K. Tomita et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 3 (2014)
429
in the Supporting Information. Assuming that 96% of mono-
mer emission with 360 nm excitation originates from the
opened form like Figure 1C, the estimated monomer emission
measured with 342 nm excitation and 4 times integration
consists of 57% of the photofragmentation products of the
disulfide bonding. During 5 min of fluorescence measure-
ment, the intensities of the excimer fluorescence at 474 nm
were decreased 2.7 « 1.0% regardless of the concentration of
acetate ion (Figure S8). This result suggests that the addition
of acetate itself does not induce nor accelerate the cleavage of
disulfide.
The Stern-Volmer plot for the two peaks (Figure S10) show-
ed a significant downward bend. Under this circumstance, the
Hindered Access Model can explain the result.32 In this model,
there are two fluorophores with different accessbility to the
quencher. The quenching is linearized with the equation below:
I0
1
1
1
¼
þ
ð1Þ
ðI0 ꢀ IÞ f KSV ½Qꢁ
f
where I0 and I are the fluorescent intensities obtained from the
BPyME and its complex with [Q] M of acetate ion. KSV is the
Stern-Volmer constant, and f is the fraction of the accessible
fluorophore. Values of KSV and f obtained from Figure 3C were
1.57 © 105 M¹1 and 0.55, respectively, for monomer emissions
The other part of the monomer emission observed sug-
gests the presence of conformers with the trans form of the
thioureido moiety or the opened form of the molecular
framework represented by the conformer in Figure 1C. The
excimer emission of molecules with two pyrenyl groups is
influenced by the conformation of its molecular framework.
For example, 1,n-bis(1-pyrenylcarboxy)alkanes show a differ-
ent steric effect from that of 1,n-bis(1-pyrenyl)alkanes.27,28
From the difference in conformation between methylene and
ester, the lower rotational energy of the 1-pyrenylcarboxy
group increases the frequency of the collision of chromophores
to form dimer. The difference comes from the strong steric
effect (intrachain H/H-repulsion of methylene) of 1,n-bis-
(1-pyrenyl)alkanes. As for BPyME, the ratio of the quantum
yield of excimer/monomer Φ¤/Φ equals 6.81 calculated from
the spectra with 342 nm excitation, and 18.1 with 360 nm
excitation (the result of peak separation is shown in Figure S9).
The small deviation comes from the difference in the rate of
photofragmentation for each wavelength of excitation light.
The Φ¤/Φ of the BPyME with 342 nm excitation is still >1.86
times larger than that of 1,10-bis(1-pyrenylcarboxy)decane in
methylcyclohexane at 20 °C. This result suggests that the small
difference in the rotational energy of each bond21,29 largely
influences the average conformation of sequentially changing
conformers. The receptor reported by Dahan et al.15 also has
two (1-pyrenylmethyl)thiourea groups at terminals of the linear
molecular framework, though BPyME has a larger Φ¤/Φ.
This difference also suggests that the combination of disulfide
between two thiourea groups is preferable for the collision of
chromophores resulting in intramolecular excimer formation.
The Benesi-Hildebrand plot30 (B-H plot) for the fluores-
¹1
and 1.52 © 105 M and 0.69, respectively, for excimer emis-
sions. In spite of the homogeneous solution of BPyME, the half
of the fluorophore for the monomer emission is hindered from
the quencher. Therefore the f value seems to represent the
BPyME-acetate complex that contains two fluorophores (one
is accessible and the other is inaccessible) per one acetate
ion. The molecular orbitals calculated with DFT-B3LYP/
6-311++G(d,p) are helpful for discussing the hindered fluo-
rophore of BPyME-acetate complex. Before complexation,
molecular orbitals on either pyrenyl ring can be considered
independent of the other pyrenyl ring (Figure S11).
After formation of the complex with acetate ion, this circum-
stance is not changed, suggesting that another pyrenyl ring on
the other side can behave as a hindered fluorophore during the
exciplex formation. In the case of the excimer emission, all
fluorophores are accessible. Therefore the fraction f was higher
than that of the monomer emission.
From the fluorescence experiment described above, the con-
formational change in the complex formation of the acetate ion
and BPyME is suggested as Scheme 2. BPyME forms the 1:1
complex with acetate ion accompanied by the rotation of a 1-
pyrenylmethylthioureirene group. This conformational change
induces dissociation of intramolecular excimer. However most
of the monomer fluorescence is suppresed by the exciplex
formation. To verify the “disulfide bonding assisted” stable 1:1
complex formation and the function of anion recognition by
the rotation of end group, UV-visible absorption spectra of
4-nitrophenyl-substituted receptors with diethyl disulfide or
hexamethylene framework were measured.
¹
cence spectra of BPyME-CH3COO is shown in Figure 3B.
Experimental Validation of the Role of the Disulfide
Linear relation against inverse of the n-th power of the
concentration of guest molecule means the host-guest associ-
ation reaction is 1:n stoichiometry. Therefore, the peak at 474
nm of BPyME, which mainly consisted of the intramolecular
excimer emission, was quenched in association with the for-
mation of the complex with the ratio of receptor:anion = 1:1.
Bonding.
UV-visible absorption spectra of BNE in the
¹
presence of CH3COO as TBA salts are shown in Figure 4A.
Because of the electron-withdrawing nitro substitution, a strong
absorption of the intramolecular charge transfer (ICT) at 338.5
nm showed a clear peak shift to 367 nm via the appearance of a
suspending peak at 352.5 nm. The B-H plots for absorption
spectra33 shown in Figure 4B indicate that the complexation
was 1:1 for dilute acetate and 1:n (n > 1) for concentrated
acetate. The peak shift means that the donor ability of the
thiourea moiety is enhanced by the acetate ion.34 The sta-
bility constant for the 1:1 complex of BNE with the acetate
The ratio of the intensities at 474 nm over 376 nm (I474/I376
)
was plotted versus the concentration of acetate ion is inserted
in Figure 3B (inserted). Decrease in the ratio suggests the
binding of acetate ion associates excimer dissociation, though
the exciplex emission of the group of 3-(1-pyrenylmethyl)-
thioureido-acetate complexes rises at 472 nm in parallel with
the excimer quenching. The value of I474/I376 > 2 corre-
sponds to the higher binding constant of BPyME and acetate
ion (1.5 © 105 M¹1) than that of 3-pyrenylmethyl-1-methyl
thiourea (5.7 © 103 M¹1).31
ion was calculated as 2.5 © 106 M by the fitting procedure
¹1
(Figure S12) 0-7 © 10¹5 M. This stability constant is 5 times
larger than that of a monosubstituted thiourea derivative,35 1-
methyl-3-(4-nitrophenyl)thiourea, 5.0 © 105 M¹1. This differ-
ence indicates that two ionophores of BNE orient cooperatively
Tomita, Kentaro
