Synthesis of a [3.3]biphenylophane, and its photophysical and photochemical properties studied by emission and transient absorption measurements

Article abstract of DOI:10.1016/j.cplett.2008.06.066

[3.3](4,4′)Biphenylophane (BPP) is synthesized, and the photophysical and photochemical properties are studied by means of emission and transient absorption measurements. BPP emits excimer fluorescence at 295 and 77 K, and phosphorescence from the locally-excited (LE) triplet state at 77 K. Based on the transient absorption spectra of BPP, it is found that the excimeric triplet state of BPP is produced along with the LE triplet at 295 and 77 K. The triplet excimer of BPP is shown to be formed via intersystem crossing from the singlet excimer state, and concluded to be non-phosphorescent.

Full text of DOI:10.1016/j.cplett.2008.06.066

Chemical Physics Letters 460 (2008) 474–477
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Synthesis of a [3.3]biphenylophane, and its photophysical and photochemical
properties studied by emission and transient absorption measurements
*
Minoru Yamaji , Takehiko Tsukada, Haruo Shizuka, Jun Nishimura
Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
a r t i c l e i n f o
a b s t r a c t
[3.3](4,40)Biphenylophane (BPP) is synthesized, and the photophysical and photochemical properties are
studied by means of emission and transient absorption measurements. BPP emits excimer fluorescence at
295 and 77 K, and phosphorescence from the locally-excited (LE) triplet state at 77 K. Based on the tran-
sient absorption spectra of BPP, it is found that the excimeric triplet state of BPP is produced along with
the LE triplet at 295 and 77 K. The triplet excimer of BPP is shown to be formed via intersystem crossing
from the singlet excimer state, and concluded to be non-phosphorescent.
Article history:
Received 21 May 2008
In final form 23 June 2008
Available online 27 June 2008
This Letter is dedicated to the memory of
the late Associate Professor Yukihiro Okada
at Gunma University, Japan.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
lene [26]. In contrast, a large number of reports have appeared
on inter- and intra-molecular triplet excimers of carbazolyl chro-
Since the first report of excimer fluorescence from pyrene in
solution [1], a large number of studies of inter- and intra-molecular
excimer fluorescence from aromatic molecules have been reported
from the viewpoints of relationship between molecular configura-
tions and photophysical properties [2]. The preferred molecular
configurations of singlet excimers have been revealed to be paral-
lel-arranged chromophores by using diarylalkanes, known as the
Hirayama’s n = 3 rule [3]. Conversely, existence of triplet excimers
has been alleged in the last three decades [4–46]. Based on theoret-
ical and experimental studies of inter- and intra-molecular triplet
excimer of naphthalene derivatives in solution, Lim concluded that
the geometry of the naphthalene triplet excimer completely differs
from that of the corresponding singlet excimer [14]. They proposed
the most favored geometry of triplet excimer of naphthalenes is an
L-shaped arrangement of two naphthalene moieties whose long
axes are parallel and short axes constitute an angle of ca. 109°. It
seems that the geometry of the chromospheres is closely related
to nature of singlet and triplet excimers. Nickel and Rodriguez Pri-
eto, however, could not reproduce any excimeric phosphorescence
mophores and dinaphthyl compounds containing heteroatoms in
fluid and rigid media [19–25,27,28,30–32,34–38]. It seems that
the triplet excimer issue is still far from being settled.
Previously, investigations of intra-molecular excimers have
been carried out using dimeric chromophores linked with a single
oligomethylene chain. These molecules are expected to have a
favorable conformation for excimer interactions by molecular mo-
tion in fluid media. They are, therefore, not suitable for excimeric
phosphorescence measurements in a glassy matrix at low temper-
ature. In some cases, measurements of transient absorption by la-
ser photolysis techniques at room temperature have been carried
out to characterize triplet excimeric states [7,12,14,22,32,40,46]
since the absorption is not affected by the presence of small quan-
tities of strongly luminescent impurities. To make clear relation-
ship between molecular conformations and properties of singlet
and triplet excimers and to perform emission measurements free
of ambient temperature, we have prepared cyclophanes having
two aromatic chromophores, such as benzene, naphthalene and
phenanthrene, tethered with more than one oligomethylene chains
with various lengths, and studied the photophysical and photo-
chemical properties by measurements of emission and transient
absorption [6,33,39,42–45]. By using syn-[3.4](1,5)naphthaleno-
phane whose naphthalene moieties are forced in a parallel and
face-to-face geometry with two oligomethylene chains, we found
formation of the triplet excimer in transient absorption at 77 and
295 K although no excimeric phosphorescence was observed at
77 K [43]. Thus, we concluded that triplet excimer in a parallel-
sandwich conformation is non-phosphorescent. On the other hand,
Benten et al. recently reported observation of excimer phosphores-
cence from triply bridged carbazolophanes in a parallel-sandwich
structure at 77 K [46]. It seems that triplet excimer is still to be
argued.
of naphthalene and 1,n-di-a-naphthalenes in solution observed by
Lim et al., and attributed to the sensitized phosphorescence of
biacetyl-like impurities of the solvent [13,17]. Locke and Lim re-
sponded to this criticism with additional results in an attempt to
eliminate possible impurities for their reported emission, and reas-
serted the assignment of the excimer emission and transient
absorption to the naphthalene triplet excimer [15,18]. After this
argument, new negative results on triplet excimer formation were
reported on solutions of 1-bromonaphthalene [16] and naphtha-
* Corresponding author. Fax: +81 277 301212.
E-mail address: yamaji@chem.gunma-u.ac.jp (M. Yamaji).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.06.066

M. Yamaji et al. / Chemical Physics Letters 460 (2008) 474–477
475
In contrast to vigorous studies of excimers consisted of benzene,
naphthalene or phenanthrene ring, few reports on excimers of
biphenyl moieties can be found in the literatures. Pulse and -ray
c
radiolysis techniques enabled observation of excimer fluorescence
of biphenyl in fluid media at low temperature [4,9]. Zachariasse et
al. used 1,3-dibiphenylpropane to obtain thermodynamic parame-
ters for the excimer formation equilibrium upon photoexcitation
based on lifetimes of the excimer fluorescence in fluid media
[11]. [2.2](4,40)Biphenylophane was employed for measurements
of magnetic resonance and optical spectra at low temperature
[5,10]. Therein, the LE and excimer fluorescence spectra were ob-
tained along with LE phosphorescence. However, the presence of
the triplet excimer state was not recognized.
In the present work, we have synthesized a [3.3](4,40)bipheny-
lophane (BPP) which accords to the Hirayama rule. The emission
and transient absorption properties are investigated at 295 and
77 K. The fluorescence spectrum is excimeric irrespective of ambi-
ent temperature although only LE phosphorescence was obtained
at 77 K. Transient absorption measurements reveal the formation
of the triplet excimer at 295 and 77 K. Formation and decay pro-
cesses of the excimers are kinetically discussed.
Fig. 2. Absorption and emission spectra of 4,40-dimethyl biphenyl (DMBP) (a) and
[3.3](4,40)biphenylophane (BPP) (b) in cyclohexane at 295 K (solid) and in MP at
77 K (dash).
2. Results and discussion
BPP is absent at 295 and 77 K while 1,3-di(biphenyl)propane gives
both LE and excimer fluorescence at room temperature [11]. The
lifetime and quantum yield of excimer fluorescence of BPP at
295 K were determined to be 45.2 and 0.28 ns, respectively. The
phosphorescence spectrum of BPP whose progressions are similar
to those of DMBP is located at 30-nm longer wavelength region
than that of DMBP. Phosphorescence from BPP was absent in the
700–800 nm wavelength region. The triplet energy of BPP was
determined to be 60.5 kcal molꢀ1 from the 0–0 origin of the LE
phosphorescence spectrum at 77 K. The lifetime of the phosphores-
cence (2.7 s) from BPP at 77 K indicates that the electronic charac-
Fig. 1 shows the molecular structure of [3.3](4,40)biphenylo-
phane (BPP) optimized by the MM2 calculations. Oligomethylene
chains link the biphenyl skeletons at 4- and 40-positions. The ben-
zene chromophore of the biphenyl moiety is kept parallel to the
opposite one.
Fig. 2 shows absorption and emission spectra of 4,40-dimethyl-
biphenyl (DMBP) and BPP in cyclohexane at 295 K and in MP at
77 K. The shape of both the absorption spectra are similar to each
other, and the absorption peaks locate at the same wavelength of
254 nm. An absorption shoulder with a weak intensity at 290 nm
can be seen in the absorption spectrum of BPP, indicating that
there is a weak interaction between the biphenyl moieties in BPP
in the ground state. The fluorescence and phosphorescence spectra
of DMBP are typical of LE singlet and triplet biphenyl. The fluores-
cence of BPP at 400 nm is ascribable to the single excimer state
[11]. It is noteworthy that the locally-excited (LE) fluorescence of
ter of triplet BPP is of a p,p
* type. The photophysical parameters for
DMBP and BPP obtained in the present work are listed in Table 1.
Decomposition of BPP in degassed cyclohexane was not found
upon irradiation at 254 nm with a low-pressure mercury lamp.
Fig. 3 shows transient absorption spectra obtained at 500 ns
after 266-nm laser photolysis of DMBP and BPP. The absorption
Fig. 1. The molecular structure of [3.3](4,40)biphenylophane (BPP) optimized by MM2 calculations.

476
M. Yamaji et al. / Chemical Physics Letters 460 (2008) 474–477
Table 1
77 K (see Fig. 3c). These observations indicate that the triplet exci-
Lifetimes (sf), quantum yields (Uf) and rate constants (kf) of locally-excited (LE) and
mer state of BPP is formed irrespective of ambient temperatures.
During 266-nm laser photolysis of BPP, an increase in absor-
bance at 595 nm, which is due to the triplet excimer of BPP, was
observed (see inset in Fig. 3b). The formation rate of the triplet
excimer of BPP was obtained to be 2.2 ꢁ 107 sꢀ1, i.e., the lifetime
for the formation was 45 ns at 295 K. On the other hand, we have
obtained the decay lifetime of excimer fluorescence of BPP being
45.2 ns at 295 K. This agreement between the formation and decay
lifetimes within experimental errors (5%) indicates that the triplet
excimer state of BPP is produced via intersystem crossing from the
singlet excimer. As with the time-profile of transient absorption at
450 nm in the early time-domain for LE triplet, we were unable to
analyze it because of the presence of an intense signal due to the
excimer fluorescence in the transient absorption.
excimer (Ex) fluorescence of DMBP and BPP at 295 K,a and triplet energies (TN) and
lifetimes (s
p) of phosphorescence determined at 77 Kb
Compound
sLf E
(ns)
sEf x
(ns)
ULf E
UEf x
kfc
TN
sp
(s)
(107 sꢀ1
)
(kcal molꢀ1
)
DMBP
BPP
15.2
–
–
45.2
0.22
–
–
0.28
1.4
6.2
64.0
60.5
4.2
2.7
a
In cyclohexane.
In methylcyclohexane–isopentane (3:1 v/v).
Obtained by the equation, kf
b
c
¼
Uf sꢀf 1
.
Triplet-sensitization of BPP by benzophenone was carried out in
acetone as the solvent by transient absorption measurement at
295 K. Triplet energy can be readily transferred from triplet benzo-
phenone to BPP, expecting the formation of triplet BPP since the
triplet energy of benzophenone (69.1 kcal molꢀ1 [47]) is greater
than that of triplet BPP (60.5 kcal molꢀ1). After depletion of trip-
let–triplet absorption of benzophenone produced upon 355-nm la-
ser excitation of a benzophenone/BPP system, we obtained a
transient absorption spectrum identical to that (Fig. 3b) obtained
upon direct excitation of BPP. Thus, this fact convinces that the
transient absorption spectrum of BPP having the band peaks at
450 and 595 nm originate from the triplet manifold.
We have observed the similar features in emission and transient
absorption for syn-[3.4](1,5)naphthalenophane (NPP) in a parallel-
sandwich structure [43]. Fluorescence from NPP was only excimer-
ic irrespective of the ambient temperature while the phosphores-
cence spectrum at 77 K was of LE triplet. The transient
absorption of the excimeric naphthalene was located in the longer
wavelength region than the LE triplet. Based on the results of com-
putations performed for singlet and triplet states of naphthalene
dimer having various orientations, it was concluded for triplet
excimers with inversion symmetry, such as a parallel-sandwich
conformation, the radiative transition probabilities in such triplet
excimers are greatly reduced [41]. Analogously, with the present
BPP, the photophysical character of non-emissive triplet exci-
mer can be accounted in terms of a highly symmetric structure
of BPP.
Fig. 3. Transient absorption spectra obtained at 500 ns after 266 nm laser pulsing in
4,40-dimethyl biphenyl (DMBP) (a) and [3.3](4,40)biphenylophane (BPP) (b) in
degassed cyclohexane at 295 K and in MP glass of [3.3](4,40)biphenylophane (BPP)
(c) at 77 K. Inset; the temporal absorbance change at 595 nm for [3.3](4,40)biph-
enylophane (BPP) at 295 K.
In conclusion, we synthesized [3.3](4,4)biphenylophane
(33BPP) for the first time, and investigated the photophysical and
photochemical properties by means of emission and transient
absorption measurements. The fluorescence was excimeric only
at 295 and 77 K while the phosphorescence spectrum at 77 K
was of LE triplet. From the transient absorption spectrum, it was
found that the triplet excimer state of 33BPP is formed via intersys-
tem crossing from the singlet excimer state with a rate of
1.4 ꢁ 107 sꢀ1 at 295 K. The triplet excimer of 33BPP is concluded
to be non-phosphorescent because of the reduced radiative transi-
tion probabilities due to the inversion symmetry of the molecular
structure.
band at 380 nm with a decay lifetime of 150 ls is ascribable to the
triplet-triplet absorption of DMBP (Fig. 3a). As with BPP, the
absorption bands having the maxima at 450 and 595 nm (see Fig.
3b) decayed with lifetimes of 130 and 150 ls, respectively. The de-
cay rates of these absorption bands were accelerated in the pres-
ence of the amount of dissolved oxygen, indicating that these
absorption bands are due to the triplet states. Furthermore, the dif-
ference in the decay lifetime indicates that these absorption bands
originate from different triplet states of BPP. From the similarity of
the shape of the 450-nm band to that of triplet DMBP, it can be as-
signed to be due to the LE triplet state of BPP. The large red-shift of
the absorption peak of LE triplet BPP compared with that of triplet
DMBP may derive from the difference in the molecular configura-
tions of the excited biphenyl moieties. It is known that the angle
between the phenyl rings of biphenyl changes from 23° in the
ground state to 0° in the lowest excited singlet state (S1) [11].
When the triplet state of BPP is produced via the S1 state, the
biphenyl moiety may remain to be planer even in the triplet state
of DMBP. Conversely, the phenyl rings of BPP may remain to be
non-planar in the triplet state due to the rigid molecular structure
of BPP. On the other hand, the 595-nm band, thus, should be as-
cribed to the excimeric triplet state of BPP. These absorption bands
of LE and excimer triplets were observed even in a rigid matrix at
3. Experimental section
33BPP was prepared according to the following processes.
3.1. 4,40-Di(hydroxymethyl)biphenyl (2)
THF solution (400 ml) of 4,40-biphenyldicarboxylic acid di-
methyl ester (1, 3.0 g) with LiAlH4 (0.5 g) was refluxed at 75 °C
for 12 h. After filtration of the precipitate, the solution was
evaporated. The compound was recrystallized from ethanol. Yield
90%.

M. Yamaji et al. / Chemical Physics Letters 460 (2008) 474–477
477
3.2. 4,40-Di(bromomethyl)biphenyl (3)
which one can take a transient absorption spectrum with one laser
pulse.
THF solution (150 ml) of 2 (3.0 g) with PBr3 (15 ml) was stirred
at 20 °C for 12 h. The solution was extracted with benzene. After
the solvent was evaporated, the compounds were separated by col-
umn chromatography (SiO2, benzene), and recrystallized from ace-
tone. Yield 90%.
Acknowledgements
M.Y. and T.T. are greatly grateful to the late Associate Professor
Yukihiro Okada at Gunma University, Japan for his kind technical
instruction in their organic synthesis work. This work was partially
supported by KAKENHI (a Grant-in-Aid for Scientific Research (C);
No. 19550005) from Japan Society for the Promotion of Science
(JSPS).
3.3. 2,17-Dioxo[3,3](4,40)biphenylophane (4)
A mixture of n-Bu4NI (0.52 g), CH2Cl2 (140 ml) and 30% aqueous
NaOH solution (17 ml) was reflux. To this mixture, CH2Cl2 solution
(140 ml) of 3 (2.9 g) and toluenesulfonylmethyl isocyanide (Tos-
MIC, 1.66 g) was added dropwise over a period of 2 h. For addi-
tional 3 h, the solution was refluxed, and washed with water
after cooled. The organic layer was evaporated. To the residue in
CH2Cl2 (100 ml), conc. HCl was added and stirred for 1 h at room
temperature. After usual work-up, the residue was chromato-
graphed on silica gel with benzene to afford 4. Yield <1%.
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3.4. [3.3](4,40)Biphenylophane (BPP)
A mixture of 4 (0.1 g), KOH (2.1 g), 100% hydrazine hydrate
(3.2 ml) and diethylene glycol (14 ml) was heated at 120 °C for
13 h and then 200 °C for 3 h (Wolff–Kishner–Huang–Minglon pro-
cedure). The cooled reaction mixture was poured into HCl (10%,
50 ml), and extracted with CH2Cl2 (50 ml ꢁ 4). This solution was
evaporated in vacuo to give BPP. Yield 90%. Anal. Calcd: C, 92.74;
H 7.26. Found: C, 92.41; H, 7.27. The structure of BPP was con-
firmed by NMR spectroscopy. 1H NMR (300 MHz, CDCl3) d 6.89
(d, J = 10 Hz, 8H) 6.80 (d, J = 10 Hz, 8H), 2.80 (m, 8 H), 2.20 (m, 4H).
4,40-Dimethyl biphenyl (DMBP) from TCI was recrystallized
from hexane for purification. Acetonitrile (ACN), acetone (Ac),
cyclohexane (CH, spectrophotometric grade from Aldrich) and a
mixture (MP) of methylcyclohexane (MCH) and isopentane (IP)
(3:1 v/v) from Fluka were used as the solvents. Ac and ACN were
distilled for purification while CH, MCH and IP were used as
supplied.
Absorption and emission spectra were recorded on a U-best 50
spectrophotometer (JASCO) and a Hitachi F-4010 fluorescence,
respectively. It was confirmed that the excitation spectra of the
emission from DMBP and BPP agreed well with the corresponding
absorption spectra. The absolute fluorescence quantum yield was
determined by comparing the corrected fluorescence spectra of
DMBP and BPP with that of naphthalene in ethanol which is known
to have a fluorescence quantum yield of 0.23 [48]. The fluorescence
lifetime was determined by the single-photon counting method
with an FL-900CDT spectrophotometer (Edinburgh Analysis Instru-
ment, UK). The details of the photoncounting system have been de-
scribed elsewhere [49].
All the samples in a quartz cell with a 1 cm path length were de-
gassed by several freeze–pump–thaw cycles on a high vacuum line.
The concentration of DMBP and BBP for 266 nm laser photolysis
was adjusted to achieve the optical density at 266 nm being ca.
0.7. Transient absorption measurements were carried out at
295 K. Third and fourth harmonics (355 and 266 nm) of a Nd3+:YAG
laser (JK Lasers HY-500; pulse width 8 ns) and a XeCl excimer laser
(308 nm; Lambda Physik, Lextra 50) at 308 nm were used for flash
photolysis. The details of the detection system for the time profiles
of the transient absorption have been reported elsewhere [50]. The
transient data obtained by laser flash photolysis was analyzed by
using the least-squares best-fitting method. The transient absorp-
tion spectra were taken with a USP-554 system from Unisoku with
[48] D.F. Eaton, Pure Appl. Chem. 60 (1988) 1107.
[49] M. Yamaji, Photochem. Photobiol. Sci. 7 (2008) 711.
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7014.