Accessing the long-lived near-IR-emissive triplet excited state in naphthalenediimide with light-harvesting diimine platinum(ii) bisacetylide complex and its application for upconversion

Article abstract of DOI:10.1039/c1dt10679a

Room temperature near-IR phosphorescence of naphthalenediimide (NDI) was observed with NN Pt(ii) bisacetylide complex (Pt-NDI) in which the NDI was connected to Pt(ii) center via acetylide. Pt-NDI shows intense absorption of visible light and long-lived NDI-localized excited state (³IL) (τ_T = 22.3 μs). Pt-NDI was used as a triplet sensitizer for upconversion.

Full text of DOI:10.1039/c1dt10679a

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Volume 40 | Number 36 | 28 September 2011 | Pages 9053–9328
ISSN 1477-9226
COVER ARTICLE
Zhao, Guo et al.
Accessing the long-lived near-IR-emissive triplet excited state in naphthalenediimide with light-harvesting
diimine platinum(II) bisacetylide complex and its application for upconversion

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Accessing the long-lived near-IR-emissive triplet excited state in
naphthalenediimide with light-harvesting diimine platinum(II) bisacetylide
complex and its application for upconversion†
Yifan Liu, Wanhua Wu, Jianzhang Zhao,* Xin Zhang and Huimin Guo*
Received 16th April 2011, Accepted 5th May 2011
DOI: 10.1039/c1dt10679a
Room temperature near-IR phosphorescence of naph-
thalenediimide (NDI) was observed with NŸN Pt(II)
bisacetylide complex (Pt-NDI) in which the NDI was con-
nected to Pt(II) center via acetylide. Pt-NDI shows intense
absorption of visible light and long-lived NDI-localized
excited state (3IL) (sT = 22.3 ls). Pt-NDI was used as a triplet
sensitizer for upconversion.
Population of long-lived triplet excited states of transition metal
complexes upon visible light excitation is significant for applica-
tions in photovoltaics,1 photoinduced charge/energy transfer,2
photocatalysis,3 molecular probes and optical power limiting,
etc.4 For example, it has been demonstrated that long-lived
Scheme 1 Structures of Pt-NDI and ligand NDI-C CH. The known
complex Pt-Ph is used as model compound.
triplet excited state is helpful to improve the oxygen sensing
property,4d the efficiency of photovoltaic devices,5 and triplet–
triplet-annihilation (TTA) based upconversion.6
to the minor heavy atom effect. Attempts have been made to
Typically transition metal complexes were used to access the
long-lived triplet excited state, such as Ru(II) diimine complexes,7
PtII/PdII porphyrin complexes,8 more recently the PtII acetylide
complexes and the cyclometallated PtII complexes, etc.9 The heavy
atom effect of the transition metal will facilitate the intersystem
crossing (ISC), thus the triplet excited state of the complexes can
be populated upon photoexcitation via S0→S1→T1 (direct S0→T1
transition is forbidden). Concerning this aspect, the diimine PtII
bisacetylide complexes are particularly interesting, due to their
readily tunable photophysical properties simply by changing the
acetylide ligands attached to the PtII center, which is synthetically
feasible.9 However, the typical diimine PtII bisacetylide complexes,
such as dbbpy PtII bisphenylacetylide (dbbpy = 4,4¢-di(tert-butyl-
2,2¢-bipyridine, Scheme 1, Pt-Ph), show very poor absorption in
the visible range and the lifetime of the T1 state is short (for Pt-Ph,
t = l.3 ms).10b
prepare light-harvesting molecular arrays by attaching organic
chromophores to NŸN (or NŸNŸN) PtII acetylide scaffolds, such
as benzyl, naphthal, pyrenyl, perylene, naphthalimide or boron-
dipyrromethene (BODIPY) moieties.10,11 However, the UV-vis
absorption of these dyads are still limited to the UV/blue region
and in some cases the photophysics of the respective components
collapsed in dyads. For example, perylenediimide (PDI) was
attached to PtII via acetylide bond (Pt-PDI).12 Pt-PDI shows
absorption at 573 nm (molar extinction coefficient e = 68 000 M-1
cm-1). However, this complex is non-phosphorescent and the
lifetime of its 3IL excited state (tT = 0.246 ms) is shorter than the
model complex Pt-Ph.12 As aforementioned, longer triplet excited
state lifetimes are desired for applications. However, to the best of
our knowledge, no PtII bisacetylide–organic chromophore dyads
have been reported to show intense absorption in the visible region,
and at the same time, show long-lived triplet excited states.
On the other hand, to establish the intraligand triplet excited
state (3IL) is in particular interesting because the 3IL state usually
shows prolonged lifetime compared to the 3MLCT state, due
Naphthalenediimide (NDI) is used as the light-harvesting
chromophore in solar cells and molecular probes, due to its intense
absorption in the visible region or high fluorescence quantum
yield.13 But its room temperature (RT) phosphorescence has not
been reported. Herein we report a naphthalenediimide (NDI)
PtII bisacetylide complex, in which NDI aromatic ring is directly
connected to PtII via –C C– bond (Scheme 1, Pt-NDI). Intense
absorption in the green/yellow region, long-lived triplet excited
state (tT = 22.3 ms), and RT near-IR emission (lem = 784 nm,
UP = 0.2%) was observed. To the best of our knowledge, these
State Key Laboratory of Fine Chemicals, School of Chemical Engi-
neering, Dalian University of Technology, Dalian, 116024, P. R. China.
E-mail: zhaojzh@dlut.edu.cn, guohm@dlut.edu.cn; Fax: + 86 411 8498
6236; Tel: + 86 411 8498 6236
† Electronic supplementary information (ESI) available: Preparation of
the complexes and detail of the upconversion experiments. See DOI:
10.1039/c1dt10679a
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9085–9089 | 9085
©

are the longest absorption and emission wavelengths observed for
diimine PtII bisacetylides complexes.10b Moreover, it is for the first
time that RT NDI phosphorescence was observed. We used Pt-
NDI as a triplet sensitizer for TTA upconversion and significant
upconversion quantum yield was observed.
Naphthaldianhydride was used as starting material for the
synthesis of Pt-NDI (see ESI for details†). Bromination by
dibromoisocyanuric acid leads to the 2,6-dibromonaphthalene
bisanhydrides. Then reaction with 2-ethylhexan-1-amine gives the
bisboromodiimide, an ether chain was introduced to improve the
solubility of the diimide. The acetylide (–C CH) was introduced
by Sonogashira coupling (see ESI for detail†).
Pt-NDI is phosphorescent at RT upon excitation at either
583 nm or 420 nm (Fig. 2). The emission is centered at 784 nm,
which is much red-shifted compared to the emission of the free
NDI-C CH ligand (lem = 561 nm). The large Stokes shift and the
quenching of the emission by O2 indicated the phosphorescence
nature of the emission band. Another emission band at 603 nm
was also observed and was tentatively assigned to the fluorescence
of the ligand (this emission band can not be quenched by O2).
Similar fluorescence/phosphorescence dual emission was recently
reported for a cyclometallated PtII complex with large ligands.9d
It should be noted that PtII complexes which give emission in
the near-IR region is rare.10b,10e To the best of our knowledge,
Pt-NDI is the PtII acetylide complex that gives the longest
emission wavelength at RT. Previously Pt-PDI was reported to
show visible-light harvesting effect, but no phosphorescence was
observed.12
The absorptions of Pt-NDI, NDI acetylide ligand (NDI-
C
CH) and the model complex Pt-Ph were compared (Fig. 1). Pt-
Ph gives absorption at 424 nm, with a moderate e of 5800 M-1 cm-1,
which is due to the MLCT/LLCT transitions (metal-to-ligand-
charge-transfer, and ligand-to-ligand charge transfer).10b For Pt-
NDI, this absorption band is clearly discernable. Furthermore,
Pt-NDI shows intense absorption in the green/yellow region,
centered at 581 nm (e = 31 300 M-1 cm-1). To the best of our
knowledge, Pt-NDI is the PtII bisacetylide complex that shows the
most red-shifted absorption wavelength.10 By comparison with
the absorption of the NDI-C CH ligand, the intense absorption
band at 583 nm is clearly attributed to the p–p transition of the
NDI moiety. The red-shifted absorption of the complex compared
to the free ligand NDI-C CH is due to the strong electronic
communication between the PtII and the acetylide, and is typical
for NŸN PtII bisacetylide complexes.12
Fig. 2 Normalized excitation and emission spectra (lex = 583 nm) of
Pt-NDI. In toluene (1.0 ¥ 10-5 M, 20 ◦C).
Electron withdrawal acetylide in NŸN PtII bisacetylide com-
plexes usually lead to blue-shifted emission compared to Pt-Ph
(lem = 567 nm),9,10b but Pt-NDI shows emission at 784 nm, which
is red-shifted by 217 nm vs. Pt-Ph. Thus the near-IR emission of
Pt-NDI is due to the NDI ligand centered 3IL excited state. This is
supported by the nanosecond time-resolved transient absorption
spectra (vide infra), the DFT calculations of the S0–T1 energy
gap of NDI-C C–H ligand (744 nm, ESI†) and the spin density
isosurface of Pt-NDI (vide infra).
Fig. 1 UV-vis absorption spectra of Pt-NDI, Pt-Ph and the ligand
NDI-C CH. Inset is the photograph of the solutions of Pt-Ph and
Pt-NDI. In toluene (1.0 ¥ 10-5 M; 20 ◦C).
The RT phosphorescence quantum yield (UP) of Pt-NDI is
0.2%. Two possible factors may contribute to this small UP value.
First, the product of the triplet excited state upon photoexcitation
may be non-efficient. Secondly, the energy gap law may play a
role in the weak emission of Pt-NDI, because a small energy gap
(long emission wavelength) usually leads to low emission efficiency,
due to the significant non-radiative decay of the triplet excited
state.10 Considering that the NDI-C CH is directly attached to
the PtII ion, which is efficient for p-conjugation, thus we propose
Previously we prepared a PtII bisacetylide complex containing
the naphthalimide (NI) acetylide, which shows absorption at a
much shorter wavelength of 429 nm (blue-shifted by 154 nm
compared to Pt-NDI).11 Recently a PtII complex was reported with
the NDI attached to the phen ligand of a NŸN PtIICl2 complex (via
amide C–N bond). But that complex shows absorption at 380 nm.14
Thus it is clear that attachment of NDI to PtII via non-p-conjugated
linker won’t induce a red-shifted absorption. A further example
is that NDI linked to PtII via the imide N-position shows UV-vis
absorption at 430 nm.2a Thus we propose that direct metallation of
the organic chromophore (the p-conjugated framework as the C
donor of the C–Pt bond) is necessary for accessing the red-shifted
absorption.
3
that the 1IL→ IL ISC is efficient,12 which was proved by our
later TTA upconversion studies. The photophysical properties
of the Pt-NDI, Pt-Ph and NDI-C CH ligand are summarized
(Table 1).
The population of the 3IL excited state of Pt-NDI upon
photoexcitation was proved by the nanosecond time-resolved
transient difference absorption spectra (Fig. 3). Upon pulsed
laser excitation at 532 nm, a significant bleaching at 580 nm was
9086 | Dalton Trans., 2011, 40, 9085–9089
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The Royal Society of Chemistry 2011
©

Table 1 Photophysical parameters of the PtII complexes and the ligand.
In toluene. 20 ◦
C
a
labs
eb
lem
U
t
Pt-NDI
Pt–Ph
583
424
3.13
0.58
0.96
603/784
567
561
0.2% (784 nm)c
42.4%c
22.3 mse
1.3 mse
NDI-C C–H 531
7.3%d
11.3 nsf
a 1.0 ¥ 10-5 M. b Molar extinction coefficient at the absorption maxima.
e: 104 M-1 cm-1. c [Ru(dmb)3][PF6]2 as standard (U = 0.074 in CH3CN)
d With quinoline sulfate as standard (U = 0.547 in 0.05 M sulfuric acid).
e Triplet state lifetimes, measured by transient absorption. 5.0 ¥ 10-5 M.
f Fluorescence lifetime.
Fig. 4 Isosurfaces of spin density of Pt-NDI and the model complex
Pt-Ph at the optimized T1 state (isovalue: 0.0004). Calculation was
performed at B3LYP/6-31G/LANL2DZ level with Gaussian 09W. Please
note that the alkyl chains of Pt-NDI were simplified as methyl groups to
reduce the computation time.
and the dbbpy ligand, which is in agreement with the previous
assignment of the T1 state of Pt-Ph as 3MLCT/3LLCT (ligand to
ligand charge transfer).10b
As a further proof of the population of triplet excited state of
Pt-NDI upon excitation, and as an application of the visible-light
3
harvesting and long-lived IL excited state, Pt-NDI was used as
triplet sensitizer for TTA upconversion (Fig. 5).6,16 Upconversion is
important for photovoltaics, photocatalysis, non-linear photonics
and molecular probes, etc.16a Among the other methods, such as
upconversions based on rare earth nanoparticles,16e TTA upcon-
version is interesting because of its low excitation power (lower
than the solar light power density), tunable excitation/emission
wavelength, high upconversion quantum yield. One of the key
steps in TTA upconversion is the triplet–triplet-energy-transfer
(TTET) from the sensitizer to the triplet acceptor (see ESI†
for Jablonski diagram). Currently the triplet sensitizers for TTA
upconversion are limited to the RuII polyimine complexes or
the PtII/PdII porphyrin complexes.6,16 However, it is very difficult
for these sensitizers to be chemically modified to optimize the
photophysical properties for upconversion purposes, such as the
excitation/emission wavelength, the energy levels and the lifetimes
Fig. 3 Nanosecond time-resolved transient absorption difference spectra
of Pt-DNI. (a) Transient absorption with different delay times. (b) Decay
trace at 580 nm. 20 ◦C. In deaerated toluene after pulsed excitation at
532 nm.
found, which clearly indicates the depletion of the ground state
of the NDI-C C- ligand in Pt-NDI. Transient absorptions in
400 nm–500 nm and beyond 600 nm were observed. These features
are different from that of Pt-Ph, which is with charge transfer state
(from PtII to the dbbpy, MLCT).15 Thus the transient absorption
spectra of Pt-NDI demonstrated that the triplet excited state
is localized on NDI ligand (3IL). The excited state lifetime of
Pt-NDI ascertained by transient absorption kinetics at 580 nm
is 22.3 ms, much longer than the typical NŸN PtII bisacetylide
complexes (usually less than 5.0 ms).10b A perylenediimide (PDI)
containing PtII bisacetylide complex was reported with a slightly
blue-shifted absorption at 573 nm, but high e value (68 000 M-1
cm-1),12 however, the lifetime of the PDI-localized 3IL excited state
3
is shorter (0.246 ms).12 With NDI, we prolonged the IL triplet
excited state lifetime by up to 90-fold.
In order to prove the emission of Pt-NDI is due to the NDI
3
acetylide localized IL state from a theoretical perspective, we
carried out DFT calculations. First we studied the electronic
structure of the T1 state of NDI ligand, and found a S0–T1 energy
gap of 744 nm (see ESI†), which is in line with the experimentally
observed phosphorescence of Pt-NDI at 784 nm. Furthermore, the
isosurface of the spin density of the complex clearly demonstrated
that the frontier orbitals of T1 state are localized on NDI subunit
(which is coplanar with the diimine PtII coordination plan, another
NDI ligand is tilted by 24◦ to the NŸN PtII coordination plan,
see ESI for details†), the dbbpy moiety makes no contribution to
the overall spin density (Fig. 4). This conclusion is corroborated
with the steady state and the nanosecond time-resolved transient
Fig. 5 Upconversions with Pt-NDI as triplet sensitizer and perylene
as triplet acceptor. (a) Emission of the upconverted emission. Results
of Pt-Ph were included for comparison. Inset: phosphorescence of the
sensitizers without perylene. Excited by 532 nm laser (5 mW). The asterisks
indicated the scattered laser. The y scale of the two figures are the same to
compare the emission intensity. (b) The photographs of the emissions
of the sensitizers alone and the upconversion. In deaerated toluene.
c[complexes] = 1.0 ¥ 10-5 M. c [perylene] = 2.0 ¥ 10-4 M. 20 ◦C.
3
absorption spectra of the Pt-NDI. Thus we conclude that IL
excited state is responsible for the RT near-IR emission of Pt-NDI
and the transient absorptions. For the model complex Pt-Ph, the
spin density isosurface is localized on phenylacetylides, PtII atom
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 9085–9089 | 9087
©

of the triplet excited states, etc. Thus, new triplet sensitizers with
readily tunable photophysical properties are highly desired.
Perylene was used as triplet acceptor due to its appropriate T1
state energy level (1.53 eV, 810 nm), which must be appropriately
lower than the T1 state energy level of the triplet sensitizer Pt-NDI
(approximated as 1.58 eV, 784 nm).
Weak emission was observed for Pt-NDI upon laser excitation
at 532 nm. In the presence of perylene, however, strong blue
emission at 440 nm–600 nm was observed, which is due to the
upconverted emission of perylene (Fig. 5a). Irradiation of perylene
alone at 532 nm does not produce the fluorescence emission, thus
Acknowledgements
We thank the NSFC (20972024 and 21073028), Fundamental
Research Funds for the Central Universities (DUT10ZD212
and DUT11LK19), the Royal Society (UK) and NSFC (China)
(China-UK Cost-Share Program, 21011130154), Ministry of Ed-
ucation (SRFDP-200801410004 and NCET-08-0077) for financial
support.
Notes and references
˚
verified the upconversion. The upconversion quantum yield (UUC
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for Jablonski diagram) requires quenching of the triplet excited
state of the sensitizer (Pt-NDI) to produce the triplet excited state
of the acceptor (perylene), and the intensity ratio of quenched
phosphorescence and upconversion fluorescence should be at least
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at the excited state (3IL) that otherwise non-emissive sensitized
the TTA upconversion. Thus our results propose that weakly
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a long-lived IL triplet excited state and RT near-IR emission.
It is also the first time that phosphorescence of NDI has been
observed. The complex was used as a triplet sensitizer for triplet–
triplet-annihilation based upconversion and the upconversion
quantum yield (UUC) is 9.5%. We propose that weakly phos-
phorescent, or non-phosphorescent complexes can be used as
triplet sensitizers. Our results pave the way for preparation and
application of visible-light harvesting PtII complexes with long-
3
lived IL excited state for photovoltaics, photocatalysis, or more
generally, any other appropriate photophysical processes that re-
quire triplet excited state (not necessarily emissive) as the initiating
species.
9088 | Dalton Trans., 2011, 40, 9085–9089
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©

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