NMR spectra (vide supra).
However, upon excitation of the lowest lying n → * transition,
the electron density in the thione non-bonding (n) orbital
decreases, shown in Figure 5b, which substantially reduces the
proton-accepting strength of the sulfur and hence weakens the
H-bond. Therefore, as for the phosphorescence of DM-7HIT,
the corresponding emitting state may no longer only be the
which are in good agreement with the observed dual
phosphorescence peaks (Table S10). When the angle of O-H--
--S was gradually varied from T (n*) to a local minimum
T’ (n*), we estimated the barrier of the T (n*) → T’ (n*) H-
bond on/off flipping to be as small as ~1.0 kcal/mol (Figure
S32).
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
1
1
1
Based on these analyses, we thus propose the most acceptable
mechanism incorporating a fast pre-equilibrium between the
intramolecular H-bonded T
supported by the computational result indicating that the H-
bonded T (n*) state of DM-7HIT is only 1.7 kcal/mol higher
in energy than that of the non-H-bonded T’ (n*) (herein, prime
indicates the non-H-bonded species) states
1
(n*) state. This mechanism is
T
1
(n*) and T’ (n*) states, which is essentially an
1
1
intramolecular H-bond on/off switching process, exhibiting
solution-phase RTPs of 550 nm and 685 nm with identical
decay time constants (vide supra). The above H-bond flipping
mechanism is supported by the fact that the 685 nm
phosphorescence of DM-7HIT with the vibronic spectral
feature is nearly identical to that of the phosphorescence of
DM-7MIT, which lacks H-bond (Figure 2a, b). Additionally,
this mechanism rationalizes the observation of a single
phosphorescence band in the solid crystal and PE film, which,
according to the mechanism, can be attributed to the H-bonded
1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
T
1
(n*) emission because of the inhibition of –OH flipping to
the T’ (n*) state by the solid matrix. We also carried out the
1
steady-state spectra of DM-7HIT in different solvents and the
results are shown in Figure S33. With increasing solvent
polarity, the non-H-bonded emission becomes predominant.
This phenomenon, driven by the stabilization of hydroxyl group
of non-H-bonded species by polar solvent molecules, also
supports the proposed mechanism of H-bond flipping.
Remarkably, the S
due to the external H-bonds with methanol molecules, which
0
makes the non-H-bonded species (S’ in Figure 5a) partially
exist in ground state.
2
emission can also be observed in methanol
Figure 5. a, The schematic energy diagram of the mechanism of
H-bonding hopping for DM-7HIT. The energy gaps between the
corresponding states of the H-bonded and the non-H-bonded forms
are calculated values and the emission wavelengths were taken
from the observed maxima. An animation of the H-bond flipping is
provided as a supplementary video. b, The calculated HOMO (cyan
color) and LUMO (orange color) of H-bonded and non-H-bonded
DM-7HIT.
Figure 6. Comparison of the emission spectra of DM-7HIT
(orange filled area), DTDM-7HIT (red) and DMADM-7HIT
(blue) in cyclohexane at room temperature. ex: 380 nm.
The strategic design and synthesis of DTDM-7HIT (see
Figure S5 for X-ray structure) and DMADM-7HIT (Scheme
1) further supported the proposed mechanism. DTDM-7HIT is
the derivative of DM-7HIT with a tert-butyl substituent at the
ortho (C6) and para (C4) position. Chemically, we expect that
the population of non-H-bonded species in the triplet state will
be suppressed (cf. the H-bonded species) due to steric hindrance
from the bulky tert-butyl group at C6, which decreases the
degrees of freedom of the non-H-bonded species. Supportively,
the DTDM-7HIT in cyclohexane (Figure 6 and Figure S34)
in cyclohexane. Considering the uncertainty of the current
computational approach, it is reasonable to expect a thermal
equilibrium between T
1
(n*) and T’ (n*) in a nonpolar
1
solution. Additionally, the difference in the emission peaks (550
and 685 nm) was calculated to be ~10 kcal/mol, which is
consistent with the calculated difference between the H-bonded
S
0
and non-H-bonded S’
Moreover, based on total energy differences, the calculations
showed that the vertical transitions of T (n*)→ S and
T’ (n*) → S’ occur at 607 nm and 764 nm, respectively,
0
species (9.8 kcal/mol, vide supra).
showed a dramatic decrease in the 685 nm T’ (n*)
phosphorescence relative to that of DM-7HIT (Figure 2a).
1
1
0
In yet another approach, for DMADM-7HIT, which is
synthesized by inserting a dimethylaminomethyl moiety ortho
1
0
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an almost negligible small rotation barrier, have nearly the same
to the hydroxyl group of DM-7HIT via a Mannich reaction
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(Scheme 1), formation of an O-H-----N H-bond is competitive
with O-H-----S H-bond formation (Figure 7). Nevertheless, the
computational studies indicate that the DMADM-7HIT
conformer having the O-H-----S H-bond is more stable than the
O-H-----N H-bonded conformer by 2.65 kcal/mol and is the
dominant species in the ground state. This is also evidenced
experimentally by DMADM-7HIT and DM-7HIT having the
energy for DTDM-7HIT. The theoretically predicted near 1:1
thermodynamic population for the two triplet excited states
corresponds to the much more evenly distributed emission
intensities of the two phosphorescent bands for DTDM-7HIT,
shown in Figure 6. These results provide indisputable evidence
to support the H-bond flipping mechanism established in DM-
7HIT.
same S
Figure 2a). However, despite O-H-----S H-bonded DMADM-
HIT being the dominant species in the ground state, upon
0 1
→ S (n*) absorption at 530 nm (Figure S34 and
Based on the above analyses, we then measured the initial
relaxation kinetics to elucidate the corresponding H-bond
flipping dynamics of DM-7HIT. To avoid any additional
relaxation processes, the excitation wavelength was tuned to
7
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
excitation, only the 681 nm phosphorescence peak belonging to
the non-O-H-----S H-bonded form can be observed (Figure 6).
The absence of the 550 nm phosphorescence peak
corresponding to
4
00 nm, which is near the onsetof the S
0 2
→ S (*)
absorption adjacent to the S → S (n*) transition (inset of
0
1
Figure 2a.). First, we used time-correlated single-photon
counting (TCSPC) measurements with a femtosecond laser
source (180 fs, 1 kHz repetition rate) and a multichannel plate
as the detector. After convolution, the system response was
estimated to be ~25 ps. However, as shown in Figure S37, the
results indicated that both the 550 and 685 nm emission bands
consist of an ultrafast early decay component that is beyond
the system response (< 25 ps). To gain further insight into the
dynamics of the H-bond flipping, we then applied a
femtosecond up-conversion technique, and the results are
shown in Figure 8 with the pertinent time-resolved data
presented in Table 2.
When monitoring at an emission wavelength of 580 nm, two
decay components were observed in a 10 ps window, and the
components were fitted to be 0.3 ps and 3.4 ps. Notably, the
phosphorescence decay components that appear in Figure 8
cannot be resolved in this time regime because they are
forbidden and hence their initial
Figure 7. The schematic energy diagram of the mechanism of H-
bonding hopping in DMADM-7HIT. The energy gaps between the
corresponding states of the H-bonded and the non-H-bonded forms
are calculated values, and the emissions in wavelengths are taken
from the observed peak wavelengths.
the O-H-----S H-bonded form indicates that in the triplet
emitting state of DMADM-7HIT, the equilibrium shifts
completely to the O-H----N H-bonded species because of the
much weaker O-H-----S H-bond in the T (n*) state. This
1
phenomenon is further supported by the computational studies
involving the optimization of the geometry of both the O-H----
-
N and O-H-----S H-bond species under the n* configuration,
and these studies concluded that the O-H-----N H-bonded
T’ (n*) state is more stable than the O-H-----S H-bonded
Figure 8. The early dynamics decay curve of DM-7HIT at 550 nm
(blue) and 740 nm (red).
1
intensities are rather low. Upon monitoring at 720 nm, a rise
component of 0.3 ps followed by a 3.4 ps decay can be
T
1
(n*) state by 9.3 kcal/mol (see Figure 7).
The substituent effect on triplet stabilization can be more
observed. The results reveal precursor-successor-type kinetic
clearly seen from computational approach shown in Figure
S32. Compared to DM-7HIT, the T’ (nπ*) state of the
53-55
behaviour under a thermal equilibrium.
These results,
1
together with the microsecond time-resolved data (Figure 3a),
are sufficient to understand the relaxation dynamics behind the
DMADM-7HIT is significantly lowered due to the O-H-----N
stabilization, whereas the incorporation of a bulky tert-butyl
group (DTDM-7HIT) raises the energy of the T’ (nπ*) state of
1
the H-flipping product due to the steric repulsion. The n*
triplet excited states before and after the H flipping, involving
dual RTP signals. The lowest lying excited state (S
HIT is a sulfur non-bonding (n) → * transition, which
drastically weakens the O-H----S H-bond and results in fast O-
H bond flipping from S (n*) to a non-H-bonded S’ (n*) state
1
) for DM-
7
1
1
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with a ~300 fs time constant, which is of the same magnitude as
the torsional motion. Similar to the equilibrium observed
between the T (n*) and T’ (n*) states, it is also reasonable to
expect thermal interconversion between the S’ (n*) and
(n*) states, which give the same decay time of 3.4 ps. Based
on the computational studies, the energy of T (*) state is in
between those of the S (n*) and T (n*) states (Figure 5).
Therefore, the flipping of the n → * orbital should greatly
Pi-Tai Chou, e-mail: chop@ntu.edu.tw
Kew-Yu Chen, e-mail: kyuchen@fcu.edu.tw
Elise Y. Li, e-mail: eliseytli@ntnu.edu.tw
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
1
1
Author Contributions
S
1
Z.-Y. Liu, J.-W. Hu, C.-H. Huang contributed equally.
2
1
1
ACKNOWLEDGMENT
5
6
enhance the rate of S
rationalizing the fast 3.4 ps decay time constant.
supported by the calculated spin-orbit coupling (SOC) integral
1
(n*) → T
2
(*) intersystem crossing,
We appreciate the Ministry of Science and Technology (MOST
107-2628-M-002-017, MOST 107-2113-M-035 -003, and MOST
106-2113-M-003-010-MY3), Taiwan, for their generous support.
5
7-59
This is also
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
of 32 cm between the S
1
(n*) and T (*) states, which is
2
much larger than that between S(n*) and T(n*) states
REFERENCES
5
7
calculated in previous report. (see experimental in SI) Prior to
the decay of either the T’ (n*) or T (n*) state, fast H-bond
on/off switch occurs between T’ (n*) and T (n*) in a pre-
equilibrium, rendering dual T’ (n*, 685 nm) and T (n*, 550
1
.
Copley, M. J.; Marvel, C. S.; Ginsberg, E., Hydrogen
Bonding by S-H. Vll. Aryl Mercaptans. J. Am. Chem. Soc. 1939, 61,
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1
1
1
1
3
1
1
nm) RTP with the identical phosphorescence lifetime.
6
CONCLUSION
In summary, we have presented a seminal study on a
photoinduced intramolecular H-bond on/off reaction along a
pre-existing sulfur H-bond. The flipping of the H-bond in the
n* state originates from the n → * transition, causing an
electron deficiency in the non-bonding orbital of the S atom and
hence drastically reducing its proton-accepting strength and
weakening the H-bond. This photoinduced H-bond on/off
reaction should be universal for molecules having an n*
configuration for their lowest lying excited state, in which the
non-bonding (n) orbital is involved in the H-bond. Thione-type
H-bonds are an example of such species because of thione’s low
electron binding energy and hence lower ionization energy for
the non-bonding electron, making this phenomenon applicable
to many of the other thiobases that have recently been
4
.
calculations on the intramolecular hydrogen bond in
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energy of intramolecular hydrogen bonds. J. Phys. Chem. A 2006, 110,
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Jabłoński, M.; Kaczmarek, A.; Sadlej, A. J., Estimates of the
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Khalilinia, E.; Birgan, M. N., O-H···S intramolecular hydrogen bond
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Posokhov, Y.; Gorski, A.; Spanget-Larsen, J.; Duus, F.;
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.
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bases and formation of hydrogen bonding pair
.
Nevertheless, this is not believed to be a unique case. Small
organic molecules possessing carbonyl groups that serve as
proton acceptors to form intramolecular or intermolecular H-
bonds may have an n* configuration as the lowest state, and
these molecules would therefore be expected to undergo similar
H-bonding on/off flipping phenomena. Overall, the H-bonding
on-off flipping creates two excited-state isomers that may have
drastically different photophysical properties, as evidenced by
the thiones studied above; thus, this represents previously
unrecognized photochemistry. The photoinduced H-bonding
on/off flipping and the resulting dual room-temperature
phosphorescence of the sulfur-containing intramolecular H-
bonds may thus open a new chapter in the chemistry of
hydrogen bonds.
2
Govindarajan, S., Gene Designer: a synthetic biology tool for
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