Observation of intramolecular dimer radical anion of 1,1-diarylmethanols bearing electron withdrawing groups at room temperature

Article abstract of DOI:10.1016/S0009-2614(00)01089-7

Transient absorption measurement of radical anions of 1,1-diarylmethanols bearing electron-withdrawing groups in dimethyl-formamide at room temperature has revealed that symmetric compounds form intramolecular dimer radical anions as an intermediate of th

Full text of DOI:10.1016/S0009-2614(00)01089-7

3 November 2000
Chemical Physics Letters 330 (2000) 97±102
www.elsevier.nl/locate/cplett
Observation of intramolecular dimer radical anion of
1,1-diarylmethanols bearing electron withdrawing groups
at room temperature
Nobuyuki Ichinose *, Junpei Hobo, Sachiko Tojo, Tetsuro Majima
The Institute of Scienti®c and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received 12 June 2000; in ®nal form 4 September 2000
Abstract
Transient absorption measurement of radical anions of 1,1-diarylmethanols bearing electron-withdrawing groups in
dimethyl-formamide at room temperature has revealed that symmetric compounds form intramolecular dimer radical
anions as an intermediate of their reductive C±O bond dissociation giving diarylmethyl radicals. The dimer radical
anions showed an absorption band in the near infrared region (NIR) (1000±1600 nm) characteristic to their charge
resonance (CR) properties. On the contrary to 1,1-diarylmethanols, corresponding radical anion of 1,3-diarylpropan-1-
ol showed no CR band. It is suggested that formation of dimer radical anions is energetically favorable for diaryl
compounds with one-atom spacer but unfavorable for them with three-atom spacer being di€erent from the n ˆ 3 rule
for the favorable formation of intramolecular excimers. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction
examples for inter- and intra-molecular dimer rad-
ical anions have been reported in room temperature
solutions and even at low temperature [12±16].
We now report an intramolecular dimer radical
anion of 1,1-diarylmethanols bearing electron-
withdrawing groups formed as an intermediate of
the C±O bond dissociation upon attachment of
solvated electron (es ) during pulse radiolysis in
N,N-dimethylformamide (DMF) solution. It has
been also suggested that the 1,3-bichromophoric
structure is not favorable for the dimer radical
anion formation unlike excimer formation known
as the `n ˆ 3 rule' [17].
Formation of dimer radical cations in solution
has been studied intensively by pulse radiolysis and
laser ¯ash photolysis [1±5]. Optical absorption
spectroscopy a€ords information on the dynamics,
structure, and stability of dimer radical cations, in
particular, on the charge resonance (CR) energy
from the absorption spectra in the near infrared
(NIR) region of 800±2000 nm [2,6±8]. This ab-
sorption is a characteristic feature of the electronic
interaction in dimer radical cations and also ob-
served for some intramolecular radical pairs [9±11].
On the contrary to dimer radical cations, fewer
2. Experimental
* Corresponding author. Fax: +81-6-6879-8499.
E-mail addresses: ichinose@sanken.osaka-u.ac.jp (N. Ichi-
nose), majima@sanken.osaka-u.ac.jp (T. Majima).
N,N-Dimethylformamide (DMF, Wako in®nity
pure grade) and 1,1-diphenylmethanol (1a, Tokyo
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 8 9 - 7

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N. Ichinose et al. / Chemical Physics Letters 330 (2000) 97±102
Chemicals) were used as received. 2-Methyl
tetrahydrofuran (MTHF, Tokyo Chemicals) was
distilled over CaH2 before use. Diarylmethanols,
1,1-bis(4-cyanophenyl)methanol (1b), 1,1-bis(4-
methoxycarbonylphenyl)methanol (1c), 1-(4-cyano-
phenyl)-1-phenylmethanol (1d), and 1-(4-meth-
oxycarbonylphenyl)-1-phenylmethanol (1e) were
synthesized by reduction of the corresponding
benzophenones with NaBH4 in methanol. These
diarylmethanols were recrystallized from hexane.
1,3-bis(4-cyanophenyl)propan-1-ol (3) was pre-
pared from reduction of 1,3-bis(4-cyanophenyl)-2-
propene-1-one. Synthetic details will be reported in
a full length paper.
band width of 10 nm) and its intensity was moni-
tored with a fast InGaAs PIN photodiode equip-
ped with an ampli®er (Thorlabs, PDA255) and
digital oscilloscope (Tektronix, TDS-380P). In-
frared and visible absorption spectra of c-irradi-
ated 77 K MTHF matrices of 1 ꢀ5:0  103 M† in a
sealed quartz cuvette ꢀ0:1  1  4 cm3† were re-
corded on a Shimadzu UV-3100PC spectro-
photometer. The irradiated samples were further
irradiated with visible±NIR light (>600 nm) from
a 500 W Xe lamp to activate the matrix-trapped
electron. After the light irradiation trapped elec-
tron was captured by substrate molecule and dis-
appearance of the corresponding absorption
around 1200 nm was observed. Fluorescence
spectra of the compounds were measured in
MTHF at room temperature on a JASCO FP-550
spectro¯uorometer equipped with an analogue-
digital converter (Nippon Filcon J. J. JOKER J-1)
and were transferred to an MS-DOS personal
computer (NEC PC-9801 NL).
Transient absorption spectra of argon-purged
2
DMF solution of 1 ꢀ2:0  10 4±1:0  10 M† in a
quartz cuvette ꢀ1  1  4 cm3† were obtained up-
on the electron pulse irradiation (28 MeV, 8 ns,
0.7 kGy) with a gated multichannel spectrometer
system (UNISOKU, TSP-601-02). A focussed Xe
arc (300±2000 nm), which was synchronized with
the electron pulse was passed through the sample
as a monitor light for the transient absorption
measurement. Time-pro®les of the transient ab-
sorption at the UV±visible region were measured
with a monochromator (CVI, DK-240) equipped
with a photomultiplier (Hamamatsu R928) as re-
ported before [18]. The monitor ligh for the
measurement of time-pro®les of the transient
absorption in infrared region was passed through
an interference ®lter (CVI, transmittance of 40%,
3. Results and discussion
Pulse radiolysis of DMF leads to ionization of
DMF giving es , which shows an absorption in the
NIR region of 1200±1600 nm [19]. It has been re-
ported that diphenylmethanol (1a) reacts with es to
give diphenylmethyl radical Ph2CHáꢀ2a† [20].
Transient absorption spectrum of 1a showed a

N. Ichinose et al. / Chemical Physics Letters 330 (2000) 97±102
99
peak at kmax ˆ 330 nm assigned to 2a immediately
after an electron pulse irradiation during pulse
radiolysis of its argon-purged DMF solution
ꢀ5:0  103 M†. No transient absorption of 1a ra-
dical anion ꢀAr2CHOHá ; 1aá † was observed. On
the other hand, introduction of electron-with-
drawing groups on the phenyl ring retarded the C±
O bond dissociation by stabilizing the radical anion
as an intermediate. The transient absorption spec-
trum with a peak at kmax ˆ 328 nm was observed
immediately after an electron pulse, and decayed
with the formation of peaks at kmax ˆ 362 and 530
nm during pulse radiolysis of 1,1-bis(4-cyanophe-
nyl)methanol (1b) (Fig. 1). The absorption spec-
trum with the peak at 328 nm was assigned to local
excitation band of the 4-cyanophenyl group radical
anion in Ar2CHOHá ꢀ1bá † from its similarity to
the spectrum of 4-cyanotoluene radical anion,
while the peak at kmax ˆ 362 nm was to Ar2CHá
ꢀ2b†. The absorption around 530 nm was assigned
to a by-product from 1bá because the rate for its
formation was ®rst-order (the rate constant of
1
3:0  104 s † and independent of the concentra-
tion of 1b or 1bá . Since the rate is slower by one
order than that for the 2b formation, this species
would not be important (see below). Radical an-
ions 1c±eá together with radical 2c±e were simi-
larly observed both for symmetric and unsymmetric
compounds 1c±e. For 1 bearing methoxycarbonyl
groups (1c and 1e), transient absorption was as-
signed to 1cá and 1eá on the basis of the absorp-
tion spectra of benzoic acid esters [21]. Radical
anions 1b±eá decayed slowly to give the corre-
sponding diarylmethyl radicals (2b±e) obeying a
Fig. 1. Transient absorption spectra observed during the pulse
radiolysis of the DMF solutions of 1a, (a), and 1b, (b) ([1] ˆ
5:0 Â 103 M).
1
®rst-order kinetics ꢀkr ˆ ꢀ2 3†  105 s † and
their lifetimes were independent of the mono- or di-
substitution.
second-order rate constants kea for 1b±e were
The rate for electron attachment ꢀkea; M 1s
†
1
nearly
di€usion-controlled
rate
ꢀꢀ3 Æ 1† Â
1010
M
1s † and were higher than that of 1a
1
was measured from the decay of transient ab-
sorption of es monitored at 1550 nm. The decay of
es in pure DMF was analyzed with a single ex-
ponential function with a rate constant of ke
ꢀꢀ4 Æ 1†  109 M 1s †. However, the decay curve
observed at 1550 nm for 1b,c showed a long tail
with a half lifetime of 10±12 ls but not for 1d,e
(Fig. 2). The rise and lifetime of 1b; cá monitored
at 330 nm agreed well with the fast and slow
components of the decay observed at 1550 nm.
Apparently, the transient absorption observed at
1550 nm was assigned to an absorption band of
1b, cá at this region. The time-resolved absorption
1
1
ꢀke ˆ 5:3  106 s † except for the initial time
range shorter than 20 ns, during which the decay
seemed to involve a fast recombination process.
The absorption at 1550 nm decayed completely
within 800 ns. The decay rate of es increased with
the concentration: decay rate ˆ kea‰1Š ‡ ke. The

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N. Ichinose et al. / Chemical Physics Letters 330 (2000) 97±102
Fig. 2. (a) Short range decay curves of the transient absorption at 1550 nm observed during the pulse radiolysis of DMF (none, dashed
line) and 1a,b and 1d (dotted, bold-solid, and solid lines, respectively) in DMF ([1] ˆ 5:0  103 M); (b) long range decay curves of the
transient absorption at 1550 nm observed during pulse radiolysis of 1b and 1d (bold-solid and solid lines, respectively) in DMF ([1] ˆ
5:0 Â 103 M). Both short and long range decay curves of 1b involve the decay of the CR band of 1bá as well as the decay of es ;
(c) transient absorption spectra in the NIR region obtained from decay curves during pulse radiolysis of 1b in DMF.
spectra of es in DMF and 1bá in the range of 800±
1600 nm were obtained from the decay curves
measured at every 100 nm step. While the peak
wavelength of the absorption spectrum of es was
at around 1400 nm, that of 1bá was at >1600 nm.
We measured the absorption spectra of 1b±eá by
low-temperature (77 K) c-radiolysis in MTHF.
Although the NIR absorption spectra peaked at
1780 and 1930 nm were observed for 1b and 1c,
respectively, no peaks were observed in this region
for 1d,e (Fig. 3). From the similarity of the struc-
tureless feature of the NIR sbsorption spectra to
those reported for CR band of dimer radical an-
ions [12,14±16], we assigned the NIR absorption to
the CR band of the intramolecular dimer radical
anions of 1b; cá . From the peak wavelengths, CR
energies (DHCR) were determined to be 8.0 and 7.4
1
kcal mol for 1bá and 1cá , respectively. The

N. Ichinose et al. / Chemical Physics Letters 330 (2000) 97±102
101
either by pulse radiolysis or by c-radiolysis of 3.
Although 3 has an unsymmetrical structure, such
unsymmetry does not inhibit the excimer forma-
tion [17]. Yamamoto et al. attempted to observe
intramolecular dimer radical anion of naphthalene
chromophore with 1,3-di-b-naphthylpropanes, but
they observed only monomer-like radical anion of
naphthalene chromophore [22]. We also failed
in observing the CR band of radical anion of
1,3-di-a-naphthylpropane and 1,3-di-1-pyrenyl-
propane upon c-radiolysis in 77 K MTHF matrix
[23]. Pulse radiolysis of 1,3-di-1-pyrenylpropane in
tetrahydrofuran [24] and 1,3-di-b-naphthylpropan-
1-ol in DMF [23] at room temperature are only
monomeric radical anions of pyrene and naph-
thalene chromophore without showing the CR
band, although pulse radiolysis of these com-
pounds in 1,2-dichloroethane gave the corre-
sponding intramolecular dimer radical cations. On
the other hand, Yamamoto et al. assumed intra-
molecular dimer radical anion of methylterephth-
alate chromophore in the kinetic study on the
anion radical transfer from methylterephthalate to
1,4-dicyanobenzene where the rate was slower for
diester of 1,2-ethanediol than that of 1,3-propane-
diol [25]. These suggest that the formation of in-
tramolecular dimer radical anions may di€er from
the Hirayama rule for intramolecular excimer for-
mation probably owing to the repulsion energy
higher than DHCR. Although the motion of two
chromophores would be highly restricted in the 1,1-
diarylmethanol structure, we observed excimer
¯uorescence of 1b in MTHF at room temperature.
Introduction of electron-withdrawing group might
enhance both excimer and dimer radical anion
formation since 1,1-diphenylmethane and 1,1-
diphenylmethanol do not show excimer ¯uores-
cence. However, this cannot explain the absence of
the intramolecular dimer radical anion of 3. An-
other possibility is that the 1,1-diarylmethyl struc-
ture is e€ective for formation of the intramolecular
dimer radical anions being di€erent from the in-
tramolecular singlet excimers as suggested for the
triplet excimers of naphthalene chromophore [26].
While the sandwich-pair is preferred for the singlet
excimer, the L-shaped conformation as in the most
symmetric conformation of 1,1-di-a-nephthyl-
methane and cis-6b,12b-dihydro-acenaphth(1,2-a)
Fig. 3. NIR absorption spectra of observed after c-radiolysis of
1a±e in 77 K MTHF matrices (5:0 Â 103 M). The spectra were
subtracted by those before irradiation.
absence of the CR band for the radical anions of
mono-substituted alcohols 1d,e ꢀ1d; eá † in the
77 K c-radiolysis also supported the assignment of
the intramolecular dimer radical anions of 1b; cá
(Chart 2).
It is well known that two aromatic chromo-
phores pendent on 1,3-positions of alkyl chain
form an intramolecular excimer favorably (the
Hirayama rule or n ˆ 3 rule) [17]. We have exam-
ined formation of the intramolecular dimer radical
anion of 1,3-bis(4-cyanophenyl)propan-1-ol (3) by
pulse radiolysis and c-radiolysis and formation of
the intramolecular excimer by ¯uorescence mea-
surement. Although the excimer ¯uorescence was
observed in MTHF at room temperature, no NIR
absorption due to the CR interaction was observed

102
N. Ichinose et al. / Chemical Physics Letters 330 (2000) 97±102
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for the naphthalene triplet excimer [26] and the 1,1-
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In conclusion, intramolecular dimer radical
anions 1b; cá have been observed at room tem-
perature for the ®rst time during pulse radiolysis of
1b,c in DMF. The absence of the kinetic e€ect of
the intramolecular dimer radical anion formation
on the C±O bond dissociation behavior will be
further discussed in a full length paper.
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Acknowledgements
[13] T. Majima, S. Tojo, S. Takamuku, J. Phys. Chem. A 101
(1997) 1048.
We are grateful for the gift of benzophenone-
4,40-dicarboxylic acid (DKDA) and benzophe-
none-4-carboxylic acid (4-benzoylbenzoic acid,
4-BZA) from Nihon Noyaku as precursors of 1c
and 1e. We thank all the member of the Radiation
Laboratory of the ISIR, Osaka University for the
operation of the linear accelerator and 60Co c-ray
radiation source. This work was partly supported
by a Grant-in-Aid for Scienti®c Research (Nos.
09226223, 10142237, 09450319, and 09875209)
from the Ministry of Education, Science, Sport
and Culture of Japan.
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