Long-lived platinum(ii) diimine complexes with broadband excited-state absorption: Efficient nonlinear absorbing materials

Article abstract of DOI:10.1039/c2dt31267k

Platinum(ii) diimine complexes with naphthalimide substituted fluorenylacetylide ligands are synthesized and characterized. The complexes exhibit long-lived ³ILCT or ³ILCT/³MLCT/ ³LLCT excited states (τ = ～20-30

Full text of DOI:10.1039/c2dt31267k

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Long-lived platinum(II) diimine complexes with broadband excited-state
absorption: efficient nonlinear absorbing materials†
Rui Liu,a Alexander Azenkeng,b Yuhao Lia and Wenfang Sun*a
Received 12th June 2012, Accepted 24th August 2012
DOI: 10.1039/c2dt31267k
substituents on the acetylide ligands increases.5 However, the
Platinum(II) diimine complexes with naphthalimide substi-
nature of the lowest excited state of these complexes could be
switched from an MLCT/LLCT state to the arylacetylide ligand
localized π,π* state or mixed MLCT/LLCT/π,π* when the
π-conjugation of the aryl group on the acetylide ligand is
extended6 or a strong electron-withdrawing or donating substitu-
ent is introduced on the aryl group.7 Such a change will not
only enhance the ground-state absorption but also influence
the excited-state absorption. Therefore, it is critical to choose
the appropriate acetylide ligand that can tune the energy
of the lowest excited state, but retain the broadband excited-state
absorption and long excited-state lifetime of the Pt diimine
complexes.
To utilize the 2nd strategy, our group incorporated the ben-
zothiazolyl fluorenylacetylide ligand, which was first applied by
the Schanze group in Pt phosphine complexes to show two-
photon absorption in the near-IR region,5d into Pt bipyridyl com-
plexes. We discovered that the complex containing this ligand
not only exhibited remarkable reverse saturable absorption (i.e.
transmission decreases with increased incident energy due to the
stronger excited-state absorption than ground-state absorption) in
the visible spectral region but also large TPA in the near-IR
region.4 Therefore, it is a very promising broadband nonlinear
absorbing material.
Intrigued by this exciting result, we are pursuing further
enhancement of the nonlinear absorption of the Pt(II) diimine
acetylide complexes via substitution of the benzothiazolyl group
by a naphthalimidyl (NI) group. Our rationale for choosing this
group is motivated by the seminal work of the Zhao group on a
Pt diimine complex with ethynyl naphthalimide ligands, which
has an extremely long-lived NI-based lowest triplet excited state
(τT = 124 μs in MTHF) and broadband excited-state absorp-
tion.3,8 By inserting the fluorene group in the acetylide ligand,
we anticipate slightly red-shifting the MLCT/LLCT band to
extend the reverse saturable absorption region; meanwhile,
fluorene derivatives have been widely used as building blocks
for TPA materials.5d,9 We envision that the TPA of the new com-
plexes could be enhanced. In this work, four Pt(II) complexes
containing naphthalimide substituted fluorenylacetylide ligands
(Pt-1–Pt-4, Chart 1) were synthesized and the ground-state and
excited-state properties and nonlinear absorption were investi-
gated. Pt-3 and Pt-4 lack the low-lying MLCT/LLCT states and
are used to compare to the properties of Pt-1 and Pt-2.8 The
branched-alkyl chains were introduced on the ligands to improve
the solubility and prevent intermolecular π–π stacking. As
tuted fluorenylacetylide ligands are synthesized and charac-
terized. The complexes exhibit long-lived 3ILCT or
3ILCT/3MLCT/3LLCT excited states (τ = ∼20–30 μs) and
broadband triplet transient absorption in the visible-NIR
region. Nonlinear transmission experiments at 532 nm
demonstrate that these complexes are efficient nonlinear
absorbing materials.
Pt(II) diimine complexes with long-lived excited-states have
attracted great attention in recent years because of their potential
applications in low-power upconversion,1 solar energy conver-
sion (via photo-induced charge separation),2 oxygen sensing,3
and nonlinear optics.4 For nonlinear optical applications,
materials with large excited-state absorption and long-lived
excited states are of particular interest. In order to utilize the
excited-state absorption, the excited state has to be populated
first via one-photon or two-photon absorption (TPA) upon exci-
tation. However, the absence of ground-state absorption of the
reported Pt(II) diimine complexes above 600 nm prevents acces-
sing the excited state via one-photon absorption. To remedy this
deficiency, two strategies are proposed: (1) one can modify the
molecular structure to bathochromically shift the low-energy
absorption band to the desired spectral region; or (2) one
can introduce a two-photon absorbing motif to the molecule to
populate the excited state via TPA. Although both strategies are
feasible, the second approach has the advantage of keeping the
light color of the material and further enhancing the nonlinear
absorption via TPA in addition to the excited-state absorption.
Previous studies on Pt diimine arylacetylide complexes
demonstrated that the energy of the lowest excited state,
namely the metal-to-ligand charge transfer (1MLCT)/ligand-to-
ligand charge transfer (1LLCT) states in most cases, decreases
when the electron-withdrawing ability of the substituent on the
diimine ligand increases or the electron-donating ability of the
aDepartment of Chemistry and Biochemistry, North Dakota State
University, Fargo, ND 58108-6050, USA. E-mail: Wenfang.Sun@ndsu.edu;
Fax: (+1) 701-231-8831
bEnergy and Environmental Research Center, University of North
Dakota, Grand Forks, ND 58202-9018, USA
†Electronic supplementary information (ESI) available: Synthetic
scheme, experimental details, characterization data, TDDFT calculation
results, UV-Vis absorption and emission spectra and data for the ligands
and the Pt(II) complexes in different solvents, ns time-resolved TA
spectra of Pt-1–Pt-4. See DOI: 10.1039/c2dt31267k
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reported in this letter, these complexes exhibit long-lived triplet
excited states and broadband excited-state absorption, and have
been demonstrated to be among the best reverse saturable absor-
bers for ns laser pulses at 532 nm.
and mixed with some dπ(Pt) → π*(naphthalimide) and π(CuC)
→ π*(CuC) characters. This assignment is supported by the
DFT calculation results for Pt-1 and Pt-2 (Fig. 2, Table 2, and
ESI, Fig. S2, S3 and Tables S1, S2†) discussed below. Compared
to Pt-3 and Pt-4, the visible absorption band in Pt-1 and Pt-2 is
slightly red-shifted and broadened, which is more salient in Pt-1
The UV-Vis absorption spectra of the complexes in a CH2Cl2
solution (1 × 10−5 mol L−1) are presented in Fig. 1a, and the
band maxima and molar extinction coefficients for each complex
are compiled in Table 1. The structured absorption bands below
1
than Pt-2. With reference to the DFT calculation results, MLCT
(dπ(Pt)
→
π*(bipyridyl))/1LLCT (ligand-to-ligand charge
1
370 nm are assigned to the intraligand (IL) π,π* transitions of
transfer from π(fluorenylacetylide) to π*(bipyridyl)) transitions
also contribute to the visible absorption band in Pt-1, but not to
the band in Pt-2. This could explain the longer tail in Pt-1 that is
absent in Pt-2. The charge transfer nature of the visible absorp-
tion band is reflected by the negative solvatochromic effect
(ESI, Fig. S4, S5†). Notably, the tail in complex Pt-1 is also red-
shifted compared to its analogue reported in the literature, which
has no fluorene component in the acetylide ligands.3,8 The red-
shifted tail in Pt-1 and Pt-2 in the visible region is critical for
broadening the reverse saturable absorption spectral region,
which requires the population of excited states via one-photon
absorption.
1
the acetylide ligands. Compared to the π,π* transitions in their
respective ligands (ESI, Fig. S1†), these bands exhibit a batho-
chromic shift, indicating the delocalization of the ligand-centered
molecular orbitals via interactions with the Pt dπ orbitals. The
broad structureless absorption bands above 370 nm are domi-
nated by the 1ILCT (intra-ligand charge transfer from the ethynyl-
fluorene component to the naphthalimide component) transition,
Density-functional theory (DFT) calculations and time-
dependent density functional theory (TDDFT) calculations were
performed for complexes Pt-1 and Pt-2 in CH2Cl2 in order to
understand the nature of the ground and excited electronic states
(Fig. 2, Table 2, and ESI, Fig. S2, S3 and Tables S1, S2†).
The calculation results indicate that the HOMO and HOMO − 1
orbitals are essentially fluorenylacetylide and Pt metal based for
both Pt-1 and Pt-2, while the LUMO is almost exclusively π*
(naphthalimide) based for Pt-2, and delocalized on π*(naphthal-
imide) and π*(bipyridine) for Pt-1. The electron density distri-
butions on the LUMO + 1 are similar to that of the LUMO for
Pt-2, with the dominant contribution from the π*(naphthal-
imide); but the LUMO + 2 is dominated by the π*(bipyridyl).
For Pt-1, LUMO + 2 resembles that of LUMO, with the domi-
nant contribution from the π*(bipyridine) and π*(naphthal-
imide); but the LUMO + 1 is exclusively localized on the π*
(naphthalimide).
Chart 1 Chemical structures of Pt complexes Pt-1–Pt-4.
The TDDFT calculation results show that the 1st excited state
for Pt-1 occurs at 375 nm, which has the dominant contributing
configuration involving the HOMO → LUMO and HOMO →
LUMO + 2 orbital pairs; while the 2nd excited state that involves
the HOMO − 1 → LUMO and HOMO − 1 → LUMO + 2
orbital pairs as the dominant configurations appears at 361 nm.
For Pt-2, the 1st excited state involving the HOMO → LUMO
Fig. 1 (a) UV-Vis absorption spectra and (b) normalized emission
spectra (c = 1.0 × 10−5 M) of Pt complexes in CH2Cl2, 25 °C, λex is
417 nm for Pt-1 and Pt-2, 395 nm for Pt-3 and 419 nm for Pt-4.
Table 1 Photophysical parameters of Pt-1–Pt-4
λabs/nm
λem/nm
(τem/ns; Φem
λT1–Tn/nm (τTA/μs; εT1–Tn/
(ε/104 L mol−1 cm−1 a
)
)
104 M−1 cm−1; ΦT)d
σ0 e/10−18 cm2
σex f/10−18 cm2
σex/σ0
ΦTσex/σ0
b
Pt-1
Pt-2
Pt-3
Pt-4
337 (9.10), 404 (5.04)
349 (9.17), 436 (8.20)
345 (8.52), 398 (5.32)
353 (8.75), 433 (7.94)
589 (c, 0.012)
535 (21.6, 6.24,0.11)
565 (29.6, 6.89, 0.091)
535 (31.2, 5.77, 0.084)
565 (27.3, 6.19, 0.083)
0.78
1.54
—
256
242
220
191
328
157
—
37.4
14.2
—
500 (c, 0.051), 646 (c)
494 (90, 0.118)
503 (80, 0.122)
0.52
367
30.2
a Electronic absorption band maxima and molar extinction coefficients in CH2Cl2 at room temperature. b Room temperature emission band maxima
and decay lifetimes measured in CH2Cl2 at a concentration of 1 × 10−5 mol L−1. A degassed aqueous solution of [Ru(bpy)3]Cl2 (Φem = 0.042, excited
at 436 nm) was used as the reference. c Too weak to be measured. d ns TA band maximum, triplet extinction coefficient, triplet excited-state lifetime
and quantum yield. Measured in CH3CN. SiNc in C6H6 was used as the reference (ε590 = 70 000 L mol−1 cm−1, ΦT = 0.20). e Ground-state (σ0)
absorption cross section at 532 nm, measured in CH3CN. f Excited-state absorption cross section (σex) at 532 nm, measured in CH3CN.
12354 | Dalton Trans., 2012, 41, 12353–12357
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Fig. 2 Contour plots of the four highest occupied molecular orbitals (HOMOs) and four lowest unoccupied molecular orbitals (LUMOs) for Pt-2 in
CH2Cl2.
Table 2 Excitation energies (eV), wavelengths (nm), oscillator strengths, main contributing transitions, and the associated configuration coefficients
of five low-lying electronic states of complex Pt-2 in CH2Cl2 obtained at the CAM-B3LYP/LANG631//B3LYP/LANG631 level of theory. All
calculations include corrections due to solvent effects using the PCM model
Excitation energy
Coefficient of main
Sn
1
eV
nm
f
Main contributing transitions
contributing configuration
3.02
3.08
409
402
3.362
0.966
HOMO → LUMO
0.40
0.35
0.40
0.37
0.58
0.56
0.35
0.28
0.28
HOMO − 1 → LUMO + 1
HOMO − 1 → LUMO
HOMO → LUMO + 1
HOMO → LUMO + 2
HOMO − 1 → LUMO + 2
HOMO → LUMO + 3
HOMO − 2 → LUMO
HOMO − 3 → LUMO + 1
2
3
4
5
3.35
3.47
3.83
370
356
324
0.021
0.224
0.175
and HOMO − 1 → LUMO + 1 orbital pairs as the dominant
configurations appears at 409 nm; while the 2nd excited state
(involving the HOMO − 1 → LUMO and HOMO → LUMO + 1
orbital pairs as the dominant configurations) occurs at 402 nm.
According to the electron density distributions on HOMO − 1,
HOMO, LUMO, LUMO + 1, and LUMO + 2, the 1st and 2nd
excited states should be predominantly attributed to
consistent with the nature of the lowest singlet excited state of
Pt-2 described above. However, Pt-2 exhibits an additional
emission band at 646 nm, which is quite sensitive to oxygen
quenching (ESI, Fig. S12†). Considering the large Stokes shift
and sensitivity to oxygen quenching, this band can be ascribed
to phosphorescence from the 3ILCT excited state. In contrast,
Pt-1 exhibits a broad structureless emission at 589 nm, which is
sensitive to oxygen quenching and shows a positive solvato-
chromic effect (ESI, Fig. S13†). With reference to the TDDFT
calculation results for Pt-1, which ascribes the lowest singlet
1
1ILCT/1MLCT/1LLCT transitions for Pt-1 and ILCT transition
for Pt-2, which correspond to the broad, structureless absorption
band between 360 nm and 500 nm for Pt-1 and the 380–520 nm
band for Pt-2, respectively. It is noted that the UV-Vis absorption
band maxima and the values predicted by calculations are gener-
ally consistent with each other and the trend for Pt-1 and Pt-2 is
also in line with the experimental result. This supports our initial
assignment of the absorption bands discussed in the preceding
paragraph.
1
excited state of Pt-1 to be ILCT/1MLCT/1LLCT, and with the
assumption that the singlet/triplet splitting may not be large, we
attribute the emission of Pt-1 to the 3ILCT/3MLCT/3LLCT
excited states. It is noted that the nature of the emitting state for
Pt-1–Pt-4 is significantly altered compared to their analogue
reported in the literature without the fluorene component in the
3
To understand the excited state characteristics that are closely
related to nonlinear absorption, emission of Pt-1–Pt-4 in a
degassed CH2Cl2 solution was studied. Excitation of these com-
plexes at their respective visible charge-transfer band at room
temperature results in structureless blue-to-orange luminescence
(Fig. 1b). The emission of Pt-2, Pt-3, and Pt-4 exhibits relatively
small Stokes shifts and short lifetimes (<5 ns). The emission
energy is quite similar to the fluorescence from their respective
acetylide ligands (ESI, Fig. S6†). In addition, similar to that
observed in the ligand, the emission exhibits a positive solvato-
chromic effect (ESI, Fig. S7–S11†). Therefore, the observed
acetylide ligands, which possesses long-lived π,π* phosphore-
scence from the acetylide ligand.3,8
Transient difference absorption (TA) spectroscopy is a critical
technique to understand the excited-state absorption. One can
not only identify the spectral region where excited-state absorp-
tion is stronger than ground-state absorption from the spectral
feature, but also obtain the lifetime of the excited state from the
decay of the transient absorption. Fig. 3a displays the triplet TA
spectra of the Pt complexes at the zero time delay in a degassed
CH3CN solution. Pt-1–Pt-4 exhibit similar spectral features,
with bleaching occurring below 450 nm and broad absorption
bands in the visible to the NIR region. Similar to that observed
in their UV-Vis absorption spectra, the absorption bands in Pt-2
1
emission for Pt-2, Pt-3, and Pt-4 can be assigned to the ILCT
fluorescence from the acetylide ligand. This assignment is also
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the salient red-shift of the visible absorption band and the
stronger NIR absorption band in Pt-1 compared to those in Pt-3,
which is in line with that observed from the respective UV-Vis
absorption spectra. For Pt-2, the TA feature is essentially identi-
cal to that of Pt-4. Thus the transient absorbing excited state for
3
Pt-2 and Pt-4 is assigned to the ILCT state. The same attribu-
tion is applied to Pt-3 considering the similar TA features
between Pt-3 and Pt-2 and Pt-4. These assignments are sup-
ported by the similar TA features of Pt-2–Pt-4 to those of their
corresponding ligands 1-L and 2-L in CH3CN (ESI, Fig. S26,
S27†), in which the excited state giving rise to the TA is believed
to be the 3ILCT state (the lowest triplet excited state in polar sol-
vents). It should be pointed out that the lifetimes for Pt-1–Pt-4
are much longer than that for the Pt complex bearing benzothia-
zolyl fluorenylacetylide ligands reported by our group earlier.4a
The long-lived triplet excited state and the broadband excited-
state absorption imply that reverse saturable absorption (RSA)
could occur from these complexes in the visible spectral region.
Reverse saturable absorption (RSA) is a nonlinear absorption
phenomenon that the absorptivity of the molecule increases with
increased incident energy due to stronger excited-state absorption
with respect to the ground-state absorption. In order for RSA to
occur, the molecule has to exhibit larger excited-state absorption
cross-section (σex, which describes the probability of a molecule
at a lower excited state to absorb one photon of the excitation
light to be populated to a higher excited state, and is proportional
to the excited-state molar extinction coefficient via equation σ =
2303ε/NA) than the ground-state absorption cross section (σ0,
i.e. the probability of a molecule at the ground state to absorb
one photon to be populated to the singlet excited state). Namely,
the ratio of σex/σ0 has to be larger than 1; and the larger the
σex/σ0 ratio, the stronger the RSA. To demonstrate the RSA, a
nonlinear transmission experiment at 532 nm was carried out in
CH2Cl2 solutions (Fig. 3b) using ns laser irradiation for Pt-1,
Pt-2, and Pt-4. Pt-3 has no ground-state absorption at 532 nm
and thus the excited state cannot be populated via one-photon
absorption at this wavelength. As a result, RSA cannot be
observed and the nonlinear transmission experiment was not
carried out for Pt-3. The concentration of each complex solution
was adjusted to obtain a linear transmission of 80% in a 2 mm
cuvette. To evaluate the effect of the naphthalimide substituent
compared to the benzothiazole substituent on the nonlinear
absorption, a Pt complex bearing benzothiazolyl fluorenylacety-
lide ligands reported by our group earlier4a was also measured
under the same conditions. With increased incident energy, the
transmission of these Pt complexes decreases drastically, which
clearly manifests the RSA at 532 nm. The strength of the RSA
follows this trend: Pt-1 > Pt-4 > Pt-2 > Pt complex in ref. 4a.
Among these, Pt-1 exhibits the strongest RSA, with the trans-
mission decreasing to 15% when the incident energy reaches
∼440 μJ. The RSA of these complexes at 532 nm is stronger
than the Pt complex bearing benzothiazolyl fluorenylacetylide
ligands reported by our group earlier.4a
Fig. 3 (a) Triplet transient difference absorption spectra of Pt com-
plexes at zero delay after the 355 nm pulsed-laser excitation in a
degassed CH3CN solution, A355 = 0.4 in a 1 cm cuvette; (b) nonlinear
transmission curves for Pt complexes in CH2Cl2 for 4.1 ns laser pulses
at 532 nm. The radius of the beam waist at the focal point was ∼96 μm.
The linear transmission for all samples was adjusted to 80% in a 2 mm
cuvette.
and Pt-4 are obviously red-shifted compared to those in Pt-1
and Pt-3 due to the extended π-conjugations in the acetylide
ligand. The lifetimes obtained from the decay of the TA
(Table 1) are ∼20–30 μs, which are remarkably different from
the emission lifetimes and are much longer than that for a typical
triplet metal-to-ligand charge-transfer excited state. This indi-
cates that the excited state giving rise to the TA is from the
triplet excited state. Considering the nature of the lowest singlet
excited state for Pt-1 and Pt-2 discussed earlier, which is
1ILCT/1MLCT/1LLCT and 1ILCT, respectively, and assuming
the singlet and triplet excited state splitting is not large, the
triplet excited state giving rise to the observed TA spectra for
Pt-1 and Pt-2 could be 3ILCT/3MLCT/3LLCT and 3ILCT,
respectively. This notion is consistent with the assignment of the
lowest triplet excited state for these two complexes based on the
phosphorescence of Pt-1 and Pt-2. The involvement of
3MLCT/3LLCT characters in Pt-1 but not in Pt-2 is reflected by
Because all of the complex solutions have the same linear
transmission to guarantee the same population at the singlet
excited state upon excitation, the observed nonlinear trans-
mission difference should be attributed to the different ratios of
the excited-state absorption cross section to that of the ground-
state (σex/σ0). In addition, because RSA of ns laser pulses are
12356 | Dalton Trans., 2012, 41, 12353–12357
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dominated by the triplet excited-state absorption, the triplet
excited-state quantum yield (ΦT) should also play a role. There-
fore, the key parameter that determines the RSA of ns laser
pulses is the ΦTσex/σ0 value of a molecule. The ground-state
absorption cross sections at 532 nm for these complexes are
deduced from the ε values obtained from their UV-Vis absorp-
tion using the conversion equation σ = 2303ε/NA, where NA is
the Avogadro constant. The triplet excited-state absorption cross
sections at 532 nm are estimated from the ΔOD at zero time
delay of the ns TA spectrum and the εT1–Tn at the TA band
maximum (Table 1). The results are compiled in Table 1. The
ΦTσex/σ0 values for these complexes at 532 nm follow this
trend: Pt-1 > Pt-4 > Pt-2. The stronger RSA of Pt-1 than Pt-2 is
a result of the extended π-conjugation in the acetylide ligands,
which red-shifts both the ground-state and excited-state absorp-
tion spectra. Consequently, the ground-state absorption cross
section at 532 nm increases while the triplet excited-state absorp-
tion decreases, leading to the reduced ratio of ΦTσex/σ0 and
decreased RSA in Pt-2 compared to that in Pt-1. The stronger
RSA of Pt-4 than Pt-2 is mainly attributed to the reduced σ0 at
532 nm for Pt-4, leading to a larger ratio of ΦTσex/σ0 for Pt-4
than for Pt-2. It is worth noting that the estimated σex/σ0 values
for these complexes at 532 nm are much larger than most of the
reverse saturable absorbers reported in the literature at 532 nm
(see ESI, Table S6†).10 (It should be noted that comparison of
the bulk RSA of different molecules reported in the literature
that are measured under different experimental conditions is
meaningless because different concentrations, thicknesses of
samples, different wavelengths or experimental setups will affect
the RSA results drastically. A better way to evaluate the perform-
ance of a reverse saturable absorber is to compare the intrinsic
molecular parameter σex/σ0.) Although the σex/σ0 values for
Pt-1, Pt-2 and Pt-4 are somewhat smaller than those for the Pt
complex with benzothiazolyl fluorenylacetylide ligands,4a the
values reported in this work are estimated values using different
methods from that reported previously for the Pt complex with
benzothiazolyl fluorenylacetylide ligands.4a Because the esti-
mated σex values using the method briefed in this work are
generally smaller than the σex values obtained via fitting of the
Z-scan data using a five-level model (which is the case for
obtaining the σex values for the Pt complex with benzothiazolyl
fluorenylacetylide ligands),4a the σex values and the σex/σ0 and
ΦTσex/σ0 ratios reported in this paper for Pt-1, Pt-2 and Pt-4 are
the bottom-line values for these complexes. The true values will
be reported later by conducting Z-scan measurements and fitting
the Z-scan data using a five-level model.4a Nevertheless, the
much longer lifetimes of Pt-1, Pt-2 and Pt-4 could compensate
for the somewhat smaller σex/σ0 values, leading to the observed
stronger RSA for these complexes than the Pt complex bearing
benzothiazolyl fluorenylacetylide ligands.4a From the compari-
son of the σex/σ0 values listed in Table S6†, the RSA of Pt-1,
Pt-2 and Pt-4 should be much stronger than the other reverse
saturable absorbers reported in the literature if the RSA exper-
iments are carried out under identical conditions. Therefore, they
are among the best reverse saturable absorbers for ns laser pulses
at 532 nm. Moreover, these complexes could potentially exhibit
large two-photon absorption in the near-IR region considering
the charge-transfer nature of the lowest excited state of these
complexes (this will be studied and reported in the near future).
They are thus very promising broadband nonlinear absorbing
materials as well.
In conclusion, the Pt(II) diimine complexes bearing naphthali-
mide substituted fluorenylacetylide ligands exhibit strong
1
1ILCT or mixed ILCT/1MLCT/1LLCT absorption bands in the
visible spectral region, as well as long-lived 3ILCT or
3ILCT/3MLCT/3LLCT excited states (τ = ∼20–30 μs) and broad-
band triplet TA in the visible-NIR region. Upon ns laser pulse
irradiation at 532 nm, strong RSA occurs. Thus, these Pt com-
plexes are among the most promising nonlinear absorbing
materials. Moreover, their long-lived triplet excited states make
them potential candidates for upconversion, oxygen sensing, and
photovoltaic applications.
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