Solvent and excitation wavelength effects on photocleavage of N-(9-anthroyloxy)-9-fluorenylideneamine as a precursor of 9-anthroyloxyl radicals. A mechanistic study

Article abstract of DOI:10.1016/S0040-4039(00)01476-3

Photocleavage of the N-O bond of N-(9-anthroyloxy)-9-fluorenylideneamine takes place efficiently in acetonitrile in the excited singlet state attributed to the fluorenylidene moiety. This made it possible for the first time to directly observe 9-anthroyloxyl radicals. However, the efficiency decreases remarkably in benzene in which the lowest excited singlet state is attributed to the anthroate moiety. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01476-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8353–8357
Solvent and excitation wavelength effects on photocleavage
of N-(9-anthroyloxy)-9-fluorenylideneamine as a precursor
of 9-anthroyloxyl radicals. A mechanistic study
Yasuo Saitoh,a Katsunori Segawa,a Hiroki Itohb and Hirochika Sakuragia,*
aDepartment of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
bDepartment of Material and Biological Chemistry, Faculty of Science, Yamagata University,
Yamagata 990-8560, Japan
Received 3 July 2000; revised 28 August 2000; accepted 1 September 2000
Abstract
Photocleavage of the NꢀO bond of N-(9-anthroyloxy)-9-fluorenylideneamine takes place efficiently in
acetonitrile in the excited singlet state attributed to the fluorenylidene moiety. This made it possible for
the first time to directly observe 9-anthroyloxyl radicals. However, the efficiency decreases remarkably in
benzene in which the lowest excited singlet state is attributed to the anthroate moiety. © 2000 Elsevier
Science Ltd. All rights reserved.
Keywords: esters; fluorescence; photochemistry; radicals and radical reactions; solvents and solvent effects.
1. Introduction
Diaroyl peroxides are useful precursors of aroyloxyl radicals and have been used for studies
on various kinds of benzoyloxyl and naphthoyloxyl radicals.1–10 Anthroyloxyl radicals are
attractive species to study, since they have a more expanded conjugated ring system and the
carbonyloxyl moiety suffers varying extents of steric hindrance depending on its attached ring
position. Among some anthroate esters of aromatic and aliphatic ketoximes employed as
precursors, only fluorenone oxime esters have been found to produce the anthroyloxyl radicals
efficiently in polar solvents. According to the previously proposed triplet mechanism,11–13
fluorenone oxime esters should be less reactive than acetophenone and benzophenone oxime
esters. Firstly, we have undertaken to clarify the mechanism of photocleavage of the radical
precursors in polar solvents and herein describe the results as studied from the viewpoints of
solvent and excitation wavelength effects.
* Corresponding author. Tel: 0298-53-4239; fax: 0298-53-6503; e-mail: sakuragi@staff.chem.tsukuba.ac.jp
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01476-3

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2. Photocleavage of the oxime ester
N-(9-Anthroyloxy)-9-fluorenylideneamine (9AFA, Scheme 1) exhibits absorption bands
around 300 and 365 nm due to the fluorenylidene and anthroate moieties, respectively (Fig. 1).
9AFA was photolyzed with 366 nm stationary light in a low concentration (1×10−3 mol dm−3)
in acetonitrile under argon at ambient temperature to give 9-anthroic acid (AA; 0.92 mol/mol
oxime ester) and fluorenone azine (FAz; 0.42) as products. Similar irradiation of 9AFA at 300
nm gave almost the same results. The formation of FAz indicates intermediacy of 9-fluorenyli-
deneaminyl radicals,11 which must be generated together with 9-anthroyloxyl radicals (9-
AnthCO2’) via homolysis of the NꢀO bond of 9AFA. The quantum yields for photocleavage of
9AFA (1–3×10−4 mol dm−3) were determined to be 0.10 with 302 or 365 nm light in acetonitrile,
and 0.04 and 0.01 with 302 and 365 nm light, respectively, in benzene. It is noticeable that the
quantum yield is dependent on the solvents and excitation wavelengths. In the above concentra-
tion range the quantum yields were not affected.
Scheme 1.
Figure 1. Absorption, fluorescence and fluorescence excitation spectra of 9AFA in acetonitrile. The spectra were
normalized at a band maximum except for the fluorescence spectrum measured on 300 nm excitation; this spectrum
was depicted relative to the fluorescence spectrum measured on 364 nm excitation
3. Direct observation of the radical species
Pulsed laser photolysis of 9AFA (5×10−4 mol dm−3) at 308 nm under argon in acetonitrile at
ambient temperature exhibited transient absorption bands at 400–500 and 500–800 nm, as

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shown in Fig. 2. Similar laser photolysis of 9AFA at 360 nm gave almost the same spectrum.
The longer-wavelength band exerted essentially identical decay time constants of 6–7 ms at 580,
630 and 720 nm, and can be reasonably assigned to 9-AnthCO2’ in comparison with the spectra
of 2,6-dimethylbenzoyloxyl and other aroyloxyl radicals.2–8 For this band, molecular oxygen
neither reduced spectral intensity nor affected spectral and decay profiles. These facts not only
support the above assignment of the oxygen-centered radicals, but also indicate that excited
triplet states are not involved in the radical formation.
Figure 2. Transient absorption spectra observed on 308 nm pulsed laser excitation of 9AFA in acetonitrile under
argon
The shorter-wavelength band may be ascribed to a plural species, such as 9AFA triplets and
a long-lived intermediate which is formed concurrently with the disappearance of 9-AnthCO2’.
Biacetyl-sensitized excitation of 9AFA (5×10−4 mol dm−3) with 444 nm laser pulses in aceto-
nitrile exhibited an absorption band around 430 nm (lifetime (~T) 7.8 ms), which was assigned to
T–T absorption of the anthroate moiety (ET 177 kJ mol−1).14 However, no bands assignable to
the radicals were detected in the 500–800 nm region. These observations also indicate that the
9AFA triplets are not responsible for the NꢀO bond cleavage. This is in contrast to the
photocleavage of some acetates and benzoates of aromatic ketoximes, which proceeds in the
triplet state.11–13 The anthroate triplet energy may be insufficient for the bond cleavage.13
Behavior of 9-AnthCO2’ and detailed analysis of the shorter-wavelength band will be reported
elsewhere.
Laser photolysis of 9AFA (5×10−4 mol dm−3) at 308 nm in benzene also exhibited two bands,
one attributable to 9-AnthCO2’ at 500–800 nm and the other mainly due to 9AFA triplets
around 430 nm; the latter was much stronger in intensity than the former.
4. Roles of excited singlet states played in the photocleavage
The fluorescence spectra of 9AFA (1×10−4 mol dm−3) are structureless and very similar to
those of methyl 9-anthroate (M9A, 1×10−4 mol dm−3) in acetonitrile (Fig. 1) and benzene,
respectively, on excitation at 364 nm. The fluorescence quantum yield (ƒf) and lifetime (~f) of
9AFA were, however, much lower and shorter, respectively, than those of M9A, as determined
in acetonitrile (in benzene) to be ƒf=0.31 (0.69) and 0.01 (0.05) and ~f=8.3 (12.0) and 0.60 (7.7)

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ns for M9A and 9AFA, respectively. The results were not affected up to a substrate concentra-
tion of 1×10−4 mol dm−3. These results indicate that the anthroate singlet is efficiently quenched
by the fluorenylidene moiety. The interaction between the two chromophores was confirmed by
quenching of the M9A fluorescence by N-methoxy-9-fluorenylideneamine (MFA); linear Stern–
Volmer plots of fluorescence lifetimes gave quenching rate constants (kq) of 1.5×1010 and 8.3×109
mol−1 dm3 s−1 in acetonitrile and benzene, respectively. The Stern–Volmer plots of fluorescence
intensities provided much larger values of the quenching rates, suggesting a possibility of static
interaction in the ground state.
The fluorescence excitation spectrum of 9AFA monitored at 462 nm shows no clear peak
around 300 nm in acetonitrile, though the absorption spectrum exhibits the band due to
fluorenylidene moiety in this region (Fig. 1). A very weak fluorescence spectrum observed on 300
nm excitation in acetonitrile was more than 20 nm shifted to the longer wavelengths compared
to that observed on 364 nm excitation, and can be ascribed to the fluorenylidene moiety.
Actually, MFA exhibited a weak fluorescence in a similar region on 300 and 361 nm excitation.
In benzene, however, the excitation spectrum of 9AFA exhibited a weak but clear band around
300 nm, and the excitation at 300 nm gave the same fluorescence spectrum as did the 364 nm
excitation.
These facts indicate that the relative singlet energy levels of the two chromophores in 9AFA
change with the polarity of solvents; the lowest excited singlet of the fluorenylidene moiety
(S2(F)) is higher in energy than, or similar to, that of the anthroate moiety (S1(A)) in benzene
(Fig. 3). In acetonitrile, however, the lowest excited singlet of the fluorenylidene moiety (S1(F))
is clearly lower than that of the anthroate moiety (S2(A)) and may be responsible for the NꢀO
cleavage. This order of the singlets is consistent with other spectral behavior; the fluorescence
excitation spectrum is slightly shifted in the 350–400-nm region to the shorter wavelengths
compared to the absorption spectrum (Fig. 1). This can be interpreted in terms of location of a
weak absorption band due to the fluorenylidene moiety extending to slightly longer wavelengths
than that of the anthroate moiety, coupled with the low emission efficiency of S1(F). Inspection
of absorption and fluorescence spectra of M9A and MFA indicates that the difference in the
singlet energy between M9A and MFA is larger in acetonitrile than in benzene though the
singlet level of M9A is higher than that of MFA in both solvents.
Figure 3. Schematic energy diagrams of 9AFA in polar and nonpolar solvents

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Thus, the irradiation of 9AFA at 300 nm (ca. 400 kJ mol−1) in acetonitrile results in the
production of a higher excited singlet state of the fluorenylidene moiety (probably S3(F); Fig. 3)
followed by its internal conversion to the lowest excited singlet state, S1(F) (ES ca. 290 kJ mol−1,
as estimated from absorption and fluorescence spectra), which undergoes NꢀO bond cleavage to
give the radicals. The weak fluorescence results predominantly from S1(F). On the contrary, the
irradiation at 365 nm produces the S2(A) state (ca. 300 kJ mol−1 from absorption and
fluorescence spectra), which undergoes fluorescence emission or energy transfer to the fluorenyli-
dene moiety to give S1(F), leading to the NꢀO bond cleavage. In benzene, however, the S2(F)
state produced by 300 nm irradiation followed by internal conversion undergoes predominantly
energy transfer to the anthroate moiety to produce S1(A), which undergoes fluorescence
emission or intersystem crossing to give eventually the T1(A) state; thus the NꢀO cleavage is less
efficient.
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