Revealing the internal heavy chalcogen atom effect on the photophysics of the dibenzo[a,j]phenazine-cored donor-acceptor-donor triad

Article abstract of DOI:10.1039/d1tc02635f

A new twisted donor-acceptor-donor (D-A-D) multi-photofunctional organic molecule comprising phenoselenazine as the electron donor (Ds) and dibenzo[a,j]phenazine (DBPHZ) as the electron acceptor (A) has been developed. The developed selenium-incorporated D-A-D compound features multi-color polymorphism, distinct mechanochromic luminescence, chemically-stimulated luminochromism, thermally-activated delayed fluorescence, and room-temperature phosphorescence. The internal heavy atom effect on the photophysical properties of the D-A-D system has been investigated through a comparison with the physicochemical properties of a previously developed sulfur analogue and a tellurium analogue.

Full text of DOI:10.1039/d1tc02635f

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ARTICLE
Revealing Internal Heavy Chalcogen Atom Effect on the
Photophysics of Dibenzo[a,j]phenazine-Cored Donor–Acceptor–
Donor Triad
Received 00th January 20xx,
Accepted 00th January 20xx
Shimpei Goto,a Yuya Nitta,a Nicolas Oliveira Decarli,b Leonardo Evaristo de Sousa,c Patrycja
Stachelek,d Norimitsu Tohnai,a Satoshi Minakata,a Piotr de Silva,*c Przemyslaw Data*be, and Youhei
Takeda*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new twisted donor–acceptor–donor (D–A–D) multi-photofunctional organic molecule comprising of phenoselenazine as
the electron-donors (Ds) and dibenzo[a,j]phenazine (DBPHZ) as the electron-acceptor (A) has been developed. The
developed selenium-incorporated D–A–D compound is featured with multi-color polymorphism, distinct mechanochromic
luminescence, chemically-stimulated luminochromism, thermally-activated delayed fluorescence, and room-temperature
phosphorescence. The internal heavy atom effect on the photophysical properties of the D–A–D system has been
investigated through the comparison with the physicochemical properties of a previously developed sulfur analogue and a
tellurium analogue.
catalysis sciences,5 polymer sciences,6 and materials sciences.7
INTRODUCTION
Selenium (Se) and Tellurium (Te), that are heavier congeners of
Sulfur (S), have a very similar and lower electronegativity (cP
with Sulfur (Se: 2.55, Te: 2.10, S: 2.58), respectively, and they
have slight larger covalent radius than that of sulfur (Se: 1.17 Å,
Te: 1.38 Å, S: 1.04 Å). Owing to such electronic and structural
In this context, planar
decorated with Se and/or Te at the main
frameworks or at the peripherals of the -systems have
attracted considerable attention as functional materials for
organic electronics. In addition to the intrinsic planar and rigid
p
-conjugated molecules and polymers
p
-conjugated
p
)
p
-systems, the presence of Se/Te atoms results in not only
strong intermolecular -orbital interactions but also
p
perturbation, the replacement of the
S atoms of an
electrostatic interactions (i.e., chalcogen-bonding) in the solid
states,8 allowing for efficient charge carrier transport. Thus,
some classes of compounds, such as seleno-/telluroarenes,
seleno-/tellurophenes, and their oligomeric and polymeric
organosulfur compound with Se or Te atoms leads to a drastic
change in reactivity and physicochemical property. Such
“internal heavy atom effect” have fascinated scientists, and
indeed a myriad of functional organo-selenium and -tellurium
compounds have been developed. Nowadays, they can find
diverse applications in a range of fields such as biology,1
medicinal chemistry,1,2 chemical biology,3 synthetic chemistry,4
derivatives,
have
emerged
as
promising
organic
semiconducting materials, and they find applications in organic
field-effect transistors (OFETs),9 organic photovoltaics
(OPVs),10 and organic thermoelectronics (OTEs).11
In stark contrast, organoselenium and organotellurium
compounds displaying luminescence in the solid states have
been less developed, and thereby the optoelectronic
applications thereof are in their infancy.12 The incorporation of
selenium and other heavy atoms such as halogens into a
luminogenic scaffold often generates the pair of singlet excited
a. Department of Applied Chemistry, Graduate School of Engineering, Osaka
University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan.
E-mail: takeda@chem.eng.osaka-u.ac.jp
b. Faculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100
Gliwice, Poland.
E-mail: przemyslaw.data@polsl.pl
c. Department of Energy Conversion and Storage, Technical University of Denmark,
Anker Engelunds Vej 301, 2800 Kongens Lyngby, Denmark.
E-mail: pdes@dtu.dk
state 1(n,
and 3(n,
orbit coupling (SOC) constant (
and Te (ca. 11.3 kcal/mol),13 the SOC matrix element (HSOC) for
the S1–T1 transition of a Se- or Te-containing -conjugated
p
p
*) (S1) and triplet excited state 3(
*). In conjunction with the intrinsically large spin-
SO) of Se (ca. 5.07 kcal/mol)
p,p p,p*)
*) (T1) or 1(
d. Durham University, Chemistry Department, South Road, Durham DH1 3LE, United
Kingdom.
z
e. Centre of Polymer and Carbon Materials, Polish Academy of Science, M. Curie-
Sklodowskiej 34, 41-819, Zabrze, Poland.
p
Electronic Supplementary Information (ESI) available: experimental procedures for
the syntheses of materials, spectroscopic data of new compounds, single crystal X-
ray crystallographilc data, UV-Vis absorption and photoluminescence spectra,
powder X-ray diffraction (PXRD) analysis data, differential scanning calorimetry
(DSC) profiles, thermogravimetric analysis (TGA) profiles, cyclic voltammograms,
theoretical calculation details, and the copies of NMR spectra of new compounds.
See DOI: 10.1039/x0xx00000x
compound becomes large, allowing the spin-forbidden S–T
transitions.14 Therefore, upon photo-excitation, the T1 state of
an organoselenium/tellurium molecule is promptly populated
through a rapid intersystem crossing (ISC) from the S1 state.15
With
a
well-studied
planar
p-conjugated
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organoselenium/tellurium frameworks, the singlet-triplet replacing Se for S of a TADF compound TXO1-TPA on ISC and
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DOI: 10.1039/D1TC02635F
energy splitting (DEST) is relatively large comparable with other rISC processes by developing a D–p–ASe type compound
acene analogues, and as mentioned-above, the intermolecular SeXO1-TPA (Fig. 1b).17 The detailed kinetic analysis revealed
interactions are strong in the solid states. As the results of that the Se affects both of ISC and rISC rates, enhancing the ISC
intrinsic large SOC induced by Se and Te, ISC from the S1 to T1 more than 250 times and the rISC 22 time faster than those of
is allowed, and thereby T1 state in the solids would be just the sulfur TADF emitter. Given these apparently contradicting
depopulated through non-irradiative pathways such as results, the position of Se atom on
thermal internal conversion (IC) via bond rotations and significantly affect the photophysical properties of the
molecular vibrations, and energy diffusion. selenium-embedded compounds. Most recently, Kim and Lee
Over the last few years, the internal heavy atom effects on developed DSe– –Dse type metal-free RTP compounds PSe1,
optoelectronic properties of triplet-harvesting PSe2, and PSe3 (Fig. 1c) for OLEDs. The authors claimed that
organoselenium emitters based on twisted D– * transitions involving differentially-oriented p orbitals
–A16,17 and D– the n-
–D18 scaffolds displaying thermally activated delayed on the Se atom (pz→pxy) significantly enhance the HSOC
fluorescence and room-temperature between the S0 and T1, thereby accelerating the radiation from
(TADF)19
phosphorescence (RTP)20 have been reported (Fig. 1). In 2019, the T1 state.18 Zhao and Huang reported the effect of
de Sa Pereira, Kukhta, and Lee investigated the effect of chalcogen atoms in the chalcogenophene– -phenylcarbazoles
p-electron systems
p
the
p
p
p
p
replacing Se for S atom of a D–
photophysics through
spectroscopic analysis of DSe–
p–A TADF emitter on the on the optoelectronic properties and revealed dual emission
a
sophisticated time-resolved behavior of fluorescence and RTP.21 He and co-workers
p
–A type compound PSeZ-TRZ developed a D–ASe–D type organic phosphorescent compound
(Fig. 1a) in comparison with that of its sulfur TADF emitter PTZ- for afterglow imaging and photodynamic therapy by taking the
TRZ.16 The analysis of the complicated decays of PSeZ-TRZ in advantage of long-lived triplet excited states.22 Therefore,
non-polar polymer matrix Zeonex® rationally explained the investigation on the effect of replacing Se/Te for S atoms of
independent radiative pathways of two conformers, i.e., quasi- TADF/RTP-active compounds on their photophysical properties
axial (ax) and quasi-equatorial (eq) (Fig. 1a). Most importantly, would provide profound insights into understanding the design
replacing Se for S does not affect the reverse ISC (rISC) in principles for balancing conflicting factors
DEST and SOC to
significant degree but so does room-temperature realize novel chalcogen-containing organic photofunctional
phosphorescence (RTP) efficiency of the donor. On the other materials.
hand, Credgington and Seferos investigated the effect of
Herein we disclose the development of novel twisted
donor-acceptor-donor (D–A–D) organic emitters PSeZ-DBPHZ-
a) DSe–π–A
b) ASe–π–D
O
Se
Ar
N
PSeZ
and
PTeZ-DBPHZ-PTeZ
based
on
a
new
Se
organoselnium/tellurium scaffold (DSe–A–DSe and DTe–A–DTe,
respectively) (Fig. 1d). Most importantly, the investigation of
their physicochemical properties revealed that PSeZ-DBPHZ-
PSeZ is multi-photofunctional material featured with
polymorphs-dependent multi emission colors, single-molecular
white emission (SMWE), mechanochromic luminescence (MCL),
chemical stimuli-responsive emission change, and distinct
delayed emission in matrices and organic light-emitting diodes
(OLEDs) (Fig. 1d). The comparison of the experimental
properties and quantum chemical calculation data of the series
of D–A–D compounds clarified that the conformational
preference arising from the “ax” and “eq” donors is strongly
correlated with the chalcogen atoms and thus governs their
photophysical properties.
N
N
Se
N
N
quasi-axial = “ax”
Ar
O
N
Se
N
PSeZ-TRZ
2019, de Sa Pereira, Kukhta & Lee
SeXO1-TPA
2020, Credgington & Seferos
quasi-axial = “eq”
c) DSe–π–DSe
N
N
Se
N
N
Se
Se
N
N
Se
Se
Se
PSe1
2020, Kim & Lee
PSe2
PSe3
d) DSe–A–DSe and DTe–A–DTe (This Work)
N
N
N
N
N
N
N
N
Se
Se
Te
Te
RESULTS AND DISCUSSION
Design and Synthesis
PSeZ-DBPHZ-PSeZ
PTeZ-DBPHZ-PTeZ
• RTP in films and OLEDs
• polymorphs-dependent multi-color emission
• single-molecular white emission (SMWE)
• mechanochromic luminescence (MCL)
Previously, we have developed multi-photofunctional organic
materials displaying thermally activated delayed fluorescence
(TADF), mechanochromic luminescence (MCL),23 and room-
temperature phosphorescence (RTP) properties, based on a
donor–acceptor–donor (D–A–D)24–29 or a D–A–D–A scaffold,30
utilizing dibenzo[a,j]phenazine (DBPHZ)31 as the key acceptor
unit. In 2017, we reported the first example of TADF-active
multi-color-changing MCL material (PTZ-DBPHZ-PTZ) based on
• acid/base- and redox-responsive emission change
• RTP in films and OLEDs
Fig. 1 a)–c) Structures of previously reported TADF- and
RTP-active organoselenium compounds; d) Structures of
newly developed D–A–D type organoselenium and
tellurium compounds.
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the design concept of conformation-dictated regulation of preparation of those polymorphs, we found out that the
View Article Online
DOI: 10.1039/D1TC02635F
luminescence properties, where the ax- and eq-conformer recrystallization from a CHCl3/n-Hex solution exclusively gave
arising from the boat-chair geometry of phenothiazine plays polymorph Y, while CH2Cl2/AcOEt solvent system provided
the key role in giving divergent ground and excited state polymorph O. The characterization of these polymorphs
energies.26a To reveal the effect of replacing Se/Te for S of PTZ- obtained from different solvent systems were confirmed by
DBPHZ-PTZ, D–A–D compounds on the photo-functionality, the identification of experimental and simulated powder X-ray
PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ were designed (Fig. diffraction (PXRD) patterns. Unfortunately, we were not able
1d). PSeZ-DBPHZ-PSeZ was successfully synthesized through a to find solvent system to exclusively provide polymorph G
Pd-catalyzed Buchwald-Hartwig double amination of 3,11- after extensive attempts. The PL spectra of polymorphs Y and
dibromo-dibenzo[a,j]phenazine
(Br2-DBPHZ)31
with O clearly show significant difference in emission profiles (Ylem
Y
O
O
phenoselenazine (PSeZ) in a good yield (the upper equation in 546 nm, FPL 0.02; lem 666 nm, FPL 0.03) (Fig. 2b). Due to
Scheme 1). The product was further purified through the scarcity of the amounts of the G polymorphs, the PL
recrystallization from CH2Cl2/n-Hex solution to give yellow spectra were not collected. To investigate the origin of the
powdery solids. The product was fully characterized by H, 13C, significant difference in emission colors of those polymorphs,
1
&
77Se NMR, X-ray crystallography, mass spectroscopy, IR the X-ray crystallographic analyses were conducted (Fig. 2c–k).
spectroscopy, and elemental analysis (for the details, see the Unexpectedly, the emission difference stems from not only the
Synthetic Procedures and Spectroscopic Data in the ESI). PTeZ- different conformational geometries (i.e, G: ax-ax conformer; Y
DBPHZ-PTeZ was also synthesized through an amination of and O: eq-ax conformers, Fig. 2c–e) but also the different
Br2-DBPHZ with phoenotellurazine (PTeZ) (the bottom packing modes (Fig. 2f–k). In all the polymorphs, anti-parallel
equation in Scheme 1; see the Synthetic Procedures and dimeric pairs were found with close interplane distance (dinter
)
Spectroscopic Data in the ESI). In stark contrast to PSeZ, the between the two adjacent DBZPH units (dinter 3.27–3.47 Å),
known synthetic method for PTeZ is far from practical, due to which cancel out dipole moments (Fig. 2i–k, Fig. S1–S3). Close
the use of toxic mercury reagent.32 Therefore, we decided to look at the packing structures of Y and O revealed the distinct
establish an up-dated synthetic method for PTeZ by modifying difference in dinter and the degree of overlap of the
p unit of
our method for phosphorous-bridged diarylamine the acceptor (DBPHZ) (Fig. 2j and k). From the comparison with
(dihydrophenophosphanizine).27 As the results, the cyclization the packing structure of previously reported sulfur-analogue
of dilithiated N-Boc diarylamine with TeCl4 followed by the PTZ-DBPHZ-PTZ in orange emitting single crystal (eq-ax
reduction of the resulting Te(IV) center successfully gave PTeZ
(for the detail procedures, see the ESI). Incidentally, during the
preparation of this manuscript, the Patureau group also
reported a robust synthetic method for PTeZ using Te(0) as the
tellurium source.33 It is noted that PTeZ-DBPHZ-PTeZ
represents the first example of D–A type emissive compounds
that have phenotellurazine (PTeZ) as the electron donor.
[Pd(PtBu3)2] (5 mol%)
NaOtBu (2.6 equiv)
PSeZ-DBPHZ-PSeZ
1,4-dioxane
110 °C, 18 h
71%
N
N
+
H
N
Chl
Br
Br
[Pd(dppf)Cl2•CH2Cl2] (15 mol%)
NaOtBu (2.6 equiv)
Br2-DBPHZ
PSeZ (Chl = Se)
PTeZ (Chl = Te)
PTeZ-DBPHZ-PTeZ
toluene
110 °C, 24 h
62%
(2.2 equiv)
Scheme 1 Synthesis of PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-
PTeZ.
Polymorphisms
Fig. 2 a) A photograph of concomitant polymorphs G, Y,
and O under the irradiation of a UV lamp (l = 365 nm); b)
PL spectra of Y and O (lex = 340 nm; values in parentheses
indicate the PLQY determined with an integral sphere);
molecular structures of PSeZ-DBPHZ-PSeZ in c) G, d) Y, and
e) O; packing structures of PSeZ-DBPHZ-PSeZ in f) G, g) Y,
and h) O; dimeric pairs of PSeZ-DBPHZ-PSeZ with the
closest dinter value in i) G, j) Y, and k) O polymorphs. The
Through our attempts to recrystallize PSeZ-DBPHZ-PSeZ from
various solvent systems, we serendipitously found out that the
molecule forms 3 different concomitant polymorphs (denoted
as G, Y, and O; for the detailed X-ray crystallographic data, see
the Table S1–S3 and Fig. S1–S3) grown from the same n-
Hex/EtOAc solution. Notably, those polymorphs displayed
distinctly different photoluminescence colors (green, yellow,
and orange) under the irradiation of a UV light (Fig. 2a). As the
results of screening of solvent systems for selective
pale pink region indicates the overlap area of p-moieties.
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conformation, lem 640 nm),26a it turned out that the packing the conformational preference results from an interplay
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DOI: 10.1039/D1TC02635F
structure of the selenium D–A–D compound in polymorph O is between Pauli, electrostatic, and orbital relaxation factors
very similar to that of PTZ-DBPHZ-PTZ in the orange-emitting (Table S9). All these terms are strongly coupled to the
crystal, where the overlap of the p-unit in the dimeric pair and equilibrium geometry. Nevertheless, substituting S with Se and
dinter (3.46 Å) is almost the same. Taken together, we can Te in the D–A–D scaffold at a fixed geometry always leads to
conclude that the difference in emission profiles among the the relative stabilization of the axial conformer by Pauli
polymorphs of PSeZ-DBPHZ-PSeZ are not only determined by repulsion and its destabilization by electrostatic and orbital
the molecular conformations but also the intermolecular relaxation. Given that the destabilizing Pauli energy accounts
electronic interactions.34
In contrast to the selenium D–A–D compound, any single
crystals of tellurium containing compound PTeZ-DBPHZ-PTeZ
suitable for X-ray crystallographic analysis were not obtained
even after many attempts, due to powdery nature of the
compound.
Steady-State Photophysical Properties in Solutions
To clarify the single molecular photophysical properties of the
D–A–D compounds, the steady-state UV-vis absorption and
photoluminescence spectra of dilute solutions (c = 10–5 M) in
various solvents were acquired (Fig. 3, Table 1, Fig. S4, and
Table S4). The Se-containing compound PSeZ-DBPHZ-PSeZ in
dichloromethane displays
a quite different absorption
spectrum from the superposition of the individual spectra of D
(PSeZ) and A (DBPHZ) (2D +A, Fig. 3a). The disappearance of
the vibronic absorptions (ca.
l 300 nm and 380–420 nm)
derived from the acceptor unit indicates a strong electronic
coupling between the D and A units (Fig. 3a). This implies a
quite different conformation of the Se compound from its
sulfur analogue, where eq-eq conformer dominates and
efficiently electron-decoupled absorption spectra displays.26a
In a similar manner with our previously developed DBPHZ-
cored D–A–D compounds,25–29 an intramolecular charge-
transfer (ICT) transition at around
3a). However, when compared to the absorption spectra of
the sulfur analogue, the molar coefficient (ca. 25,000–
28,000) of the ICT band of the selenium compound is much
larger than that of sulfur compound (ca.
10,000).26a
A similar trend in the absorption spectra was observed for
tellurium-containing compound PTeZ-DBPHZ-PTeZ, where
more distinct vibronic structure in the lowest-energy
absorption regime ranging from 420 to 480 nm was dominated
than the selenium compound (Fig. S4 and Table S4). This
distinct difference caused by the Se and Te replacement for S
atom suggests the ax-ax conformer is more dominated than
sulfur analogue in solutions, which was supported by the
theoretical calculations (vide infra). The comparison of the
conformational population of the D–A–D compounds with
each chalcogen-bridged diarylamine donor (S, Se, and Te)
l 460 nm was observed (Fig.
e
Fig. 3 a) Steady-state UV-vis absorptions of DCM solutions
(c = 10–5 M) of PSeZ (cyan dotted line), DBPHZ (pink dotted
line), 2D+A (moth solid line), and PSeZ-DBPHZ-PSeZ (black
solid line); b) Conformational population of D–A–D
compounds with S, Se, and Te donors in vacuum at 300 K
calculated based on a Boltzmann weighted average of each
conformer’s equilibrium structure energy; c) Steady-state
UV-vis absorption and photoluminescence (PL) spectra of
e
solutions (c = 10–5 M,
lex = 400 nm) of PSeZ-DBPHZ-PSeZ
prepared from various solvents. The inset photographs
indicate the solutions taken under the irradiation of a UV
light (l = 365 nm).
Table 1 Summary of steady-state photophysical properties
of PSeZ-DBPHZ-PSeZ in solutions (c = 10–5 M).
calculated by
a Boltzmann weighted average of each
conformer’s equilibrium structure energy is illustrated in Fig.
3b. The effect of Se replacement for S in the D–A–D scaffold
includes the switching of the majority conformer from eq-eq
(ca. 59% contribution for the sulfur compound) to ax-ax (ca.
55% contribution for the selenium compound; ca. 98%
contribution for the tellurium compound; Table S5–7).
Importantly, energy decomposition analysis (EDA)35 shows that
a
solvent
Toluene 453
THF
CHCl3
DMF
l
abs (nm)
e
(M–1cm–1)
l
em (nm)
FPL
25,800
28,200
28,400
24,900
480, 644
493
504
0.04
0.01
0.02
0.01
454
456
457
520
aDetermined with an integrating sphere.
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for electronic repulsion, the thermodynamic preference of ax- dielectric response of the solvent; the calculated emission
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ax conformers over eq-eq conformers in the selenium and wavelength in toluene is 558 nm (2.22 eV) for eq-ax and 608
tellurium-containing D–A–D compounds would be rationalized nm for eq-eq (2.04 eV) (Table S10 and S11). In a polar THF
by less transannular electronic repulsion between the lone solvent, the CT emission are predicted at 656 nm (1.89 eV) and
pairs on the N and the chalcogen atoms in the boat-chair 720 nm (1.72 eV), respectively, but these states are most likely
donor unit.
The absorption spectra of PSeZ-DBPHZ-PSeZ were not
relaxing non-radiatively.
Tellurium-incorporated D–A–D compound PTeZ-DBPHZ-
affected by solvent polarity, suggesting the almost constant PTeZ shows fluorescence with a low quantum yield (FPL 0.01–
distribution of the conformers (dotted lines, Fig. 3c). The weak 0.15) in diluted organic solvents, showing positive
effect of solvent polarity on the conformer distribution is also solvatochromic luminochromism (Fig. S4 and Table S4). In
confirmed by theoretical calculations as shown in Table S7. contrast to PSeZ-DBPHZ-PSeZ, the tellurium compound did not
The difference in the absorption spectra between S, Se, and show dual emission in toluene, which is in good agreement
Te-containing D–A–D compounds is also fully explained by with the domination of ax-ax conformer in this solvent (Fig. 3b)
different conformer stability. The theoretical spectra as suggested by the theoretical calculation (vide infra).
calculated by weighting contributions from all the conformers
agree with the experiments in terms of both relative band External Stimuli-Responsive Luminescence Properties
positions and their intensities (Table S10, S11, and Fig. S5).
The photoluminescence behavior of PSeZ-DBPHZ-PSeZ under
external stimuli was then investigated (Fig. 4). The as-prepared
Not only the absorption, the impact of the Se replacement
was drastic in photoluminescence (PL) spectra as well. Most
sample is a powdery crystalline sample, which shows sharp
importantly, the toluene solution of PSeZ-DBPHZ-PSeZ
diffraction patterns in PXRD measurement (Fig. S9a). Upon the
displayed a dual emission peaked at
l 480 nm and 644 nm,
irradiation of UV light, the solid emits yellow PL (lem 576 nm,
PL 0.04) (the inset photograph, Fig. 4a). Upon grinding the
thereby giving white emission (the photograph in Fig. 3c). This
is a remarkable contrast with the sulfur compound: it only
F
solid with a pestle and a mortar, the emission color drastically
turned to deep-red, displaying a broad Gaussian emission
gives a broad Gaussian-type CT emission at around
l 657 nm
in toluene.26a This should be associated with the dominance of
the orthogonally-structured conformers (eq-eq and eq-ax) in
the sulfur compound, which generates distinctly charge-
separated CT excited states and relaxed in a polar solvent. The
Se replacement affects the thermodynamic stability of the
conformers, and thereby the admixture of the greenish LE
spectrum (lem 671 nm,
4a). The ground sample was found amorphous from the PXRD
analysis (Fig. S9a). At glance, this mechanochromic
FPL 0.09). (the inset photograph, Fig.
a
emission at around
l 480 nm from the dominant ax-ax
conformer and the CT emissions from eq-ax (~38%) and eq-eq
(~8%) conformers should yield white emission. Recently, single
molecular white emitters (SMWEs) have emerged as promising
materials for optoelectronic and sensor applications.36
Nevertheless, the rational design of SMWEs is difficult, due to
the energy transfer problems, for instance. Therefore, the
introduction of PSeZ donors in D–A–D scaffold allows for
strategic design of SMWEs by mixing ax-ax and eq-based
conformers to balance energetically-different emissions. As
the polarity of solvent increases, the LE emission at around
480 nm in toluene gradually red-shifted, while the CT emission
at around 650 nm in toluene was totally vanished (Fig. 3c). This
polarity-dependence would support the significant difference
in CT natures of the two emissions observed in toluene: the
bluer emission is LE with a weak CT nature yielded from ax-ax
conformer, while redder emission is derived from a stabilized
CT excited state with a highly twisted structure (twisted
intramolecular charge-transfer: TICT state) from eq-ax and eq-
eq conformers. Due to the distinct charge separation, the
redder emission is highly susceptible to solvent polarity, and in
more polar solvent than toluene, the lower CT excited state is
dissipated through atomic vibrations and rotations, following
the energy gap law. Theoretical calculations reveal that strong
stabilization of the S1 state in eq-ax and eq-eq conformers is
due to the planarization of the equatorial donor unit in the
excited state (Fig. S6). Such CT state is further stabilized by the
Fig. 4 a) Steady-state PL spectra of powder solids (as
prepared), ground, and heated samples; b) Steady-state
UV-vis absorption and PL spectra of solutions (c = 10–5 M,
l
ex = 400 nm) of PSeOZ-DBPHZ-PSeOZ prepared from
various solvents. The inset photographs indicate the
solutions taken under the irradiation of a UV light (
nm).
l = 365
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luminescence (MCL) behavior is very similar to that of its sulfur character showing larger positive solvatochromic VsihewiftAr(tiFcliegO. n4libne)
analogue.26a However, the response against thermal and vapor than that of Se(II) species (Fig. 3c), altDhOouI:g10h.10th39e/De1lTeCc0t2ro63n5iFc
treatment of the ground Se-compound was found quite donating ability of the phenoselenazine unit should be
different from its sulfur atom (Fig. 4a). Upon heating the red- decreased.
A
possible rationale for explaining these
emitting ground sample on a hot plate at 200 °C for 15 min, contradicting spectra change would that the effect of the Se-
the emission color reverted back to yellow, which was clearly oxidation caused drastic change in conformational preference
seen on the PL spectra (Fig. 4a). The PXRD patterns of the from ax-ax to eq-based conformers, where the D and A units
heated sample were found almost superimposed with those of are almost completely decoupled, and thereby the charge-
the as-prepared powder sample (Fig. S9a), confirming the separated excited states are stabilized by the perpendicular D–
recovery of the initial state. The fuming of the ground sample A–D scaffold. This senario was supported by the
with organic vapor such as AcOEt and CHCl3 also caused the conformational population by the theoretical calculation (eq-
similar recovery of yellow emission (Fig. S10). The PXRD eq: 48%, eq-ax: 38%, ax-ax: 14%, see the Table S8).
analysis supported the recovery of the initial state (Fig. S9a).
In contrast to the behavior of the selenium compound,
The deep-red emitting amorphous state was reproduced by PTeZ-DBPHZ-PTeZ did not show photoluminescence in the
grinding the heated and fumed samples. In contrast, the solid state, probably due to the rapid ISC and IC accelerated by
amorphous state of the sulfur analogue PTZ-DBPHZ-PTZ shows larger SOC in the condensed state, which could be caused by
a much smaller blue-shift for heating (621 cm–1) and fuming intermolecular heavy atom effect. Although the D–A–D
(1920 cm–1) than that for selenium-compound (2458 cm–1), tellurium compound does not show PL in the aggregated solid
and the deep-red emitting (ground sample) state does not states, the compound exhibits blue-greenish PL in matrices
revert back to yellow-emitting state.26a Therefore, the Se with
replacement for S atoms allows for high reversibility of the phosphorescence (RTP) (for the details, see the Time-Resolved
two-emitting states. To obtain deeper insights on this unique Spectroscopy Section). More interestingly, the
a long lifetime, indicating the room-temperature
phenomenon, we compared the PXRD patters of the yellow- phenotellurazine donor (PTeZ) shows yellow (lem 556 nm, FPL
emitting crystalline sample with those predicted from the 0.09) and orange (lem 591 nm, FPL 0.19) photoluminescence
single-crystals of G, Y, and O (Fig. S9b). The comparison (PL) in the solid states (Fig. S13a) and Zeonex® matrix (Fig.
suggested that the eq-ax conformer would be the conformer S13b), respectively. The unexpected orange emission in the
contained in the yellow-emitting powder solids. Differential matrix was characterized with a long lifetime (
scanning calorimetry (DSC) of each solid sample revealed that under vacuum, Fig. S13c), which becomes shorter under the
the only the ground orange-emitting sample shows exothermic effect of air ( avr 5.7 s, Fig. S13c). Given this emission peak
tavr 11.7 µs
t
µ
peaks at around 159 and 198 °C (Fig. S11), suggesting the almost matches with that of phosphorescence spectra
ground sample is meta-stable state. Therefore, we can acquired at 77 K (Fig. S13b), the orange emission from PTeZ
conclude that the enhanced reversibility of the initial state should be RTP. Although the details of the origin of this unique
would be associated with the thermodynamic stability of the RTP awaits further analysis, the large SOC, which was
eq-ax conformer over eq-eq conformer in the aggregate states. supported by the theoretical calculation (<S0|HSOC|T1> 13.51
Although this thermodynamic conformational preference cm–1, Table S14), should accelerate the irradiation from the T1
appears to be inconsistence with that in solutions (Fig. 3b), to the S0 state. It should be noted that PTeZ represents a new
there could be many factors controlling the thermodynamic entry to organotellurium RTP family.37
preference of conformers in the solid states such as packing
forces and intermolecular interactions.
Time-Resolved Spectroscopic Analysis
In addition to the reversible MCL behavior in response to
various stimuli, the selenium compound shows response to
chemical stimuli. For example, the photoluminescence (PL) of
the as-prepared PSeZ-DBPHZ-PSeZ turns off upon the
treatment of vapor of trifluoroacetic acid (TFA), while the
yellow PL turns on by fuming the solid with basic triethylamine
(TEA) vapor (Fig. S12a). This turn/on-off cycle is reversible and
repeatable (Fig. S12b). The selenium compound also shows
redox-responsive emission color change through the reversible
manipulation of the oxidation state of the Se atoms (i.e., Ar2Se
Initial steady-state photoluminescence analysis revealed the
possibility of room-temperature phosphorescence (RTP) of
both D–A–D compounds. Nevertheless, the full time-resolved
analysis was indispensable to fully proof the photophysical
mechanism. For that purpose, we investigated the
photophysical properties of the solid-state layers of PSeZ-
DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ in both a non-polar
polymeric host Zeonex® and small molecular host CBP [4,4’-
bis(N-carbazolyl)-1,1’-biphenyl] (Fig. 5). As supporting work,
the analysis of pure donors (PSeZ and PTeZ) in Zeonex® matrix
was also investigated (Fig. S14). The layers were deposited on
sapphire substrate and analyzed in broad range of
temperatures from 10 K up to 300 K. The short-lived emission
of PSeZ-DBPHZ-PSeZ in Zeonex® matrix observed at 5.1 ns
could be associated with emission from S1 (1LE) state and is
similar at low (80 K, black line in Fig. 5a) and room-
temperature (300 K, blue line in Fig. 5a). At further time delay,
⇄
Ar2Se=O) (Fig. 4b, for the details of the redox procedures,
see the ESI). It is worth mentioning that such Se-oxidation
caused drastic change in absorption spectra, where the
vibronic absorptions ascribed to the acceptor unit (ca. labs 390
and 420 nm) are manifested and the CT absorption at around
450 nm is decreased (Fig. 4b) when compared to those of
PSeZ-DBPHZ-PSeZ (Fig. 3c). The photoluminescence of the
oxidized form (PSeOZ-DBPHZ-PSeOZ) displays more CT
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View Article Online
DOI: 10.1039/D1TC02635F
Fig. 5 Time-resolved spectroscopic analysis of PSeZ-DBPHZ-PSeZ and PTeZ-DBPHZ-PTeZ in matrices: Photoluminescence
spectra of a) PSeZ-DBPHZ-PSeZ and b) PTeZ-DBPHZ-PTeZ in Zeonex®; c) PSeZ-DBPHZ-PSeZ and d) PTeZ-DBPHZ-PTeZ in CBP;
Photoluminescence dacay of e) PSeZ-DBPHZ-PSeZ and f) PTeZ-DBPHZ-PTeZ in Zeonex®; g) PSeZ-DBPHZ-PSeZ and h) PTeZ-
DBPHZ-PTeZ in CBP.
the emission is shifted to lower energies, and the similarity is coincidental. The T1 state of the dominant ax-ax
phosphorescence is observed at low temperature (red line in conformer of PTeZ-DBPHZ-PTeZ is localized on the acceptor
Fig. 5a). As the temperature rises, the delayed emission is (3LEA), and the donor retains its bent structure (Fig. 7f), while
hypsochromically shifted, and the spectrum becomes broader the T1 state of PTeZ has a planarized geometry which stabilizes
(green and purple lines in Fig. 5a). Based on theoretical the excited state (Fig. S7). In the ax-ax conformer of the D-A-D
calculation, the ΔEST gap of PSeZ-DBPHZ-PSeZ in an ax-ax form compound, such planarization is not possible, due to the steric
is larger (490 meV) than the gap between T1 and T2 states repulsion between D and A units. As the results, the energies
(ΔET1-T2 = 400 meV) (Fig. 7). Given these energy alignments and of both triplet emissions of D and D–A–D compounds appear
the SOC between the S0–T2 states (1.56 cm–1), the temperature to be very close to each other.
activated process (Fig. 5a and e) could be related with the
More difficult processes to evaluate are involved in the
processes comprising of ISC (from S1 to T1) followed by thermal photophysical properties of the investigated compounds inside
reverse internal conversion (from T1 to T2) and the photo- CBP host, which was used for the fabrication and evaluation of
irradiation from the T1 and/or T2 states.29 Given the PSeZ- OLED devices (Fig. 5c, d, g, and h). The emissions of both D–A–
DBPHZ-PSeZ is admixture of ax-ax and eq-ax conforms (Fig. 3b), D compounds at shortest times are similar to those in Zeonex®
the energy transfer from the triplet excited state of ax-ax matrix and should be ascribed to the localized excited singlet
conformer to eq-ax conformer follwed by TADF emission from state (1LE) in both low (80 K, black lines in Fig. 5c and d) and
the 1CT state possibly also occur.
room temperature (300 K, blue lines in Fig. 5c and d). With
Slightly different and complex behavior was observed for both compounds, at low temperatures and long delay times,
the PTeZ-DBPHZ-PTeZ in Zeonex® (Fig. 5b). The emissions at the phosphorescence is observed in the lower energy region
short delay times (in the ns region) are related with the than those in for Zeonex® layers, but the intensity is not that
emission from the S1 (black and blue lines in Fig. 5b). As for the significant, suggesting it could be the emission from the lowest
long-delayed emission (phosphorescence), the emission CT state. On one hand, the thermally activated emission
slightly shifted to the higher energy regime with the rise of processes are on the same energy levels like the process from
temperature (red and green lines in Fig. 5b). Interestingly, the lowest 3LE state, but the spectra are broader like the
while the energy level and the spectrum shape of the emission related to charge transfer process (Fig. 5c, d, g, and
phosphorescence from T1 of PSeZ-DBPHZ-PSeZ are similar to h). Such complex behavior is not usually observed, but as
those of previously studied DBPHZ-cored D–A–D mentioned before, we probably observe the emission from T1
compounds,24-29 the phosphorescence of PTeZ-DBPHZ-PTeZ and a higher triplet state (T2).29 The change of the host more
appears to be much more similar to the phosphorescence of affects the CT excited state energies to the lower level than LE
donor (PTeZ) (Fig. S14b). But, the calculation suggests that states, thereby in CBP host both D–A–D compounds display
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almost the same energy level. Fitting lifetimes of the D–A–D
compounds and the donors are shown in Fig. S15 for reference,
although the development of theoretical models to explain
complex dynamics of such weak TADF and RTP emitters is
undergoing.
View Article Online
DOI: 10.1039/D1TC02635F
Electrochemical Properties
The electrochemical behavior of the donors and D–A–D
compounds in dichloromethane was investigated with cyclic
voltammetry (CV) (Fig. S16). The cyclic voltammogram of PSeZ-
DBPHZ-PSeZ displayed a reversible oxidation and reduction
ox
red
curves at
E
= +0.31 V and
E
= –1.71 V (vs. Fc/Fc+
onset
onset
redox couple), respectively. This electrochemical behavior
looks typical for DBPHZ-cored D–A–D compounds (Fig.
S15a).24–29 On the other hand, the tellurium compound PTeZ-
DBPHZ-PTeZ showed irreversible electrochemical redox
behavior (Fig. S16b), indicating the less electrochemical
instability than the sulfur26a and selenium analogues. Such
tendency in the electrochemical analysis was previously
reported in thiophene, selenophene, and tellurophene
analogues, where the decomposition of the tellurophene ring
was observed during the oxidation process.38 When compared
to the donors (PSeZ and PTeZ), distinct positive shift in the
oxidation potential for the both D–A–D compounds by >0.3 V
was observed (Fig. S16). The ionization potential (IP) and
electron affinity (EA) for the PSeZ-DBPHZ-PSeZ and PTeZ-
DBPHZ-PTeZ estimated from the CV experiment were
estimated to be 5.41/3.39 eV and 5.38/3.35 eV, respectively.39
When compared to the IP/EA of the sulfur analogue PTZ-
DBPHZ-PTZ (5.33/3.38 eV)26a and oxygen analogue POZ-
DBPHZ-POZ (5.36/3.38 eV),25 the Se/Te replacement effect for
S atoms in the D–A–D scaffold on electrochemical redox
characters is very small. This can be explained by the kinetic
process: the conformer with the lowest oxidation/reduction
potential (eq-eq) is faster electrochemically oxidized/reduced
on the electrode than other conformers (eq-ax and ax-ax) with
higher oxidation/reduction potentials. What is also interesting,
Fig.
6
The characteristics of the OLED devices: a)
Electroluminescence spectra; b) Current density-bias
characteristics; c) EQE-current density characteristics; d)
Luminance-current density characteristics.
performances and photophysical results of the fabricated
OLED devices were investigated and compared in order to
evaluate emissive pathways that boost the efficiency
properties. Most importantly, dual-emissive property of the
emitters was manifested in both OLED devices (Fig 6a). Such
behavior is not usual, but taking into the time-resolved
spectroscopic analysis data (Fig. 5), the higher-energy emission
at around lem 500 nm in both cases would be related with the
electroluminescence from the S1 (1LE) state and the lower-
energy emission at around lem 600 nm with the RTP from the
3LE state. In other words, fluorescence and room-temperature
phosphorescence (RTP) processes are involved in the
electroluminescence.40 In both cases, the RTP contribution in
overall emission, which was estimated from the comparing the
area of emission spectra acquired in aerated and degassed
conditions, is higher than 90%, and therefore the mostly
related factor for the efficiency of the device should be the
phosphorescence process. The highest external quantum
both
D–A–D
compounds
undergo
oxidative
electropolymerization process after the third oxidation peak
(Fig. S17a and b). It is also noted that only PTeZ donor forms
polymer on the electrode (Fig. S17c).
OLED Fabrication and Characterization
To investigate the feasibility of fabricating OLED devices with efficiency (EQE) of the OLEDs was obtained with PSeZ-DBPHZ-
the developed emitters, the thermal stability of the emitters PSeZ (4.35%), where the device with tellurium derivative PTeZ-
was checked. The themogravimetry analysis (TGA) revealed DBPHZ-PTeZ showed a lower EQE of 1.89% (Fig. 6c). These EQE
their high thermal stability [PSeZ-DBPHZ-PSeZ: Tg (5 wt%) values may not be that high, when compared with the OLED
383 °C under N2 gas; PTeZ-DBPHZ-PTeZ: Tg (5 wt%) 412 °C devices with existing heavy-atom-containing RTP organic
under N2 gas] (Fig. S18 and S19). This indicates the feasibility of emitters. But, if we assume that the overall PLQY for those
thermal sublimation process of OLED devices under vacuum. compounds is less than 10%, it would suggest we obtained as
The investigated compounds were used as dopant in the CBP highest efficiency as possible. On the other hand, the devices
matrix. The following two OLED structures were deposited exhibit quite high luminance as for selenium and tellurium
using high-vacuum thermal evaporators, Dev1: ITO/NPB (40 derivatives with up to 10,000 cd/m2 for PSeZ-DBPHZ-PSeZ and
nm)/10% PSeZ-DBPHZ-PSeZ in CBP (25 nm)/TPBi (50 nm)/LiF more than 4,000 cd/m2 for PTeZ- DBPHZ-PTeZ-based device.
(1 nm)/Al (100 nm); Dev2: ITO/NPB (40 nm)/10% PTeZ-DBPHZ-
PTeZ in CBP (25 nm)/TPBi (50 nm)/LiF (1 nm)/Al (100 nm). The
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followed by eq-ax (38%) and eq-eq (7%) (Table S5). In contrast,
View Article Online
only two PTeZ-DBPHZ-PTeZ conformersDOarI:e10e.1x0p3e9/cDte1TdC0t2o63b5eF
present at room temperature, namely ax-ax (98%) and eq-ax
(2%) (Table S6). As such, we noticed that one of the effects of
heavy atom substitution from S to Se and Te is a progressive
dominance of ax-ax conformations (Fig. 3b), due to the
deepening of the potential energy surface in this particular
geometry. Importantly, conformational preference is not
significantly affected by solvents, as shown in Table S7.
Excited state calculations in toluene revealed that for all
PSeZ-DBPHZ-PSeZ conformers the T1 states correspond to a
localized excitation (3LEA) state of the acceptor unit. On the
other hand, S1 states correspond to charge transfer (CT) states
in all but the ax-ax conformer, as evidenced by the natural
transition orbitals (NTOs) presented in Fig. 7a, b, and c.
Theoretical Calculations
To obtain deeper insights into the developed emitters,
theoretical calculations were performed at the density
functional theory level for the PSeZ-DBPHZ-PSeZ and PTeZ-
DBPHZ-PTeZ compounds (see the ESI for details). Ground state
geometry optimizations of the different conformers associated
with a weighting procedure using Boltzmann factors at 300 K
allowed us to conclude that all the three PSeZ-DBPHZ-PSeZ
conformers (i.e., ax-ax, ax-eq, and eq-eq) should coexist at
room temperature under vacuum, with the ax-ax
conformation being the most stable (55% probability),
Fig. 7 Natural transition orbitals (NTOs) for the S1 and T1 states along with corresponding energy levels for the three
conformations of a) eq-eq, b) eq-ax, and c) ax-ax conformers of PSeZ-DBPHZ-PSeZ in toluene; d) eq-eq, e) ax-eq, and f) ax-ax
conformers of PTeZ-DBPHZ-PTeZ in toluene; Blue and red arrows correspond to the spin-orbit couplings (SOCs) between S1–Tn
and Tn–S0, respectively.
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However, Fig. 7d, e, and f show that for eq-ax and ax-ax PTeZ- In conclusion, we have developed
Journal Name
new family of
a
View Article Online
DBPHZ-PTeZ conformers, both S1 and T1 states correspond to photofunctional dibenzo[a,j]phenazine-cDoOreI:d10t.1w03is9t/eDd1TCd0o2n6o3r5–F
localized excitations in the acceptor portion of the molecule, acceptor–donor (D–A–D) compounds having Se and Te
whereas the eq-eq conformer presents an S1 state with CT bridging atoms in the electronic donors. Of the developed
character and a T1 state that is localized in the donor unit. In compounds, selenium-containing D–A–D compound PSeZ-
particular, this difference is reflected in the significantly higher DBPHZ-PSeZ displays unique multi-photofunctionality of
oscillator strength associated with S0-S1 transitions of the polymorph-dependent
multi-photoluminescence,
single
PTeZ-DBPHZ-PTeZ molecule. In fact, the S0-S1 transitions for molecular white emission, various external-stimuli-responsive
both the eq-eq and eq-ax conformations of PSeZ-DBPHZ-PSeZ luminochromism, and unusual dual emission of fluorescence
are predicted to be forbidden (zero oscillator strength). These and room-temperature phosphorescence in thin films and
characteristics, in addition to the aforementioned population OLED devices. In contrast, the tellurium analogue PTeZ-
of the different conformers, account for the observed DBPHZ-PTeZ are non-emissive in the solid states, probably due
differences in the absorption spectrum of the S, Se and Te- to the accelerated ISC and the following IC. Despite the
containing D–A–D compounds.
quenching in the condensed phase, the tellurium compound
Calculated emission energies from the S1 state in the ax-ax shows RTP in host matrices and OLED devices. Also,
conformers of both compounds predict fluorescence at around unexpectedly we have found out that the donor PTeZ
2.5 eV (Table S12), in agreement with experimental results (Fig. represents a new family of RTP-displaying organotellurium
3c and Fig. S4b). A lower energy band in PSeZ-DBPHZ-PSeZ compounds. From the careful comparison of the
fluorescence spectrum is also predicted at 2.2 eV to result organoselenium/tellurium compounds with the lighter
from CT emission from the eq-ax conformer (Table S12). Such chalcogen analogues, the most striking effect of replacing with
second band is not observed for PTeZ-DBPHZ-PTeZ, however Se and Te for S in the DBPHZ-cored D–A–D twisted scaffold
(Table S12). This behavior stems from the fact that the donor involves the drastic increase in the preference of ax-oriented
unit of the eq-ax Se compound undergoes planarization in the conformers over the eq ones, due to transannular electronic
S1 state (Fig. S6), resulting in a lower energy CT state that is interaction. This conformational change strongly affects the
further strongly stabilized by a polarizable environment.41 Such absorption and photoluminescence profiles in solutions, the
planarization is not seen in the Te compounds, for which the S1 reversibility in external-stimuli-induced phase transition, and
emission energy in the eq-ax conformer is much larger (2.81 excited states energy parameters such as
DEST, DETT, and spin-
eV, Fig. 7e). orbit coupling (SOCs) that govern the fate of the mechanisms
Phosphorescence energies calculated for both D–A–D involved in the excited states. The knowledge obtained
compounds are also very similar and predicted to lie around through this research would pave the new avenue for multi-
2.0 eV (Table S13). This is due to the fact that T1 states in the photofunctional
ax-ax conformation of both molecules correspond to localized materials.
excitations in the acceptor unit (Fig. 7c and f). For the pure
organoselenium
and
organotellurium
PSeZ and PTeZ donor fragments, phosphorescence energies
are predicted at 2.3 eV and 1.9 eV, respectively (Table S14). As
such, the experimentally observed similarity in
Acknowledgements
Y.T. acknowledges a Grant-in-Aid for Scientific Research on
Innovative Areas “π-System Figuration: Control of Electron and
Structural Dynamism for Innovative Functions” (JSPS KAKENHI
Grant Number JP15H00997 & JP17H05155) and “Aquatic
Functional Materials: Creation of New Materials Science for
Environment-Friendly and Active Functions” (JSPS KAKENHI
Grant Number JP19H05716) from the MEXT (Ministry of
Education, Culture, Science and Technology, Japan), a Grant-
in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number
JP20H02813), and the Research Grant from the Japan Prize
Foundation, and the Continuation Grants for Young
Researchers from the Asahi Glass Foundation, and the
Research Grant in the Natural Science from the Mitsubishi
Foundation. Y.T. and S.M. acknowledge NIPPOH CHEMICALS
for supplying N,N-diiodo-5,5-dimethylhydantoin (DIH). P. de S.
and L. E. de S. acknowledge support by a research grant
(00028053) from VILLUM FONDEN. P.D. and N.O.D.
acknowledges the Polish National Science Centre funding,
grant no. 2018/31/B/ST5/03085. P.D. and N.O.D.
acknowledges the supporting awards from the Rector of the
phosphorescence energies between the Te-containing D–A–D
compound and the PTeZ fragment does not necessarily imply
that phosphorescence in PTeZ-DBPHZ-PTeZ stems from the
donor unit.
Spin-orbit coupling calculations have also been performed
for all molecules in toluene using the T1 geometry (Fig. 7; The
NOTs for T2 and T3 are shown in Fig. S8). These calculations
revealed that S0-T1 spin-orbit couplings are affected mostly by
substitution of the Se atoms for Te rather than by
conformation. This can be seen from the overall larger
couplings calculated for PTeZ-DBPHZ-PTeZ conformers when
compared to PSeZ-DBPHZ-PSeZ. For the ax-ax conformation,
which is the most likely to be present, S1-T1 couplings are
found to be around 0.7 cm–1 for both molecules. The S1-T1
transition in this conformer is allowed, partially due to the fact
that it corresponds to a transition between two different LE
states (n-p* and p-p*).
Conclusions
Silesian
University
of
Technology
(04/040/BKM20/0124, 04/040/RGJ21/0149). Y.T., N.O.D., P. de
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16 D. de Sa Pereira, D. R. Lee, N. A. Kukhta, K. H. Lee, C. L. Kim,
S. and P.D. acknowledge the EU’s Horizon 2020 for funding the
OCTA project under grant agreement No 778158.
View Article Online
A. S. Batsanov, J. Y. Lee and A. P. Monkman, J. Mater. Chem.
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