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the major 2PA band is higher than its one-photon excited
were further investigated on their high-order MPP fluores-
state, which is related with the excited virtual state in the 2PA
process, and is consistent with the 2PA selection rule for the
octupolar (and quadrupolar) molecule.[4b,c,24] Like their one-
photon absorption spectra, the sharp peaks were observed in
their 2PA spectra with extending the rigid p-conjugated
molecular skeletons, especially for SpL-3. It means that the
transition dipole moments (in the transitions from the ground
state to the intermediate state and from the intermediate state
to the excited state) are significantly enhanced at some
specific energy levels of the cross-shaped spiro-fused ladder-
type materials with highly extended and rigid p-conjugated
molecular skeletons.[4c,25] The similar phenomena were also
observed in SpL(2)Ày (y = 1–3) quadrupolar molecules (Fig-
ure S4). For comparison, the distributions of 2PA at the first
excited state of SpL(1)Àx (x = 1–3) dipolar molecules are
relatively high. Again, with increasing the p-conjugated
lengths of ladder-type skeletons, the 2PA distribution at the
higher excited state has increased in SpL(1)Àx (x = 1–3).
Note that due to the selection rules, the pathways of
resonance enhancements and molecular transitions for one-
photon and multiphoton processes are generally different.[4b]
The phenomenon of a higher excited state is more obvious in
the high-order MPA process.
As illustrated in Figure 1b and Table S1, the maximum s2
of all samples, which were extracted throughout the 2PA
spectra, are significantly enhanced as increasing the p-
conjugated lengths of the ladder-type skeletons in spiro-fused
molecules. A recorded large s2 of 9188 GM was obtained in
SpL-3, which has a cross-shaped molecular configuration with
an extended p-conjugation. The 2PP PL spectra exhibit the
same peaks, and more likely low energy band emission
(0–1 and 0–2 emission bands) in comparison with the one-
photon-excited fluorescence (Figure 1c; Figure S5). Com-
pared with their linear counterparts, SpL(1)Àx and
SpL(2)Ày (x, y = 1–3), the unification of an extremely high
s2 (up to ca. 9.2 103 GM) and brilliant luminescence
emission (PLQY: ꢁ 86%) in SpLÀz (z = 1–3) make them
ideal candidates as high-order MPP upconversion emitters.
The logarithmic power-dependent plots of the relative
2PP fluorescence intensity of SpL(1)Àx, SpL(2)Ày, and
SpLÀz (x, y, z = 1–3) solution samples as a function of input
power intensity excited at 800 nm afford straight line relation-
ships with slopes in the range of 1.829–2.109, 2.045–2.150, and
1.919–1.964, respectively (Figure 1d; Figure S6), demonstrat-
ing a strong quadratic dependence of excited fluorescence on
input power intensity, and thus corroborating the 2PA-
induced frequency-upconverted fluorescence. Meanwhile,
the nonlinear transmission of input power density exists in
the 2PA process. Figure 1e and Figure S7 show that the
transmitted intensities of 800 nm laser pulses follow the
expected behaviors in solution samples of SpL(1)Àx,
SpL(2)Ày, and SpLÀz (x, y, z = 1–3), further verifying the
2PA processes. The nonlinear 2PA coefficients (b) extracted
from the fitting curves for SpL-1, SpL-2, and SpL-3 are
calculated as 0.051, 0.070, and 0.059 cm/GW, respectively,
when excited at 800 nm femtosecond pulses.
cence properties. The strong high-order MPP fluorescence
responses and characteristics were studied with excitation
wavelengths varying from 900 nm to 2200 nm. For compar-
ison, we finely modulate the laser beam area and pump
energy intensity to avoid the possible presence of third-
harmonic generation (THG) from laser beam, which impact
the MPA-induced fluorescence intensity and data analysis,
especially for the power dependence measurement.[26] The
PLE spectra covering the whole excitation wavelength region
of SpL(2)Ày and SpLÀz (y, z = 1–3) solutions are summar-
ized in Figure 2a (Figure S8). The MPA-induced fluorescence
emission intensity of SpL-3 solution is in fact several orders
(ꢀ 8) of magnitudes lower than that of 2PP, and no MPP PL
signal was detected when the excitation wavelength exceeds
2200 nm. Figure 2b depicts the slopes from power depend-
ence measurement as a function of excitation wavelength of
SpL-3 solution; the 2PA processes could be clearly observed
with slopes changing from 1.844 to 1.905 with the wavelength
range of 600–900 nm. The cubic dependence of fluorescence
on input power intensity is observed in the wavelength region
of 900 nm to 1100 nm. Here the wavelength of one-photon
absorption band is shorter than a half of the excitation
wavelength (larger than 950 nm). Therefore, the 3PA pro-
cesses could be confirmed in this excitation wavelength
region. When the excitation wavelengths exceed 1100 nm, it is
difficult to ascertain whether the frequency-upconverted
fluorescence is a pure high-order MPA process, since the
individual excitation mechanism from 3- to 6-PA cannot be
established from the power dependence measurement. De-
tailed information is available in Figure S9 (Figure 2c). We
can approximately divide the excitation wavelength regions
for 3- to 6-PA processes into 900–1400 nm, 1100–1520 nm,
1520–1760 nm, and 1720–2200 nm, respectively. In the over-
lapped wavelength regions, the large variation in power-
dependent index indicates the manifold excitation contribu-
tion to the MPA process, consistent with our pervious
report.[16c] The manifold excitation contribution to the MPA
process is attributed to the overlap of energy levels in
different MPA processes. Figure 2c shows the log-log plots of
fluorescence emission intensity versus input power intensity
of SpL-3 solution at the selected excitation wavelengths
(990 nm, 1440 nm, 1540 nm, and 1820 nm, respectively), with
slope gradients of 3.198, 4.384, 5.185, and 5.882, correspond-
ing to the typical 3- to 6-PA processes.
The 3PA cross-sections, s3, were measured in the range of
900–1400 nm with (L)-Ph(4)-NPh as a reference standard
(Figure 2d; Figure S10).[15] The 3PA processes of SpL(2)Ày
and SpLÀz (y = 1–3, z = 1, 2) were also confirmed by the
power dependence and nonlinear transmission measurements
at 990 nm (Figures S11 and S12). Consistently, the maximum
s3 increase with extending the conjugation length in these
series (Figure 2e). A record-high intrinsic s3 of 2.39
10À74 cm6 s2 photonÀ2 at 940 nm for SpL-3 was obtained.[4d,27]
From the fitted curves of non-linear transmission, the 3- to
6-PA processes of SpL-3 could be further confirmed at the
selected wavelengths of 990 nm, 1440 nm, 1540 nm, and
1820 nm, respectively (Figure 2 f). The 3-, 4-, 5, and 6-PA
coefficients (g, d, f, and w) of SpL-3 were evaluated as
Due to their larger 2PA cross-sections as compared with
SpL(1)Àx (x = 1–3), SpL(2)Ày and SpLÀz (y, z = 1–3) series
Angew. Chem. Int. Ed. 2021, 60, 10007 –10015
ꢀ 2021 Wiley-VCH GmbH
Jiang, Yi
