Extending the scope of the carbonyl facilitated triplet excited state towards visible light excitation

Article abstract of DOI:10.1039/c8cp01023d

A series of extended π-conjugated benzophenone analogs was synthesized through a facile Lewis-acid catalyzed Friedel-Crafts reaction in order to exploit the integral triplet state properties of benzophenone. Extending the π-conjugated plane of the phenyl ring of benzophenone allowed tuning of the excitation wavelength from the far-UV end (～260 nm) to the visible spectrum (～446 nm). Compared to benzophenone, significant red-shifts in the absorption (up to 450 nm in solution) with high photostability were observed for the synthesized benzophenone analogs. As is evident from the density functional theory calculations, expansion of the ring size of the aromatic part of the benzophenone analogs induces a decrease in the HOMO-LUMO gap. The considerable extension of the electron density to the carbonyl group in the LUMO substantiates the triplet nature associated with the benzophenone analogs. By virtue of the properties of the carbonyl functionality, an apparent increase in the triplet quantum yield (Φ_T = 5.4% to 87.7%) was observed for the benzophenone analogs when compared to the corresponding bare polyaromatic hydrocarbon. The spin orbit coupling was computationally estimated for the benzophenone analogs to propose pathways for the observed intersystem crossing process. The plausibility to photoexcite the aromatic-ring-fused benzophenone frameworks for triplet activation in the visible range opens the door for a new class of materials for photonic application.

Full text of DOI:10.1039/c8cp01023d

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Extending the Scope of Carbonyl Facilitated Triplet
Excited State towards Visible Light Excitation†
Shinaj K. Rajagopal,‡ Nagaraj K.,‡ Somadrita Deb, Vinayak Bhat, Devika Sasikumar, Ebin
Sebastian and Mahesh Hariharan*
School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram
(IISERꢀTVM), Maruthamala P. O., Vithura, Thiruvananthapuram 695551, Kerala, India.
Abstract. A series of extended πꢀconjugated benzophenone analogs was synthesized through
the facile Lewisꢀacid catalyzed FriedelꢀCrafts reaction in order to exploit the integral triplet state
properties of benzophenone. Extending the πꢀconjugated plane of the phenyl ring of
benzophenone allowed to tune the excitation wavelength from farꢀUV end (~260 nm) to the
visible spectrum (~446 nm). Compared to benzophenone, significant redꢀshifts in the absorption
(up to 450 nm in solution) with high photostability were established for the synthesized
benzophenone analogs. As evident from the density functional theory calculations, expansion of
ring size at the aromatic part in benzophenone analogs induces a decrease in the HOMOꢀLUMO
gap. The considerable extension of electron density to the carbonyl group in the LUMO
substantiates the triplet nature associated in the benzophenone analogs. By virtue of the
properties of carbonyl functionality, an apparent increase in the triplet quantum yield (ΦT = 5.4%
to 87.7%) was observed for benzophenone analogs when compared to the corresponding bare
polyaromatic hydrocarbon. The spin orbit coupling was computationally estimated for the
benzophenone analogs to propose pathways for the observed intersystem crossing process. The
plausibility to photoexcite the aromaticꢀringꢀfused benzophenone frameworks for triplet
activation in the visible range opens the door for a new class of materials for photonic
application.
1

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DOI: 10.1039/C8CP01023D
Keywords: Benzophenone • Spinꢀorbit coupling • Intersystem Crossing • Triplet Excited State
• Phosphorescence
Introduction
Triplet states are important intermediates in material chemistry and invite many potential
optoelectronic applications.1 Triplet excited states of conjugated organic molecules play a major
role in organic light emitting diodes (OLEDs) through harvesting of triplet excitons that
enhances the electroluminescence efficiency.2–4 Formation of triplet excitons evade deleterious
charge recombination in organic photovoltaics (OPVs).5–10 Longꢀlived triplet excited states can
potentially give rise to larger diffusion length for advanced solar cells.11 Any urge to identify
conjugated organic molecules which can efficiently generate triplet states for improved
performance of OLEDs, OPVs and other electronic devices can be considered noteworthy.12–15
Strategies to generate triplet excitons in conjugated organic molecules through chemical
modification is still an emerging area.16–21 Incorporation of heavy atoms (such as I,22,23 Br,24,25
S,26 etc.17) and auxochrome functionalities (such as carbonyl,27,28 nitro,29–31 etc.) can induce
efficient triplet formation in conjugated organic molecules through enhanced spinꢀorbit coupling.
Likewise, the outꢀofꢀplane vibrations in nonꢀplanar frameworks comprehend efficient vibronic
coupling to populate the triplet excited states.32 Recent approaches have been developed to
enhance triplet yield by a singlet fission process in various chromophoric systems.33
Rapid intersystem crossing (ISC) to populate the triplet excited states in organic
chromophores have been extensively investigated by timeꢀresolved spectroscopic methods.34–37
In this regard, benzophenone (BP) is a prototype molecule which has been widely investigated
for triplet state properties.38,39 A high triplet quantum yield (ΦT) has been inferred in
benzophenone which upon excitation into the singlet excited state exhibits an efficient
intersystem crossing (ISC). The characteristic high triplet yield of benzophenone can classify
2

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them as a potential photosensitizer for generating singlet oxygen in photodynamic therapy.
However, benzophenone bears the ability to efficiently generate the triplet excited states by
absorbing in the ultraviolet (UV) region. Incorporating the favorable property of benzophenone
for efficient ISC and extending the excitation wavelength to the visible region are essential to
harvest maximum photons from the solar radiation. Our longstanding interest in excited state
dynamics40–42 prompted us to explore the triplet state properties of benzophenone through
extending the conjugation at the phenyl ring and/or replacing the phenyl groups by bridging the
carbonyls with πꢀconjugated groups (Fig. 1).
Fig. 1 Synthesis scheme and chemical structure of benzophenone (BP) analogs.
EXPERIMENTAL DETAILS
Materials and Methods
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General Informations: All chemicals were purchased from chemical suppliers and used as
received, without further purification. All reactions were carried out in standard oven dried
glasswares. Solvents were dried and distilled by standard procedures. TLC analyses were
performed on precoated aluminum plates of silica gel 60 F254 plates (0.25 mm, Merck). IR
spectra were recorded on a Shimadzu IR Prestigeꢀ21 FTꢀIR spectrometer as neat KBr pellets for
all the derivatives. CHN analyses were carried out on an Elementar vario MICRO cube
Elemental Analyzer. All values recorded in elemental analyses are given in percentage.
Reference standard used for elemental analysis is 4ꢀaminobenzenesulphonic acid (sulphanilic
acid). High Resolution Mass Spectra (HRMS) was recorded on Agilent 6538 Ultra High
Definition (UHD) AccurateꢀMass QꢀTOFꢀLC/MS system using either atmospheric pressure
chemical ionization (APCI) or electrospray ionization (ESI) mode. Proton nuclear magnetic
resonance (1H NMR) and carbon nuclear magnetic resonance (13C NMR) spectroscopy were
performed on 500 MHz and 125 MHz with Bruker AVANCEꢀIII spectrometer respectively.
1
Chemical shifts of H NMR spectra are reported in units of parts per million (ppm) downfield
from 1,1,1,1ꢀtetramethylsilane (TMS) (δ = 0.0) and relative to the signal of chloroformꢀd (δ =
7.260, singlet). Multiplicities were given as s (singlet); br s (broad singlet); d (doublet); t
(triplet); q (quartet); dd (doublet of doublets); m (multiplets), etc. The number of protons (n) for
a given resonance is indicated by nH. Carbon nuclear magnetic resonance spectra (13C NMR) are
reported in units of parts per million (ppm) downfield from 1,1,1,1ꢀtetramethylsilane (TMS) (δ =
0.0) and relative to the signal of chloroformꢀd (δ = 77.160, t). Absorption spectra were recorded
on Shimadzu UVꢀ3600, emission and phosphorescence on Horiba Jobin Yvon Fluorolog 3
spectrofluorimeter. The solution state fluorescence quantum yields were determined by using
optically matched solutions of BP analogs and their respective standards. Quinine sulfate
4

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dissolved in 0.5 M H2SO4 (ΦF = 0.546)43 is used as the standard for AP, AN, PyP and PyPy;
pyrene dissolved in cyclohexane (ΦF = 0.32)44 is used as the standard for NP, NN and NN’,
perylene in cyclohexane (ΦF = 0.94)44 and nile red in dioxane (ΦF= 0.7)45 are used as reference
for AA and PerP respectively.
General procedure for synthesis of BP analogs:46 Anhydrous AlCl3 (2 eqv.) dissolved in dry
DCM was added to the stirred solution of polyaromatic hydrocarbon (Naphthalene, Anthracence,
Pyrene and Perylene) in dry DCM (1 eq.) at ambient conditions. The corresponding acyl chloride
(Benzoyl chloride, Naphthoyl chloride and 1ꢀPyrene carbonyl chloride) (1 eqv.) was then added
dropwise to this stirring mixture maintained at 0º C, which is then stirred for additional 3 hours
at room temperature. After the completion of the reaction, the reaction mixture was poured into
10% HCl. The resulting suspension was extracted with DCM, washed with NaHCO3 solution,
dried over Na2SO4 and purified through column chromatography (Silica gel, Pet ether, DCM) to
afford the desired product. (For the detailed syntheses and charaterization of the BP analogs see
ESI†)
Nanosecond transient absorption measurements (nTA):40 Nanosecond laser flash photolysis
experiments of the argon purged solutions were carried out in an Applied Photophysics Model
LKSꢀ60 laser kinetic spectrometer using the third harmonic (355 nm, pulse duration ≈10 ns) of a
Quanta Ray INDIꢀ40ꢀ10 series pulsed Nd:YAG laser as the excitation source. Triplet states of
BP analogs in toluene were confirmed by performing the experiments with oxygen purged
solutions. Triplet quantum yields obtained upon direct photoexcitation at 355 nm were
determined using [Ru(bpy)]C12 in methanol as standard (ΦT = 100 %), with nonsaturating laser
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intensities. Equal volume of 0.2 mM solution of βꢀcarotene was added to optically matched
solutions of reference and the sample. The equation for the triplet quantum yield is given by,
ꢂꢃꢄ
ꢇꢈꢁ
∆ꢅꢁ
∆ꢅꢂꢃꢄ
ꢆꢇꢁꢈꢁ
ꢆꢇꢁꢈꢁ − ꢆꢉꢁ
ꢆ
− ꢆꢉꢂꢃꢄ
ꢂꢃꢄ
ꢇꢈꢁ
Φꢀꢁ = Φꢂꢀꢃꢄ
×
×
×
(1 )
ꢆ
Where, Φꢀꢁ and Φꢂꢃꢄdenote the triplet quantum yield of the sample and reference respectively;
ꢀ
∆ꢅꢁ and ∆ꢅꢂꢃꢄare transient absorption intensity of βꢀcarotene in sample and reference
respectively; ꢆꢇꢁꢈꢁ and ꢆꢉꢁ are decay rate of sample transient species before and after the addition
ꢂꢃꢄ
ꢇꢈꢁ
of βꢀcarotene. ꢆ
and ꢆꢉꢂꢃꢄ are decay rate of reference transient species before and after the
addition of βꢀcarotene.
Femtosecond transient absorption technique (fTA): Spectraꢀphysics Mai Tai Oscillator (80
MHz, 800 nm) was used as seed for a SpectraꢀPhysics Spitfire Ace Regenerative amplifier (1
KHz, 5.5 mJ). A fraction of the amplified output was used for the generation of the 400 nm
pump pulse. The residual 800 nm pulse was passed through a delay line within an Excipro pumpꢀ
probe spectrometer from CDP Systems. A rotating CaF2 plate (2 mm thickness) was used in the
generation of white light from the delayed 800 nm pulses. The continuum of white light was
further split into two – one as the probe and the other as the reference pulse. The transient
absorption spectrum was recorded by a dual diode array detector with a detection window of 200
nm and an optical delay line resolution of 1.6 fs. The sample solution was prepared in a rotating
sample cell with 1.2 mm path length. IRF was determined by solvent (10 % benzene in
methanol) two photon absorption. The energy per pulse incident on the sample was attenuated
employing 80% neutral density filter. fTA measurement on AP in toluene was carried out by
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exciting the sample at 400 nm, 200 nJ, ~100 fs pulses. Global analyses of the fTA spectrum was
carried out using Glotaran (version 1.2).47
X-ray Crystallography: Highꢀquality specimens of appropriate dimensions were selected for
the Xꢀray diffraction experiments. Crystallographic data collected are presented in the
supplementary information. Single crystals were mounted using oil (Infineum V8512) on a glass
fiber. All measurements were made on a CCD area detector with graphite monochromated Mo
Kα radiation. The data was collected using Bruker APEXII detector and processed using APEX2
from Bruker. All structures were solved by direct methods and expanded using Fourier
techniques. The nonꢀhydrogen atoms were refined anisotropically. Hydrogen atoms were
included in idealized positions, but not refined. Their positions were constrained relative to their
parent atom using the appropriate HFIX command in SHELXLꢀ97. The full validation of CIFs
and structure factors were performed using the CheckCIF utility and found to be free of major
alert level.
Computational methods: Groundꢀstate structure optimisation, with toluene as solvent, and
vibrational frequencies were computed using density functional theory (DFT) at PCM/ωB97Xꢀ
D/TVZP level of theory.48,49 Vertical excitation energies and oscillator strengths were calculated
by employing time dependent DFT (TDꢀDFT) at PCM/ωB97XꢀD/TVZP. All computations were
performed with the Gaussian 09.50 The spin orbit coupling was evaluated with a linear response
timeꢀdependent density functional theory using the PySOC code.51
Results and discussions
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Syntheses and crystal structure analyses
Conventional Lewis acid catalyzed FriedelꢀCrafts aroylation at ambient conditions rendered
benzophenone (BP) analogs namely naphthalenꢀ1ꢀyl(phenyl)methanone (NP), di(naphthalenꢀ1ꢀ
yl)methanone (NN), naphthalenꢀ1ꢀyl(naphthalenꢀ2ꢀyl)methanone (NN’), anthracenꢀ9ꢀ
yl(phenyl)methanone (AP), anthracenꢀ9ꢀyl(naphthalenꢀ1ꢀyl)methanone (AN), di(anthracenꢀ9ꢀ
yl)methanone (AA), phenyl(pyrenꢀ1ꢀyl)methanone (PyP), di(pyrenꢀ1ꢀyl)methanone (PyPy) and
perylenꢀ3ꢀyl(phenyl)methanone (PerP) with moderate to high yields (31ꢀ75 %) (Fig. 1). Single
crystals of NP, NN, NN’, AN, AA and PyP suitable for Xꢀ ray diffraction (XRD) analysis were
obtained through slow evaporation of hexane/chloroform mixtures at ambient conditions. The
XRD analysis of NN, NN’, AA established the formation of solvent free monoclinic crystal
system with space group P21/n, P21, and P21/c respectively, whereas NP and AN crystallized in
orthorhombic crystal system with P212121 and Pca21 space groups. PyP exhibited a solvent free
triclinic crystal system with Pꢀ1 space group (Fig. 2, Table S1; ESI†). The dihedral angle
between the carbonyl group and the corresponding arenes in NN, NN’ and AA crystals is 17.9º,
21.2º and 46.5º respectively (Fig. S1; ESI†). Compounds containing dissimilar arenes displayed
two torsional angles with respect to the carbonyl group. Torsional angles of 67.3º and 25.6º were
identified for crystalline AN relative to the carbonyl group which could be due to the twist of
anthracene and naphthalene respectively.
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Fig. 2 Single crystal Xꢀray structure of I) NP, II) PyP, II) NN, IV) NN’, V) AN, VI) AA.
Steady state absorption, emission and frontier molecular orbital analyses
UVꢀVis absorption spectra of BP analogs (Fig. 3, Fig. S2 & Table S2; ESI†) were recorded
in toluene. The absorption maxima of the BP analogs spanned from the UV region, 305 nm for
NP, to the visible region, 446 nm for PerP. A significant redꢀshift of ~141 nm observed in the
absorption maximum of PerP relative to NP could be attributed to the extension in the πꢀ
conjugation of the benzophenone core.
Fig. 3 UVꢀVis absorption spectra of BP analogs in toluene.
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Density functional theory (DFT) calculations were performed to correlate the observed redꢀ
shift in the electronic absorption spectrum (Table S3 and Fig. S3; ESI†). Though marginally
blueꢀshifted with respect to the experimental data, the calculated λmax agree well with the trend
observed in the experimental data with increase in the size of the aromatic core. The vertical
excitation energies computed from the TDꢀDFT calculation revealed that the first vertical
transition (S0→S1) is described by HOMO→LUMO excitation, with appreciable oscillator
strength in the presented benzophenone analogs (f = 0.0123ꢀ0.7110; see Fig. 4C and Table S4;
ESI†). Careful analysis of the frontier molecular orbitals indicates that the LUMO is more
delocalized with modest extension to the carbonyl group, while a significant electron density on
oxygen atom of the carbonyl group hint towards the nonꢀbonding nature of the molecular orbital.
(Fig. 4A and 4B, Fig. S4 & S5; ESI†). The lowꢀintensity short wavelength band
(HOMO→LUMO) emerge as a result of nꢀπ* transition of the carbonyl group. The prominent
long wavelength bands were assigned to have contributions from πꢀπ* transition.
Fluorescence spectra recorded on photoexcitation of BP analogs in toluene at 305ꢀ450 nm,
exhibited emission bands centered at around 380ꢀ630 nm (Table 1, Fig. S6; ESI†). The solution
state fluorescence quantum yield (ΦF) of the BP analogs were determined using optically
matched solutions of the sample and the respective standards (see experimental details). The
estimated φF using pyrene in cyclohexane as the reference (ΦF = 32%)44 for NP, NN, NN’ were
1.9, 2.1 and 0.9 % respectively. Similarly AP, AN, PyP and PyPy demonstrated a ΦF of 0.3, 0.2,
2.7 and 25.3 % respectively, by matching with quinine sulphate in 0.05 M H2SO4 (ΦF =
54.6%).43 Perylene in cyclohexane (ΦF = 94%)44 was used as a reference to measure the ΦF of
AA and the obtained quantum yield was 1.7%. The highest fluorescence quantum yield among
the BP analogs was exhibited by PerP with ΦF of 35%, quantified using nile red in dioxane (ΦF =
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70%)44
as
the
reference.
Fig. 4 The frontier molecular orbitals A) HOMO B) LUMO of PerP; C) Energy level diagram
for BP analogs as calculated using TDꢀDFT (PCM/ωB97XꢀD/TZVP).
Nanosecond transient absorption measurements
In order to get insights into the intrinsic triplet character of benzophenone upon πꢀ
conjugation, nanosecond transient absorption measurements (nTA) of the BP analogs in toluene
were carried out by exciting the samples at 355 nm (Fig. 5 and Fig. S7; ESI†). Upon
photoexcitation (λexc = 355 nm), BP analogs NP, NN, and NN’ exhibited positive absorption
peaks in the spectral region 450 nm to 600 nm. The observed band corresponds to the tripletꢀ
triplet absorption and is in excellent agreement with the naphthalene T1 transition.52 Crespoꢀ
Hernández and coꢀworkers have identified a similar observation in the nanosecond broadband
transient absorption of nitronaphthalene derivatives.29 Observed triplet band exhibited a lifetime
(τT) of 0.2, 0.3 and 0.3 ꢀs for NP, NN and NN’ respectively (Table 1, Fig. S8; ESI†). Triplet
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quantum yield (ΦT) measurement of BP analogs were performed by employing tripletꢀtriplet
energy transfer method using βꢀcarotene as a triplet sensitizer.53 The calculated ΦΤ values were
60.1, 87.6 and 63.4% for NP, NN and NN’ respectively (Table 1). The nTA measurements on
AP, AN and AA displayed absorption maximum at around 430ꢀ450 nm (Fig. 5 & S7; ESI†). A
pronounced negative absorption was observed at 350 nm, corresponding to the steady state
absorption of anthracene (A) unit which manifests the ground state depopulation of A. The
transient absorption
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Fig. 5 nTA spectra of A) AA, B) AP and C) PyPy in toluene with the respective kinetic traces
monitored at 450 nm (λex = 355 nm).
Table 1. Triplet lifetime (τΤ) and triplet quantum yield (ΦΤ) in toluene using tripletꢀtriplet energy
transfer method.
Φꢊꢋ (%)
Φꢍꢀ (%)
Φꢎꢀ (%)
ꢌꢀꢈ (ꢀs)
10
Compound
BP
NP
NN
NN’
AP
AN
~ 0
1.9
2.1
0.9
0.3
0.2
1.7
2.7
25.3
35.0
100
60.1
87.6
63.4
86.7
75.8
77.4
48.2
69.2
5754
0.2
0.3
0.3
0.7
0.9
1.1
1.8
2.6
1.9
8055
7255
AA
PyP
PyPy
PerP
3855
~ 0
5.4
b
c
afluorescence quantum yield; triplet life time; triplet quantum yield
of corresponding BP analogs. dtriplet quantum yield of bare
chromophore taken from literatures.
signals observed around 430ꢀ450 nm were readily quenched by the dissolved oxygen which
strongly suggests the existence of triplet excited state (3A*; Fig. S9; ESI†). The positive
absorption band around 430ꢀ450 nm thus, could be due to the triplet absorption of anthracene
(3A*)56 possessing triplet lifetime (τT) of 0.7, 0.9 and 1.1 ꢀs for AP, AN and AA, respectively
(Fig. 5, Table 1 & Fig. S8; ESI†). Estimated quantum yields of 86.7, 75.8 and 77.4%,
respectively for the aroylated anthracene, showed 5ꢀ20% enhancement in ΦT compared to the
corresponding bare chromophore, A (ΦT = 72%). Similar features of triplet absorption were
observed in pyrene containing analogs, PyP and PyPy with absorption maximum at 500ꢀ520 nm
and triplet lifetime of 1.8 and 2.6 ꢀs respectively (Fig. 5, Table 1 & Fig. S7; ESI†). The
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absorption band at 500ꢀ520 nm could be attributed to the triplet absorption of pyrene (3Py*) with
ΦT of 48.2% (PyP) and 69.2% (PyPy). The estimated φT for pyrene containing analogs exhibited
an increase in the triplet quantum yield by 27ꢀ82% than the bare pyrene which has a reported
triplet quantum yield of 38% (ΦT). PerP was characterized by a weak transient absorption band
at 500 nm with life time of 1.9 ꢀs and ΦT of 5.4% (Table 1, Fig. S7; ESI†). The corresponding
bare chromophore, perylene exhibits negligible triplet quantum yield.
Triplet formation by visible light excitation
The concrete evidence for visible light excitation populating the triplet states in the BP
analogs was provided by femtosecond transient absorption (fTA) spectroscopy (Fig. 6AꢀC). The
representative PyP (ΦT = 69.2%) was photoexcited at 400 nm using 100 fs laser pulse. The
transient absorption band for PyP in toluene initially decaying at 530 nm indicates the absorption
of the higher energy singlet state (S2ꢀSn transition), as reported by Foggi and coꢀworkers.57 The
decay of the peak at 530 nm is followed by the appearance of a singlet excited state (S1)
corresponding to the absorption band at 460 nm (S1ꢀSm transition).18 The singlet state S1 then
undergoes ISC process to populate the triplet state (Tn) represented by the absorption peak at 510
nm.18,42
Time-gated emission measurements
To better understand the emissive nature of the triplet states, timeꢀgated emission
measurement of BP analogs was performed by photoexciting the deoxygenated ethanolic
solutions at the respective absorption maxima. The emission profile obtained at 77 K for NN and
NN’ after a delay time of 0.05 ms were red shifted (ca. 130ꢀ150 nm; Fig. 6D) when compared to
their fluorescence spectra. While, the spectral profile of AA, PyPy and PerP at 77 K obtained
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after a 0.05 ms delay matched their respective fluorescence emission spectra.58 Radiative decay
of the triplet excited states has been reported in the cases of BP and NP whereas AP and PyP
exhibit no phosphorescence.59 The phosphorescence spectra of NN’ range from 475 nm to 675
nm and that of NN span from 475 nm to 650 nm (Fig. 6D).
0.20
0.15
0.10
0.05
0.00
1.00
0.75
0.50
0.25
0.00
S
S
A)
C)
B)
D)
2
1
n
T
450
500
550
600
450
500
550
600
Wavelength, nm
Wavelength, nm
1.00
0.75
0.50
0.25
0.00
1.00
0.75
0.50
0.25
0.00
NN
NN'
S
S
(18.5 ps)
2
(0.36 ns)
(long )
1
n
T
500
550
600
650
700
0.00
0.25
0.50
0.75
1.00
1.25
1.50
Wavelength, nm
τ, ns
Fig. 6 Shows the A) fTA spectrum of PyP in toluene upon excitation at 400 nm; B) Evolution
associated spectra corresponding to the spectra shown in Fig. 6A; C) population profile of spectra
shown in Fig. 6A; D) phosphorescence emission spectra of NN and NN’ in ethanol.
Triplet populating pathways and spin orbit coupling
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To gain insight into the pathways populating the triplet manifolds in BP analogs via the
spin orbit coupling (SOC) mechanism,60,61 the SOCs were calculated using the PySOC code
developed by Thiel and coꢀworkers.51 The SOCs were evaluated at the FranckꢀCondon geometry.
In BP, the large SOC value (Table S5; ESI†) for S1/T1 (13 cmꢀ1) and a significant SOC for S1/T2
(3 cmꢀ1) could efficiently populate the triplet manifold by the ultrafast ISC process as reported.38
A similar pathway for populating the triplet could exist in the other carbonyl derivatives,
however, governed by ElꢀSayed’s rule.62 S1ꢀT2ꢀT1 or S1ꢀT3ꢀT1 could be the pathway
populating the triplet in NP. The SOC for S1/T2 (10 cmꢀ1) or S1/T3 (12 cmꢀ1) could be sufficient
for promoting the ISC in NP. The near degenerate S1 and T4 with a SOC of 11 cmꢀ1 and 9 cmꢀ1 in
NN and NN’ respectively could allow ISC to generate the triplet states (Fig. S10; ESI†). In AP,
quasiꢀdegenerate T2/T3/T4 and a small ꢁESꢀT (0.1ꢀ0.2 eV) could promote the population of the
triplet states from S1. The lower triplet quantum yield in AN and AA, in comparison to AP, could
be attributed to the low SOCs in the range of 0.3 cmꢀ1 to 5 cmꢀ1 for S1/T3 and S1/T4 respectively.
Despite the low SOC between the states, a small value of ꢁESꢀT could promote the ISC process in
AN and AA. In case of PyP and PyPy, the S1ꢀT4 pathway could dominate the ISC process.
However, the nature of the S1ꢀT4 transition is allowed in PyPy, S1(nꢀπ*)ꢀT4(πꢀπ*) and
forbidden in PyP, S1(nꢀπ*)ꢀT4(nꢀπ*), which could explain the lower triplet quantum yield in
PyP compared to PyPy (Fig. S10; ESI†). Similarly, the low quantum yield of triplet in PerP
could be attributed to a low SOC for S1/T2 and also the forbidden nature of the S1/T2 transition.
In addition, the deviation from planarity observed in the BP analogs could considerably alter the
ISC process in such systems.63
Conclusions
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In conclusion, visible lightꢀharvesting benzophenone analogs that can be used as organic
triplet generators were designed and synthesized through a facile aroylation reaction. Efforts to
extend the πꢀconjugation of the phenyl ring(s) on benzophenone (BP) allowed to tune the
photoabsorption from UV to the visible region. Considerable expansion in the πꢀconjugation of
the phenyl ring led to a 186 nm (e.g. PerP) red shifted absorption when compared to that of BP,
which is corroborated by DFT calculations. Population of longꢀlived triplet excited states were
confirmed by nanosecond time resolved absorption which exhibited an increase in the triplet
quantum yield in BP analogs (ꢁΦT = 5.4 to 31.2%) when compared to their respective bare
polyaromatic hydrocarbons. The visible light excitation populating the triplet excited states in
the BP analogs was confirmed by femtosecond timeꢀresolved absorption spectroscopic
technique. Furthermore, the computationally predicted spin orbit couplings in the BP analogs aid
in proposing the triplet population pathways that substantiates with the experimentally observed
trends in triplet quantum yield. Though the analogs have a lower triplet quantum yield than the
parent BP, the strategy of multiple aroylation of the BP analogs could possibly increase the
triplet quantum yield and also result in the redꢀshift of the UVꢀVis absorption of the analogs, as
observed earlier.42 The enhanced triplet activation through photoexcitation in the visible range in
benzophenone analogs will thus make them desirable materials for triplet excitons generators for
organic photovoltaic devices.
1
†Electronic Supplementary Information (ESI) available: Experimental details, H and 13C
NMR spectra of BP analogs as well as characterization data. Computational details, fluorescence
and nanosecond timeꢀresolved spectra for BP analogs. CCDC 1559462ꢀ1559466, 1440151.
‡These authors contributed equally.
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Authors information
Corresponding Author
*MH; email: mahesh@iisertvm.ac.in
ORCID: Mahesh Hariharan: 0000ꢀ0002ꢀ3237ꢀ6235
Acknowledgements
M. H. acknowledges Department of Science & TechnologyꢀNanomission for the support of this
work, DSTꢀSR/NM/NSꢀ23/2016 (C). N. K. thanks CSIR for financial assistance. Authors thank
Mr. Alex P. Andrews for Xꢀray crystal structure analysis and Ms. Devika Priyadarshini for
partial assistance in the purification of compounds. V.B. thanks DSTꢀINSPIRE for financial
support.
Notes and references
The authors declare no competing financial interest.
1
2
S. K. Lower and M. A. ElꢀSayed, The Triplet State and Molecular Electronic Processes in
Organic Molecules, Chem. Rev., 1966, 66, 199–241.
M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R.
Forrest, Highly efficient phosphorescent emission from organic electroluminescent
devices, Nature, 1998, 395, 151–154.
3
R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D.
D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund and W. R. Salaneck,
Electroluminescence in conjugated polymers, Nature, 1999, 397, 121–128.
A. Köhler and H. Bässler, Triplet states in organic semiconductors, Mater. Sci. Eng. R
Reports, 2009, 66, 71–109.
G. Lakhwani, A. Rao and R. H. Friend, Bimolecular Recombination in Organic
Photovoltaics, Annu. Rev. Phys. Chem., 2014, 65, 557–581.
A. Rao, P. C. Y. Chow, S. G??linas, C. W. Schlenker, C. Z. Li, H. L. Yip, A. K. Y. Jen, D.
4
5
6
18

Page 19 of 23
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C8CP01023D
S. Ginger and R. H. Friend, The role of spin in the kinetic control of recombination in
organic photovoltaics, Nature, 2013, 500, 435–439.
7
S. Westenhoff, I. A. Howard, J. M. Hodgkiss, K. R. Kirov, H. A. Bronstein, C. K.
Williams, N. C. Greenham and R. H. Friend, Charge Recombination in Organic
Photovoltaic Devices with High OpenꢀCircuit Voltages, J. Am. Chem. Soc., 2008, 130,
13653–13658.
8
9
Y. Shao and Y. Yang, Efficient Organic Heterojunction Photovoltaic Cells Based on
Triplet Materials, Adv. Mater., 2005, 17, 2841–2844.
S. R. Yost, E. Hontz, S. Yeganeh and T. Van Voorhis, Triplet vs Singlet Energy Transfer
in Organic Semiconductors: The Tortoise and the Hare, J. Phys. Chem. C, 2012, 116,
17369–17377.
10
H. Ohkita, S. Cook, Y. Astuti, W. Duffy, M. Heeney, S. Tierney, I. McCulloch, D. D. C.
Bradley and J. R. Durrant, Radical ion pair mediated triplet formation in polymer–
fullerene blend films, Chem. Commun., 2006, 0, 3939–3941.
11
12
M. J. Y. Tayebjee, A. A. GrayꢀWeale and T. W. Schmidt, Thermodynamic Limit of
Exciton Fission Solar Cell Efficiency, J. Phys. Chem. Lett., 2012, 3, 2749–2754.
E. R. Bittner, V. Lankevich, S. Gelinas, A. Rao, D. A. Ginger and R. H. Friend, How
disorder controls the kinetics of triplet charge recombination in semiconducting organic
polymer photovoltaics, Phys. Chem. Chem. Phys., 2014, 16, 20321–20328.
J. Zhao, K. Xu, W. Yang, Z. Wang and F. Zhong, The triplet excited state of Bodipy:
formation, modulation and application, Chem. Soc. Rev., 2015, 44, 8904–8939.
J.ꢀS. Yang and C.ꢀJ. Lin, Fate of photoexcited transꢀaminostilbenes, J. Photochem.
Photobiol. A Chem., 2015, 312, 107–120.
13
14
15
J. A. SanchezꢀRodriguez, A. Mohamadzade, S. Mai, B. Ashwood, M. Pollum, P.
Marquetand, L. Gonzalez, C. E. CrespoꢀHernandez and S. Ullrich, 2ꢀThiouracil
intersystem crossing photodynamics studied by wavelengthꢀdependent photoelectron and
transient
absorption
spectroscopies,
Phys.
Chem.
Chem.
Phys.,
,
DOI:10.1039/C7CP02258A.
16
17
N. Adarsh, M. Shanmugasundaram, R. R. Avirah and D. Ramaiah, Aza BODIPY
Derivatives: Enhanced Quantum Yields of Triplet Excited States and the Generation of
Singlet Oxygen and their Role as Facile Sustainable Photooxygenation Catalysts, Chem.
Eur. J., 2012, 18, 12655–12662.
P. Spenst, R. M. Young, M. R. Wasielewski and F. Wurthner, Guest and solvent
modulated photoꢀdriven charge separation and triplet generation in a perylene bisimide
cyclophane, Chem. Sci., 2016, 7, 5428–5434.
18
19
R. A. Vogt, M. A. Peay, T. G. Gray and C. E. CrespoꢀHernández, ExcitedꢀState Dynamics
of (Organophosphine)gold(I) Pyrenyl Isomers, J. Phys. Chem. Lett., 2010, 1, 1205–1211.
S. Jockusch, M. Porel, V. Ramamurthy and N. J. Turro, CIDEP from a Polarized Ketone
Triplet State Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous
Solution, J. Phys. Chem. Lett., 2011, 2, 2877–2880.
20
21
A. I. Burshtein and K. L. Ivanov, The crucial role of triplets in photoinduced charge
transfer and separation, Phys. Chem. Chem. Phys., 2002, 4, 4115–4125.
M. C. Sajimon, D. Ramaiah, C. H. Suresh, W. Adam, F. D. Lewis and M. V George,
Photochromic Dibenzobarrlenes:ꢁ LongꢀLived Triplet Biradical Intermediates, J. Am.
Chem. Soc., 2007, 129, 9439–9445.
22
S. Henkel, P. Costa, L. Klute, P. Sokkar, M. FernandezꢀOliva, W. Thiel, E. Sanchezꢀ
19

Physical Chemistry Chemical Physics
Page 20 of 23
View Article Online
DOI: 10.1039/C8CP01023D
Garcia and W. Sander, Switching the Spin State of Diphenylcarbene via Halogen
Bonding, J. Am. Chem. Soc., 2016, 138, 1689–1697.
23
24
25
26
27
C. Zhang and J. Zhao, Triplet excited state of diiodoBOPHY derivatives: preparation,
study of photophysical properties and application in tripletꢀtriplet annihilation
upconversion, J. Mater. Chem. C, 2016, 4, 1623–1632.
G. G. Giachino and D. R. Kearns, Nature of the External Heavy Atom Effect on
Radiative and Nonradiative Singlet–Triplet Transitions, J. Chem. Phys., 1970, 52, 2964–
2974.
H. Saigusa and T. Azumi, Internal heavy atom effect on the triplet spin sublevels of the
lowest triplet state of naphthalene. I. Radiative and nonradiative decays of the spin
sublevels of 1 halonaphthalenes, J. Chem. Phys., 1979, 71, 1408–1413.
Y. Chen, J. Zhao, L. Xie, H. Guo and Q. Li, Thienylꢀsubstituted BODIPYs with strong
visible lightꢀabsorption and longꢀlived triplet excited states as organic triplet sensitizers
for tripletꢀtriplet annihilation upconversion, RSC Adv., 2012, 2, 3942–3953.
Y. Niko, Y. Hiroshige, S. Kawauchi and G. Konishi, Additional Insights into
Luminescence Process of Polycyclic Aromatic Hydrocarbons with Carbonyl Groups:
Photophysical Properties of Secondary NꢀAlkyl and Tertiary N,NꢀDialkyl Carboxamides
of Naphthalene, Anthracene, and Pyrene, J. Org. Chem., 2012, 77, 3986–3996.
H. J. van Ramesdonk, B. H. Bakker, M. M. Groeneveld, J. W. Verhoeven, B. D. Allen, J.
P. Rostron and A. Harriman, Ultrafast Intersystem Crossing in 9,10ꢀAnthraquinones and
Intramolecular Charge Separation in an AnthraquinoneꢀBased Dyad, J. Phys. Chem. A,
2006, 110, 13145–13150.
28
29
R. A. Vogt, C. Reichardt and C. E. CrespoꢀHernández, ExcitedꢀState Dynamics in Nitroꢀ
Naphthalene Derivatives: Intersystem Crossing to the Triplet Manifold in Hundreds of
Femtoseconds, J. Phys. Chem. A, 2013, 117, 6580–6588.
30
31
S. Murudkar, A. K. Mora, P. K. Singh and S. Nath, Origin of Ultrafast Excited State
Dynamics of 1ꢀNitropyrene, J. Phys. Chem. A, 2011, 115, 10762–10766.
J. S. Zugazagoitia, E. ColladoꢀFregoso, E. F. PlazaꢀMedina and J. Peon, Relaxation in the
Triplet Manifold of 1ꢀNitronaphthalene Observed by Transient Absorption Spectroscopy,
J. Phys. Chem. A, 2009, 113, 805–810.
32
X. Cheng, F. Li, S. Han, Y. Zhang, C. Jiao, J. Wei, K. Ye, Y. Wang and H. Zhang,
Emission behaviors of unsymmetrical 1,3ꢀdiarylꢀβꢀdiketones: A model perfectly
disclosing the effect of molecular conformation on luminescence of organic solids, Sci.
Rep., 2015, 5, 9140.
33
34
M. B. Smith and J. Michl, Singlet Fission, Chem. Rev., 2010, 110, 6891–6936.
Y. Harada, C. Okabe, T. Kobayashi, T. Suzuki, T. Ichimura, N. Nishi and Y. Z. Xu,
Ultrafast Intersystem Crossing of 4ꢀThiothymidine in Aqueous Solution, J. Phys. Chem.
Lett., 2010, 1, 480–484.
35
36
37
M. Richter, P. Marquetand, J. GonzálezꢀVázquez, I. Sola and L. González, Femtosecond
Intersystem Crossing in the DNA Nucleobase Cytosine, J. Phys. Chem. Lett., 2012, 3,
3090–3095.
R. P. Sabatini, T. M. McCormick, T. Lazarides, K. C. Wilson, R. Eisenberg and D. W.
McCamant, Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar
Hydrogen Production, J. Phys. Chem. Lett., 2011, 2, 223–227.
O. Yushchenko, G. Licari, S. MosqueraꢀVazquez, N. Sakai, S. Matile and E. Vauthey,
Ultrafast IntersystemꢀCrossing Dynamics and Breakdown of the Kasha–Vavilov’s Rule of
20

Page 21 of 23
Physical Chemistry Chemical Physics
View Article Online
DOI: 10.1039/C8CP01023D
Naphthalenediimides, J. Phys. Chem. Lett., 2015, 6, 2096–2100.
38
39
M. Marazzi, S. Mai, D. RocaꢀSanjuán, M. G. Delcey, R. Lindh, L. González and A.
Monari, Benzophenone Ultrafast Triplet Population: Revisiting the Kinetic Model by
SurfaceꢀHopping Dynamics, J. Phys. Chem. Lett., 2016, 7, 622–626.
S. Aloïse, C. Ruckebusch, L. Blanchet, J. Réhault, G. Buntinx and J. P. Huvenne, The
benzophenone S1(n,π*) →T1(n,π *) states intersystem crossing reinvestigated by ultrafast
absorption spectroscopy and multivariate curve resolution, J. Phys. Chem. A, 2008, 112,
224–231.
40
41
K. Nagarajan, A. R. Mallia, V. S. Reddy and M. Hariharan, Access to Triplet Excited
State in CoreꢀTwisted Perylenediimide, J. Phys. Chem. C, 2016, 120, 8443–8450.
A. M. Philip, A. R. Mallia and M. Hariharan, Prolonged Charge Separated States in
Twisted Stacks of AllꢀCarbon Donor and Acceptor Chromophores, J. Phys. Chem. Lett.,
2016, 7, 4751–4756.
42
S. K. Rajagopal, A. R. Mallia and M. Hariharan, Enhanced intersystem crossing in
carbonylpyrenes, Phys. Chem. Chem. Phys., 2017, 19, 28225–28231.
D. F. Eaton, No Title, Pure Appl. Chem., 1988, 60, 1107 – 1114.
I. B. Berlman, in Handbook of Fluorescence Spectra of Aromatic Molecules (Second
Edition), Academic Press, 1971, pp. 107–415.
43
44
45
46
47
D. L. Sackett and J. Wolff, Nile red as a polarityꢀsensitive fluorescent probe of
hydrophobic protein surfaces, Anal. Biochem., 1987, 167, 228–234.
H. Vollmann, H. Becker, M. Corell and H. Streeck, Beiträge zur Kenntnis des Pyrens und
seiner Derivate, Justus Liebigs Ann. Chem., 1937, 531, 1–159.
J. J. Snellenburg, S. P. Laptenok, R. Seger, K. M. Mullen and I. H. M. van Stokkum,
Glotaranꢁ: A Java ꢀBased Graphical User Interface for the R Package TIMP, J. Stat.
Softw., 2012, 49, 1–22.
48
49
50
J.ꢀD. Chai and M. HeadꢀGordon, Longꢀrange corrected hybrid density functionals with
damped atom–atom dispersion corrections, Phys. Chem. Chem. Phys., 2008, 10, 6615.
A. Schäfer, C. Huber and R. Ahlrichs, Fully optimized contracted Gaussian basis sets of
triple zeta valence quality for atoms Li to Kr, J. Chem. Phys., 1994, 100, 5829–5835.
M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson and E. Al., Gaussian 09, revision
A.02, 2009.
51
X. Gao, S. Bai, D. Fazzi, T. Niehaus, M. Barbatti and W. Thiel, Evaluation of SpinꢀOrbit
Couplings with LinearꢀResponse TimeꢀDependent Density Functional Methods, J. Chem.
Theory Comput., 2017, 13, 515–524.
52
53
X. Cai, M. Hara, K. Kawai, S. Tojo and T. Majima, Naphthalene in the higher triplet
excited state, Chem. Commun., 2003, 222–223.
C. V Kumar, L. Qin and P. K. Das, Aromatic thioketone triplets and their quenching
behaviour towards oxygen and diꢀtꢀbutylnitroxy radical. A laserꢀflashꢀphotolysis study, J.
Chem. Soc. Faraday Trans. 2 Mol. Chem. Phys., 1984, 80, 783–793.
R. B. Cundall and W. Tippett, Intersystem crossing in irradiated benzene and toluene,
Trans. Faraday Soc., 1970, 66, 350–362.
A. R. Horrocks and F. Wilkinson, Triplet State Formation Efficiencies of Aromatic
Hydrocarbons in Solution, Proc. R. Soc. London. Ser. A. Math. Phys. Sci., 1968, 306,
257–273.
54
55
56
E. J. Land, Absorption spectrum of the triplet state of duroquinone, Trans. Faraday Soc.,
21

Physical Chemistry Chemical Physics
Page 22 of 23
View Article Online
DOI: 10.1039/C8CP01023D
1969, 65, 2815–2822.
F. V. R. Neuwahl and P. Foggi, Direct Observation of S –S Internal Conversion in
57
58
2
1
Pyrene by Femtosecond Transient Absorption, Laser Chem., 1999, 19, 375–379.
S. Y. Lee, T. Yasuda, Y. S. Yang, Q. Zhang and C. Adachi, Luminous Butterflies:
Efficient Exciton Harvesting by Benzophenone Derivatives for FullꢀColor Delayed
Fluorescence OLEDs, Angew. Chemie Int. Ed., 2014, 53, 6402–6406.
C. Parkanyi, E. J. Baum, J. Wyatt and J. N. Pitts, Physical properties and chemical
reactivity of alternant hydrocarbons and related compounds. XVI. Electronic absorption
and phosphorescence spectra of aryl phenyl ketones, J. Phys. Chem., 1969, 73, 1132–
1138.
59
60
61
N. Ohmori, T. Suzuki and M. Ito, Why does intersystem crossing occur in isolated
molecules of benzaldehyde, acetophenone, and benzophenone?, J. Phys. Chem., 1988, 92,
1086–1093.
S. Yabumoto, S. Sato and H. Hamaguchi, Vibrational and electronic infrared absorption
spectra of benzophenone in the lowest excited triplet state, Chem. Phys. Lett., 2005, 416,
100–103.
62
63
M. A. ElꢀSayed, Triplet state. Its radiative and nonradiative properties, Acc. Chem. Res.,
1968, 1, 8–16.
K. Schmidt, S. Brovelli, V. Coropceanu, D. Beljonne, J. Cornil, C. Bazzini, T. Caronna,
R. Tubino, F. Meinardi, Z. Shuai and J. L. Brédas, Intersystem crossing processes in
nonplanar aromatic heterocyclic molecules, J. Phys. Chem. A, 2007, 111, 10490–10499.
22

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