The Hammett correlation between distyrylbenzene substituents and chemiluminescence efficiency providing various ρ-values for peroxyoxalate chemiluminescence of several oxalates

Article abstract of DOI:10.1016/j.jphotochem.2012.12.006

Peroxyoxalate chemiluminescence (PO-CL) was investigated using eight oxalates with various phenol moieties and the distyrylbenzene (DSB) fluorophores with various substituents. The ρ-values in the Hammett correlation between the substituent constants (σ_p⁺) of the DSBs and the singlet chemiexcitation yields (Φ_S) for the PO-CL reactions varied from -0.50 to -1.01 depending on the oxalate structure, and the reactive oxalates tended to afford the higher absolute ρ-values but with a few exceptions. Based on the CIEEL mechanism, these experimental observations suggest that the aryloxy groups still remain in the 1,2-dioxetanones (DOTs), which are the postulated high-energy intermediates, and control the electronic properties of DOTs as electron-acceptors. The LUMO energies of the DOTs calculated by the ab initio method with a B3LYP/6-31g(d) basis set reveal that the lower the DOT-LUMO energies, the higher the absolute ρ-values were provided for the corresponding oxalates, as predicted by the frontier molecular orbital (FMO) theory. Thus, the chemical species interacting with the DSBs would be not unitary and will be DOTs.

Full text of DOI:10.1016/j.jphotochem.2012.12.006

Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The Hammett correlation between distyrylbenzene substituents and
chemiluminescence efficiency providing various ꢀ-values for peroxyoxalate
chemiluminescence of several oxalates
Takayuki Maruyama, Susumu Narita, Jiro Motoyoshiya∗
Applied Chemistry Course, Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Peroxyoxalate chemiluminescence (PO-CL) was investigated using eight oxalates with various phenol
Received 1 October 2012
Received in revised form
27 November 2012
moieties and the distyrylbenzene (DSB) fluorophores with various substituents. The ꢀ-values in the
Hammett correlation between the substituent constants (␴ +) of the DSBs and the singlet chemiexci-
p
tation yields (˚S) for the PO-CL reactions varied from −0.50 to −1.01 depending on the oxalate structure,
and the reactive oxalates tended to afford the higher absolute ꢀ-values but with a few exceptions. Based
on the CIEEL mechanism, these experimental observations suggest that the aryloxy groups still remain
in the 1,2-dioxetanones (DOTs), which are the postulated high-energy intermediates, and control the
electronic properties of DOTs as electron-acceptors. The LUMO energies of the DOTs calculated by the ab
initio method with a B3LYP/6-31g(d) basis set reveal that the lower the DOT-LUMO energies, the higher
the absolute ꢀ-values were provided for the corresponding oxalates, as predicted by the frontier molec-
ular orbital (FMO) theory. Thus, the chemical species interacting with the DSBs would be not unitary and
will be DOTs.
Accepted 3 December 2012
Available online 20 December 2012
Keywords:
Peroxyoxalate chemiluminescence
Hammett correlation
High-energy intermediate
Frontier molecular orbital theory
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
fluorophores to the HEI, such as the DOTs or DOD, takes place
to form a pair consisting of the carbon dioxide radical anion and
Since Chandross first discovered the peroxyoxalate chemi-
luminescence (PO-CL) reaction [1], much attention has been paid
to this CL and chemists have tried to elucidate its mechanism for
a long time [2–25] because of high efficiency [23] and usefulness
in analytical chemistry [26,27]. Among the several peroxides pro-
posed so far as the high-energy intermediates (HEIs) capable of
generating the excited fluorophores during their decomposition,
the four-membered cyclic peroxides, such as 4-aryloxy-4-hydroxy-
1,2-dioxetane-3-one (DOT) and 1,2-dioxetne-3,4-dione (DOD),
have been the most frequently proposed [3,4,6,7,9,10,14,19,21,22].
However, these intermediates have never been captured nor iso-
lated. There have been studies observing labile intermediates by
NMR using the highly reactive oxalates [28,29]. In addition, the
EPR detection of a biradical as a trace of the formed DOD dur-
ing the reaction of the oxalyl chloride and hydrogen peroxide
spectra was very recently reported [30]. CIEEL (chemically ini-
tiated electron exchange luminescence) mechanism [31,32] was
applied to the PO-CL [33,34], and explained experimental evidences
[35–40]. According to this mechanism, an electron transfer from the
the fluorophore radical cation during the decomposition of the
HEI, which is followed by a back electron transfer to produce the
excited fluorophores (Scheme 1). In spite of a steady accumula-
tion of knowledge and many recent efforts made by a number of
chemists, identification of the HEIs interacting with fluorophores
still remains unclear.
It is important to examine the effect of the oxalate structures on
the CL behavior [22], because there have been only a few reports in
which a number of oxalates were employed, and new aspects can
be obtained by comparing the CL behavior of the different oxalates,
if the structure of the HEI as the electron-acceptors is not unitary
and might depend on the oxalate structures. On the other hand,
the 1,4-distyrylbenzene (DSB) derivatives are strongly fluorescent
[41,42] and good emitters among the numerous fluorophores so far
used for the PO-CL reactions [38,39] due to availability by the sim-
ple synthetic method and capability of modulating their electronic
properties by the substituents. During our continuous studies of the
DSB enhanced PO-CL, some remarkable electronic effects on the CL
efficiency have been observed. Therefore, it is expected that there is
a possibility of identification of the HEIs when the CL behavior using
various DSBs and oxalates is investigated in detail. In this paper, we
report the Hammett correlation between the DSB substituents and
the behavior of the DSB enhanced PO-CL reactions employing the
∗
Corresponding author. Tel.: +81 268 21 5402; fax: +81 268 21 5391.
E-mail address: jmotoyo@shinshu-u.ac.jp (J. Motoyoshiya).
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.12.006

T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
223
HO
ArO
O
O O
C C OOH
O
O
O O
H2O2
ArOH
and/or
ArO
ArO
C C OAr
O
O
O
O
ArOH
Ar = aryl groups
DOT
DOD
Flu.
HO
ArO
O
O
O
(fluorophore)
CO2
Flu.
Flu.
+
CO2
and/or
intermolecular
ArOH
O
O
O
O
electron transfer
*
Flu.
(ground state)
+
h
Flu.
(excited state)
+
CO2
back electron
transfer
Scheme 1. A CIEEL process of peroxyoxalate chemiluminescence containing the diexetanones (DOTs) or dioxetanedione (DOD) high-energy intermediates.
eight aryl oxalates and give consideration to the HEIs in connec-
tion with the interpretation of the observed CL behavior using the
frontier molecular orbital (FMO) theory.
were carried out by a photon-counting method using a Hamamatsu
Photonics R464 photomultiplier connected to a photon-counting
unit (C3866) and a photon-counting board M8784, and the luminol
CL [˚CL = 0.09 in dimethyl sulfoxide, Lit. J. Lee, H. H. Seliger, Pho-
tochem. Photobiol. 15 (1972) 227–237] was used as the standard
in DMSO for the calibration of the photomultiplier tube.
2. Experimental details
All commercially available chemicals and solvents were of the
highest purity or purified by the standard method. A series of
the oxalates employed in the present study, all of which are
known compounds, are listed in Table 1. They were prepared
from the reaction of oxalyl chloride and two equivalents of the
corresponding phenols in the presence of triethylamine in ben-
zene, while the unsymmetrical oxalate 4-CPPO was prepared from
the reaction of oxalyl chloride and one equivalent phenol in the
presence of triethylamine followed by the addition of another p-
chlorophenol. DNPO [bis(2,4-dinitrophenyl) oxalate] was supplied
by Tokyo Chemical Industries (TCI).
2.1. Materials
Melting points were determined using a hot stage microscope
apparatus and were uncorrected. Proton nuclear magnetic res-
onance 1H and 13C NMR spectra were measured on a Bruker
AVANCE-400 at 400 MHz for 1H NMR, 100 MHz for 13C NMR,
respectively, in CDCl3. The chemical shifts (ı) are reported in ppm
downfield from TMS as internal standard or from the residual sol-
vent peak for 1H and 13C NMR. Coupling constants (J) are reported
in Hz. Low and high resolution mass spectra (MS) were recorded by
the JEOL JMS-K9 spectrometers. Elemental analysis was recorded
on a Perkin-Elmer 2400CHN elemental analyzer. Analytical TLC
was carried out on precoated silica gel 60F-254 plates (E. Merck).
Column chromatography was performed on silica gel (E. Merck).
Absorption and fluorescence spectra were recorded on a U-3310
spectrometer (Hitachi) and on a RF-5000 spectrometer (Shimadzu),
2.1.1. 1,4-Bis(2-ethylhexyloxy)-2,5-
bis(diethylphosphonomethyl)benzene
2-Ethylhexyl bromide (35 g, 180 mmol) was added dropwise to
a solution of hydroquinone (10 g, 91 mmol) and potassium hydrox-
ide (13 g, 230 mmol) in ethanol (80 mL) and stirred for 2 h at
respectively. The measurements of the total CL quantum yield (˚CL
)
Table 1
Structures of oxalates used in this study and pKa of phenols as leaving groups.
Oxalate
Abbreviation
X1
X2
X3
pKa of phenol
Diphenyl oxalate
DPO
TCPO
H
H
H
Cl
H
Cl
H
H
H
9.91
6.00
7.85
6.69
9.29
8.41
3.87
bis(2,4,6-Tricholorophenyl) oxalate
bis(2,4-Dichlorophenyl) oxalate
bis(2,6-Dichlorophenyl) oxalate
bis(4-Chlorophenyl) oxalate
bis(2-Chlorophenyl) oxalate
bis(2,4-Dinitrophenyl) oxalate
Cl
Cl
Cl
H
Cl
NO2
Cl
Cl
H
Cl
H
2,4-DCPO
2,6-DCPO
4-CPO
2-CPO
DNPO
NO2
4-Chlorophenyl phenyl oxalate
4-CPPO
–

224
T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
80 ◦C. After the removal of the solvent and the addition of ether,
the inorganic by-products were removed by washing with sat-
urated NaHCO3 aqueous solution and brine. The organic phase
was dried over anhydrous Na2SO4. After the Na2SO4 was fil-
tered out and the solvent was distilled away, the crude product
was purified by column chromatography on silica gel (eluant: n-
hexane) to afford 1,4-bis(2-ethylhexyloxy)benzene as a colorless
liquid. Yield: 50% (15 g); 1H NMR (400 MHz, CDCl3) ı 0.87–0.94
(m, 12H, CH3), 1.29–1.53 (m, 16H, CH2 ), 1.66–1.72 (m, 2H,
CH ), 3.78 (d, 4H, CH2O , J = 5.8 Hz), 6.82 (s, 4H, Ar H). After
a solution of 1,4-bis(2-ethylhexyloxy)benzene (9.5 g, 28 mmol)
and paraformaldehyde (1.9 g) in hydrobromic acid (48%, 60 ml)
and acetic acid (90 ml) was stirred for 17 h at 70 ◦C, the solu-
tion was cooled to room temperature for 2 h. The deposited
crystals were filtered and washed by water and n-hexane to
afford 1,4-bis(2-ethylhexyloxy)-2,5-bis(bromomethyl)benzene as
colorless crystals. Yield: 82% (12 g); 1H NMR (400 MHz, CDCl3) ı
0.89–0.97 (m, 12H, CH3), 1.32–1.61 (m, 16H, CH2 ), 1.72–1.78
(m, 2H, CH ), 3.87 (d, 4H, CH2O, J = 5.6 Hz), 4.52 (s, 4H, CH2Br),
6.85 (s, 2H, Ar H). A mixture of 1,4-bis(2-ethylhexyloxy)-2,5-
bis(bromomethyl)benzene (11 g, 21 mmol) and triethyl phosphite
(8.8 g, 53 mmol) was stirred for 1.5 h at 130 ◦C with removing by-
product (bromoethane) using distilling receiver. The crude product
was purified by column chromatography on silica gel (eluant:
hexane/ethyl acetate = 5/1) to afford 1,4-bis(2-ethylhexyloxy)-2,5-
bis(diethylphosphonomethyl)benzene as a colorless liquid. Yield:
96% (13 g); 1H NMR (400 MHz, CDCl3) ı 0.88–0.94 (m, 12H, CH3),
1.23 (t, 12H, OCH2CH3, J = 7.1 Hz), 1.29–1.56 (m, 16H, CH2 ),
1.67–1.73 (m, 2H, CH ), 3.22 (d, 4H, Ph CH2 P, J = 20.2 Hz), 3.81
(d, 4H, OCH2 , J = 5.6 Hz), 3.96–4.06 (m, 8H, OCH2CH3), 6.94 (s,
2H, Ph H). This compound was used for the preparation of the
following distyrylbenzenes (1a–g) without further purification.
(100 MHz, CDCl3) ı 11.76, 14.54, 23.54, 24.68, 29.70, 31.38, 40.23,
55.73, 72.22, 110.54, 114.55, 121.91, 127.17, 128.04, 128.43, 131.36,
151.47, 159.55; MS (m/z) 598 (M+); Anal. Calcd for C40H54O4: C,
80.22; H, 9.09. Found: C, 80.18; H, 9.28.
2.1.4. 1,4-Bis(2-ethylhexyloxy)-2,5-bis(p-methylstyryl)benzene
(1c)
This compound was prepared in
a manner similar to
the procedure described above using 1,4-bis(2-ethylhexyloxy)-
2,5-bis(diethylphosphonomethyl)benzene (1.0 g, 1.6 mmol) and
p-tolualdehyde (0.40 g 3.3 mmol) and tBuOK (0.61 g, 5.5 mmol).
Yield: 35% (0.31 g) as light yellow crystals; mp 113–114 ◦C; 1H NMR
(400 MHz, CDCl3) ı 0.92 (t, 6H, CH3, J = 7.0 Hz), 0.99 (t, 6H, CH3,
J = 7.5 Hz), 1.29–1.66 (m, 16H, CH2 ), 1.75–1.87 (m, 2H, CH ),
2.35 (s, 6H, CH3), 3.94 (d, 4H, OCH2 , J = 5.5 Hz), 7.07–7.14 (m,
6H, CH CH , Ph H), 7.16 (d, 4H, Ph H, J = 8.1 Hz), 7.42 (d, 2H,
Ph H, J = 8.0 Hz), 7.45 (d, 2H, CH CH , J = 16.4 Hz); 13C NMR
(100 MHz, CDCl3) ı 11.78, 14.56, 21.69, 23.56, 24.70, 29.73, 31.40,
40.25, 72.24, 110.69, 123.01, 126.81, 127.25, 128.94, 129.81, 135.73,
137.63, 151.58; MS (m/z) 566 (M+); Anal. Calcd for C40H54O2: C,
84.75; H, 9.60. Found: C, 84.59; H, 9.83.
2.1.5. 1,4-Bis(2-ethylhexyloxy)-2,5-distylylbenzene (1d)
This compound was prepared in a manner similar to the
procedure described above using 1,4-bis(2-ethylhexyloxy)-2,5-
bis(diethylphosphonomethyl)benzene (0.50 g, 0.79 mmol) and
benzaldehyde (0.18 g 1.7 mmol) and tBuOK (0.31 g, 2.8 mmol).
Yield: 50% (0.21 g) as yellow green crystals; mp 86–87 ◦C; 1H NMR
(400 MHz, CDCl3) ı 0.92 (t, 6H, CH3, J = 7.0 Hz), 0.99 (t, 6H, CH3,
J = 7.5 Hz), 1.30–1.66 (m, 16H, CH2 ), 1.74–1.90 (m, 2H, CH ),
3.96 (d, 4H, OCH2 , J = 5.5 Hz), 7.10–7.19 (m, 4H, CH CH
,
Ph H), 7.24 (t, 2H, Ph H, J = 7.4 Hz), 7.35 (t, 4H, Ph H, J = 7.6 Hz),
7.46–7.56 (m, 6H, CH CH , Ph H); 13C NMR (100 MHz, CDCl3) ı
11.77, 14.55, 23.56, 24.70, 29.73, 31.40, 40.24, 72.24, 110.79, 123.98,
126.90, 127.27, 127.80, 129.09, 138.47, 151.66; MS (m/z): 538
(M+); Anal. Calcd for C38H50O2: C, 84.71; H, 9.35. Found: C, 84.56;
H, 9.60.
2.1.2. 1,4-Bis(2-ethylhexyloxy)-2,
5-bis(p-dimethylaminostyryl)benzene (1a)
A solution of 1,4-bis(2-ethylhexyloxy)-2,5-bis(diethylphospho-
nomethyl)benzene (0.50 g, 0.79 mmol) and p-dimethylamino-
benzaldehyde (2.5 g, 1.7 mmol) in tetrahydrofuran (THF) (5 mL)
was added dropwise to a solution of tBuOK (0.31 g, 2.8 mmol) in
THF (10 ml) and stirred for 2 h at room temperature. After the
removal of the solvent and the addition of 1 M hydrochloric acid
for neutralization, the deposited crystal was filtered and washed
by cooled hexane to afford yellow crystals. Yield: 54% (0.26 g);
mp 159–160 ◦C; 1H NMR (400 MHz, CDCl3) ı 0.92 (t, 6H, CH3,
J = 7.0 Hz), 0.99 (t, 6H, CH3, J = 7.5 Hz), 1.28–1.67 (m, 16H, CH2 ),
1.74–1.88 (m, 2H, CH ), 2.98 (s, 12H, N(CH3)2), 3.94 (d, 4H,
OCH2 , J = 5.5 Hz), 6.72 (d, 4H, Ph H, J = 8.8 Hz), 7.05 (d, 2H,
2.1.6. 1,4-Bis(2-ethylhexyloxy)-2,5-bis(p-chlorostyryl)benzene
(1e)
This compound was prepared in a manner similar to the
procedure described above using 1,4-bis(2-ethylhexyloxy)-2,5-
bis(diethylphosphonomethyl)benzene (0.50 g, 0.79 mmol) and
p-chlorobenzaldehyde (0.23 g 1.7 mmol) and tBuOK (0.31 g,
2.8 mmol). Yield: 43% (0.20 g) as yellow green crystals; mp
104–106 ◦C; 1H NMR (400 MHz, CDCl3) ı 0.91 (t, 6H, CH3,
J = 7.0 Hz), 0.99 (t, 6H, CH3, J = 7.5 Hz), 1.28–1.65 (m, 16H, CH2 ),
1.75–1.85 (m, 2H, CH ), 3.94 (d, 4H, OCH2 , J = 5.5 Hz), 7.03–7.14
(m, 4H, CH CH , Ph H), 7.31 (d, 4H, Ph H, J = 8.5 Hz), 7.39–7.50
(m, 6H, CH CH , Ph H); 13C NMR (100 MHz, CDCl3) ı 11.76,
14.55, 23.54, 24.69, 29.70, 31.40, 40.20, 72.23, 110.77, 124.55,
127.11, 127.94, 128.01, 129.25, 133.35, 136.95, 151.67; MS (m/z)
606 (M+); Anal. Calcd for C38H48Cl2O2: C, 75.10; H, 7.96. Found: C,
74.97; H, 8.07.
CH CH , J = 16.4 Hz), 7.09 (s, 2H, Ph H), 7.30 (d, 2H, CH CH
,
J = 16.4 Hz), 7.42 (d, 4H, Ph H, J = 8.8 Hz); 13C NMR (100 MHz, CDCl3)
ı11.77, 14.56, 23.55, 24.68, 29.71, 31.36, 40.26, 40.93, 72.21, 110.29,
112.95, 119.79, 127.17, 127.21, 127.89, 128.67, 150.34, 151.33; MS
(m/z) 624 (M+); Anal. Calcd for C42H60N2O2: C, 80.72; H, 9.68; N,
4.48. Found: C, 80.45; H, 9.87; N, 4.34.
2.1.3. 1,4-Bis(2-ethylhexyloxy)-2,5-bis(p-methoxystyryl)benzene
(1b)
2.1.7. 1,4-Bis(2-ethylhexyloxy)-2,5-bis(p-cyanostyryl)benzene
(1f)
This compound was prepared in a manner similar to the
procedure described above using 1,4-bis(2-ethylhexyloxy)-2,5-
bis(diethylphosphonomethyl)benzene (0.50 g, 0.79 mmol) and
p-anisaldehyde (2.3 g, 1.7 mmol) and tBuOK (0.31 g, 2.8 mmol).
Yield: 39% (0.19 g) as light yellow crystals; mp 86–87 ◦C; 1H NMR
(400 MHz, CDCl3) ı 0.92 (t, 6H, CH3, J = 7.1 Hz), 0.99 (t, 6H, CH3,
J = 7.5 Hz), 1.30–1.65 (m, 16H, CH2 ), 1.74–1.88 (m, 2H, CH ),
3.82 (s, 6H, OCH3), 3.94 (d, 4H, OCH2 , J = 5.5 Hz), 6.90 (d, 4H,
Ar H, J = 8.8 Hz), 7.05–7.13 (m, 4H, CH CH , Ar H), 7.36 (d,
2H, CH CH , J = 16.5 Hz), 7.46 (d, 4H, Ar H, J = 8.7 Hz); 13C NMR
This compound was prepared in a manner similar to the
procedure described above using 1,4-bis(2-ethylhexyloxy)-
2,5-bis(diethylphosphonomethyl)benzene (0.50 g, 0.79 mmol)
and p-cyanobenzaldehyde (0.21 g 1.6 mmol) and tBuOK (0.31 g,
2.8 mmol). Yield: 54% (0.25 g) as light orange crystals; mp
160–161 ◦C; 1H NMR (400 MHz, CDCl3) ı 0.92 (t, 6H, CH3,
J = 7.0 Hz), 1.00 (t, 6H, CH3, J = 7.5 Hz), 1.30–1.64 (m, 16H, CH2 ),
1.76–1.89 (m, 2H, CH ), 3.97 (d, 4H, OCH2 , J = 5.5 Hz), 7.12
(s, 2H, Ph H), 7.17 (d, 2H, CH CH , J = 16.5 Hz), 7.52–7.68 (m,

T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
225
Table 2
The Hammett substituent constants (ꢁp+) and spectral data of distyrylbenzenes (DSBs).a.
ꢂabs (nm)a
ꢂem (nm)b
˚
F
Rel. ˚F in aq. THFa,c
+
b
DSBs
R
ꢁp
1a
1b
1c
1d
1e
1f
NMe2
OMe
Me
H
Cl
CN
−1.7
−0.778
−0.311
0
0.114
0.659
0.79
419
394
391
390
396
415
444
490 (441)
444 (395)
442 (340)
442 (390)
451 (397)
484 (424)
556 (452)
0.73
0.78
0.75
0.76
0.81
0.7
1.32
1.32
1.16
1.00
1.03
1.10
0.04
1g
NO2
0.25
a
Measured in THF/H2O (3/1) (7.5 × 10−6 M).
b
c
Measured in cyclohexane using 9,10-diphenylanthracene as a standard.
Relative values based on the ˚F for 1d.
10H, CH CH , Ph H); 13C NMR (100 MHz, CDCl3) ı 11.74, 14.52,
23.51, 24.69, 29.67, 31.38, 40.14, 72.18, 110.79, 119.51, 127.12,
127.20, 127.50, 127.72, 132.90, 142.82, 151.93; MS (m/z) 588 (M+);
Anal. Calcd for C40H48N2O2: C, 81.59; H, 8.22; N, 4.76. Found: C,
81.33; H, 8.35; N, 4.74.
absorptions (Abs) and the integrated areas (S) of the emission
spectra obtained by excitation at 375 nm were used for the estima-
tion of the fluorescence quantum yields according to the following
equation (a term for the index of refraction is omitted because of
the measurements in the same solvent).
˚
˚
SDSB × AbsDPA
SDPA × AbsDSB
F DSB
=
2.1.8. 1,4-Bis(2-ethylhexyloxy)-2,5-bis(p-nitrostyryl)benzene
(1g)
F DPA
The relative fluorescence quantum yields in aqueous THF
(THF/H2O = 3/1) were estimated in a manner similar to the above
using 1d (R = H) as a standard. These values are listed in Table 2.
This compound was prepared in a manner similar to the
procedure described above using 1,4-bis(2-ethylhexyloxy)-2,5-
bis(diethylphosphonomethyl)benzene (0.50 g, 0.79 mmol) and p-
nitrobenzaldehyde (0.25 g 1.6 mmol) and tBuOK (0.31 g, 2.8 mmol).
Yield: 24% (0.12 g) as red crystals; mp 160–161 ◦C; 1H NMR
(400 MHz, CDCl3) ı 0.92 (t, 6H, CH3, J = 7.0 Hz), 1.01 (t, 6H, CH3,
J = 7.5 Hz), 1.30–1.64 (m, 16H, CH2 ), 1.80–1.91 (m, 2H, CH ),
3.99 (d, 4H, OCH2 , J = 5.5 Hz), 7.14 (s, 2H, Ph H), 7.23 (d, 2H,
CH CH , J = 16.5 Hz), 7.55–7.74 (m, 6H, CH CH , Ph H), 8.23
(d, 4H, Ph H, J = 8.8 Hz). 13C NMR (100 MHz, CDCl3) ı 11.74, 14.51,
23.50, 24.69, 29.66, 31.38, 40.14, 72.18, 111.01, 124.60, 127.17,
127.21, 127.41, 128.40, 144.82, 147.07, 152.03; MS (m/z) 628 (M+).
Although all spectra show the appropriate data for this compound,
the satisfactory elemental analysis was not obtained.
2.3. Measurement of ˚CL
For a typical run, 0.5 mL of a solution containing sodium carbon-
ate (4.0 × 10−4 M) and hydrogen peroxide (1.0 × 10−2 M) in distilled
water was added to 1.5 mL of a solution containing the oxalates
(5.0 × 10−6 M) and fluorophores (1.0 × 10−5 M) in THF in a quartz
cell placed in front of the photomultiplier at 25 ◦C. The photons
generated were counted with the photomultiplier. The average of
the values obtained by three times measurements was used for the
calculation of ˚CL
.
2.2. Fluorescence quantum yields of the DSBs (1a–g)
3. Results and discussion
9,10-Diphenylanthracene was used as a standard [˚F = 1.00 in
cyclohexane: Lit. G. Heinrich, S. Schoof, H. Gusten, J. Photochem.
3 (1974/75) 315–320)] by excitation at 375 nm. All DSBs and
9,10-diphenylanthracene (DPA) were dissolved in cyclohexane at
10−6 M and measured their absorptions at 375 nm. Thus obtained
3.1. Spectral data of distyrylbenzene (DSB) fluorophores
DSB fluorophores 1a–g with 2-ethylhexyloxy groups at the
center of the molecules for increasing their solubility in organic
solvents and with various substituents at the para, para’-positions
Fig. 1. Absorption (a) and fluorescence (b) spectra of the DSBs 1a–g. 7.5 × 10−6 M in THF/H2O (3:1).

226
T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
included in Table 2. DSBs 1b–1e show absorption maxima around
Fluorescence
390 nm and the emissions around 450 nm, while 1a, 1f and 1g with
strongly electron-donating or attracting substituents show some
red-shifts in both spectra compared to the others. The fluorescence
quantum yields (˚F) of 1a–f in cyclohexane have similar values,
and 1g (R = NO2) showed a much lower ˚F compared to the oth-
ers. For CL study, the rel. ˚Fs in aqueous THF were also measured
and 1g was found to be almost non-fluorescent [42] probably due
to the loss of the excitation energy through the hydrogen bonds of
the nitro groups in protic media.
Chemiluminescence
3.2. DSB enhanced PO-CL reactions of the eight oxalates and the
Hammett correlation between the DSB substituents and the
relative singlet chemiexcitation yields
350
400
450
500
550
600
650
Wavelength(nm)
Fig. 2. Fluorescence and chemiluminescence spectra of 1b using TCPO. Oxalate
(7.5 × 10−5 M), DSB derivatives (7.5 × 10−6 M) and H2O2 (2.5 × 10−1 M) in THF/H2O
(3:1) were used.
When TCPO was reacted with alkaline hydrogen peroxide in the
presence of 1a or 1b in aqueous THF, an intense CL was observed,
whose spectra were in agreement with the fluorescence spectra of
1a and 1b (Fig. 2). This indicated that the emitting species are the
excited 1a and 1b generated during the reactions as reported in
the previous reports, in which the PO-CL reactions of the several
oxalates and oxamates were enhanced by the various fluorescent
were prepared via the Horner–Wadsworth–Emmons reaction of
the bisphosphonate and the corresponding aromatic aldehydes.
The absorption and fluorescence spectra measured in aqueous THF
are shown in Fig. 1, and the spectral data measured both in aqueous
THF (THF/H2O = 3/1) and cyclohexane are summarized in Table 2.
The Hammett substituent constants (ꢁp+) of the DSBs are also
DSBs in a bimolecular manner, and the CL quantum yields (˚CL
were related to the electronic nature of the DSBs [38,39].
)
Table 3
Chemiluminescence quantum yields (˚CL × 104) of the PO-CL for eight oxalates enhanced by DSBs (1b–f).a
DSB
TCPO
2,4-DCPO
2,6-DCPO
4-CPO
2-CPO
DNPO
DPO
4-CPPO
1b (R = OMe)
1c (R = Me)
1d (R = H)
1e (R = Cl)
1f (R = CN)
1.12
0.55
0.33
0.27
0.11
6.30
2.38
1.50
1.00
0.25
1.01
0.61
0.32
0.29
0.10
3.63
1.36
0.62
0.50
0.15
8.68
3.97
1.79
1.58
0.41
1.06
0.54
0.17
0.11
0.04
0.29
0.16
0.05
0.04
0.02
0.72
0.30
0.14
0.11
0.04
a
[oxalate] = 3.75 × 10−6 M, [DSB] = 7.5 × 10−6 M, [H2O2] = 2.5 × 10−3 M, [Na2CO3] = 7.5 × 10−5 M.
Fig. 3. The Hammett correlation of PO-CL of the eight oxalates enhanced by DSBs (1b–f). [oxalates] = 3.75 × 10−6 M, [DSBs] = 7.5 × 10−6 M, [H2O2] = 2.5 × 10−3 M, and
[Na2CO3] = 7.5 × 10−5 M in THF/H2O (3/1).

T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
227
Table 4
11
The ꢀ-values and the pseudo-first-order rate constants of the CL decays of the eight
oxalates enhanced by 1b.
DPO
Oxalate
ꢀ-Value
Pseudo-first-order
4-CPO
decay ratea k (s−1
)
9
7
5
3
TCPO
−0.65
−0.90
−0.63
−0.90
−0.87
−1.01
−0.84
−0.84
1.18
12.56
0.38
5.40
9.28
16.27
0.65
4.28
2-CPO
2,4-DCPO
2,6-DCPO
4-CPO
2-CPO
DNPO
2,4-DCPO
2,6-DCPO
TCPO
a
K
p
DPO
4-CPPO
a
Standard deviations (R2) are described in Fig. 5.
All the PO-CL reactions using the oxalates listed in Table 1 were
completed within a few seconds under the prescribed conditions
for the present study and provided various total CL quantum yields
(˚CL) depending on the type of oxalates and fluorophores (Table 3).
Since the PO-CL reaction in the aqueous media is accompanied by
the hydrolysis of the oxalates to form the oxalic acids and phe-
nols, a large excess amount of hydrogen peroxide (ca. 700 times vs.
the oxalates) was used to undergo the peroxalate reactions before
hydrolysis.
DNPO
0.5
0.7
0.9
1.1
| |
Fig. 4. Correlation of pKa of phenols in the oxalates and ꢀ-values.
−1.01 for DNPO, (ii) between −0.9 and −0.8 for 2,4-DCPO, 2-CPO,
4-CPO, 4-CPPO and DPO, and (iii) near −0.6 for TCPO and 2,6-DCPO.
The salient feature of these ꢀ-values is that the highly activated
DNPO had the highest absolute ꢀ-values, but two oxalates, TCPO
and 2,6-DCPO, had the lowest values. Because the pKa values of
the phenols [43,44] are the tentative indicators of their electronic
nature, the pKa of the phenols forming the oxalates (Table 1) are
also related to the ꢀ-values as shown in Fig. 4, in which the oxalates
consisting of strongly acidic phenols tend to produce high absolute
ꢀ-values except for TCPO and 2,6-DCPO.
The CL decay curves of the PO-CL enhanced by 1b as the rep-
resentative fluorophore are shown in Fig. 5a and well fitted by the
single exponential decay as shown in Fig. 5b as has been observed
for the typical PO-CL [12,13,45]. The pseudo-first-order decay rate
constants (k) estimated from the logarithmic plots of the decay
curves (Fig. 5b) are listed in Table 4. The rate-determining step
for the formation of the HEIs will be the bimolecular process in
which an oxalate reacts with perhydroxy anion to give hydroperox-
ide because the CL decay rates are dependent on the phenol acidity
(Fig. 6) except for TCPO and 2,6-DCPO with much lower CL decay
rates [22]. The observed correlation between k and pKa is similar
to that between |ꢀ| and pKa (Fig. 4). The deviation for TCPO and
2,6-DCPO is probably due to the steric effect of the ortho-phenol
substituents on the both initial and the subsequent intramolecular
acyl substitutions to form the cyclic HEIs [45].
The ˚CL was highly dependent on the electronic nature of
fluorophores in the previously investigated PO-CL reactions, and,
therefore, a Hammett correlation between the ˚CL and the DSB
substituent constants would give the useful information for the
chemiexcitation of the DSBs as well as the information of the HEIs
because the DSBs would electronically interact with the HEIs to
form the polarized complexes initially as the precursors of the rad-
ical ion pairs according to the CIEEL mechanism. The relative singlet
chemiexcitation yield (rel. ˚S defined as ˚CL/rel. ˚F) for each CL
reaction was correlated with the DSB substituent constants (ꢁp+) in
terms of the Hammett relationship; namely, the plots of ꢁp+-values
versus log(˚S/˚S(H)) for all the PO-CL reactions gave straight lines
+
with negative slopes (ꢀ-values) as shown in Fig. 3. The ꢁp val-
ues were used as the substituent constants for the examination of
the present Hammett correlation because the DSBs would interact
with electron deficient HEIs to form the positively charged DSBs
that would be stabilized with electron-donating substituents by
the resonance effect. However, the DSB 1a and 1g could not be
applied to the examination of this Hammett correlation because the
former produced too high a ˚CL value that exceeded the limit of the
photon-counting system set for this study and the latter was almost
non-fluorescent under these CL conditions. It is interesting to note
that the ꢀ-values for the eight oxalates in Table 4 are apparently
classified into three groups by their ranges, (i) the smallest value of
Fig. 5. The CL decay curves and logarithm plots of the eight oxalates using 1b as the fluorophore: (a) CL decay curves (inset: normalized curves), (b) pseudo-first-order
reaction fitting of the normalized CL emission decays. [oxalate] = 3.75 × 10−6 M, [1b] = 7.5 × 10−6 M, [H2O2] = 2.5 × 10−3 M, and [Na2CO3] = 7.5 × 10−5 M in THF/H2O (3/1).
Standard deviations (R2): 0.990 for TCPO, 0.995 for 2,4-DCPO), 0.979 for 2,6-DCPO, 0.994 for 4-CPO, 0.979 for 2-CPO, 0.992 for DNPO, 0.996 for DPO, 0.994 for 4-CPPO.

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T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
20
15
10
5
HEIs electronically interacting with the DSBs; the oxalates with
the higher ꢀ-values form HEIs which make effective electronic
interactions. The strength of the electronic interactions depends
on the oxalate structures, to indicate that HEIs retain the phenol
moieties. Thus, DOD is excluded as a HEI. The DOT intermedi-
ates with the phenol ester moieties are, in principle, capable of
generating excited fluorophores by an electronic interaction with
the fluorophores [19] considering the structural similarity to the
chemiluminescent ˛-peroxy lactones [46]. The same ꢀ-value for
DPO and 4-CPPO is probably due to the structurally same HEIs,
because 4-CPPO would eliminate the 4-chlolophenol by the ini-
tial nucleophilic acyl substitution leading to the formation of an
intermediate the same as that formed from DPO.
The relative heats of formation of the phenyl O,O-hydrogen
monoperoxalate, the HEIs, such as the DOT together with DOD, and
the final products, such as carbon dioxide and phenol, calculated
by ab initio method using the B3LYP/6-31G (d) basis set, are shown
in Fig. 7 as examples. The formation of the cyclic peroxides, the
HEIs from the acyclic hydroperoxide, is an endothermic process,
while their decomposition to carbon dioxide and phenol is a very
favorable exothermic process. It is noteworthy that the conversion
from the DOT to the DOD is an endothermic process with a small
energy difference (4.64 kcal/mol), which slightly favors the DOT at
equilibrium, and the DOT can be a candidate for the HEI more than
DOD.
DNPO
2.4-DCPO
)
-1
2-CPO
(sec
k
4-CPO
TCPO
DPO
2,6-DCPO
0
3
5
7
pKa
9
11
Fig. 6. Correlation of the CL decay rates (k) and pKa of phenols in the oxalates.
3.3. Evaluation of the Hammett correlation providing various
ꢀ-values and explanation by the frontier molecular orbital (FMO)
theory
As observed in the experimental results, various ꢀ-values with
a relatively large range were produced depending on the oxalate
structures, which suggests that the initial electronic interaction is
a rate-limiting process for the chemiexcitation step and the reactive
oxalates would give the polar species in the electronic interaction
between the HEI and the DSBs. Since the electronic interaction is
much faster than the chemical reactions forming the HEI and cannot
be kinetically observed as has been documented [3,6,14,17], the ꢀ-
value will be related to the electronic interaction between the HEI
and the fluorophores. For the high absolute ꢀ-value, the preferable
electronic interaction would promote the electron exchange pro-
cess to form a radical ion pair of the DSB radical cation and carbon
dioxide radical anion in the CIEEL process. It should be noted that
the formation of the excited DSBs is the result of the back elec-
tron transfer from the radical anions to the DSB radical cations, but
the present study applying the Hammett correlation focuses on the
initial stage of the electronic interaction between the HEIs and the
DSBs, which, however, controls the DSB chemiexcitation as a result.
A possible explanation for various ꢀ-values in the Hammett cor-
relation in the present PO-CL system is based on diversity of the
The Hammett substituent constants are also regarded as the
index of the electron releasing ability and parallel to the oxidation
potentials of the compounds bearing the Hammett substituents.
Therefore, the Hammett relationship has been applied to explain
the substituent effect on the electron transfer [47–50]. The Ham-
mett correlation also has been rationalized with the FMO theory
[51], in which the magnitudes of the ꢀ-values are closely related
to the energy gaps between the HOMOs and LUMOs of the
electron-donor and electron-acceptor, respectively. When two
molecular orbitals interact, the smaller the interval between the
FMO energies, the higher the interaction energy becomes. The small
difference in the energy gap of the FMOs leads to increase contri-
bution of the electronic effect on the reaction process, resulting in
the higher ꢀ-values. This treatment with the FMO theory for the
electronic interaction between the HEI and the DSBs corresponds
to the analysis with CIEEL mechanism. As the negative sign of the
ꢀ-values in the present PO-CL system indicates the interaction of
the DSB-HOMOs and DOT- or DOD-LUMOs, the FMO theory pre-
dicts that the lower LUMO energies of DOT leads to the smaller
25
18.13
13.49
0
0
O
O
(-376.9877)
HO
O
O
+
O
O
O
O
O O
-25
-50
O
C C OOH
(-684.4535)
OH
(-307.4584)
g
y
(
k
c
a
l
/
m
o
l
)
(-684.4750)
Ener
2CO2 (-188.5809 x 2)
+
-75
(-307.4584)
OH
-91.11
-100
Fig. 7. Relative heats of the formation of the high-energy intermediates and its decomposition. The values in parentheses are the energies in atomic unit (Hartree) calculated
by the ab initio method with a B3LYP/6-31g(d) basis set.

T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
229
Fig. 8. The HOMO (DSBs) and LUMO (DOTs and DOD) energies. Calculated by the ab initio method with a B3LYP/6-31g(d) basis set.
energy difference between their LUMOs and the DSB-HOMOs in the
endothermic electron transfer, resulting in the significant change
-1.2
in the electronic interaction rates along with the changes in the
ꢁp+-values, and finally increasing the absolute ꢀ-values.
The energies of the DSB-HOMOs and the DOT-LUMOs as well
as the DOD-LUMO were also calculated and their energy levels
and the representative orbital shapes are shown in Fig. 8. For the
DOT produced from DNPO, the LUMO + 2 was selected as the orbital
interacting with the DSBs because its LUMO and LUMO + 1 are local-
ized at the phenyl ring bearing two nitro groups. For all the DOTs
2,6-DCPO
TCPO
2-CPO
-1.4
-1.6
-1.8
-2.0
DPO
4-CPO
2,4-DCPO
(LUMO + 2 for the DOT from DNPO), the LUMOs extend on the O
O
bondings, which is reasonable for the proposed CIEEL process that
begins with an electron transfer from the fluorophores to the anti-
bonding orbitals of the DOTs. There is a remarkable difference in
the shapes of the LUMOs of the DOTs and DOD, the former of which
is anti-bonding but the latter of which is bonding, but both HEIs are
ready to accept an electron as revealed by the extending LUMOs on
the O O bondings. If both HEIs hold an electron to form their radical
anions, the O O bond cleavage will readily take place. Fig. 9 shows
the relationship between the ꢀ-values and the LUMO energies of
DNPO
0.5
0.7
0.9
1.1
|
|
Fig. 9. Plots of absolute ꢀ-value vs. LUMO energy.

230
T. Maruyama et al. / Journal of Photochemistry and Photobiology A: Chemistry 252 (2013) 222–231
HO
ArO
HO
ArO
O
O
Flu.
HO
ArO
O
Flu.
Flu.
O
O
O
O
O
O
*
+
+
+
CO2
Flu.
+
+
ArOH
CO2
ArOH
2CO2
Flu.
Scheme 2. A plausible CIEEL path involving the DOTs as the electron acceptors.
the DOTs, in which the oxalates with the higher absolute ꢀ-values
tend to have lower LUMO energies except for TCPO and 2,6-DCPO.
This tendency agrees with the prediction by the FMO theory, if the
DOT is involved in the electronic interaction during the CIEEL pro-
cess. On the other hand, the deviation by TCPO and 2,6-DCPO is also
significant because they would give much higher ꢀ-values from
the viewpoint of the FMO theory conflicting with the experimen-
tal observation. The steric hindrance in the HEIs would not only
decrease the CL decay rate but also restrain the electronic inter-
actions with the DSBs, which also correlates with the importance
of the DOTs as the HEIs because such results would be expected
for the aryl group containing HEIs. When the DOTs electronically
interact with the fluorophores, it would be possible that the DOT
radical anions decompose into phenols and CO2 including its radical
anions because the ꢁ* bonding of the DOT occupied with an electron
readily breaks resulting in a ring cleavage as shown in Scheme 2.
[8] J.H. Lee, J.C. Rock, M.A. Schlautman, E.R. Carraway, Characteristics of key
intermediates generated in uncatalyzed bis(2,4-dinitrophenyl) oxalate (DNPO)
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[9] J.H. Lee, J. Je, M.A. Schlautman, E.R. Carraway, Fast peroxyoxalate chemi-
luminescence for minimized analytical separation systems, Chemical Commu-
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4. Conclusions
In the present PO-CL reactions using several oxalates and the
DSB fluorophores, the linear Hammett correlations with various
ꢀ-values between the singlet chemiexcitation yields and the DSB
substituent constants (ꢁp+) were established, and the ꢀ-values
were related to the CL decay rates and LUMO energies of the
postulated HEIs such as the DOTs. The FMO theory explains the
experimental observations by the reason that the electronic inter-
action operates with various biases depending on the oxalate
structure and that the donor–acceptor electronic interaction deter-
mines the ꢀ-value.
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Acknowledgements
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This work was supported by the Global COE from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
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