Viable access to the triplet excited state in peryleneimide based palladium complex §

Article abstract of DOI:10.1007/s12039-018-1537-x

Abstract: Triplet excited state in organic chromophores is ubiquitously significant owing to its utility in light harvesting and photovoltaic device applications. Herein, we report the enhancement in the triplet character of an innately triplet deficient peryleneimide chromophore via incorporation of a heavy atom. Palladium incorporated perylenemonoimide (PMI-Pd) was synthesized via oxidative addition of PMI-Br into Pd(0) under inert experimental conditions. The structural sanctity of the PMI-Pd and the model derivative PMI was characterized via single crystal X-ray diffraction and the close-packing was examined employing Hirshfeld surface analysis. The steady-state spectroscopic measurements of PMI-Pd in chloroform reveal an apparent perturbation in the UV-Vis absorption, fluorescence emission and lifetime characteristics. A much higher perturbation is observed in the ultrafast photoexcited processes of PMI-Pd in chloroform as envisaged via nanosecond transient absorption (nTA) measurements. The nTA measurements of PMI-Pd in chloroform reveal a significant enhancement in the triplet character of PMI-Pd as compared to the model derivative PMI. Spin-orbit coupling (SOC) mediated triplet enhancement in PMI-Pd suggest heavy atom incorporation as a viable route for accessing the triplet excited states in triplet deficient aromatic chromophores. SOC mediated triplet enhancement in innately triplet deficient organic chromophores can revive the utility of these materials for novel photovoltaic and energy storage applications. Graphical abstract: SYNOPSIS Synthesis and characterization of a perylenemonoimide palladium(II) complex (PMI-Pd) and the enhancement in the triplet excited state of PMI-Pd as compared to PMI are explored. The results suggest that the combined spin-orbit coupling mediated by the incorporation of palladium and bromine atoms results in the enhancement of the triplet character in PMI-Pd. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-018-1537-x

J. Chem. Sci. (2018) 130:137
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1537-x
REGULAR ARTICLE
Special Issue on Photochemistry, Photophysics and Photobiology
Viable access to the triplet excited state in peryleneimide based
§
palladium complex
ABBEY M PHILIP, EBIN SEBASTIAN, GOPIKA GOPAN, REMYA RAMAKRISHNAN and
∗
MAHESH HARIHARAN
School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram (IISER-TVM),
Maruthamala P.O., Vithura, Thiruvananthapuram, Kerala 695 551, India
E-mail: mahesh@iisertvm.ac.in
MS received 9 June 2018; revised 30 July 2018; accepted 1 August 2018
Abstract. Triplet excited state in organic chromophores is ubiquitously significant owing to its utility in light
harvesting and photovoltaic device applications. Herein, we report the enhancement in the triplet character of an
innately triplet deficient peryleneimide chromophore via incorporation of a heavy atom. Palladium incorporated
perylenemonoimide (PMI-Pd) was synthesized via oxidative addition of PMI-Br into Pd(0) under inert
experimental conditions. The structural sanctity of the PMI-Pd and the model derivative PMI was characterized
via single crystal X-ray diffraction and the close-packing was examined employing Hirshfeld surface analysis.
The steady-state spectroscopic measurements of PMI-Pd in chloroform reveal an apparent perturbation in the
UV-Vis absorption, fluorescence emission and lifetime characteristics. A much higher perturbation is observed in
the ultrafast photoexcited processes of PMI-Pd in chloroform as envisaged via nanosecond transient absorption
(nTA) measurements. The nTA measurements of PMI-Pd in chloroform reveal a significant enhancement in
the triplet character of PMI-Pd as compared to the model derivative PMI. Spin-orbit coupling (SOC) mediated
triplet enhancement in PMI-Pd suggest heavy atom incorporation as a viable route for accessing the triplet
excited states in triplet deficient aromatic chromophores. SOC mediated triplet enhancement in innately triplet
deficient organic chromophores can revive the utility of these materials for novel photovoltaic and energy storage
applications.
Keywords. Perylenemonoimide; palladium complex; triplet; spin-orbit coupling.
1
. Introduction
systems,6–8 its application in organic bulk
9,10
heterojunction solar cells
and incipient energy
However, PI vitiates its intense
fluorescence character upon self-assembly (viz. solid-
11–13
Peryleneimide (PI) family of chromophores have been
long-known as a lightfast vat dye and have been
explored for its versatile optical properties.1 Owing
to its remarkable fluorescence characteristics and high
photo, thermal and chemical stability, PI’s have gained
immense significance in emerging optical and
electronic device applications.3–5 Additionally, the sig-
nificant electron affinity of PI fortifies its utility as a non-
fullerene acceptor while exploring fundamental aspects
of light-harvesting in artificial donor–acceptor (D–A)
storage application.
,2
14
state) and is inherently triplet deficient. Long-lived
triplet character of a chromophore can be a vital factor
controlling the exciton migration and the efficiencies
of the D–A blends used for organic photovoltaics.1
To date, the access to the triplet excited state in
organic chromophores has been reported via bimolec-
5,16
17
ular triplet sensitization, spin-orbit coupling medi-
18–21
14,22,23
ated heavy-atom effect,
singlet fission
and
*
For correspondence
§
th
Dedicated to Professor M V George on the occasion of his 90
Birth Anniversary.
Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s12039-018-1537-x) contains
supplementary material, which is available to authorized users.
0123456789().: V,-vol

137 Page 2 of 9
J. Chem. Sci. (2018) 130:137
charge-recombination of the photoinduced radical-ion 2. Experimental
pair intermediates.24–26 Though the above-mentioned
2
.1 Materials and methods
methods can populate the triplet state, they are prone to
annihilation and the generation of efficient long-lived
triplet excited-state in innately triplet deficient chro-
mophores remains a highly investigated subject.
All chemicals were obtained from commercial suppliers and
used as received without further purification. All reactions
were carried out in oven-dried glassware prior to use and
wherever necessary, were performed under dry nitrogen in
dried, anhydrous solvents using standard gastight syringes,
cannulae, and septa. Solvents were dried and distilled by stan-
dard laboratory purification techniques. TLC analysis was
Our continual efforts in exploring the photoexcited
2
7–29
state properties of organic crystalline
/nonplanar D–
organic chro-
30–32
14,33
A assemblies,
and core-twisted
mophores motivated us to investigate the effect of a
heavy-atom on the photoexcited state properties of performed on precoated aluminium plates of silica gel 60
perylenemonoimide (PMI). The synthesis of the heavy- F254 plates (0.25 mm, Merck) and developed TLC plates
atom appended PMI (PMI-Pd) was carried via oxidative were visualized under short and long wavelength UV lamps.
Flash column chromatography was performed using silica
addition of PMI-Br to Pd(PPh ) under inert experi-
3
4
gel of 200–400 mesh employing a solvent polarity corre-
lated with the TLC mobility observed for the substance of
interest. Yields refer to chromatographically and spectroscop-
ically homogenous substances. Melting points were obtained
using a capillary melting point apparatus and are uncorrected.
IR spectra were recorded on a Shimadzu IRPrestige-21 FT-
mental conditions. The structure of PMI-Pd was further
characterized via nuclear magnetic resonance (NMR),
high-resolution mass spectrometry (HRMS) and single
crystal X-ray diffraction analysis. The consequence of
the direct attachment of the –Pd(PPh ) Br to the PMI
core was evident from the stead-state spectroscopic and
time-resolved lifetime analysis. The UV-Vis absorp-
tion and fluorescence emission spectra of PMI-Pd in
3
2
1
13
IR spectrometer as KBr pellets. H and C NMR spectra
were measured on a 500 MHz Bruker Avance DPX spec-
1
13
trometer. Internal standard used for H and C NMR is
chloroform reveal an apparent red-shift as compared to 1,1,1,1-tetramethyl silane (TMS). High-resolution mass spec-
the model derivative. The lifetime analysis of PMI-Pd tra (HRMS) were recorded on Thermo Scientific Q Exactive
in chloroform also displayed a similar perturbation as mass spectrometer using electrospray ionization (ESI, posi-
tive mode) technique.
compared to the model derivative PMI. The effect of
the heavy-atom (Pd) substitution in PMI was further
2
.1a Steady-state
spectroscopy:
Photophysical
explored employing the time-resolved spectroscopic
analysis. The femtosecond transient absorption (fTA)
measurements of both PMI-Pd and the model deriva-
tive PMI reveal the presence of the singlet excited state
at 630–640 nm. The nanosecond transient absorption
measurements of the derivatives were carried out in a
cuvette of 3 mm path length unless otherwise mentioned.
Absorption and emission spectra were recorded on Shi-
madzu UV-3600 UV-VIS-NIR and Horiba Jobin Yvon Fluo-
rologspectrometers respectively. Solution-staterelativequan-
tum yield measurements were performed using Rhodamine
(nTA) measurements of PMI-Pd and PMI reveal the
presence of the triplet excited state; however, the singlet-
to-triplet transition was not perceived. Interestingly, the
triplet formation in PMI-Pd was found to be higher out in an IBH picosecond time-correlated single photon
6
G as the standard (λex = 488 nm, reported quantum
34
yield ꢀf = 0.94). Lifetime measurements were carried
(
0.58 ± 0.01) as compared to the model derivative PMI counting (TCSPC) system. The fluorescence decay profiles
(
0.08 ± 0.005) leading to the quenching of the fluores- were de-convoluted using IBH data station software (DAS
2
6
.3), and fitted with exponential decay, minimizing the χ
cence in PMI-Pd. A similar heavy-atom (Pd) substituted
PDI derivative displayed a high fluorescence charac-
ter and a comparatively low yet significant heavy-atom
values.
Solution-state fluorescence quantum yields of PMI and
6
PMI-Pd derivatives were calculated by relative quantum yield
method as follows,
effect upon direct attachment of Pd to the PDI core. The
quenched fluorescence in PMI-Pd and the higher yield
of triplet formation suggest the combined effect of the
heavyatoms(PdandtheadjacentBr)mediatingthespin-
orbit coupling in PMI-Pd. Thus, the spin-orbit coupling
mediated heavy-atom can allow a viable access route
to achieve triplet character in integrally triplet deficient
aromatic chromophores. Triplet enhancement in PMI-
Pd as compared to the triplet deficient PMI chromophore
can renovate the utility of such chromophoric systems
ꢀ
ꢁ ꢀ
ꢁ ꢀ
ꢁ
2
Is
ODref
ODs
ns
ꢀ
s = ꢀref
(1)
Iref
nref
wherein, ꢀs and ꢀref are the quantum yields of sample and
reference respectively, Is and Iref are the area under the emis-
sion spectrum for sample and reference respectively. ODs
and ODref are the absorbances of the sample and reference
respectively at the excitation wavelength. ns and nref are the
for innovative light harvesting and energy-related appli- refractive index of the solvent in which sample and reference
cations. are taken.

J. Chem. Sci. (2018) 130:137
Page 3 of 9 137
Table 1. Crystallographic data and refinement
parameters for crystalline PMI-Pd.
Regenerative Amplifier (1 kHz, 4 mJ). A fraction of the
amplified output was used to generate 400 nm pump pulse.
Residual 800 nm pulse was sent through a delay line inside
an Excipro pump-probe spectrometer from CDP systems. A
rotating CaF2 plate (2 mm thickness) was used to generate a
continuum of white light from the delayed 800 nm pulses. The
continuum of white light was split into two and the streams
were used as a probe and reference pulses. Transient absorp-
tion spectra were recorded using a dual diode array detector
having a 200 nm detection window with an optical delay of
1.6 fs. Sample solutions were prepared in a rotating sample
cell with 4 mm path length. IRF was determined by solvent
(10% benzene in methanol) two-photon absorption and was
found to be approximately 110 fs at about 530 nm. Energy
per pulse incident on the sample is attenuated employing 80%
neutral density filter when required. fTA measurements of
PMI and PMI-Pd in CHCl3 were excited at 400 nm, 200 nJ,
Parameters
PMI-Pd
Formula
C70H BrNO2P2Pd
5
6
Formula wt
Colour, shape
Dimens, mm
Crystal system
Space group, Z
a, Å
1191.40
Red
0.24 × 0.1 × 0.08
Triclinic
P − 1
13.1460
14.7260
19.047
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
110.927
91.113
102.763
3339.3
Temp, K
296 (2)
1.185
≈
110 fs pulses, to moderate singlet-singlet annihilation
dcalcd, mg/m3
39
that occurs often in multichromophoric assemblies. The
observed kinetic components are laser intensity independent,
ruling out the assignment of kinetic components to singlet-
singlet annihilation.39
No. of reflections collected
No. of unique reflections
50862
11666
50
2
θmax, deg
No. of parameters
R1, wR2 (I > 2s(I))
R1, wR2 (all data)
Goodness of fit
694
0.0915, 0.2737
0.1352, 0.3227
1.096
2.1d Nanosecond transient absorption (nTA)
measurements: Nanosecond laser flash photolysis exper-
iments of the argon-purged solutions of PMI and PMI-Pd
were carried out in an Applied Photophysics Model LKS-
6
(
CCDC number
1845450
The crystal structure of PMI is available on
CCDC with a deposition number 900321.27
0 laser kinetic spectrometer using the second harmonic
λex = 355 nm, pulse duration ≈7 ns) of a Quanta Ray
INDI-40-10 series pulsed Nd: YAG laser as the excitation
.1b X-ray crystallography: Single crystal of PMI-Pd source. Triplet states of PMI and PMI-Pd in chloroform were
2
was obtained by slow evaporation of solvent from n-hexane: confirmed performing the experiments with oxygen purged
chloroform (1:1) homogenous mixture at room tempera- solutions. Triplet quantum yields upon direct photoexcitation
ture. High-quality specimens of approximate 0.24 × 0.1 × (355 nm) were determined using [Ru(bpy)]Cl in methanol
2
3
0
.08 mm dimension were selected for the X-ray diffraction as standard with nonsaturating laser intensities. An equal vol-
experiments. Crystallographic data collected for PMI-Pd is ume of 0.2 mM solution of β-carotene was added to optically
presented in Table 1. A single crystal was mounted using matched solutions of reference and the sample. The equation
oil (Infineum V8512) on a glass fibre. All measurements for the triplet quantum yield is given by,40
were made on a CCD area detector with graphite monochro-
s
Ref
obs
Ref
− k0
Ref
obs
ꢁ
ꢁARef
As
k
k
mated Mo Kα radiation. The data were collected using a
Bruker APEXII detector and processed using APEX2 from
Bruker. The structure was solved by a direct method and
s
Ref
obs
ꢀ
= ꢀT
×
× ks
×
(2)
T
s
− k0
obs
k
expanded using Fourier technique. The non-hydrogen atoms Where, ꢀs and ꢀT denote the triplet quantum yield of
Ref
T
s
Ref
were refined anisotropically. Hydrogen atoms were included
in idealized positions, but not refined. Their positions were
constrained relative to their parent atom using the appropri-
the sample and reference respectively; ꢁA and ꢁA are
transient absorption intensity of β-carotene in sample and
reference respectively; ks and k are decay rate of sample
s
0
obs
3
5
ate HFIX command in SHELX-97. All the programs used
during the crystal structure analysis are incorporated in the
WINGX software.36 The full validation of CIF and structure
factor of PMI-Pd was performed using the checkCIF utility
and found to be free of major alert levels. 3D structure visual-
ization and the exploration of the crystal packing of PMI-Pd
transient species before and after the addition of β-carotene.
Ref
obs
Ref
k
and k0 are the decay rate of reference transient species
before and after the addition of β-carotene.
The triplet formation rates in the two derivatives PMI and
PMI-Pd were not perceived as the fTA experiments were lim-
ited to 2.7–3.5 ns, whereas the nTA measurements provide the
signals from 10 ns onwards only. Lack of clear singlet-triplet
3
7,38
was carried out using Mercury 3.5.1.
(
S-T) transition in the femtosecond (fTA) and nanosecond
.1c Femtosecond transient absorption (fTA) transient absorption (nTA) measurements suggest the triplet
technique: Spectra-Physics Tsunami Oscillator (80 MHz, formation is slow and may form in 3.5–10 ns window, which
00 nm) was used as a seed for a Spectra-Physics Spitfire we are not able to perceive in our current experimental setup.
2
8

137 Page 4 of 9
J. Chem. Sci. (2018) 130:137
(
a)
3.33 Å
3.34 Å
3
0%
90%
10%
PMI
X-ray Structure
Close-Packing
(b)
PTCDA
PMI
PMI-Br
PMI-Pd
Scheme 1. Synthesis scheme for PMI-Pd. (a) 2,6-diiso-
◦
propylamine, imidazole, Zn(OAc) , H2O, 190 C, 24 h; (b)
2
7.57 Å
Br2, I2, Glacial acetic acid, RT, 6 h; (c) Pd(PPh3)4, 2 M
◦
K2CO3, Toluene, 110 C, 24 h.
7.73 Å
2
.1e Computational analysis: The vertical excitation
energiesandharmonicoscillatorfrequenciesofPMIandPMI-
Pd were computed using density functional theory (DFT) at
the Becke’s three parameter functional in combination with
PMI-Pd
X-ray Structure
Close-Packing
the Lee-Yang-Parr correlation functional (B3LYP) and 3-21G Figure 1. Molecularstructureandclose-packingof(a)PMI
basis set. All the computations were performed using the and (b) PMI-Pd. For clarity, the hydrogens have been omitted
Gaussian 09 Rev. A.02 and visualized using GaussView 5.0.41 in the crystal structure and the close-packing.
42
2
.1f Hirshfeld analysis : Important intermolecular
(
d, J = 8.1 Hz, 2H), 8.18 (d, J = 8.2 Hz, 1H), 8.0–8.02
interactions within the crystal structure of PMI and PMI-Pd
(m, 2H), 7.44–7.47 (m, 12H), 7.37–7.41 (m, 3H), 7.25 (d,
were identified through Hirshfeld surface analysis using Crys-
tal Explorer17.43 The Hirshfeld surface is defined as a set of
J = 7.85 Hz, 2H), 7.12–7.14 (m, 8H), 7.05–7.08 (m, 12H),
2
.66–2.70 (m, 2H), 1.11 (m, 12H); HRMS (ESI): m/z calcu-
points in 3D space where the ratio of promolecule and pro-
crystal electron densities is equal to 0.5. The exploration of
intermolecular contacts is provided by mapping normalized
contact distances (dnorm), which is a function of the closest
distance from the point to the nuclei interior (di) and exterior
+
lated for C70H BrNO2P2Pd [M+H] : 1190.2083; Found :
5
6
1
190.1963.
3
. Results and Discussion
(
2
de) to the surface as well as on the van der Waals radii (rvdw).
D fingerprint which was generated by deriving from the Hir-
3
.1 Synthesis and structure
shfeld surface by plotting the fraction of points on the surface
as the function of di and de which provide a visual summary
of intermolecular contacts within the crystal.
The title compound PMI-Pd was synthesized by reacting
PMI-Br
27,44
and tetrakis(triphenylphosphine)palladium
(
(
0) in toluene (with 2 M K CO ) under inert conditions
Scheme 1, Figure 1). The precursor 9-bromoperylene
2
3
2.2 Synthesis and structural analysis
monoimide (PMI-Br) was synthesized in accordance
Synthesis of PMI and PMI-Br were carried out following
our previous reported procedures.27,44 The synthesized PMI- with our previous reported procedure by reacting
Br was further reacted with tetrakis(triphenylphosphine) perylene-3,4,9,10-tetracarboxylic anhydride (PTCDA)
palladium(0) and 2 M K2CO3 solution in toluene (under inert with 2,6-diisopropylaniline (yielding PMI) followed
◦
conditions) at 110 C for 24 h to yield the title compound
by successive bromination in the presence of elemen-
PMI-Pd (Scheme 1). Red single crystals of PMI-Pd (1:1, n-
hexane: chloroform) was obtained by slow evaporation of
solvent from the homogenous solution at room temperature.
Crystal structure of PMI is available with CCDC (Deposi-
tion No. 900321) and the crystallographic data for PMI-Pd
derivative (1845450) is tabulated in Table S1, Supplementary
Information (SI).
27,44
tal bromine (Scheme 1).
Slow evaporation of a
27
solution of PMI (chloroform) and PMI-Pd (in 1:1,
n-hexane:chloroform) afforded decent quality crys-
tals for X-ray diffraction (Figure 1). The molecular
structure of PMI and PMI-Pd suggest a steric conges-
◦
◦
tion mediated near-orthogonal (θ = 85.8 and 86.8
respectively) nature of the 2,6-diisopropylphenyl
substituent at the imide region (Figure 1). The close-
packinginPMIrevealsthepresenceoftwonon-identical
2.3 Spectral characterization
◦
1
PMI-Pd: Yield: 10% (dark red solid); M.p. > 300 C; H stacked PMI dimers with a distance of 3.33 and 3.34 Å.
NMR (500 MHz, CDCl3): δ 8.51 (d, J = 8.1 Hz, 1H), 8.45 In contrast, the close-packing in PMI-Pd reveals the

J. Chem. Sci. (2018) 130:137
Page 5 of 9 137
steric intervention of the flanking triphenylphosphine 0.02) with a narrow Stokes’ shift and a characteristic
moiety between the two PMI cores lending a virtu- vibronic pattern archetypally seen in rylene chro-
45
ally negligible π-overlap (7.57 and 7.73 Å) between mophores. The fluorescence lifetime measurement of
the adjacent PMI-Pd dimers (Figure 1). To get addi- PMI reveals a monoexponential decay with a lifetime
tional insights into the subtle intermolecular interactions of 4.85 ns suggesting a single decay pathway for the
guiding the close-packing, Hirshfeld surface analysis singlet excited state. The UV-Vis absorption spectrum
of PMI and PMI-Pd was carried out. Hirshfeld sur- of PMI-Pd complex reveals a red-shifted (20–30 nm)
face analysis and the corresponding 2D finger plot absorptioncenteredat540nm(HOMO→LUMO)while
quantifies the % intermolecular interactions in the still maintaining the characteristic vibronic features
molecular assembly of PMI and PMI-Pd (Table S1, (Figure3, TableS2, inSupplementaryInformation). The
Supplementary Information). The ρ (%C•••H/%C•••C) red-shifted UV-Vis absorption in PMI-Pd can be a result
value obtained from the Hirshfeld analysis suggest of the perturbation exerted by the –Pd(PPh ) Br on the
3
2
1
,46
a β-motif and herringbone arrangement in the crys- PMI core. A much higher perturbation is observed in
talline assembly (Figures S1–S2, Table S1, in Sup- the fluorescence spectrum (λex = 480 nm) of PMI-Pd
plementary Information) of PMI and PMI-Pd, in chloroform as compared to bare chromophore PMI.
respectively.
The emission spectrum of the PMI-Pd in chloroform
reveals a 60 nm red-shift and a concomitant decrease
in the fluorescence quantum yield (ꢀ = 0.38 ± 0.02)
3.2 Steady-state spectroscopy
f
as compared to PMI. Moreover, the picosecond time-
To evaluate the preliminary photophysical features resolved lifetime analysis of PMI-Pd in chloroform
of PMI and PMI-Pd, steady-state UV-Vis absorption, reveal a bi-exponential decay (τ
= 1.64 and 4 ns)
f
fluorescence emission and lifetime measurements were of the fluorescence emission. The quenched emission
carried out in chloroform (Figure 2). The UV-Vis and the bi-exponential decay of the fluorescence emis-
absorption of PMI reveals a broad absorption ranging sion suggest an alternate pathway/phenomenon for the
from 400 to 590 nm with peak centered at 520 nm decay of the singlet excited state via incorporation
(
Table 2). The observed UV-Vis absorption of PMI in of the Pd moiety in PMI-Pd. The photoexcited state
chloroform corresponds to a typical π-π* (HOMO → characteristics of crystalline PMI and PMI-Pd differed
LUMO) transition similar to the earlier literature reports slightly from the solution state properties (Figure S3
1,44
(Figure 3, Table S2, in Supplementary Information).
and Table S3, in Supplementary Information). The UV-
The fluorescence spectrum (λex = 480 nm) of PMI in Vis absorption of crystalline PMI (300–650 nm) and
chloroform reveal an intense emission (ꢀ = 0.86 ± PMI-Pd (300–750) reveal red-shift as compared to that
f
Figure 2. (a) Absorption and emission spectra (λex = 480 nm) of PMI (top) and PMI-Pd (bottom);
fluorescence decay profiles of (b) PMI (λex = 439 nm, pulse width < 100 ps; λemi = 531 nm) and (c)
PMI-Pd (λex = 478 nm, pulse width < 90 ps; λemi = 600 nm).

137 Page 6 of 9
J. Chem. Sci. (2018) 130:137
Table 2. Photophysical properties of PMI and PMI-Pd in CHCl3.
Derivative λabs (nm)
λf (nm)
τf [amplitude] (ns)
ꢀf
τT (ns)
ꢀT
PMI
PMI-Pd
520
540
539, 578, 636
4.85 [100]
0.86 ± 0.02 1.58 ± 0.05 μs 0.08 ± 0.005
614, 658, 733 1.64 [10.5] 4 [89.5] 0.38 ± 0.01 0.90 ± 0.03 μs 0.58 ± 0.01
PMI
PMI-Pd
and PMI-Pd suggest the presence of the singlet excited
state, the singlet-to-triplet conversion was not perceived.
To probe the existence of the triplet excited state, nTA
spectroscopic measurements (λex = 355 nm) of PMI
and PMI-Pd were carried out in a nitrogen purged chlo-
roform solutions (Figure 4). The nTA spectra of PMI in
chloroform reveal positive transient absorption at 500–
LUMO+1
LUMO
6
00 with a peak centered at 560 nm and which decay
2
.76 eV
2
.68 eV
[
449.14 nm]
[
462.57 nm]
in a few microseconds timescale (τ = 1.58 ± 0.05 μs,
T
Figure 4a). The long decay time of the transient species
centered at 560 nm suggests the possibility of a triplet
excited state. To confirm the existence of a triplet excited
state, the nTA measurements of PMI were carried out
in oxygen purged solutions. The triplet-triplet energy
HOMO
HOMO-1
3
transfer (from PMI-Pd* to the ground state (triplet)
of oxygen) mediated decrease in the optical density
and the fast decay of the 560 nm component in the
oxygen purged solution confirms the presence of the
triplet excited state of PMI. On the other hand, nTA
measurements (λex = 355 nm) of PMI-Pd in chlo-
roform reveals a transient signal at 620 nm with a
lifetime of 0.90 ± 0.05 μs (Figure 4b). The minor
variation in the position of the triplet excited state
in PMI-Pd can be due to the difference in the triplet
energy levels of PMI-Pd as compared to PMI, aris-
Figure 3. The frontier molecular orbital analysis
B3LYP/3-21G) of PMI (left) and PMI-Pd (right), respec-
tively, carried out in Gaussian 09.
(
observed in chloroform solution. The emission spectra
of crystalline PMI and PMI-Pd revealed a similar red-
shifted trend as that of UV-Vis absorption. Interestingly,
the picosecond time-resolved lifetime analysis of crys-
talline PMI-Pd reveal a faster decay of the excited state
as compared to crystalline PMI. The faster decay of the
fluorescence lifetime in crystalline PMI-Pd suggests an
alternate pathway (possibly triplet excited state) respon-
sible for the faster decay of the singlet excited state
in PMI-Pd, which is not present in crystalline PMI
ing from a consequence of the incoming –Pd(PPh ) Br
3
2
(Figure 5). Upon oxygen purging, the positive transient
absorption at 620 nm shows an apparent quenching,
confirming the species to have a triplet origin. Inter-
estingly, the triplet quantum yield (φT = 0.58 ±
0
.01 μs) of PMI-Pd in chloroform reveal a consider-
(
Table2).
able increment as compared to PMI (φT = 0.08 ±
.005 μs). The apparent increase in the triplet yield
in PMI-Pd can be a direct consequence of the spin-
0
3.3 Transient absorption spectroscopy
Tofurtherprobethephotoexcited-stateprocessesinPMI orbit coupling mediated enhanced population of the
andPMI-Pd, ultrafasttransientabsorptionspectroscopic triplet excited state and the reduced energy differ-
measurements were carried out in monomeric chloro- ence/near iso-energetic nature of the S and T energy
1 2
form solution. The fTA (λex = 400 nm) spectra of levels (Figure 5). We believe that a tacit understand-
PMI and PMI-Pd reveal positive signal at 630–640 nm, ing of the spin-orbit coupling-mediated heavy-atom
suggesting the presence of singlet excited state which effect and the consequent enhancement in the triplet
does not completely decay within the given experimen- excited state of polyaromatic hydrocarbon can revive
tal window and is further supported by the time-resolved the design principles of novel organic materials for
fluorescence lifetimes (Table 2, Figure S4, in Supple- optoelectronic applications (Table S4, Supplementary
mentary Information). Though the fTA spectra of PMI Information).

J. Chem. Sci. (2018) 130:137
Page 7 of 9 137
Figure 4. 2D plot (top) and nTA spectra (bottom, λex = 355 nm) of nitrogen purged solutions of (a) PMI
and (b) PMI-Pd; nTA spectra (λex = 355 nm) of oxygen purged solutions of (c) PMI and (d) PMI-Pd.
analysis revealed the weak intermolecular interactions
guiding the three-dimensional packing in the crystalline
assembly. The steady-state UV-Vis absorption and emis-
ISC
ISC
Sn
Sn
sion spectra of PMI-Pd display an apparent red-shift
when compared to the model derivative PMI, from a
consequence of the effect exerted by the incoming -
PMI
f
φ = 0.38
f
φ = 0.86
Tn
Tn
Pd(PPh ) Br group. The effect of the incoming Pd group
3
2
is more pronounced in the photoexcited state processes
of PMI-Pd. The nTA spectra of PMI-Pd exhibits an
apparent increase in the triplet population from the
incorporation of the heavy atom (Pd). The spin-orbit
coupling-mediated heavy atom effect proposes a viable
route to access the triplet excited state in polyaromatic
chromophores possessing negligible triplet character.
Enhancement of the triplet character in aromatic chro-
mophores can have a progressive impact on the efficacy
of such aromatic chromophores in solar light-to-energy
conversion applications.
φ
T
= 0.08
φ
T
= 0.58
Figure 5. Jablonski diagram depicting the energy levels
and the photophysics of PMI and PMI-Pd.
4
. Conclusions
In summary, we report
a
novel arene-metal
conjugate wherein the spin-orbit coupling mediated
heavy atom effect conveys a higher triplet population in
PMI-Pd. The arene-metal conjugate was synthesized via
Supplementary Information (SI)
Crystallographic data for the structural analyses of PMI and
PMI-Pd have been deposited with the Cambridge Crystal-
lographic Data Centre bearing the CCDC Nos. 900321 and
oxidative addition of PMI-Br into Pd(PPh ) under inert
3
4
conditions. The structural characterization of the model
derivative PMI and PMI-Pd were carried out using 1845450, respectively. Copies of this information are avail-
single crystal X-ray diffraction and the close-packing able on request at free of charge from CCDC, Union Road,

137 Page 8 of 9
J. Chem. Sci. (2018) 130:137
Cambridge, CB21EZ, UK (fax: +44-1223-336-033; e-mail: 11. Sharma P, Damien D, Nagarajan K, Shaijumon M M and
deposit@ccdc.ac.uk or http://www.ccdc.cam.ac.uk). Details
of Hirshfeld analysis and close-packing (Figures S1–S2,
Table 1), TD-DFT energy levels (Table S2) and fTA (Fig-
ure S4) analysis. Supplementary Information is available at
www.ias.ac.in/chemsci.
Hariharan M 2013 Perylene-polyimide-Based Organic
Electrode Materials for Rechargeable Lithium Batteries
J. Phys. Chem. Lett. 4 3192
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2. Banda H, Damien D, Nagarajan K, Hariharan M and
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Perylene Diimides with Tunable Redox Properties for
Organic Sodium-Ion Batteries (Adv. Energy Mater.
Acknowledgements
M. H. acknowledges Kerala State Council for Science,
Technology and Environment (KSCSTE) for the support
of this work, 007/KSYSA-RG/2014/KSCSTE. The authors
thank A. P. Andrews, IISER-TVM for the single crystal X-ray
structure analyses. A.M.P., G.G., E.S. & R.R. thank IISER-
TVM, and Department of Science and Technology (DST)
INSPIRE, University Grant Commission (UGC) fellowships
for the financial assistance.
20/2017) Adv. Energy Mater. 7. https://doi.org/10.1002/
aenm.201770112
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1
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4. Nagarajan K, Mallia A R, Reddy V S and Hariharan M
2016 Access to Triplet Excited State in Core-Twisted
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