Chemiluminescence behavior of fluorescent aromatics tethered 9-methylidene-10-methylacridans involving chemiluminescence resonance energy transfer (CRET) quenching

Article abstract of DOI:10.1016/j.tetlet.2012.11.149

The chemiluminescence (CL) behavior, observed in the singlet oxygenation of three fluorescent aromatics tethered 9-methylidene-10-methylacridans bearing the pyrene, perylene, and stilbene moieties, was investigated. The CL spectrum of 9-(perylen-3′-ylidene)-10-methylacridan displayed a red-shifted emission different from the fluorescent products, while 9-(pyren-1′- ylidene)-10-methylacridan and 9-(4′-styrylbenzylidene)- 1-methylacridan produced very weak CLs. A chemiluminescence resonance energy transfer (CRET) quenching of the excited aromatic aldehydes by the acridans remaining as the unreacted reactants was found to result in these unexpected CL behaviors. Copyright

Full text of DOI:10.1016/j.tetlet.2012.11.149

Tetrahedron Letters 54 (2013) 1338–1343
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Chemiluminescence behavior of fluorescent aromatics tethered
9-methylidene-10-methylacridans involving chemiluminescence resonance
energy transfer (CRET) quenching
Takayuki Maruyama a, Hiroaki Sugishita a, Harumi Kujira a, Musubu Ichikawa b, Yoshiyuki Hattori a,
Jiro Motoyoshiya a,
⇑
a Applied Chemistry Course, Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, Ueda, Nagano 386-8567, Japan
b Functional Polymer Science 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:
The chemiluminescence (CL) behavior, observed in the singlet oxygenation of three fluorescent aromatics
tethered 9-methylidene-10-methylacridans bearing the pyrene, perylene, and stilbene moieties, was
investigated. The CL spectrum of 9-(perylen-30-ylidene)-10-methylacridan displayed a red-shifted emis-
sion different from the fluorescent products, while 9-(pyren-10-ylidene)-10-methylacridan and 9-(40-sty-
rylbenzylidene)-10-methylacridan produced very weak CLs. A chemiluminescence resonance energy
transfer (CRET) quenching of the excited aromatic aldehydes by the acridans remaining as the unreacted
reactants was found to result in these unexpected CL behaviors.
Received 19 October 2012
Revised 27 November 2012
Accepted 30 November 2012
Available online 7 December 2012
Keywords:
Chemiluminescence
Acridan
Ó 2012 Elsevier Ltd. All rights reserved.
Singlet oxygen
Chemiluminescence resonance energy
transfer (CRET)
Chemiluminescence (CL) during the thermal decomposition of
the intermediate acridan–dioxetanes generated by the singlet oxy-
genation of 9-benzylidene-10-methylacridans has been occasion-
ally investigated,1 in which the corresponding aldehydes and the
fluorescent N-methylacridone (NMA), the latter of which is the
common emitter, were formed as the decomposition products as
shown in Scheme 1. Recently, some new aspects of their CL have
been reported. For example, the CL intensity is highly dependent
on the substituents attached to the benzylidene moiety,2 a CIEEL
(chemically initiated electron exchange luminescence) is applied
to the chemiexcitation process,3 and the azacrown ether tethered
benzylideneacridan is a potential probe for metal ion sensing based
on the interaction between the emitter, the excited NMA, and the
metal-coordinated azacrowned benzaldehyde.4 The thermal dioxe-
tane decomposition, in principle, produces two carbonyl com-
pounds, one of which is excited and emits light as a visible
output if either carbonyl compound is fluorescent. Therefore, it is
interesting to explore the chemiluminescent singlet oxygenation
of 9-methylidene-10-methylacridans possessing fluorescent moie-
ties because of the possibility of observing the unprecedented CL.
In this study, we report the CL behavior observed during the sin-
glet oxygenation of 9-methylidene-10-methylacridans bearing fluo-
⇑
Corresponding author. Tel.: +81 268 5402; fax: +81 268 5391.
E-mail address: jmotoyo@shinshu-u.ac.jp (J. Motoyoshiya).
Scheme 1. Chemiluminescence reaction of 9-benzylideneacridans.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.149

T. Maruyama et al. / Tetrahedron Letters 54 (2013) 1338–1343
1339
(a)
Figure 1. Structures of 9-methylidene-10-methylacridans used in this study.
rescent moieties such as pyrene, perylene, and stilbene moieties,
revealing that a chemiluminescence resonance energy transfer
(CRET) quenching occurred and provided an unexpected CL behavior
different from the known CL of the 9-benzylidene-10-methylacridans.
The three acridans, such as 9-(pyren-10-ylidene)-10-methylacri-
dan (1a),5 9-(perylen-30-ylidene)-10-methylacridan (1b),6 and 9-
(40-styrylbenzylidene)-10-methylacridan (1c),7 were prepared as
luminophores by the Horner–Wadsworth–Emmons reaction of 9-
diethylphosphono-10-methylacridan and the corresponding aro-
matic aldehydes,2,4 whose structures are shown in Figure 1. It is
important to note that all the acridans thus prepared are non-fluo-
rescent in spite of having fluorescent moieties probably due to the
fluorescence quenching by an intramolecular photo-induced elec-
tron transfer (PeT) from the nitrogen atom in the acridan ring to
(b)
the excited fluorescent moiety.8 Although 1a–c have the fully
p-
conjugated systems, the preliminary semi-empirical calculations
suggested that the methylidene moieties were twisted (Fig. 1S in
Supplementary data) and the conjugations would be separated be-
tween the acridan and the aromatic moieties, which would lead to
the intramolecular PeT as can be seen the several fluorescence
probes with twisted structures.8b,c
Figure 3. (a) The CL spectra of 1b with varying its concentration and (b) the
These acridans 1a–c were subjected to the CL reaction by the
singlet oxygenation using a combination of hydrogen peroxide
and sodium hypochlorite in aqueous THF as the singlet oxygen pro-
ducing systems.9 The CL intensities of 1a and 1c were too weak to
normalized fluorescence spectra of the solutions after the CL reaction at
EXk = 360 nm. [1b] = 1.0 ꢀ 10ꢁ5
,
2.5 ꢀ 10ꢁ5
,
5.0 ꢀ 10ꢁ5
,
1.0 ꢀ 10ꢁ4
,
2.5 ꢀ 10ꢁ4
,
5.0 ꢀ 10ꢁ4 and 1.0 ꢀ 10ꢁ3 M in THF/H2O (1:1). (a) The pictures of the CL were shot
by the exposure of 30 s. With correction of the brightness. (b) The fluorescence were
shot under UV light irradiation (375 nm).
record their CL spectra, but 1b gave a CL strong enough to obtain
the CL spectrum as shown in Figure 2, which was different from
the fluorescence spectra of both decomposition products such as
NMA and 3-formylperylene but red-shifted compared to their
fluorescences.
Interestingly, the CL spectrum of 1b gradually shifted from 538
to 595 nm (Fig. 3a) with the increasing intensity when the concen-
tration of 1b increased without changing any other conditions.
Such a red-shifting emission was also observed in the fluorescence
spectrum of the solution after the CL reaction of 1b, in which the
intense NMA fluorescence at the low concentration gradually de-
creased while the fluorescence of 3-formylperylene simulta-
neously red-shifted from 521 to 587 nm with the increasing
concentration of 1b as can be seen in Figure 3b. This fluorescence
behavior is somewhat complicated, namely, the remarkable de-
crease in the NMA fluorescence is due to the FRET (fluorescence
resonance energy transfer) from the excited NMA to 3-formylper-
ylene as indicated by the large overlapping of the NMA fluores-
cence and the 3-formylperylene absorption as shown in Figure 4a
as well as the excitation and fluorescence spectra shown in
Figure 4b and c, where the irradiation at 360 nm, at which NMA
has a strong absorption but 3-formylperylene a much weaker
absorption, increased the fluorescence of 3-formylperylene. In addi-
Figure 2. Chemiluminescence (CL) spectra of acridan 1b and fluorescence spectra
of the degraded products, N-methylacridone (NMA) and 3-formylperylene (3-FP).
The CL spectra of 1b; [1b] = 1.0 ꢀ 10ꢁ3 M, [NaClO] = 3.75 ꢀ 10ꢁ2 M, and
[H2O2] = 1.25 ꢀ 10ꢁ2 M. The fluorescence spectra; [3-FP] = 1.0 ꢀ 10ꢁ3 M, and
[NMA] = 1.0 ꢀ 10ꢁ3 M at EXk = 360 nm in THF/H2O (1:1). The picture of the CL
was shot by the exposure of 30 s. The fluorescence were shot under UV light
irradiation (375 nm).

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T. Maruyama et al. / Tetrahedron Letters 54 (2013) 1338–1343
(a)
(a)
(b)
(b)
(c)
(c)
Figure 4. (a) The normalized absorption (A) and fluorescence (F) spectra of
N-methylacridone (NMA) and 3-formylperylene (3-FP) at EXk = 360 nm. The
concentrations were 1.0 ꢀ 10–5 M and 1.0 ꢀ 10–3 M for the absorption and fluores-
cence spectra, respectively, in THF/H2O (1:1). (b) Excitation (EX) spectra of NMA
and 3-FP. [NMA] = [3-FP] = 1.0 ꢀ 10–4 M. (c) Fluorescence spectra of NMA, 3-FP and
the mixture of NMA and 3-FP. EXk = 360 nm. [NMA] = [3-FP] = 1.0 ꢀ 10–4 M.
Figure 6. The normalized absorption spectra of the acridans 1a–c, and the
normalized fluorescence of the corresponding aldehydes. [acridan] = 1.0 ꢀ 10ꢁ5
M
and [aldehyde] = 1.0 ꢀ 10ꢁ3 M in THF/H2O (1:1).
concentration, the red-shift due to the excimer formation was ex-
cluded. On the other hand, a significant red-shift was observed with
the decreasing fluorescence intensity when 3-formylperylene and
1b were mixed in different ratios (Fig. 5), indicating that the large
red-shifts were due to the interaction of the excited 3-formylperyl-
ene and non-fluorescent 1b. The possibility of an exciplex formation
of 3-formylperylene and 1b was investigated by the measurement of
the fluorescence lifetimes of the mixed solution at various ratios of
both compounds. All solutions containing different ratios of 3-
formylperylene and 1b in THF/H2O (1/1), at which the red-shift fluo-
rescence was observed, produced lifetimes of 0.25 or 0.26 ns, the
same as that for 3-formylperylene (Fig. S2 in Supplementary data),
revealing that the exciplex formation between 3-formylperylene
and 1b was also excluded but that the excited species in the CL reac-
tion of 1b would be 3-formylperylene in spite of the difference be-
tween its fluorescence and CL spectra.
Figure 5. The fluorescence spectra with changing each ratio of the acridans (1a)
and
3-formylperylene
(3-FP)
at
EXk = 360 nm
in
THF/H2O
(1:1).
[aldehyde] + [acridan] = 5.0 ꢀ 10ꢁ4 M.
Considering the spectral study described above, we focused our
attention on the absorptions of the reactants, 1a–c, having kmax
values of 400–450 nm and found that the absorption spectrum of
1b overlapped with the blue side of the fluorescence of 3-formyl-
perylene, while those for 1a and 1c overlapped with large parts
tion, the observed red-shift is caused by some other factor still to be
determined. Because there was no significant change in the
fluorescence spectrum of 3-formylperylene while changing its

T. Maruyama et al. / Tetrahedron Letters 54 (2013) 1338–1343
1341
(a)
(c)
(b)
(d)
Figure 7. The absorption spectra of (a) Congo red, (b) Orange 1, (c) Indigo and (d) Fast green FCF, and the fluorescence spectra with changing each ratio of 3-formylperylene
(3-FP) and these dyes at EXk = 360 nm. For the absorption spectra, [dye] = 1.0 ꢀ 10ꢁ5 M. For the fluorescence spectra, [3-FP] + [dye] = 5.0 ꢀ 10ꢁ4 M in THF/H2O (1:1).
of the fluorescences of the corresponding aldehydes as shown in
Figure 6. Therefore, for 1b the excitation energy transfer8 from
the excited aldehydes to non-fluorescent reactants should cut off
a part of the fluorescence, and the residual emission led to the
apparent red-shift with their decreasing intensities. As observed
in Figure 5, for the higher ratios of the reactant 1b, more red-shifts
in the fluorescence of 3-formylperylene were observed with their
decreasing intensities. On the other hand, the effective fluores-
cence quenching would occur for 1a and 1c because of large
amounts of overlapping.
Such changes in the fluorescence spectra prompted us to ex-
plore the fluorescence shift by the excitation energy transfer dur-
ing the interaction with some non-fluorescent dyes but with
strong absorptions between 400 and 650 nm. As expected, the
fluorescence shifts of 3-formylperylene were observed by adding
four well-known organic dyes such as Congo red, Orange 1, Indi-
go, and Fast green FCF. Congo red and Orange 1 with absorptions
on the blue side of the 3-formylperylene fluorescence shifted its
fluorescence from 588 to 597 nm and from 556 to 568 nm with
the increasing concentration of the dyes as shown in Figure 7a
and b, respectively. In contrast, for other two dyes with their
absorptions on the red side of the 3-formylperylene fluorescence,
the fluorescence shifted from 543 to 539 nm by the interaction
with Indigo and from 531 to 515 nm for Fast green FCF as shown
in Figure 7c and d, respectively. Thus, the fluorescence can be not
only red-shifted, but also blue-shifted based on the choice of the
additive dyes with the absorptions overlapping the fluorescence
of the aldehyde. These observations prompted us to examine
the effect of the additive dye as a chemical filter in the CL of
1b. As illustrated in Figure 8, Fast green FCF erased the CL emis-
sion and the residual emission can be seen at both ends of the
emission spectrum when an equimolar amount of the dye toward
1b was added to the CL system.
Now the CL behavior of 1b can be explained considering the
experimental results. There are two ways for the excitation of
the carbonyl compounds, NMA, or the aldehydes by the decompo-
sition of the intermediate dioxetanes as illustrated in Scheme 2.
Although the excited 3-formylperylene might be formed because
of their singlet energies lower than NMA as found in their spectra,
the initial formation of the excited NMA is anticipated considering
the involvement of a CIEEL process for the acridan-dioxetane
decomposition as previously documented, in which the initial pro-
cess is an electron transfer from the nitrogen atom to the dioxetane
O–O bonding followed by the cleavage of O–O r⁄ bonding to form
the excited NMA and the aldehydes. There are two possibilities for
the initial electron transfer from the nitrogen atom of the acridans
or from the perylene moiety; namely, the former generates the ex-
cited NMA, while the latter directly generates the excited 3-form-
Figure 8. The CL spectra of 1b in the presence of Fast green FCF. [1b] = 1.0 ꢀ 10–3 M,
[NaClO] = 3.75 ꢀ 10–2 M, and [H2O2] = 1.25 ꢀ 10–2 M in THF/H2O (1:1).

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T. Maruyama et al. / Tetrahedron Letters 54 (2013) 1338–1343
Scheme 2. A plausible pathway of chemiluminescent reaction of 1b involving a partial CRET quenching.
ylperylene. The former is probable because the oxidation potential
for 10-methylacridan was much lower than that of perylene
(Fig. 3S in Supplementary data) and as its corroboration the HOMO
of 1b was localized at the acridan region while the LUMO at the
perylene moiety (Fig. 4S in Supplementary data). The CRET from
the excited NMA to the fluorescent aldehydes was followed by
the partial CRET quenching cutting off the part of the emission of
aldehydes and leading to the red-shift emission, in which the en-
ergy acceptor was the non-fluorescent 1b still remaining in the
reaction mixture as the unreacted reactant. The slight difference
in the emission wavelength between the CL and the fluorescence
spectra can also be explained by the different ratios of 3-formyl-
perylene and 1b in the solutions. On the other hand, the very
weak CL observed for 1a and 1c would be due to a similar circum-
stance, in which the emissions from the excited NMA, 1-formylpy-
rene, or 4-styrylbenzaldehyde were almost erased by the CRET
quenching by the reactants 1a and 1c, respectively, because of
the large overlapping of the reactant absorptions with the fluores-
cence of the corresponding aldehydes. CRET is a phenomenon that
has been usually observed in some bioluminescence10 or suitably
constructed CL systems,11 but the present case is different from
this CRET, because the energy acceptor is the non-fluorescent reac-
tants themselves used as the luminophores and act as the partial
CL quenchers.
In conclusion, the CL behavior of 9-methylideneacridans with
the pyrene, perylene, and stilbene moieties (1a–c) during the sin-
glet oxygenation was investigated, and the unexpected CL provid-
ing the very weak or red-shifted light emissions was explained by
the partial CRET quenching of the excited aldehydes, one of the
fluorescent decomposition products, by the unreacted reactants
used as the luminophores remaining in the solution. The excitation
energy transfer from the fluorescence of 3-formylperylene, the
emitter for the CL reaction of 1b, to various commercially available
dyes was also studied, in which the fluorescence not only red-, but
also blue-shifted depending on the absorption of the additive dyes.
The CRET quenching has been only slightly documented,12 but it is
important to design new luminophores. As one of the require-
ments, the luminophore with little overlapping between its
absorption and the emitter fluorescence will be suitable for the
efficient CL.

T. Maruyama et al. / Tetrahedron Letters 54 (2013) 1338–1343
1343
J = 8.3 Hz), 7.10 (d, 1H, J = 8.4 Hz), 7.15 (d, 1H, J = 8.1 Hz), 7.35 (t, 1H, J = 7.5 Hz),
7.40 (s, 1H), 7.85 (d, 1H, J = 7.9 Hz), 7.95 (d, 2H, J = 7.3 Hz), 7.97–8.02 (m, 3H),
8.06 (d, 1H, J = 9.0 Hz), 8.14 (d, 2H, J = 7.6 Hz), 8.43 (d, 1H, J = 9.0 Hz); 13C NMR
(100 MHz, CDCl3) d 34.10, 113.05, 113.47, 120.41, 120.92, 121.78, 122.68,
124.39, 125.05, 125.22, 125.34, 125.40, 125.47, 125.64, 126.29, 126.95, 127.24,
127.38, 127.64, 127.92, 128.76, 128.80, 129.57, 129.65, 130.54, 131.73, 131.89,
Acknowledgments
This work was supported by a Grant-in-Aid (19550137) and the
Global COE from the Ministry of Education, Culture, Sports, Science
and Technology of Japan.
133.85, 134.52, 141.69, 143.20; HRMS (ESI): Calcd for
408.1729, found: 408.1747.
C
31H22N([M+H]+):
6. Compound 1b: This product was prepared in a manner similar to the procedure
described above using 3-formylperylene (0.22 g, 0.78 mmol) in THF (15 ml), 9-
(diethylphosphono)-N-methylacridan (0.31 g, 0.94 mmol) and and t-BuOK
(0.18 g, 1.6 mmol) in THF (15 ml). Yield: 62% (0.22 g) as a red crystal; mp
234–236 °C; 1H NMR (400 MHz, CDCl3) d 3.51 (s, 3H), 6.56 (t, 1H, J = 7.6 Hz),
6.98–7.05 (m, 3H), 7.07–7.18 (m, 3H), 7.31–7.36 (m, 2H), 7.39–7.48 (m, 2H),
7.49 (t, 1H, J = 8.0 Hz), 7.63 (t, 2H, J = 9.0 Hz), 7.90 (dd, 1H, J = 7.9, 1.3 Hz), 7.97
(d, 1H, J = 7.9 Hz), 8.06 (d, 2H, J = 8.1 Hz), 8.17 (d, 1H, J = 7.5 Hz), 8.20 (d, 1H,
J = 7.5 Hz); 13C NMR (100 MHz, CDCl3) d 34.07, 112.98, 113.46, 120.31, 120.52,
120.57, 120.60, 120.62, 120.90, 121.70, 122.67, 124.30, 125.35, 126.82, 126.97,
126.99, 127.35, 127.87, 128.17, 128.70, 128.77, 129.01, 129.44, 129.54, 130.23,
131.83, 131.87, 132.06, 133.85, 133.88, 135.15, 136.59, 141.65, 143.01; HRMS
(ESI): Calcd for C35H23N([M]+): 457.1829, found: 457.1825.
7. Compound 1c: This product was prepared in a manner similar to the procedure
described above using 4-formyl-(E)-stilbene (0.20 g, 0.96 mmol) in THF
(15 ml), 9-(diethylphosphono)-N-methylacridan (0.32 g, 0.96 mmol) and and
t-BuOK (0.32 g, 2.9 mmol) in THF (15 ml). Yield: 58% (0.22 g) as a yellow
crystal; mp 225 °C (dec); 1H NMR (400 MHz, CDCl3) d 3.48 (s, 3H), 6.66 (s, 1H),
6.77 (t, 1H, J = 7.5 Hz), 6.98 (d, 1H, J = 8.3 Hz), 7.02–7.07 (m, 4H), 7.21–7.38 (m,
9H), 7.41 (dd 1H, J = 7.8, 1.6 Hz), 7.49 (d, 2H, J = 8.3 Hz), 7.72 (dd, 1H, J = 7.9,
1.4 Hz); 13C NMR (100 MHz, CDCl3) d 33.99, 112.84, 113.72, 120.47, 121.74,
122.59, 112.67, 123.86, 126.77, 126.88, 127.19, 127.90, 128.42, 128.44, 128.98,
129.04, 129.10, 129.28, 132.20, 135.71, 137.95, 138.36, 141.62, 143.45; HRMS
(ESI): Calcd for C29H23N([M]+): 385.1833, found: 385.1825.
Supplementary data
Supplementary data (figure 1S, 2S, and 1H and 13C NMR spectra
of 1a–c) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2012.11.149.
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