Photoinduced oxidation of diphenylbisanthene and its spectral manifestations

Article abstract of DOI:10.1023/B:JAPS.0000032873.77613.9b

The absorption and fluorescence spectra of diphenylbisanthene (DPB) in n-octane and benzene at 300 and 77 K have been investigated. The appreciable Stokes shift of the fluorescence band (～180 cm^-1 in noctane and ～440 cm^-1 in benzene) points to the nonplanar character of the aromatic skeleton of DPB, which has been confirmed by results of the optimization of the molecule geometry by the AMI quantummechanical method. It has been found that spectral manifestations of the products of photoinduced transformations of DPB molecules are observed in the UV region (200-350 nm), and it has been shown that the observed photochemical process is two orders of magnitude less effective than for diphenylhelianthrene. On the basis of the experimental data and the results of the quantumchemical calculations of the electronic spectra of molecules of DPB and endoperoxides (ZINDO/S method), it has been concluded that the end products of DPB phototransformations are its endobiperoxides in which -O-O- groups are added to the phenylsubstituted benzene rings of the aromatic skeleton.

Full text of DOI:10.1023/B:JAPS.0000032873.77613.9b

Journal of Applied Spectroscopy, Vol. 71, No. 2, 2004
PHOTOINDUCED OXIDATION OF DIPHENYLBISANTHENE
AND ITS SPECTRAL MANIFESTATIONS
*
S. M. Arabei and T. A. Pavich
UDC 535.37
The absorption and fluorescence spectra of diphenylbisanthene (DPB) in n-octane and benzene at 300 and 77
–1
K have been investigated. The appreciable Stokes shift of the fluorescence band ( 180 cm in n-octane and
–1
4
40 cm in benzene) points to the nonplanar character of the aromatic skeleton of DPB, which has been
confirmed by results of the optimization of the molecule geometry by the AMI quantum-mechanical method. It
has been found that spectral manifestations of the products of photoinduced transformations of DPB molecules
are observed in the UV region (200–350 nm), and it has been shown that the observed photochemical process
is two orders of magnitude less effective than for diphenylhelianthrene. On the basis of the experimental data
and the results of the quantum-chemical calculations of the electronic spectra of molecules of DPB and en-
doperoxides (ZINDO/S method), it has been concluded that the end products of DPB phototransformations are
its endobiperoxides in which —O—O— groups are added to the phenyl-substituted benzene rings of the aro-
matic skeleton.
Keywords: diphenylbisanthene, diphenylhelianthrene, absorption and fluorescence spectra, phototransformation,
endoperoxides, quantum-chemical calculation.
Introduction. The investigation of hypericin-like molecules with an elementary chemical structure is impor-
tant, first of all, for understanding the biological activity of many compounds of this class. The spectral and photo-
chemical properties of the nonsubstituted bisanthene molecule as a fundamental system of the hypericin type have been
the subject of many works [1–4]. Elementary symmetrically substituted bisanthenes from this point of view are un-
doubtedly of interest, since the hypericin molecule features symmetry of side groups (point group of symmetry C )
2
v
[5]. In the present paper, bisanthene substituted by phenyl groups in the meso-position — diphenylbisanthene (DPB)
(the structure is given in Fig. 1) — has been investigated. The attachment of two phenyl rings to the molecule should
lead to an interaction of the conjugate systems of the aromatic skeleton and substituents and, as a result, to a change
in the energies of the electronic states of bisanthene, and, therefore, in the spectral-luminescent properties. The interest
in phenyl-substituted hypericin-like compounds is also due to their possible practical application. For example, DPB
has found application as a pigment of the passive shutter for the Q-switching of a ruby optical quantum generator [6]
and its analog — diphenylhelianthrene (DPH) (see the structure in Fig. 2) — as a pigment of an actinometer sensitive
to the light in the visible region of the spectrum [7]. In the first case, Gorelenko et al. [6] used the high photostability
of DPB and in the second case [7] — on the contrary, the high efficiency of DPH transformations [2, 7–13].
For the first time, DPB was synthesized by G. Sauvage [8, 9], who only determined the maxima positions of
the longwave absorption bands. The information on the DPB fluorescence given in [6, 10] is limited and contradicting.
The photoinduced transformations of the diphenyl derivative of bisanthene were only mentioned in [6, 10].
The aim of the present paper was to investigate the spectral-luminescent properties of DPB, study its photo-
chemical transformations (in comparison with the analogous process for DPH), and interpret the experimental data on
the basis of quantum-chemical calculations of the molecule geometry of DPB and its endoperoxides, as well as of their
electronic spectra.
*To whom correspondence should be addressed.
Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus; 70 F. Skorina Ave.,
Minsk, 22072, Belarus; e-mail: arabei@imaph.bas-net.by. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71,
No. 2, pp. 173–178, March–April, 2004. Original article submitted August 27, 2003.
0021-9037/04/7102-0187 © 2004 Plenum Publishing Corporation
187

Experimental Procedure. The DPB was obtained by the method of [10] by irradiating diphenyldianthranyl
for 5–6 h with light from a DKsSh-500 xenon lamp (λ > 390 nm). The DPB + DPH mixture formed was separated
by chromatographing on aluminum oxide and silica gel (eluent — benzene). The diphenylanthranyl was synthesized by
sequentially obtaining source products (anthrone–dianthrone–dihydroxydiphenyldianthranene–diphenyldianthranyl). The
stages of this synthesis are described in [3, 10]. The chemical composition of DPB was confirmed by the results of
the elemental analysis and the similarity of the obtained characteristic absorption spectra (Figs. 1 and 2) to the litera-
ture data [6, 8–10].
The organic solvents were preliminarily purified by multiple distillation and dried. The concentration of the
solutions under investigation was chosen so that the optical density for the longwave absorption band of the pigment
did not exceed 0.2.
The absorption spectra were obtained on a Cary-500 Scan spectrophotometer (Varian, USA). The fluorescence
excitation spectra were recorded on an SFL-1211A spectrofluorimeter (Solar, Belarus). The fluorescence spectra were
measured at 300 and 77 K on a spectral facility assembled on the basis of a double monochromator and a recorder
operating in the photon-counting regime. For fluorescence excitation, either filtered radiation from a xenon lamp or ra-
diation from an LG-78 He–Ne laser (λ
= 632.8 nm) was used. In both cases, the excitation power density was
exc
2
1
mW/cm .
Optimization of the geometry of the investigated molecules was performed using the AMI semiempirical
method and the calculation of their electronic S ← S absorption spectra — by the ZINDO/S semiempirical method.
i
0
Results and Discussion. On the face of it, the attachment of two phenyl groups in the meso-positions of the
planar bisanthene molecule should not change the symmetry of its equilibrium configuration (D ). However, as the re-
2
h
sults of the optimization of the DPH molecule geometry have shown, the planes of the phenyl groups make an angle
o
Θ
66 with the plane of the aromatic skeleton. And the phenyl rings are tilted to one another so that their planes
o
make an angle of 48 (see the scheme in Fig. 1b). The rotation of the phenyl rings about single C—C bonds leads
to a fair nonplanar "propeller-like" deformation of the aromatic skeleton itself (the planes of the end benzene rings of
o
two anthracene fragments of the skeleton make an angle of 3 ). As a result, the equilibrium configuration of the
DPB molecule belongs approximately to the point group of the lower symmetry D . It has no symmetry planes, as the
2
bisanthene molecule does, but still has 2nd-order symmetry axes. Such a feature of the spatial structure of DPB is a
consequence of the steric interactions of the phenyl and skeletal benzene rings. Note that an analogous orientation of
the side phenyl rings is also observed for meso-tetraphenylporphin and its metal complexes. Thus, the calculations of
the geometrical structure and excited states of phenyl derivatives of porphin by different quantum-mechanical methods
o
[
14–16] yielded values of Θ
60–70 , which points to a small distortion of the planar structure of the porphyrin
macroring as a result of the steric interaction of the phenyl and pyrrole rings.
The thorough chromatographic separation of DPB and the use of freshly prepared solutions have made it pos-
sible to obtain a qualitative absorption spectrum of this compound in the visible and UV regions. Figure 1a (curve 1)
gives the absorption spectrum of DPB in n-octane at room temperature. The longwave region of the spectrum (550–
700 nm) has a form characteristic of hypericin-like molecules, and the position and intensities of the bands agree with
the literature data [6, 8–10]. The absorption spectrum of DPB in the UV region (200–350 nm) consists of a number
of intense bands (in the literature, this region of the spectrum has not been investigated). That the observed bands be-
long to the phenyl derivative of bisanthene has been confirmed by the fluorescence excitation spectra (Fig. 1b), which
simultaneously point to the purity of the compound being investigated. The fluorescence spectrum of DPB (Fig. 1a,
2
curve 5) was obtained under excitation by the radiation from a He–Ne laser (λexc = 632.8 nm, Wexc = 0.7 mW/cm ).
Analyzing the spectra obtained, it is important to note that the longwave absorption band of DPB in n-octane
at 679 nm is bathochromically shifted from the related band of nonsubstituted bisanthene [3] by 21 nm (for the ben-
–
1
zene solution the shift is 23 nm). Moreover, a marked Stokes shift of the fluorescence bands (∆ν
octane) compared to the corresponding shift for the planar molecule of bisanthene (∆ν
the benzene solution (Fig. 2), ∆ν reaches 440 cm–1 (for bisanthene in benzene ∆ν
190 cm for n-
–1
40 cm ) is observed [3]. In
340 cm–1 [3]). In [3], the opinion
was expressed that the large Stokes shift in the case of benzene was due to the fact that in the S -state of the hy-
1
pericin-like molecule a dipole moment inducing dipoles in the solvent molecule arises, which, as a result, leads to a
manifestation of universal intermolecular interactions (IMI) in such a system. The observed Stokes shift of the fluores-
cence bands and the bathochromic shift of the absorption bands of DPB are an additional indication of the aromatic
188

Fig. 1. Absorption spectra: 1) initial; 2–4) upon photoirradiation for 1 (2), 4.5
3), and 10 h (4) and fluorescence spectra (5, 6) at λexc = 632.8 nm of DPB
(
in n-octane at 300 (1–5) and 77 K (6) (a). Fluorescence excitation spectra of
DPB in n-octane at 300 K (λreg = 688 (1) and 750 nm (2)) (b), as well as a
three-dimensional image of the chemical structure of DPB (a) and its projec-
tion in the aromatic skeleton plane (b)
skeleton deformation. The appreciable deviation from the mirror symmetry of the obtained spectra of absorption (long-
wave region) and fluorescence, as well as the considerable Stokes shift of the fluorescence bands, can be due to the
restructuring of the DPB molecule in the excited electronic S -state (possibly, to the enhancement of nonplanarity upon
1
photoexcitation due to the reorientation of the phenyl groups). At the same time, it is not excluded that in the region
of the absorption band at 620 nm the band of the S ← S transition is localized, as in the case of bisanthene [4, 17].
2
0
Freezing of the n-octane solution of DPB to 77 K does not lead to the appearance of the quasi-line structure
of the fluorescence spectrum (Fig. 1a, curve b), although the bands markedly narrow. The absence of the structural
spectra, unlike the nonsubstituted bisanthene [3, 4, 17], is due to the noncoplanarity of the planes of the phenyl groups
to the aromatic skeleton plane, creating an obstacle to the isomorphous penetration of DPB molecules into the crystal-
lographic cells of n-octane. The latter does not lead to the removal of inhomogeneous broadening of the spectral bands
by limiting the number of types of impurity centers in the Shpolski matrix.
In [9], it was stated that DPB was not capable of phototransformations. However, as mentioned above, in [6,
1
0] it was revealed that prolonged irradiation leads to a slow decomposition of DPB. And Maulding [10] erroneously
argued that the arising absorption bands with a center at 400 nm corresponded to the formation of DPB photooxide
most probably, the investigated solution contained a certain quantity of DPH, to whose photooxide the new absorption
bands belonged [2, 7, 10, 12, 13]). As our experiments have shown (Fig. 1a, curves 1–4), under a prolonged irradia-
tion of the n-octane solution of DPB by the light of an SVDSh-500 mercury lamp (λ 200–500 nm, Wexc 1.6
(
exc
2
mW/cm ), the observed photochemical processes lead to the appearance of new bands in the UV region: a rather nar-
row band at 262 nm and a low-intensity background in the 300–400-nm range. In so doing, the absorption bands be-
longing to DPB completely disappear (see curve 4). The family of curves corresponding to various times of
preliminary photoirradiation of the DPB solution has a number of isobestic points (241, 279, 309, and 530 nm), which
definitively points to the formation of a binary mixture of DPB and its photoproduct.
189

Fig. 2. Absorption spectra: 1) initial; 2) upon 5-min photoirradiation of ben-
zene solution of DPB with DPH; 3) fluorescence spectrum of DPB in benzene
(
λexc = 632.8 nm) at 300 K. In insets: change in the relative fluorescence in-
tensity of benzene solution of DPB (1) at λreg = 708 nm and DPH (2) at λreg
600 nm depending on the photoirradiation time in the 200–500-nm range, as
=
well as a three-dimensional image of the chemical structure of DPH.
For quantitative estimation of the phototransformation efficiency of DPB, we investigated a benzene solution
containing DPB and DPH (nonchromatographed mixture), whose absorption spectrum is given in Fig. 2 (curve 1). Be-
sides the DPB bands (longwave band at 686 nm), in the spectrum of the nonirradiated solution intense bands of DPH
(for example, at 582 nm) and low-intensity bands of its endoperoxide with a center at 400 nm are observed [7, 12,
1
3]. Upon irradiation of the solution by the light of the mercury lamp for 5 min the DPH absorption bands disap-
peared from the absorption spectrum and the bands of its endoperoxide sharply gained in intensity (Fig. 2, curve 2).
This confirms the effective process of DPH phototransformations [2, 7–13]. In [12], such a photochemical reaction of
9
–1
–1
DPH was investigated and the rate constant of phototransformations k = (7.0 ± 0.3)⋅10 sec ⋅mole ⋅liter was meas-
ured. The inset in Fig. 2 shows the kinetics of DPH phototransformations, obtained under experimental conditions as
a change in the relative fluorescence intensity at λreg = 600 nm (fluorescence-band maximum) depending on the pho-
toirradiation time. The experiment has shown that in 15 sec the concentration of DPH molecules in the solution is
reduced by one-half. Under the same photoexcitation conditions, the relative fluorescence intensity of DPB at λreg
08 nm remains practically unchanged (see Fig. 2, curve 3, insert). Analysis of the obtained kinetic curves has shown
that the phototransformation efficiency of DPB is two orders of magnitude lower than that of DPH.
=
7
On the basis of the quantum-chemical calculation of the DPH molecule geometry it has been established that
the aromatic skeleton has no planar structure (Fig. 2). The reason for this is the steric obstacles of two "internal" hy-
drogen atoms, which force the end benzene rings of anthracene fragments to leave the plane of the aromatic system in
opposite directions. Such structural distortions disturb the π-electron conjugation in the DPH molecules, which leads to
a considerable shortwave shift of the absorption band from the DPB bands.
In [2, 7, 10, 12, 13], it has been found that phototransformations of DPH are a self-sensitized process of its
photooxidation with the formation of endoperoxides in which the oxygen bridges are attached to the phenyl-substituted
benzene rings. Moreover, in [18] it is stated (by an example of nonsubstituted helianthrene molecules) that exactly the
skeleton deformation (the going of the end benzene rings out of the plane) creates favorable conditions for the addition
of oxygen in meso-positions of the aromatic skeleton, An experimental proof of such a chemical structure of DPH en-
doperoxide is the NMR [2] and mass spectrometric [2, 10] data.
By analogy with nonsubstituted bisanthene [1, 3], helianthrene [2, 18], and DPH [2, 7, 10, 12, 13] we assume
that the process of phototransformations of DPB is also due to the formation upon photoexcitation of its endoperox-
ides. This is evidenced by the considerable retardation of the photochemical reaction of DPB in degassed solutions. To
190

additionally confirm such a mechanism, we have performed a quantum-chemical calculation (ZNDO/S semiempirical
method) of energies of the singlet states and oscillator strengths of transitions for molecules of DPB and its endoper-
oxides. Figure 3 shows the optimized molecule geometries of DPB and its most probable, as to chemical structure (see
below), endoperoxides in which the —O—O— group (groups) is attached to the benzene rings containing phenyl
groups of endomonoperoxide (Fig. 3b) and two isomers (cis- and trans-) of endobiperoxides (Fig. 3c and d). The re-
sults of the calculation of their electronic S ← S spectra are also given here (bold vertical lines).
i
0
According to the calculations performed, the addition of two phenyl fragments to the bisanthene molecule in-
creases the energy of the highest occupied molecular orbital (MO), whereas the energy of the lowest unoccupied or-
bital remains practically unaltered. This explains the experimentally observed bathochromic shift of the longwave
absorption band of DPB (∆λ 20 nm) from the corresponding bisanthene band. In so doing, the oscillator strength of
the longwave transition of DPB is somewhat greater than in the nonsubstituted bisanthene (f 1.03 and 0.81, respec-
tively). The calculation also shows that the oscillator strength of the S ← S transition of DPB is negligible (f <
2
0
–
0
.001). Despite the fact that the calculated value of the S –S energy interval turned out to be fairly large ( 6000 cm
2 1
1
), it may be stated that the experimentally observed breaking of the mirror symmetry of the longwave absorption and
fluorescence band intensities of DPB (see Fig. 1a and Fig. 2) is due to the localization of the S -level of DPB in the
2
region of the electronic-vibrational levels of the S -state, as is the case with nonsubstituted bisanthene [4, 17]. For the
1
shortwave region, good agreement between the experimental data and the results of the quantum-chemical calculation
of the electronic spectra of DPB is observed (Fig. 3a).
The formation of endomono- and endobiperoxides of DPB (Fig. 3b–d) leads, on the contrary, to a marked de-
crease in the energy of the two highest occupied MOs and an increase in two lowest vacant MOs. The calculation
shows that in the case of endomonoperoxide the longwave absorption band experiences a hypsochromic shift to 437
nm (see Fig. 3b). In so doing, the oscillator strength of this transition, as in the case of transitions that manifest them-
selves in the UV region, does not exceed a value of f ≈ 0.75. The calculation of the electronic spectra of endobiper-
oxides (Fig. 3c and d) yields a different result: an appreciable hypsochromic shift of the longwave bands (to 320 nm
for the cis- and 315 nm for the trans-isomer) and a negligible oscillator strength of the corresponding transitions
(0.001 and 0.009), as well as intense bands in the UV region (f
1–2). As a result, as is seen from Fig. 3c and d,
the calculated spectra of the two isomers of DPB endobiperoxides most closely agree with the experimentally observed
absorption spectrum of the irradiated DPB solution (see also Fig. 1a, curve 4), which can serve as a corroboration of
the dominant formation of endobiperoxides under the action of light.
Concurrently with the above calculations, we also calculated the electronic spectra of DPB endoperoxides in
which the oxygen bridges were attached to the central benzene rings of the aromatic skeleton (such structures of en-
doperoxides were supposed in [2, 11] for helianthrene and DPH). In this case, a wide discrepancy between the experi-
mental and calculated spectra is observed. Obviously, such a chemical structure of endoperoxide is unlikely, since the
high reactivity of the two meso-positions of bisanthene and its derivatives to oxygen [1] favors the formation of the
structures depicted in Fig. 3b–d. Moreover, as the results of the quantum-mechanical calculation of the geometry have
shown, in this case an excessive distortion of the aromatic skeleton plane takes place.
Analysis of the results obtained permits stating that endomonoperoxide makes no appreciable contribution to
the formation of the absorption spectrum of the irradiated DPB solution (Fig. 1, curve 4 and Fig. 3d). Its insignificant
concentration is indicative of a fast attachment by the DPB molecule of the second oxygen bridge. As is seen from
Fig. 3b, the presence of one bridge —O—O— leads to an expansion of the kink of the second oxygen-nonsubstituted
anthracene fragment, which increases the reactivity of its meso-positions [18] and favors fast formation of endobiper-
oxide.
The whole set of the data obtained and the literature data on the photoexcitation of hypericin-like compounds
indicates that in the case of DPB molecules the mechanism of self-induced phototransformations is realized. This
mechanism is reduced to the formation of endoperoxides through the interaction of DPB molecules with high-reactivity
1
singlet oxygen. In this case, O is formed through the energy transfer from the T -state of DPB to the unexcited oxy-
2
1
gen molecule. It should be noted that in [18] the formation of nonsubstituted helianthrene endoperoxide in the dark
through the realization of a unique (spin-forbidden) reaction between helianthrene in the S -state and molecular oxygen
0
3
in the ground triplet ( O ) state was revealed. And Seip and Brauer [18] thereby postulate that, as a result of the col-
2
lision of such molecules, a bound complex is formed, via which on the spin-forbidden path, through the charge trans-
191

Fig. 3. Absorption spectra of DPB (a) and its photoproducts (d) in n-octane at
00 K and calculated electronic spectra of DPB (a) and its endoperoxides (b–
3
d) (bold lines — calculated spectra), as a well as three-dimensional image of
the chemical structure of DPB (a) and its endoperoxides (b–d).
fer from helianthrene to oxygen, a witterion arises (the formation of a single bond between them). And the latter is
immediately transformed to the end product — endoperoxide. In our experiments, the dark reaction of DPB endoper-
oxide formation has not been revealed.
Conclusions. For one of the elementary representatives of hypericin-like molecules — diphenyl-substituted
bisanthene — a complex of spectral-luminescent studies has been carried out. The analysis of the obtained spectral
data and of the results of the quantum-chemical calculation of the DPB molecule geometry has permitted the conclu-
sion that its aromatic skeleton has a nonplanar structure. The spectral region of the appearance of products of photoin-
duced transformations of DPB in oxygen-containing solutions has been revealed. On the basis of the quantum-chemical
calculation of the energies and intensities of the electron S ← S transitions of DPB and its peroxides it has been
i
0
concluded that as a result of the photooxidation of the aromatic molecule, its endoperoxides, in which the oxygen
bridges are attached to the phenyl-substituted benzene rings, are mainly formed.
Part of this work was supported by the Belarusian Republic Basic Research Foundation (project F03-072).
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