N. Bieniek et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113286
cross-dimer, traced by UV/Vis spectroscopy, is shown in Fig. 5.
Both investigated dimers undergo cycloreversion upon irradiation
with 532 nm laser pulses, which can be derived from the increasing
absorption above 300 nm. HPLC analysis showed that only the mono-
meric forms result from the irradiation and no major side reactions
occur. Due to the different mechanisms and the intrinsically lower ab-
sorption probability, the photocleavage efficiency is much lower
compared to the above described single-photon reaction. The energy
needed to achieve similar conversions differs by six orders of magnitude
between these two photochemical processes (mJ-range for SPA, kJ-
range for TPA). Compared to the results achieved via SPA induced
cyclobutane cleavage, TPA cleavage of the homo-dimer is more efficient
than that of the cross-dimer. As the photophysical process of two-photon
reactions shows a quadratic dependence between monomer formation
and irradiation intensity, the cycloreversion was investigated at
different light intensities (Fig. 6) [28].
Fig. 4. Increase of NMQ and coumarin monomer concentration depending on
the applied light energy.
The reactions were carried out with laser pulses ranging from
approximately 30 mJ/pulse to 100 mJ/pulse. For both, coumarin and
quinolinone, an increased reaction rate can be observed with higher
energy densities. The relation between the initial rate constant of con-
version vm0 onomer and the photon density F2 with the initial dimer con-
A(λ) = [εNMQ⋅cNMQ
+
εcoumarin⋅ccoumarin]∙d
(1)
A(λ) describes the absorption at a given wavelength,
ε
is the ab-
sorption coefficient, c the to be determined concentration, and d the
pathlength of the cuvette. As cyclobutane cleavage of the cross-dimer
leads to the formation of one NMQ and one coumarin molecule, these
two concentrations are identical (cmono). Transforming Eq. (1) gives:
centration cd0imer, the quantum yield for TPA cleavage ΦTPA and the two-
photon absorption cross-section σTPA in Goeppert-Mayer (GM) is given
in Eq. 5.
v0monomer = 2⋅ cd0imer⋅ΦTPA
⋅
σTPA⋅F2
(5)
A = cmono⋅d⋅(εNMQ
+
εcoumarin
)
(2)
As can be derived from this equation, a double logarithmic plot of
lnvm0 onomer versus lnF should show a linear dependence between these two
parameters with an expected slope of 2. Experimental values of 1.73 in
the case of NMQ and 1.54 for the NMQ/coumarin cross-dimer were
obtained (Fig. 7). This dependence confirms the two-photon nature of
the underlying process. As the occurrence of additional absorption
processes via excited states cannot be excluded, the theoretical value of
2 for the slope is not obtained. The TPA cross-section σTPA, a parameter
A
cmono
=
(3)
d⋅(εNMQ
+ εcoumarin)
The initial optical density at 265 nm of the three unirradiated dimer
solutions was higher than 2, justifying the approximation that all of the
photons are absorbed. Taking the small values of overall conversions (<
1 %) during the SPA induced cleavage into account, we assume pseudo-
zeroth-order kinetics for cyclobutane splitting. The quinolinone homo-
dimer undergoes cyclobutane cleavage much more efficiently. We ex-
pected the cross-dimer cleavage efficiency in between those of the
homo-dimers, but the SPA cleavage rate constant and the quantum yield
for the hybrid cross-dimer are the lowest of the three species (see
Table 1). This finding cannot be explained by the cross-dimer’s ab-
sorption characteristics as the value of its absorption coefficient lies in
between those of the homo-dimers. Previous work demonstrated that
the coumarin and quinolinone homo-dimers SPA cleavage mechanisms
show large differences. The coumarin dimer was found to cleave into
two singlet ground state monomers, quinolinone dimer cleavage leads to
the formation of one ground state and one excited state quinolinone
[18]. Based on the presented results, it can be suggested that the dimer
cleavage of the hybrid cross-dimer follows a pathway similar to that of
coumarin. Substitution of only one lactone unit to a lactam does not lead
to a higher cleavage efficiency.
comparable to the absorption coefficient
ε for SPA experiments, can be
derived from Eq. 1. For the evaluation of the results, it is assumed that
the quantum yield for dimer cleavage is the same for SPA and TPA ex-
periments [12,29]. Considering the laser beam diameter, the effective
reaction volume, and the effective reaction time, the TPA cross-sections
of the investigated structures can be calculated to be 0.74 GM (NMQ
dimer), 0.39 GM (coumarin dimer), and 0.35 GM (NMQ/coumarin
cross-dimer). Comparing these values as well as the rate constants for
dimer cleavage, presented in Fig. 7(B), it is obvious that the cross-dimer
shows the lowest cleavage efficiency of the investigated structures, just
like in single-photon cleavage.
This behavior can be explained by molecular symmetry consider-
ations. Structures with a given donor-acceptor-donor
π center and high
symmetry reportedly show improved two-photon absorption charac-
teristics compared to structures lacking these features [30,31]. The
lactam function in the quinolinone homo-dimer acts as an electron
donating group, the unsaturated cyclobutane in the chromophores
center may be assigned as the electron-accepting part. In the case of the
coumarin dimer, the lactone functions have a weaker electron-donating
effect, explaining the lower two-photon-absorption cross-section as well
as the lower rate constant for dimer cleavage. The even lower values for
the cross-dimer are consistent with this explanation. An overall lower
electron density compared to the NMQ dimer as well as the missing
TPA induced cyclobutane cleavage. Irradiating dimer solutions with
intense 532 nm pulses leads to cleavage of the cyclobutane ring. For
coumarin dimers, the TPA-induced ring cleavage is described in earlier
literature [12]. The cleavage of NMQ dimer and NMQ/coumarin
Table 1
Absorption coefficients at 265 nm, zeroth-order kinetics rate constants, SPA
cleavage quantum yields and two-photon cross section for the three investigated
dimers.
symmetry within this structure leads to
two-photon-absorption process.
a rather unfavorable
NMQ homo-
dimer
coumarin homo-
dimer
NMQ/coumarin
cross-dimer
4. Conclusion
-1
ACN
-1
]
14,541
8.315⋅10ꢀ 4
0.26
2028
4.243⋅10ꢀ 4
0.13
7497
3.526⋅10ꢀ 4
0.11
ε
[L⋅mol ⋅cm
265 nm
-1
v0[mmol⋅s
ΦSPA
σTPA[GM]
]
The presented experiments on anti-head-to-head coumarin and qui-
nolinone (NMQ) homo-dimers, as well as the corresponding anti-head-
to-head quinolinone-coumarin cross-dimer, studied the influence of the
0.74 ± 0.10
0.39 ± 0.03
0.35 ± 0.05
4