48
BUDYKA et al.
urally, the isomerization reaction of the central double
A
bond is impeded in such a structure, leading to lower
quantum yields.
1.5
Another possible explanation of the inhibiting
action of the naphtholate anion is that the term of the
excited Np moiety decreases when the anion is formed
(see Fig. 4); this decrease is manifested in the bathoꢀ
chromic shift of the absorption and a bathofluoric shift
in the fluorescence spectrum. As a result, energy transꢀ
fer to the Np from the SQ moiety becomes possible, a
process that competes with the trans–cis photoiꢀ
somerization in the latter, leading to a decrease in
quantum yields.
1.0
0.5
0
1
8
Figure 4 depicts the energy diagram for SQ4Np in
different forms as calculated on the basis of experiꢀ
mental absorption and luminescence spectra. Using
the neutral form as an example, the diagram also
shows the processes that occur during excitation of
SQ4Np. The assignment of terms to a particular fragꢀ
ment is made on the basis of comparison with the
spectra of the model compounds. For the terms of the
lower singlet excited states localized on the Np–
250
300
350
400
λ
, nm
Fig. 3. Spectral changes during irradiation of an ethanolic
solution of the SQ4Np transꢀisomer with 365ꢀnm light of
–9
–2 –1
an intensity of 1.25
times of ( ) 0, ( ) 5, (
) 580 s; the last spectrum corresponds to photostationary
state PS ; ( ) the spectrum of the cisꢀisomer as calcuꢀ
lated by the Fischer method.
×
10 einstein cm
s
5
, for photolysis
(
S1_Np) and SQ– (S1_SQ) fragments, the positions of the
1
2
3
) 15, ( ) 30, (
4
) 50, ( ) 80, and
6
0–0 transition levels calculated from the crossing of
the absorption and luminescence spectra are shown.
For the terms of the higher excited states, the energies
of vertical (Franck–Condon) transitions, calculated
from the positions of the absorption band maxima, are
given.
(
7
8
365
tions of the Np and SQ fragments [1] is not manifested
in the photochemical properties of the dyad.
In the case of irradiation with light at a wavelength
of λ< 330 nm, each of the fragments can absorb a phoꢀ
As follows from the data in Table 2, the value of ϕtc
is reduced by a factor of ~1.5 in the alkaline medium
and ϕct decreases as well, although to a lesser extent.
Control experiments on the isomerization of model
styrylquinoline in an alkaline medium showed that it is
for the supramolecular dyad that the decrease in the
photoisomerization quantum yield is typical and is
due to the influence of the anionic form of the Np
moiety, which impedes the trans–cis photoisomerizaꢀ
tion reaction of the SQ moiety.
ton and pass to an excited state. Note that only one of
the moieties is excited at the intensities used (light
from a mercury lamp). Thus, the processes shown in
Fig. 4 for the two parts of the dyad occur only in the
one that has absorbed a photon, not simultaneously in
both moieties. For the both fragments of the dyad to be
“simultaneously” excited, it is necessary that the secꢀ
ond photon be absorbed during the lifetime of the
excited state, which is impossible unless a powerful
The inhibiting effect of the naphtholate anion can laser is used.
be explained as follows. Naphthol and styrylquinoline
If, depending on the photon energy, one of the
themselves exhibit weak electronꢀdonor and electronꢀ
acceptor properties, respectively; therefore, donor–
acceptor interaction between these components in the
neutral form is not apparent (spectrum of the dyad is
the algebraic sum of the spectra of model compounds
[1]). In the anionic form, the donor properties of
naphthol are significantly enhanced and, as such, can
facilitate the formation of an intramolecular charge
transfer complex in the ground state or exciplex in the
excited state, at least in those SQ4Np conformers in
higher excited (Sn) states is populated, its relaxation to
one of the lower locally excited states, S1_Np or S1_SQ
occurs then. Emission (fluorescence) at 350 nm for
the Np or 430 nm for the SQ moiety from the latter
states is observed. Processes competing with fluoresꢀ
cence occur in both moieties, namely, the energy
transfer from the Np to the SQ moiety and the photoꢀ
isomerization reaction in the SQ moiety.
In an acidic medium, the SQ moiety is protonated
which the SQ and Np moieties are in a sufficiently and the cation is produced, leading to a dramatic
close proximity to each other. Numerous studies of reduction in the S1_SQ term (Fig. 4, left), which is
aromatic complexes have shown that they can have manifested in a bathochromic shift of the LWAB by 56
different geometric structures, but involve contact nm and a bathofluoric shift of the fluorescence band
between the molecules in any case, resulting in limited by 95 nm. However, high photoisomerization quanꢀ
mobility of the components of the complex [10]. Natꢀ tum yields are retained.
HIGH ENERGY CHEMISTRY
Vol. 46
No. 1
2012
Budyka
