N-Rich electron acceptors: triplet harvesting in multichromophoric pyridoquinoxaline and pyridopyrazine-based organic emitters

Article abstract of DOI:10.1039/d0tc02520h

Controlling the nonradiative deactivation of triplet states and tuning the singlet-triplet energy gap are the major challenges in developing materials that exhibit thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP). Additionally, the conformational flexibility/rigidity around the donor-acceptor bond and the π-spacer unit significantly alters the charge transfer efficiency, subsequently impacting the relative propensity of TADF and RTP. Herein, we propose a new class of multichromophoric tridonor-acceptor (D₃-A) compounds with rigid and flexible π-spacers having N-rich pyridoquinoxaline and pyridopyrazine acceptor cores, respectively. The molecule with carbazole donorsmetato the quinoxaline nitrogen of the rigid pyridoquinoxaline core exhibits TADF, whereas the variation of the linkage position of carbazole to the pyridoquinoxaline as well as the twisted and flexible pyridopyrazine core shows predominantly RTP due to a relatively higher singlet-triplet energy gap. Increasing the donor strength with phenoxazine in the pyridopyrazine system leads to TADF and RTP simultaneously. Furthermore, we demonstrate the promising scope of the all-organic triplet harvesting materials in solid-state security encryption.

Full text of DOI:10.1039/d0tc02520h

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ARTICLE
N-rich electron acceptors: triplet harvesting in multichromophoric
pyridoquinoxaline and pyridopyrazine-based organic emitters
Bahadur Sk,a Samarth Sharma,a Anto James,a,b Subhankar Kundu,a and Abhijit Patra*a
Received 00th January 20xx,
Accepted 00th January 20xx
Controlling the nonradiative deactivation of triplet states and tuning the singlet-triplet energy gap are the major challenges
to develop materials exhibiting thermally activated delayed fluorescence (TADF) and room temperature phosphorescence
(RTP). Additionally, the conformational flexibility/ rigidity around the donor-acceptor bond and -spacer unit significantly
alters the charge transfer efficiency, subsequently impacting the relative propensity of TADF and RTP. Herein, we propose a
new class of multichromophoric tridonor-acceptor (D3-A) compounds with rigid and flexible π-spacer having N-rich
pyridoquinoxaline and pyridopyrazine acceptor core, respectively. The molecule with carbazole donors at meta to
quinoxaline nitrogen of rigid pyridoquinoxaline core exhibits TADF. Whereas, the variation of the linkage position of
carbazole to pyridoquinoxaline as well as twisted and flexible pyridopyrazine core show predominantly RTP due to relatively
higher singlet-triplet energy gap. Increasing the donor strength with phenoxazine in pyridopyrazine system leads to TADF
and RTP simultaneously. Further, we demonstrate the promising scope of all-organic triplet harvesting materials in solid-
state security encryption.
DOI: 10.1039/x0xx00000x
coupling (SOC) through the mixing of singlet and triplet
electronic excited states.27,28 Consequently, a high rate of
intersystem crossing (ISC) along with stable triplet excited
states leads to persistent or ultralong RTP in certain D-A
compounds.18,24,29
Introduction
Smart organic donor-acceptor (D-A) based multichromophores
have gained profound interest in recent years owing to the
efficient harvesting of the triplet excited states.1-9 The triplet
state could be emissive by thermally activated delayed
fluorescence (TADF) or the radiative decay at ambient
conditions, commonly known as room temperature
phosphorescence (RTP).10-15 Such emitters are making their way
in a wide variety of photonics applications like organic light-
emitting diodes (OLEDs),2,16 sensors,17 security devices,18-20
time-resolved luminescence imaging,21,22 etc. The key
challenges are to control the dynamic collisional quenching of
triplet excited state and lowering the energy gap between the
excited singlet (S1) and triplet (T1) states (ΔEST).18,23-26 The lower
ΔEST value promotes the photon upconversion from excited
triplet states to the emissive singlet states through reverse
intersystem crossing (RISC) leading to TADF.2 The excited-state
intramolecular charge transfer (ICT) in donor-acceptor (D-A)
molecules with spatially-separated the highest occupied
molecular orbitals (HOMOs) and the lowest unoccupied
molecular orbitals (LUMOs) is conducive to low ΔEST values.2
The charge transfer interactions also favour a strong spin-orbit
The charge transfer interactions depend not only on the
molecular geometry but also vary with the relative orientation
of donor and acceptor units.8,30-33 ICT significantly differs among
positional isomers with different substitution positions of
donors on the acceptor core.34 Chen et al. demonstrated the
switching between TADF and RTP in phenothiazine-
benzophenone based D-A compounds invoking the concept of
twisted intramolecular charge transfer (TICT) and planarized
intramolecular charge transfer (PICT).8 Bryce and coworkers
observed RTP in phenothiazine–dibenzothiophene-S,S-dioxide
based D-A-D pair by introducing steric hindrance on the donor
units.12 Tunable delayed emission from green to red by
increasing the number of donor units from D-A, D2-A, to D3-A
architecture by controlling the ICT effect was demonstrated by
Xie et al.35 As reported by Reineke and coworkers, additional
nitrogen atoms enhance the SOC, and subsequently, the rate of
ISC.13 Amidst several recent reports, a unified molecular design
strategy unraveling the abstruse role of the N-rich central
acceptor core along with conformational analysis in
multichromophoric organic emitters for tunable and efficient
TADF and RTP is still intriguing albeit challenging.
a. Department of Chemistry, Indian Institute of Science Education and Research
Bhopal, Bhauri, Bhopal, Madhya Pradesh, 462066, India.
E-mail: abhijit@iiserb.ac.in
Herein, we have examined the role of N-rich acceptor units
along with π-spacers in terms of flexibility/ rigidity and twisted/
planar geometry on emission properties of D3-A molecules at
different timescale. The central core of pyridoquinoxaline and
pyridopyrazine serve as facile acceptor units owing to the
multiple nitrogen centers, which also favor the ISC (Fig. 1, Table
b. Department of Chemistry, Indian Institute of Science Education and Research
Thiruvananthapuram Maruthamala PO, Vithura, Kerala 695551, India.
† Electronic Supplementary Information (ESI) available: Details of synthesis and
characterization, fluorescence decays, time-resolved emission spectra, crystal
structure and computational investigations, and supporting videos for RTP. See
DOI: 10.1039/x0xx00000x
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Fig. 1 Design strategy of triplet harvesting materials: (a) a comparative account of the intrinsic electron-accepting capability (LUMO level) of various acceptor units used for triplet
harvesting including the new acceptor cores of dibenzopyridoquinoxaline (PQ) and diphenylpyridopyrazine (PZ), estimated employing density functional theory (DFT) at the B3LYP/6-
31G(d,p) level. (b) Schematic illustration of our approach representing planar dibenzopyridoquinoxaline (T-shaped: PQT and V-shaped: PQV) and twisted diphenylpyridopyrazine
(PZV) acceptor units, and (c) the molecular structures of tridonor-acceptor (D3-A) compounds with carbazole (Cz) and phenoxazine (PO) as donor units: PQTCz, PQVCz, PZVCz and
PZVPO; D-A linkage, π-spacer, and donor strength are varied.
S1, S2, ESI†). Furthermore, the adjacent phenyl spacers also and 3,6-positions of PQ lead to T and V-shaped molecular
offer a distinctive synthetic advantage in regulating the geometry.34
molecular flexibility/ rigidity of the central core. The molecule
We introduce multiple electron donor units covalently
with carbazole donors at meta to quinoxaline nitrogen of rigid linked with central PQ and PZ core for the development of a new
pyridoquinoxaline core showed enhanced TADF due to the low series of multichromophoric D3-A molecules. Considering the
ΔEST. However, the higher ΔEST due to variation of the linkage efficient electron-donating properties, high thermal stability
position of carbazole (para to quinoxaline nitrogen) to and low-cost, carbazole and phenoxazine were chosen as the
pyridoquinoxaline and pyridopyrazine core resulted in donor units.41 Palladium-catalyzed Buchwald-Hartwig cross-
predominant RTP. The simultaneous TADF and RTP were coupling reactions (C-N coupling) of tribromo-PQT, PQV, and
achieved employing phenoxazine as donors in pyridopyrazine PZV with carbazole and phenoxazine resulted in PQTCz, PQVCz,
system.
PZVCz, and PZVPO (Fig. 1c, Scheme 1, S1-S6, ESI†). All the
compounds were characterized by NMR spectroscopic and
mass spectrometric analysis (Fig. S48-S70, ESI†).
Results and discussion
Quinoxaline is a well-known electron acceptor unit due to the
presence of two electronegative sp2 hybridized nitrogen
atoms.36-38 Phenazine also possesses similar electron-deficient
characteristics as quinoxaline.39,40 The presence of another
nitrogen-containing heterocycle to the quinoxaline unit leads to
the pyridoquinoxaline motif, with better electron accepting
capability than quinoxaline or phenazine (Table S1, ESI†).
Similarly, pyridopyrazine is more electron deficient than
pyrazine (Table S1, ESI†). The suitable inclusion of π-spacers
(phenyl groups) leads to the development of
dibenzopyridoquinoxaline and diphenylpyridopyrazine (Fig. 1a,
b). The phenyl -spacers do not alter the electron-withdrawing
capabilities of pyridoquinoxaline or pyridopyrazine, rather allow
synthetic flexibility and a range of viable options to introduce
the donor substituents. Henceforth, dibenzopyridoquinoxaline
and diphenylpyridopyrazine are abbreviated as PQ and PZ,
respectively. The significant difference between PQ and PZ core
lies in the molecular flexibility/ rigidity induced by the linkage
pattern of phenyl spacers (Fig. 1b). Donor substitutions at 2,7
Scheme 1 Schematic illustration for synthetic pathways of (a) pyridoquinoxaline based
PQTCz, PQVCz, and (b) pyridopyrazine based PZVCz and PZVPO compounds.
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Fig. 2 Time-resolved emission spectra (TRES) from nanosecond to millisecond timescale: normalized emission spectra of D3-A compounds in toluene at room temperature (a-d), in
toluene at 77 K (e-h) and in polymethyl methacrylate (PMMA) films at room temperature (RT: 22-25oC, i-l), respectively for (a, e, i) PQTCz (a Inset: spectrum at 1.5 ns depicting locally
excited state (LE) emission), (b, f, j) PQVCz, (c, g, k) PZVCz, and (d, h, l) PZVPO. Spectra were recorded using ex = 465 nm [steady state (SS), 450 W Xe lamp], ex = 468 nm [picosecond
(ps) diode laser] and 461 nm [millisecond (ms) multi-channel scanning (MCS) spectraLED] for PQTCz, PQVCz and PZVPO, respectively and ex = 420 nm (SS, 450 W Xe lamp), ex = 410
nm (ps diode laser) and 417 nm (ms MCS spectraLED) for PZVCz. The solid bars indicate the similarity of emission peak positions of D3-A compounds at different timescale in solution
at RT, 77K, and in polymer film.
D3-A compounds absorb predominantly in the UV-region S2, ESI†). The Lippert-Mataga plots, as shown in Fig. S7, ESI†,
along with a prominent broad absorption band in the visible further substantiate the ICT behaviour (Table S4, ESI†).
region (Fig. S1, S2, ESI†). The lower-energy absorption bands are
The fluorescence decays were measured in the solvents of
attributed to the extended -conjugation between the donor varying polarity. A biexponential fluorescence decay for all the
and the acceptor units.34,42,43 Whereas, the higher energy compounds at ambient conditions is possibly due to the
absorption bands are ascribed to -* transitions. Strong emission from the locally excited (LE) and the charge transfer
fluorescence with broad emission bands was observed for state (Fig. S8, Table S5-S7, ESI†).42,46 Further, the emission
PQTCz, PQVCz, and PZVCz in toluene (5 M) at room decays at 350 to 750 nm with an interval of 5 nm were
temperature (Fig. 2a-c, Table S4-S7, ESI†). A relatively weak measured to obtain the decay associated emission spectra for a
emission was noticeable for PZVPO in toluene at room particular timescale. The reconstructed time-resolved emission
temperature (Fig. 2d, strong D-A pair, dark TICT, vide infra).34,42 spectra (TRES) of PQTCz at 1.5 ns and 18.9 ns show two distinct
On increasing the solvent polarity from toluene to spectral profiles (Fig. 2a). The emission spectrum obtained at
dichloromethane, a gradual bathochromic shift in the emission 1.5 ns is similar to that of carbazole emission in toluene (Fig. 2a,
maxima was observed for all compounds (Fig. S4, Table S4, S9, 10, ESI†). It refers to the LE emission.46 The emission at 18.9
ESI†). The emission maxima were found to be independent of ns resembles the steady-state emission and is attributed to the
the excitation wavelengths (Fig. S5, ESI†). The spectral charge transfer fluorescence (Fig. 2a, S9, ESI†). Similarly, the
broadening of the emission bands with increasing solvent TRES of PQVCz, PZVCz, and PZVPO in toluene at ambient
polarity and positive solvatochromic behaviour are the two conditions are shown in Fig. 2b-d (Fig. S11-13, section V, ESI†).
characteristic features indicative of strong ICT in the D3-A
The phosphorescence spectra and associated decays of D3-
compounds.34,42,44,45 Notably, the Stokes shift in PQTCz (m- A compounds were recorded at an ultralow temperature in
substitution) is higher than PQVCz and PZVCz (p-substitution) in liquid nitrogen (77 K) using 450 W Xe lamp [steady-state (ss)],
all solvents (Table S5-S7, ESI†), which is in agreement with the picosecond (ps) diode laser and millisecond (ms) multi-channel
more facile ICT in the former.34 The largest Stokes shift (5245 scanning (MCS) spectraLED as the excitation source. The steady-
cm-1, toluene) exhibited by PZVPO among the D3-A compounds state spectra of D3-A compounds at 77 K showed emission
suggests the efficient charge transfer interactions due to bands with multiple humps (Fig. 2e-h) and were significantly
stronger donor capacity of phenoxazine over carbazole (Table different compared to those obtained at room temperature
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Fig. 3 (a) Emission spectra with varying laser power (pulse width = 5 - 7 ns) of PMMA films of PQTCz at ambient conditions. Spectra were recorded using intensified charge-coupled
device (iCCD) camera (ex = 355 nm). (b) The emission decay kinetics (ex = 355 nm, em = 570 nm) of PQTCz-PMMA film with varying laser power at ambient conditions. (c) The linear
fitting of the emission intensity of PMMA films of D3-A compounds as a function of excitation power. (d) The molecular structure of PZVPO obtained from the single crystal X-ray
diffraction analysis; significant dihedral angles depicting twisted geometry. (e) The intra- and intermolecular interactions in a single crystal of PZVPO; the strong intermolecular C–
H⋯O interactions (2.63-2.86 Å) between the phenoxazine oxygen and CH of π-spacer phenyl group is indicated. The dimer of PZVPO is further stabilized by the intermolecular
interactions between the π-spacer phenyl groups [CH⋯HC, 2.32 Å, CH···π, 2.84 Å; grey (C), blue (N), red (O). (f) The digital images of PQVCz, PZVCz, and PZVPO-doped PMMA films
at room temperature under the excitation of UV-lamp (ex = 365 nm), and the photographs after instant turning off the UV irradiation.
(Fig. 2a-d). The biexponential emission decays at 77 K at fast
We doped the D3-A compounds in the polymethyl
timescale were similar to that observed at room temperature methacrylate (PMMA) matrix to ascertain the triplet harvesting
due to the fluorescence from LE and charge transfer states (Fig. characteristics in the rigid media. PMMA films of all the
2a-d, S15a-S18a, ESI†). However, the decay measurements at compounds emit (steady-state) in the distinct regions of the
slower timescale at 77 K revealed ultralong phosphorescence visible spectrum (Fig. 2i-l), ranging from cyan (PZVCz) to green
ranging from few milliseconds to 0.5 seconds (Fig. 2e-h, Fig. (PQTCz and PQVCz) to red (PZVPO). The decays at the
S15b-S18b, ESI†).
nanosecond timescale monitored at the emission maxima
The phosphorescence emission maxima of PQTCz were indicate multiple emitting species at room temperature (Fig.
observed at 585 and 590 nm with decay time 400 and 195 ms, S26a-S29a, ESI†). It could be due to the emission from the
respectively (Fig. 2e, S15b, ESI†). PQVCz emits with peaks at 565 singlet (LE, ICT), triplet (T1) states, and aggregates (Fig. S25,
and 590 nm with decay time 465 and 221 ms, respectively (Fig. Table S10, ESI†).13,18,25,47 PQTCz film at ambient conditions
2f, S16b, ESI†). The phosphorescence emission peak maxima of exhibits emission peaks at 565 and 580 nm, with the decay time
PZVCz were noticeable at 535 and 545 nm with decay time 381 139 and 42 s, respectively (Fig. 2i, S26b, ESI†). The emission
and 225 ms, respectively (Fig. 2g, S17b, ESI†). The maximum at 565 nm obtained at μs timescale matches with the
phosphorescence decay time of PZVPO is 350 and 60 ms with a TRES at room temperature in toluene (18.9 ns), indicating TADF
peak centered at 585 and 595 nm, respectively (Fig. 2h, S18b, in polymer film (Fig. 2i and 2a). The TRES measured using
ESI†), which are very close to the fluorescence bands observed sensitive gated intensified charge-coupled device (iCCD)
in toluene at room temperature (585 and 610 nm, Fig. 2d). The camera further ascertain the TADF of PQTCz film (Fig. 3a, S31a,
multiexponential phosphorescence decays suggest more than ESI†).8,48 Unlike the toluene solution at ambient conditions or
one triplet charge-transfer state (3CT) due to the presence of 77 K, PQVCz film displays distinct longer component emission at
multiple donor units.12 The lower ΔEST values (<0.5 eV) of D3-A 525 and 550 nm with the decay time 414 and 135 s,
compounds based on experimental data indicate the possible respectively demonstrating RTP (Fig. 2j, S27b, S30, ESI†).
thermally activated RISC (Fig. S20, Table S8, ESI†). The long-lived
We anticipated that the D3-A compounds with flexible PZ
emission decay at room temperature with the oxygen-free core would exhibit long-lived emission. The peaks at 510 and
condition of D3-A compounds in toluene revealed the 545 nm with a respective decay time of 85 and 11 s of PZVCz
prominent delayed emission characteristics of PQTCz (Fig. S21, film (Fig. 2k, S28b, ESI†) resemble with phosphorescence peaks
S22, ESI†). However, no longer component emission was at 77 K (Fig. 2g). Thus, PZVCz in PMMA film exhibits RTP.
observed for PQVCz, PZVCz, and PZVPO under the same Similarly, PZVPO film shows long-lived decays with peaks at 570
conditions.
and 590 nm having decay time 18.9 and 3.8 μs, respectively (Fig.
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Fig. 4 Molecular-level picture of triplet harvesting in D3-A compounds: (a) PQTCz, (b) PQVCz, (c) PZVCz, and (d) PZVPO. The TDDFT-calculated electron density differences in the S1
퐸푆
and Tn states with an isosurface ( 0.0004) and ISC channels involving Tn states with the same transition configurations as in S1 (blue) within the energy range of 1  0.3 eV are
shown. H and L represent the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. Normalized emission spectra, and
schematic energy profile diagrams of D3-A compounds; the prompt fluorescence (PF), delayed fluorescence (DF), phosphorescence (PH at 77K) and room temperature
phosphorescence (RTP denoted as PH’) are displayed. (e) Demonstration of timescale encoding for security application of PZVCz-embedded PMMA film in combination with a cyan
emitting 6,7,8,9-tetrapropylpyrido[1,2-a]indole-10-carbaldehyde (TPIC, ~10 ns) fluorophore. On excitation (ex = 365 nm), the patterned security feature is unrecognizable due to
the strong cyan fluorescence from TPIC and PZVCz. The encrypted feature of ‘2020’ constructed with phosphorescent PZVCz can be identified easily by the naked eye after the
excitation is switched off; photographs were captured using a Canon EOS camera with 18-55 mm lens.
2l, S29b, ESI†). Further, the laser power-dependent emission region while the transition from S0 to S1 or T1 states.
spectra using iCCD camera and decays were recorded to Additionally, the plots of electron density differences between
elucidate the long-lived components of D3-A compounds in a S1 and T1 states indicate that varied distribution of electron
rigid polymer matrix (Fig. 3a, 3b, S32-S35, ESI†). A linear density in donors and acceptor units of PQTCz, PQVCz, and
correlation of emission intensity with laser power for all the PZVCz (Fig. S44a, ESI†). Whereas no such electron density
compounds signifies the unimolecular decay processes (Fig. 3c), differences were observed for PZVPO (Fig. S44a, ESI†). Because
ruling out the triplet-triplet annihilation. The long-lived decay in the singlet and triplet states are distributed throughout the
PZVPO film at ambient conditions considering crystal structure molecule in PZVPO (Fig. S44a, S44b, ESI†). The molecular
analysis (Fig. 3d, 3e), and quantum chemical calculations is orbitals involved in various electronic transitions within the
퐸
presumably due to mixed RTP and delayed fluorescence (vide energy range of 1  0.3 eV and their contributions are shown
푆
infra).
in Fig. 4a-4d (vide infra). Further, the natural transition orbital
Tunable fluorescence and RTP with the afterglow feature of (NTO) analysis was carried out to comprehend the electronic
D3-A compounds in polymer films were demonstrated through transitions.49 There was no direct overlap between the highest
the encryption of FML (Fig. 3f, written using PQVCz: F, PZVCz: occupied natural transition orbitals (HONTOs) and the lowest
M, and PZVPO: L). Greenish- orange, cyan, and red fluorescence unoccupied natural transition orbitals (LUNTOs) of D3-A
to yellowish-red, yellowish-green, and very weak red RTP, were molecules at S1 and T1 states (Fig. S45, ESI†). The results indicate
observed, respectively, from PQVCz, PZVCz, and PZVPO the facile excited-state charge transfer in D3-A molecules, as
embedded PMMA films (Fig. 3f). The fascinating observations, suggested by Chen and coworkers.49 Thus, the similar nature of
as mentioned above, were rationalized through time- isosurfaces and transition configurations of excited singlet and
dependent density functional theory (TDDFT) calculations, triplet states of D3-A compounds allow facile spin-orbit coupling
coupled with crystal structure investigations (Fig. 3d, 3e, 4a-d, leading to intersystem crossing (Fig 4a-d).
S38-45, Table S12-S16, ESI†). The plots of electron density
It is evident that the incorporation of multiple N atoms
differences of excited singlet (S1) and triplet (Tn) states with favors spin-forbidden singlet-triplet transitions according to El-
respect to the ground state (S0) are shown in Fig. 4a-4d. The Sayed’s rule.8,23 The computed energy levels of the lowest
퐸
퐸
= 1.99 eV) excited states
푇1
electron density shifts from the orange region to the cyan singlet ( = 2.12 eV) and triplet (
푆1
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of PQTCz are very close to the values obtained from the dye, 6,7,8,9-tetrapropylpyrido[1,2-a]indole-10-carbaldehyde
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experimental data (Fig. 4a, S20, Table S8, S14, ESI†). The lower (TPIC) under ambient conditions (Fig. 4e). TPIC exhibits similar
ΔEST (0.13 eV) due to the strong contribution from the charge strong fluorescence (max = 490 nm,  ~ 10 ns) as that of PZVCz
transfer excitations featuring the decoupled HOMO-LUMO without having any long-lived decay components.58 The color-
levels enables the photon upconversion through RISC (Fig. 4a, changing events were recorded using an image capturing Canon
S38, S43, ESI†). Thus, a prominent TADF was observed in the EOS camera (Fig. S47, ESI†). The film upon UV irradiation (ex
=
PQTCz film. Whereas, a higher ΔEST (0.29 eV) and a greater 365 nm) showed intense cyan fluorescence. A distinctly
number of energy transition channels (S1 to T1, T2, and T3) were contrasting greenish-yellow phosphorescence of PZVCz was
observed for PQVCz leading to phosphorescence at ambient observed by the naked eye after turning off the UV light source
conditions (Fig. 4b, S39, S43, Table S15, ESI†). Similarly, PZVCz (Fig. 4e, S47, ESI†). Thus, the encrypted information (‘2020’)
exhibits higher ΔEST (0.26 eV) and multiple ISC channels from PZVCz is visible multiple times through switching on/off
between S1 and Tn (Fig. 4c, S40, S43, Table S15, ESI†), and the UV light (see video file, ESI†).
phosphorescence was observed from PZVCz film.
The molecular structure of PZVPO obtained from crystal
structure analysis revealed a highly twisted geometry (Fig. 3d,
Conclusions
S46, Table S13, S16, ESI†). The dihedral angles between the
central PZ core and the phenyl linker groups are 144.77o and
118.79o, while the dihedral angle between the -spacers and
phenoxazine donor unit is close to 80o. The donor units are
connected directly to the acceptor core exhibiting a dihedral
angle of 127.35o. Furthermore, a pair of CH···π (2.84 Å) and
CH···HC (2.32 Å) intermolecular interactions between π-spacer
aromatic phenyl groups generate a dimer like twisted structure
along a axis (Fig. 3e). These strong intermolecular interactions
in the crystals of PZVPO could reduce energy loss through
nonradiative relaxation channels.20,50-52 Additionally, like
PQTCz, the similar nature of transition configurations and
isosurfaces of S1 and T1 in PZVPO increase the probability of ISC
leading to high triplet yield (Fig. 4a, 4d, S43, Table S15,
ESI†).15,27,53,54
The DFT-optimized excited-state structures of both PZVCz
and PZVPO indicate the twisted geometry (Table S13, ESI†). A
partial twist in the excited-state structure of PZVCz with the
dihedral angles 57, -123, and -135 was observed for π1-D1,
π2-D2, and PZ-D3, respectively. Whereas a more twisted and
near orthogonal D-A geometry was observed for PZVPO in the
excited state with the dihedral angles 91, 91, and -91 for π1-
D1, π2-D2, and PZ-D3, respectively (Table S13, ESI†). The
introduction of the strong donor phenoxazine in PZVPO led to
more twisted structure and small ΔEST as compared to carbazole
based PZVCz.55 Similarly, the large twist angle in phenoxazine
based D-A pair as compared to carbazole as a donor was
reported by Oi and coworkers.56 Further, the twisted structure
of PZVPO is due to the steric hindrance of the hydrogen atoms
at 1- and 9-positions of phenoxazine (Scheme S7). Whereas, the
steric factor is less effective in the case of hydrogens at 1- and
8-positions of carbazole in PZVCz.56 Thus, the very low ΔEST
value (0.02 eV) owing to the strong charge transfer interactions
coupled with long-lived triplet state contributed to the delayed
fluorescence and radiative triplet emission in the rigid matrix at
ambient conditions in PZVPO (Fig. 4d, S36d, Table S14, ESI†).
Temperature-dependent steady-state and time-resolved
emission spectra analysis unambiguously ascertain the long-
lived decay processes of D3-A compounds (Fig. 4a-d, S36, S37,
Table S11, ESI†).16,57
In summary, we developed a new class of multichromophoric
tridonor-acceptor compounds with rigid and flexible -
spacers based on pyridoquinoxaline and pyridopyrazine
acceptor core, respectively. A strategy for tuning the thermally
activated delayed fluorescence and room temperature
phosphorescence
by controlling
the charge
transfer interactions through the variation of molecular
geometry, as well as the relative orientation of donor and
acceptor units, was demonstrated. Strong charge transfer and
the lower singlet-triplet energy gap upon the m-substitution of
donors on rigid pyridoquinoxaline core facilitate reverse
intersystem crossing leading to delayed fluorescence. Whereas,
the relatively weaker charge transfer and low lying stable triplet
states by the p-substitution of donors on both
pyridoquinoxaline and flexible pyridopyrazine acceptor core,
led to predominantly room temperature phosphorescence
(Table S17, ESI†). The potential afterglow feature of D3-A
compounds was shown for real-time security data encryption
and identification. The present study elucidating the subtle role
of donor-acceptor linkages coupled with the rigidity and
flexibility of the new N-rich central acceptor cores lays a
promising foundation for further development of all-organic
materials for triplet harvesting.
Conflicts of interest
The authors declare the following competing financial interest.
A
patent
application
has
been
filed
on
‘Dibenzopyridoquinoxaline based derivatives’ with A. Patra, B.
Sk, M. Sarkar, and S. Kundu as inventors.
Acknowledgments
The infrastructural support from IISERB, financial support from
BRNS, DAE (No. 37(2)/14/06/2016BRNS/37020), and Council of
Scientific and Industrial Research (CSIR), New Delhi, (No.
01(2878)/17/EMR-II) is gratefully acknowledged. BSk and SK
thank IISERB and UGC, respectively, for fellowship. We thank
Ms. Ashima Negi and Dr. A. Gupta at IISERB for helping to record
decays using iCCD camera, Dr. Jogendar Singh at IISERB, and Dr.
J. A. Mondal at BARC for fruitful discussion.
Further, we developed a facile information encryption
pattern employing PZVCz thin film along with a cyan emitting
6 | J. Name., 2012, 00, 1-3
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28 L. Huang, L. Liu, X. Li, H. Hu, M. Chen, Q. Yang, Z.VMiewa AarnticdleXO.nJliinae,
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DOI: 10.1039/D0TC02520H
Graphical Abstract - Table of Content
“N-rich electron acceptors: triplet harvesting in multichromophoric pyridoquinoxaline and
pyridopyrazine-based organic emitters”
By Bahadur Sk, Samarth Sharma, Anto James, Subhankar Kundu and Abhijit Patra*
The role of rigid and flexible N-rich electron acceptor cores for the harvesting of triplet states through
TADF and RTP was elucidated in a series of all-organic tridonor-acceptor compounds.