Long-lived triplet excited state in a platinum(ii) perylene monoimide complex

Article abstract of DOI:10.1039/C8DT02496K

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Full text of DOI:10.1039/C8DT02496K

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Long-lived triplet excited state in a platinum(II)
perylene monoimide complex†
Cite this: Dalton Trans., 2018, 47,
15071
James E. Yarnell,
Arnab Chakraborty, Mykhaylo Myahkostupov,
Katherine M. Wright and Felix N. Castellano
*
We report the synthesis and solution based photophysical properties of a new Pt(II)-terpyridine complex
coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural
arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introdu-
cing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived
PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet
state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the
observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards
triplet excited state character. Additionally, the coincidence of the excited state absorbance difference
spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evi-
dence for this triplet state assignment, which was further supported by TD-DFT calculations.
Received 18th June 2018,
Accepted 29th September 2018
DOI: 10.1039/c8dt02496k
rsc.li/dalton
Pt(II) terpyridyl complexes emissive at room temperature in
most instances.33,34
Introduction
The search for new molecular square planar Pt(II) based
In general, the lowest energy absorption band in Pt(II) ter-
chromophores continues as their unique photophysical pro- pyridine complexes bearing an acetylide ligand can be attribu-
perties render them promising for incorporation into numer- ted to dπ(Pt) → π*(trpy) metal-to-ligand charge transfer (MLCT)
ous applications. These include singlet oxygen photosensitiza- mixed with π(C-CR) → π*(trpy) ligand-to-ligand charge transfer
tion,1,2 photocatalytic hydrogen production,3–9 electrolumines- (LLCT) transitions.35–37 These types of molecules are also
cence,10 optical power limiting,4,11–14 extrinsic luminescence known to induce strong spin–orbit coupling facilitating rapid
probes,4,15–17 among others.18–25 Since the discovery of the intersystem crossing (kisc > 1011 −1) to the triplet charge trans-
s
first platinum diimine diacetylide complex Pt(phen) fer excited state.38,39 In many instances, the π → π* transitions
(CuCPh)2, by Che and co-workers in 1994, the number of originating from the terpyridine and acetylide ligands are very
complexes using a polyimine ligand with arylacetylide deriva- high in energy so the molecular photophysical properties are
tives has markedly expanded.26,27 The incorporation of a rigid completely dictated by the triplet MLCT/LLCT excited states,
terpyridine (trpy) ligand significantly decreases the rate of resulting in charge transfer based photoluminescence. By
non-radiative decay as a result of the D4h to D2d distortion in introducing larger π-conjugated systems through the acetylene
the excited state when compared to the bipyridine linkage, such as pyrene, the energy of the ligand-centered (LC)
analogues.28–32 Unfortunately, this effect is nullified since the 3(π → π*) excited-state is lowered to the point where they are
square planar geometry around the Pt(II) atom necessarily dis- either isoenergetic with or lower than the triplet charge trans-
torts when coordinated to the terpyridine ligand, resulting in fer state. In these instances, the Pt(II) charge transfer states
the metal-centered (MC) excited states being lower in energy can internally sensitize the low energy 3LC excited state
which creates an additional nonradiative pathway for excited localized on the acetylide moiety. Throughout the literature,
state deactivation.28–32 To limit the accessibility of MC excited numerous examples of this type of excited state behaviour have
states in these chromophores, a strong field arylacetylide been reported utilizing stilbene,40 naphthalene,41 anthra-
subunit as the ancillary ligand is rather effective, rendering cene,34 pyrene,42 perylene,43,44 perylenediimide,32,43,45 among
others.46–48
The addition of highly absorbing organic chromophores to
the acetylide moiety have the added benefit of greatly increas-
ing the visible absorption cross-sections of the metal complex.
Chromophores including perylene have been recognized for
Department of Chemistry, North Carolina State University, Raleigh, North Carolina,
27695-8204, USA. E-mail: fncastel@ncsu.edu
†Electronic supplementary information (ESI) available: Additional static spectra,
time-resolved spectra, synthetic details, and structural characterization data. See
their use as photostable dyes and pigments since their initial
DOI: 10.1039/c8dt02496k
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discovery in the early 1900s.49 Since that time, their uses have Although phosphorescence from this triplet state was not
expanded to many other photofunctional molecular species observed, highly efficient quenching of this excited state by
including optical switches,50–52 organic light emitting oxygen and observation of singlet oxygen emission near
diodes,53–55 organic field effect transistors,52,56,57 laser ∼1270 nm pointed to its triplet character. The coincidence of
dyes,58–60 photovoltaic cells,56,61–63 etc. due to their desirable the excited state absorbance difference spectra from the sensi-
visible light-harvesting capabilities,54,64–70 variation of color tized PMI ligand (1) using a triplet donor and the Pt-PMI
through facile synthetic methods,71–73 and high fluorescence complex (2) provided strong evidence for this triplet state
quantum yields approaching 100%.74,75 The covalent attach- assignment, which was further supported by TD-DFT
ment of perylene derivatives to a square planar Pt(II) frame- calculations.
work results in much higher light absorption compared to
typical Pt(II) charge transfer chromophores, although the
nature of the attachment and the involvement of charge trans-
fer excited states significantly impact intersystem crossing
rates. There are a few examples of perylene derivatives being
Experimental
Reagents and chemicals
σ-bonded directly to the Pt(II) metal center and, in most cases, All synthetic manipulations were performed under inert nitro-
the intersystem crossing rates in these molecules are unusually gen atmosphere using standard Schlenk techniques. All
slow, resulting in a significant amount of unquenched fluo- reagents were purchased from Sigma-Aldrich or Alfa Aesar and
rescence from the perylene chromophore with Φf values up to were used as received. Spectroscopic samples were prepared
78%.76,77 The sluggish intersystem crossing rates are also using spectrophotometric grade solvents and were deaerated
observed when other organic chromophores such as anthra- using the freeze–pump–thaw technique.
cene, are cyclometalated to platinum(II) metal centers.78 In
N-(2,5-Di-tert-butylphenyl)-perylene-3,4-dicarboxylic imide,
other cases, where the perylene derivative is attached to the PMI. Perylene monoimide (PMI) was synthesized using modi-
Pt(II) through an acetylide connector, the intersystem crossing fied literature procedure (Scheme 1).80 The product was iso-
rates become much faster, with little to no residual fluo- lated in 16% yield. 1H NMR (400 MHz, CDCl3) δ 8.63 (d, J =
rescence
from
the
perylene
chromophore
being 8.0 Hz, 2H), 8.42 (t, J = 7.3 Hz, 4H), 7.90 (d, J = 8.1 Hz, 2H),
observed.32,43–45 The position of the heavy atom attachment to 7.67–7.57 (m, 3H), 7.46 (dd, J = 8.6, 2.3 Hz, 1H), 7.04 (d, J =
the perylene chromophore can also affect intersystem crossing 2.3 Hz, 1H), 1.34 (s, 3H), 1.30 (s, 3H).
rates, as documented by Dreeskamp and coworkers.79
9-Bromo-N-(2,5-di-tert-butylphenyl)-perylene-3,4-dicarboxylic
In this manuscript, we report the synthesis and photo- imide, PMI-Br. PMI-Br was synthesized using modified litera-
physical properties of a new Pt(II)-terpyridine complex attached ture procedure (Scheme 1).81 N-Bromosuccinimide (534 mg,
to
a PMI chromophore through an acetylene linkage, 3.0 mmol) was added to a stirring solution of PMI (1.36 g,
(9-ethynyl-N-(2,5-di-tert-butylphenyl)-perylene-3,4-dicarboxylic 2.67 mmol) in anhydrous DMF (90 mL) at room temperature
imide)(4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine)platinum(II) under nitrogen. The reaction mixture was heated to 40 °C for
hexafluorophosphate (2) (Fig. 1). The photophysical behavior 48 hours. After cooling to room temperature, the reaction
of 2 was fully characterized using a combination of steady- mixture was poured into water (500 mL). The precipitated
state and time-resolved spectroscopy, and compared against a product was collected by filtration, washed with water (2 ×
free ligand, PMI-CCH (1) (Fig. 1). The strong fluorescence from 50 mL) and dried under vacuum to afford 1.04 g (66%) of
the PMI chromophore was quantitatively quenched by PMI-Br as yellow solid. 1H NMR (400 MHz, CDCl3) δ 8.64 (t, J =
introducing metal character into its lowest energy singlet state, 7.1 Hz, 2H), 8.46 (dd, J = 12.9, 7.8 Hz, 2H), 8.39 (d, J = 8.1 Hz,
resulting in a long-lived LC (PMI-localized) triplet state. 1H), 8.30 (d, J = 8.5 Hz, 1H), 8.22 (d, J = 8.2 Hz, 1H), 7.90 (d, J =
8.0 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 7.59 (d, J = 8.6 Hz, 1H),
7.46 (d, J = 8.6 Hz, 1H), 7.03 (d, J = 2.4 Hz, 1H), 1.33 (s, 9H),
1.30 (s, 9H).
9-(Trimethylsilylethynyl)-N-(2,5-di-tert-butylphenyl)-perylene-
3,4-dicarboxylic imide, PMI-CCTMS. PMI-CCTMS was syn-
thesized using commonly employed Sonogashira cross-coup-
ling conditions (Scheme 1).82–84 The product was isolated in
90% yield. 1H NMR (400 MHz, CDCl3) δ 8.65 (dd, J = 8.1,
3.7 Hz, 2H), 8.52–8.39 (m, 4H), 8.36 (d, J = 8.0 Hz, 1H), 7.81 (d,
J = 7.9 Hz, 1H), 7.72 (t, J = 7.9 Hz, 1H), 7.59 (d, J = 8.6 Hz, 1H),
7.49–7.43 (m, 1H), 7.03 (d, J = 2.4 Hz, 1H), 1.33 (s, 9H), 1.30 (s,
9H), 0.38 (s, 9H).
9-Ethynyl-N-(2,5-di-tert-butylphenyl)-perylene-3,4-dicarboxylic
imide, PMI-CCH (1). Following a typical deprotection pro-
cedure, PMI-CCTMS (0.198 g, 0.327 mmol) was added under
ambient conditions to a vigorously stirred solution of K2CO3
Fig. 1 Structures of target chromophores used in this study: PMI-CCH
(1) and Pt-PMI (2).
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Scheme 1 Synthetic route used to prepare the PMI-CCH (1) ligand. Reagents and conditions: (i) Perylene-3,4,9,10-tetracarboxylicanhydride, 2,5-di-
tert-butyl-aniline, zinc acetate, imidazole, and water were heated to 190 °C for 22 hours. (ii) N-Bromosuccinimide was added PMI in anhydrous DMF
and heated to 40 °C for 48 hours. (iii) PMI-Br, Pd(PPh3)4, and CuI were added to a 1 : 1 mixture of anhydrous dichloromethane/diisopropylamine
along with trimethylsilylacetylene and heated to 60 °C for 36 h under nitrogen. (iv) PMI-CCTMS was added to K2CO3 in a mixture of water, methanol,
and CHCl3 and stirred at RT overnight.
(0.456 g, 3.3 mmol) in a mixture of water (4 mL), methanol cedure (Scheme 2).85 The product was isolated in 80% yield
(20 mL) and chloroform (40 mL).83,84 The reaction mixture was and used in the next step without further purification.
stirred overnight at room temperature. The reaction mixture
Chloro(4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine) platinum(II),
was evaporated to dryness, the obtained residue was washed [Pt(tbut3-trpy)Cl](PF6). [Pt(tbut3-trpy)Cl](PF6) was synthesized
with water (2 × 25 ml) and dried under vacuum. There was using a typical literature procedures (Scheme 2). The product
observed ∼8 nm blue shift in the main absorption band con- was isolated in 93% yield and used without further purifi-
firming that the deprotection was complete (Fig. S1†). The cation. 1H NMR (CD3CN): 8.82 (d, 2H), 8.30 (s, 4H), 7.79
product was isolated as deep red solid in 73% yield (0.127 mg). (d, 2H), 1.54 (s, 9H), 1.46 (s, 18H).86
1H NMR (CDCl3): 8.66 (d, 2H), 8.46 (m, 4H), 8.39 (d, 1H), 7.86
(9-Ethynyl-N-(2,5-di-tert-butylphenyl)-perylene-3,4-dicarboxylic
(d, 1H), 7.73 (t, 1H), 7.58 (d, 1H), 7.44 (d, 1H), 7.03 (s, 1H), 3.68 imide)(4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine)platinum(II) hexa-
(s, 1H), 1.33 (s, 9H), 1.26 (s, 9H). 13C NMR (CD2Cl2) δ 165.33, fluorophosphate, [Pt(tbut3-trpy)PMI](PF6) (2). [Pt(tbut3-trpy)Cl]
150.85, 144.79, 137.42, 136.96, 134.81, 134.20, 132.39, 132.14, (PF6) (200 mg, 0.26 mmol), PMI-CCH (155 mg, 0.29 mmol),
132.02, 131.46, 130.56, 130.28, 129.78, 129.36, 129.30, 128.57, CuI (25 mg, 0.13 mmol), and diisopropylamine (15 mL) were
128.23, 127.99, 127.13, 126.51, 124.68, 123.31, 122.84, 122.26, added to anhydrous dichloromethane (15 mL) under nitrogen.
122.02, 121.45, 121.16, 85.53, 81.69, 36.01, 34.75, 32.02, 31.56. The reaction mixture was stirred at room temperature for
ESI-HRMS. Found: m/z 534.2435 (MH+). Calcd for C38H32NO2: 24 hours (Scheme 2). Volatiles were removed under vacuum
m/z 534.2433. Anal. Calcd (found) for C38H31NO2·0.8CHCl3: C, and the product was purified by alumina column chromato-
74.07 (74.24); H, 5.09 (5.19); N, 2.23 (2.24). ATR-FTIR: 412, 479, graphy eluting with acetonitrile–toluene (1 : 1) to afford a
1
532, 612, 643, 700, 734, 753, 806, 1179, 1246, 1356, 1575, 1589, purple solid in 42% yield (140 mg). H NMR (CD2Cl2): 9.29 (d,
1654, 1699, 2864, 2952, 3304 cm−1
.
2H), 8.67 (d, 1H), 8.43 (d, 2H), 8.27 (d, 1H), 8.16 (s, 3H), 8.09
Bis(dimethyl-sulfoxide)platinum(II) dichloride, Pt(DMSO)2Cl2. (m, 4H), 7.76 (d, 1H), 7.67 (m, 2H), 7.53 (dd,2H), 7.45 (dd, 1H),
Pt(DMSO)2Cl2 was synthesized using a common literature pro- 7.28 (s, 1H), 1.61(s, 9H), 1.45(s, 18H), 1.31(s, 9H), 1.27(s, 9H).
Scheme 2 Synthetic route used to prepare the Pt-PMI complex (2). Reagents and conditions: (i) Potassium tetrachloroplatinate(II) was dissolved in a
mixture of DMSO and deionized water, and the reaction mixture was stirred for 3 hours at room temperature. (ii) Pt(DMSO)2Cl2 was added to a solu-
tion of 4,4’,4’’-tri-tert-butyl-2,2’:6’,2’’-terpyridine in a mixture of dichloromethane/methanol (1 : 2), and the reaction mixture was refluxed for
24 hours. (iii) [Pt(tbut3-trpy)Cl](PF6), along with PMI-CCH, CuI, and diisopropylamine was added to anhydrous dichloromethane under N2 and stirred
at room temperature for 24 hours.
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13C NMR (CD2Cl2) δ 168.46, 167.60, 165.42, 165.40, 158.71, Nanosecond transient absorption spectroscopy
155.75, 154.74, 150.74, 144.86, 137.24, 137.10, 135.05, 134.58,
Nanosecond transient absorption measurements were carried
131.54, 131.52, 131.00, 130.97, 130.29, 130.03, 129.49, 129.36,
128.65, 128.39, 127.60, 127.38, 127.05, 126.75, 126.50, 124.46,
123.99, 123.74, 122.63, 121.14, 121.08, 120.86, 120.66, 109.75,
102.68, 37.72, 36.75, 36.00, 34.80, 32.05, 31.68, 30.70, 30.47.
out on a LP920 laser flash photolysis system (Edinburgh
Instruments) described previously.90 Samples were prepared to
have optical densities between 0.3 and 0.5 at the excitation
wavelength (530 nm, 3.0 mJ per pulse) and deaerated using the
freeze–pump–thaw technique in 1 cm path length quartz
optical cells. All flash photolysis experiments were performed
at room temperature. The reported difference spectra and
kinetic data are the average of 50 laser shots. The ground state
absorption spectra were taken before and after each experi-
ment to ensure there was no sample degradation. The transi-
ent kinetic data were evaluated using the fitting routines avail-
able in OriginPro 2016.
ESI-HRMS. Found: m/z 1127.4709 (M
−
PF6)+. Calcd for
C65H65N4O2194Pt: m/z 1127.4734. Anal. Calcd (found) for
C65H65F6N4O2PPt·1.75CH2Cl2·CH3CN: C, 56.40 (56.65); H, 4.92
(5.20); N, 4.78 (4.52). ATR-FTIR: 557, 608, 732, 755, 832, 1244,
1287, 1352, 1562, 1589, 1613, 1654, 1695, 2103, 2866, 2958 cm−1
.
General techniques
1H and 13C NMR spectra were recorded on a Varian Inova 400
NMR operating at working frequencies of 400 and 100 MHz for
1H and 13C, respectively. Optical steady-state absorption Time-correlated single photon counting (TCSPC)
spectra were measured on Cary 60 UV-vis and Shimadzu
Time-resolved fluorescence experiments were conducted using
UV-3600 spectrophotometers. Steady-state photoluminescence
a time-correlated single photon counting (TCSPC) spectro-
spectra were recorded on Edinburgh FS 920 fluorescence
meter (Edinburgh Instruments, LifeSpec II). Fluorescence
spectrometer. Quantum yield measurements were performed
signals were measured using a microchannel plate photomulti-
on deaerated samples using rhodamine 6G in ethanol as a
plier tube (Hamamatsu R3809U-50) in
a Peltier-cooled
standard (λem 510 nm, Φ = 0.94).87 Singlet oxygen quantum
yield measurements were conducted in toluene and the photo-
luminescence (1200–1350 nm) was collected using the
Edinburgh FS 920 fluorescence spectrometer equipped with
the Hamamatsu NIR detector. Singlet oxygen quantum yield
values were reported relative to ZnTPP (ΦΔ = 0.93 in air).88
High-resolution electrospray mass spectrometry was carried
housing. A Ti:Sapphire laser (Chameleon Ultra II, Coherent)
was utilized as the excitation light source. For fluorescence
intensity decay measurements, the Chameleon laser was tuned
to 1080 nm, pulse picked to a 4 MHz repetition rate (Coherent
9200 Pulse Picker), and frequency doubled (APE-GmbH SHG
Unit) to afford sample excitation, λex = 540 nm.
out by the Michigan State University Mass Spectrometry Core, Density functional theory calculations
East Lansing, MI. Elemental analysis data was measured by
Atlantic Microlab, Inc., Norcross, GA.
Calculations on 1 and 2 were performed using the Gaussian 09
software package (revision D.01)91 and the computation
resources of the North Carolina State University High
Performance Computing Center. Geometry optimizations were
performed on the ground state and the lowest energy triplet
state using the B3LYP,92,93 CAM-B3LYP,94 M06,95 and PBE0
functionals,96 with the SSD ECP for the Pt core electrons,97 the
Def2-SVP basis set on all atoms.98 The polarizable continuum
model (PCM) was used to simulate the effects of the dichloro-
methane solvent environment for all calculations.99 The GD3
dispersion correction was applied to all optimized struc-
tures.100 Frequency calculations were performed on all ground
state and triplet state structures and no imaginary frequencies
were obtained for any of the optimized geometries. Time
dependent DFT calculations were performed on the optimized
ground-state geometries using the CAM-B3LYP, B3LYP, M06,
and PBE0 functionals mentioned previously. The energy and
oscillator strength were computed for each of the 50 lowest
singlet excitations and 10 lowest triplet excitations.
Ultrafast transient absorption spectroscopy
Time-resolved transient absorption measurements were per-
formed at the NCSU Imaging and Kinetic Spectroscopy
(IMAKS) Laboratory in the Department of Chemistry described
previously.89 Briefly, the 800 nm laser pulses were produced at
a 1 kHz repetition rate by a mode-locked Ti:sapphire laser
(Coherent Libra). The pulse width was determined to be
(fwhm) 105 fs using an autocorrelator. The output from the
Libra was split into pump and probe beams. The pump beam
was directed into a parametric amplifier (Coherent OPerA
Solo) to generate the 525/550 nm excitation. The probe beam
was delayed in a 6 ns optical delay stage and then focused into
a sapphire crystal for white light continuum generation
between 425 and 800 nm. The pump beam was focused into
an 800 μm spot on the sample and overlapped with the probe
beam (∼200 μm). The relative polarizations of the pump and
probe beams were set at the magic angle of 54.7°. Samples
under investigation were contained in 2 mm path length
quartz cuvettes, and each solution was stirred continuously
throughout the course of the experiment. The ground state
absorption spectra were taken before and after each experi-
Results and discussion
Electronic structure calculations
ment to ensure there was no sample degradation. The transi- Density functional theory (DFT) and TD-DFT calculations were
ent kinetic data was evaluated using the fitting routines avail- performed on 1 and 2 to provide insight into the nature of the
able in OriginPro 2016.
photoexcited states present in these molecules. The ground
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state structures were optimized using the M06, PBE0, and
B3LYP hybrid functionals, as well as the range corrected
CAM-B3LYP functional, using Grimme’s D3 dispersion correc-
tion. The optimized ground state structures of the PMI model
were very similar across all four functionals, showing consist-
ent results. The optimized ground state structure of 2 was also
shown to vary minimally with selected functionals, although
some small changes in the dihedral angle between the Pt-trpy
and PMI were observed. The plane consisting of the Pt-trpy
was found to vary between 35.7–40.9° with respect to the PMI
chromophore, depending on functional used for the DFT cal-
culation (ESI, Fig. S18†). Because of the acetylene spacer
between the two moieties, the rotational energies are likely
very small, therefore at room temperature in solvent, the di-
hedral angle likely varies a great deal, resulting in a range on
conjugation across the two parts of the molecule. The bonding
environment around the platinum was also compared across
functionals (ESI, Fig. S18†). The distorted square planar
environment around the Pt(II) in 2 was shown to be consistent
regardless of hybrid functional used. The optimized triplet
excited-state structure (and environment around the Pt(II)
centre) showed little change as well, signifying that little metal
charge transfer was involved in the optimized triplet state.
The molecule orbital diagrams for the frontier orbitals of 1
and 2 are presented in Fig. 2. In both 1 and 2, the HOMO−1
resides mostly on the phenyl ring attached to the imide and the
HOMO resides mainly of the PMI chromophore. In 2, there is a
small portion of platinum contribution to the HOMO. In 1, the
LUMO, LUMO+1, and LUMO+2 all reside on the PMI as π* con-
tributions. Complex 2 displays similar PMI π* character in the
LUMO and LUMO+1 but these molecular orbitals also have sig-
nificant contribution from the terpyridine aromatic system,
with the LUMO+2 consisting of solely terpyridine π* character.
Fig. 2 Frontier orbital diagram constructed for 1 and 2. Calculations
performed at M06-D3/Def2-SVP/SDD level of theory.
Absorption and photoluminescence spectroscopy
The electronic spectra of 1 and 2 overlaid with the calculated
vertical transitions are presented in Fig. 3. The low energy tran-
sitions of 1 observed in 450–550 nm range can be attributed to
the π–π* transitions of the PMI chromophore as demonstrated
by the natural transition orbital analysis located in the ESI.†
By introducing the Pt-trpy moiety to the PMI-acetylide as in 2,
the intense π–π* transition localized on the PMI red-shifts to
the 475–575 nm range, likely as a result of charge transfer
character being mixed with the π–π* transition localized on
the PMI, in addition to strong σ donation accompanied by
strong π back bonding from the Pt(II) metal center.32,39,43,45,101
This low energy band also experiences an increase in absorp-
tivity when compared to the model. Similar to the previously
reported Pt(II)-acetylide complexes, the increase in the inten-
sity of this transition can be explained by the presence of
additional charge transfer transitions which are hidden under
the intense π–π* transitions localized on the PMI, forming low
energy inter-ligand charge transfer (ILCT) transitions.32,44
Since there was no significant solvatochromic effect observed
in the absorption spectra of 2 (Fig. S10†), it is likely that the
Fig. 3 Electronic spectra of 1 (A) and 2 (B) in dichloromethane at room
temperature. Vertical transitions (blue lines) are calculated at the TD-DFT//
oscillator strength is due to the PMI ligand-localized tran- M06-D3/Def2-SVP/SDD level, based on optimized ground state geometries.
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sitions. The higher energy transitions observed around 325 nm
are attributed to the π–π* transitions localized on the terpyri-
dine ligand as previously reported.33,46,102
The TD-DFT (M06/Def2-SVP/SDD) calculations help support
some of the assignments given to the electronic transitions
observed in 1 and 2 with select information on the low-lying
singlet excited states presented in Table 1. In both molecules,
there is a good agreement with the position and intensity of
the low-lying S0 → Sn transitions observed experimentally and
those calculated by TD-DFT. In 1, the S0 → S1 transition has a
large oscillator strength ( f = 0.91) and can be approximated as
a one-electron HOMO → LUMO transition. The S0 → S2 and
S0 → S3 transitions are both significantly higher in energy
(∼1 eV) and have weak oscillator strength, thus not contribut-
ing much to the overall electronic spectra. In 2, the S0 → S1
and S0 → S2 transitions are very similar in how the excited
state density is localized since these transitions can be
approximated as one-electron HOMO → LUMO and HOMO →
LUMO+1, respectively, and LUMO and LUMO+1 are similar in
appearance (Fig. 2). The oscillator strength of these two tran-
sitions taken together ( f = 1.31) results in nearly a 50%
increase of absorptivity compared to 1, thus correlating well
with the experimental data. The S0 → S3 transition can be best
approximated with HOMO → LUMO+2, but it has little oscil-
lator strength and therefore does not contribute significantly
to the overall electronic spectra. By analysing the transition
density matrix, the natural transition orbitals (NTOs) can be
created to visualize the nature of the transitions (Fig. 4).103
The low-lying transitions of 2 confirm the ILCT transition
assignments shown above, with S0 → S1 and S0 → S2 consisting
of π–π* transition localized on the PMI with substantial charge
transfer to the terpyridine. The S0 → S3 transition consists
only of PMI to trpy ILCT character.
Fig. 4 Natural transition orbitals (NTOs) for select S0 → Sn excitations
of 2 determined at the TD-DFT//M06-D3/Def2-SVP/SDD level of theory.
λ is the fraction of the hole-particle contribution to the excitation.
Fig. 5 Photoluminescence spectra of optically matched solutions of 1
and 2 at room temperature in dichloromethane (OD = 0.1 at 500 nm).
Low temperature.
The photoluminescence spectra of 1 and 2 are presented in
Fig. 5 with additional spectroscopic information provided in
Table 1 Selected electronic excitation energies (eV), corresponding
oscillator strengths ( f ), and main configurations of the low-lying excited
states of chromophores in this study
Table 2. The free ligand, 1, had very intense photo-
luminescence with an emission maximum at 551 nm.
The stokes’ shift, in addition to the emission quantum yield
(ΦPL = 0.85) and the emission decay (τ = 4.67 ns) are consistent
with the emission originating from the S1 excited state. When
the PMI was attached to the Pt(II)-terpyridine, the photo-
luminescence energy red-shifts slightly, similar to what is
observed with the electronic spectra. The shoulder that
appears at 550 nm was due to a minute amount of highly emis-
sive, but difficult to remove, impurity that was still present in
the sample even after several additional re-purification steps
using column chromatography. In addition to the shift in
energy, the emission decay time constant was reduced to
<100 ps, implying that the S1 excited state population was
being quenched by an additional decay pathway not observed
in 1. Furthermore, the intensity of the prompt fluorescence
was nearly quantitatively quenched (ΦPL < 0.01). With the
covalent attachment of the Pt(II) moiety to the PMI and
Electronic
Molecule transitionsa (eV)
Energyb
fc
Composition CId
1
S0 → S1
S0 → S2
S0 → S3
2.381
3.264
3.514
0.9092 H → L
0.0000 H−1 → L
0.0041 H−6 → L
H−4 → L
0.7058
0.7012
0.1328
0.5417
0.3303
0.1121
0.2249
0.6904
0.1014
0.6947
0.1052
0.6969
H−3 → L
H−2 → L
H → L+3
0.9450 H → L
H → L+1
0.3641 H → L+1
0.0048 H−2 → L+2
H → L+2
2
S0 → S1
2.156
S0 → S2
S0 → S3
2.369
2.602
a Calculated at the TD-DFT//M06-D3/Def2-SVP/SDD level, based on
optimized ground state geometries. b Only the low-lying excited states
were presented. c Oscillator strength. d CI coefficients are absolute
values.
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Table 2 Photophysical data of the chromophores in this study
λabs max (nm)
Estimated oscillator
strength, f
λem max (nm)
λem max (nm)
Molecule
(ε, M−1 cm−1 a
)
(298 K)a
(77 K)b
ΦΔ (298 K)c
Φem (298 K)a,d
PMI-CCH (1)
Pt-PMI (2)
516 (27 000), 491 (25 600)
553 (43 500), 526 (44 100)
0.38
0.71
551
592
534
575
0.17
0.91
0.85
<0.01
a Room temperature measurements were made using dichloromethane as the solvent. b 77 K measurements were made using 2-methyl THF as
the solvent. c Singlet oxygen quantum yield measurements were performed using aerated toluene using ZnTPP in toluene as a standard
(ΦΔ = 0.93 in air).88 d Quantum yield measurements were performed on deaerated samples using rhodamine 6G in ethanol as a standard
(λem 510 nm, Φ = 0.94).87
additional metal character in the HOMO, the intersystem at 670 nm (Fig. S2†). Over the course of the first 2 ps, both the
crossing pathway is expected to become the predominant ground state bleach and excited state absorbance blue-shift,
decay pathway from the S1 excited state, similar to the other likely as a result of vibrational cooling. Using the first time
examples of Pt(II) acetylides attached to highly emissive constant obtained from the transient decays at 550 nm and
chromophores.43,45 When cooling the molecules to 77 K, the 670 nm, the observed vibrational cooling therefore occurs
fluorescence peaks in both molecules blue shifted slightly between 770 and 980 fs. From this point forward in time,
(∼15 nm) and the vibronic nature of each spectra sharpened. there was no meaningful change in the difference spectra of 1,
Although there was no long-lived phosphorescence observed in aside from the intensity, and a clean repopulation of the
2 at room temperature or 77 K, the observation of singlet ground state was observed. In both transient decay fits, last
O2 (1O2) photoluminescence centred at ∼1270 nm in aerated time constant was fixed to that of the fluorescence intensity
solutions (Fig. 6) indirectly supports the formation of a triplet decay (τ = 4.67 ns). The transient fit at 670 nm revealed a third
excited state. The generation of singlet O2 was found to be time component (τ = 170 ps) that contributes less than 9% to
extremely efficient in aerated toluene (ΦΔ = 0.91) where it was the overall decay. Since there are no significant changes to the
quantified relative to ZnTPP.88 The lack of observed phosphor- observed difference spectra, this time constant likely correlates
escence using perylene-based chromophores with low energy to solvent reorganization and/or small geometry changes in
ligand localized triplet states has been well documented in the molecule.
many relevant studies,43–45 however, recent examples of Ru(II)
As presented in Fig. 7A, the initial excited state difference
and Ir(III) PDI complexes have shown that ligand-localized spectra of 2 at 500 fs was similar to that of 1, where the ground
phosphorescence originating from a perylene chromophore is
possible.104–106
Transient absorption spectroscopy
Ultrafast transient absorption measurements of 1 and 2 were
carried out in dichloromethane solutions using 525 and
550 nm excitation, respectively. The excited state difference
spectrum of 1 at 500 fs has a ground state bleach at 530 nm
with a shoulder at 490 nm and a broad excited state absorption
Fig. 6 Near-infrared emission spectra (assigned to 1O2 emission) of
optically matched (∼0.1 OD at 550 nm) ZnTPP and 2 in aerated toluene
Fig. 7 Excited-state absorption difference spectra of 2 in dichloro-
at room temperature.
methane following 550 nm pulsed excitation (105 fs fwhm).
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state bleach appears at 560 nm with a shoulder at 525 nm and
the excited state absorbance has a maximum at 675 nm but
now this feature extends into the NIR. Over the course of
15 ps, a new excited state feature grows in at 620 nm while the
previously described excited state feature from 650 nm to NIR
decreases in intensity. Tracking the kinetics of the two bands
reveal that these two features are linked as the growth at
615 nm is best fit to a biexponential (τ1 = 200 30 fs; τ2 = 2.4
0.2 ps) while the decay at 700 nm is fit to a similar biexponen-
tial (τ1 = 470 45 fs; τ2 = 2.8 0.2 ps). The ground state bleach
also experiences a change in shape as the shoulder at 525 nm
disappears over the same time period. The kinetic analysis at
this wavelength also reveals a best fit to a biexponential func-
tion (τ1 = 240 30 fs; τ2 = 3.0 0.2 ps). The excited state fea-
tures observed at 15 ps qualitatively match the excited state
difference spectra obtained by Hayes and co-workers when
studying a zinc-porphyrin-PMI dyad, which were assigned to
the 3PMI excited-state.107 After 15 ps, the spectral features
observed (Fig. 7B) remain constant within the measured time
window of 6 ns.
Fig. 8 Excited-state absorption difference spectra of 2 in dichloro-
methane following 530 nm pulsed excitation (7 ns fwhm). The sample
was deaerated using the freeze–pump–thaw method.
Using global fit analysis (ESI, Fig. S5†), the excited state fea-
tures observed in the fastest excited-state difference spectrum below), it is straightforward to assign the excited state as being
(τ = 171 fs) of 2 closely match that of the singlet excited state ligand localized on the PMI moiety.
of 1 with a broad excited state absorbance at 675 nm, which
Nanosecond transient absorption spectroscopy was also
can be assigned as the depletion of the initially populated performed on 1, although in neat solution only the prompt
singlet excited state. Computational data shows significant emission and excited state difference spectra originating from
d-orbital contribution to the low energy singlet transitions so the singlet excited state were observed. To study the triplet
this change in features is likely due to intersystem crossing. state of the PMI ligand, the sensitizer thioxanthone was used
The second excited state difference spectrum (t = 2.3 ps) to sensitize the triplet state on the PMI through diffusional
reveals changes to both the ground state bleach and excited energy transfer. A solution of thioxanthone in deaerated di-
state absorbance from the prompt features, likely due to the chloromethane was excited at 350 nm. The excited state differ-
population of an intermediate triplet charge transfer state. The ence spectra at 1 µs exhibited a strong excited state absorbance
final spectrum obtained from the global fit analysis remains at 640 nm, corresponding to the thioxanthone triplet state
constant through the rest of the experiment and the transient (Fig. S8†). After adding a portion of 1 to this solution, a second
features match to what was previously assigned to the 3PMI measurement at 10 µs with selective excitation of the thiox-
excited-state. All put together, the ultrafast data reveals fast anthone revealed a bleach feature at 525 nm and an excited
intersystem crossing (τ = 150–300 fs) from the initially popu- state absorbance at 570 nm, corresponding to the PMI triplet
lated singlet state to a high energy triplet excited state, Tn, fol- excited state. The features of the triplet PMI difference spectra
lowed by internal conversion to the long-lived, ligand localized nearly quantitatively match those of the ultrafast spectra of 2
triplet state (τ = 2.3–2.7 ps).
A nanosecond transient absorption study of 1 and 2 was lower energies, confirming the triplet assignment discussed
carried out in deaerated dichloromethane solutions (λex
above. Additionally, spin density calculations were performed
at 6 ns, as well as the nanosecond spectra, albeit shifted to
=
530 nm, 3.0 mJ per pulse, 7 ns fwhm). As presented in Fig. 8, on the optimized triplet excited state structure (M06-D3/Def2-
3
the excited state difference spectra of 2 in the nanosecond to SVP/SDD) of both 1 and 2 confirming the LC assignment as
microsecond time domains show a ground state bleach at much of the triplet density was localized on the PMI moiety
550 nm and an excited state absorbance at 605 nm. The fea- (Fig. 9). In the case of 2, the platinum d-orbitals contribute
tures in these spectra qualitatively match the difference 1.39% to the LSOMO and 0.66% to the HSOMO of the opti-
spectra presented in Fig. 6B, signifying that the nature of the mized triplet state.108
excited state observed previously has not changed between the
Fig. 10 depicts the excited state decay processes observed in
picosecond and microsecond time domains. Both transient 2. As illustrated by the DFT calculations, the initially populated
features observed decrease in intensity with single exponential excited state (when excited at 550 nm) is likely ligand localized
time constants (τ at 550 nm = 8.4 µs, τ at 600 nm = 8.51 µs), on the PMI moiety with some amount of ligand-to-ligand
indicating clean repopulation of the ground state (Fig. S7†). charge transfer character from the PMI to the terpyridine. The
With the excited state lifetimes extending into the micro- initial difference spectrum obtained by ultrafast spectroscopy
seconds time regime and the excited state difference spectra changes quickly with growth and decay features corresponding
matching that of the triplet sensitized ligand (discussed to fast intersystem crossing (τ = 150–300 fs) followed by
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absorption spectroscopy. Density functional calculations con-
firmed experimental determinations regarding the triplet
nature of the excited states in 2. The combined finding here
suggests that the triplet excited states in perylenemonoimide
chromophores can be readily accessed using Pt(II)-acetylide
bonding motifs.
Conflicts of interest
There are no conflicts to declare.
Fig. 9 Isosurfaces of spin density of 1 and 2 at the lowest triplet energy
(isovalue = 0.0004). Calculation was performed at M06-D3/Def2-SVP/
SDD level of theory.
Acknowledgements
This work was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Award
Number DE-SC0011979. J. E. Y. was supported by the Air Force
Institute of Technology (AFIT).
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