Impact of Methoxy Substituents on Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in All-Organic Donor-Acceptor Systems

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Cite This: J. Org. Chem. XXXX, XXX, XXX −XXX

Impact of Methoxy Substituents on Thermally Activated Delayed

Fluorescence and Room-Temperature Phosphorescence in All-

Organic Donor−Acceptor Systems

Jonathan S. Ward,^†Roberto S. Nobuyasu,^‡Mark A. Fox,^†Juan A. Aguilar,^†David Hall,^†

Andrei S. Batsanov,^†Zhongjie Ren,^§Fernando B. Dias,^‡and Martin R. Bryce*^,†

‡

^†Department of Chemistry and Department of Physics, Durham University, Durham DH1 3LE, U.K.

^§Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Chemical Resource

Engineering, Beijing University of Chemical Technology Beijing 100029, China

S

* Supporting Information

ABSTRACT: Thermally activated delayed ﬂuorescence (TADF)

and room-temperature phosphorescence (RTP) are known to

occur in organic D−A−D and D−A systems where the donor

group contains the phenothiazine unit and the acceptor is

dibenzothiophene-S,S-dioxide. This study reports the synthesis

and characterization of one new D−A and four new D−A−D

systems with methoxy groups on the phenothiazine to examine

their eﬀect on emission properties in the zeonex matrix. X-ray

analysis and highly specialized NMR techniques were used to

characterize asymmetric methoxy-substituted derivative 3b, which is chiral at N because of an extremely high ﬂipping barrier at

the phenothiazine N atom. Based on hybrid-density functional theory computations, the methoxy substituents tune the relative

stabilities of the axial conformers with respect to equatorial conformers of the phenothiazine units, depending on their

substitution position. This conformational eﬀect signiﬁcantly inﬂuences both TADF and RTP contributions compared to the

parent D−A−D system. It is also demonstrated that the equatorial forms of D−A−D and D−A systems in zeonex exhibit

TADF. Additionally, the methoxy groups promote luminescence in D−A−D systems where only axial conformers exist. This

work reveals further design opportunities for more eﬃcient TADF and RTP molecules.

triplet (³LE) state results in highly eﬃcient reverse intersystem

INTRODUCTION

■

crossing (RISC).³⁰⁻³⁴A fast RISC rate also results in shorter

Luminescent organic compounds are used for a wide variety of

emission lifetimes because of excitons spending less time in the

applications including security devices,¹organic light-emitting

long-lived triplet states.²Additionally, it is suggested that a short

diodes (OLEDs),²⁻⁷chemical sensing,⁸and bioimaging.⁹The

emission lifetime can result in longer device performance

tuning of emission color in conjugated organic molecules can be

because of less electroluminescence quenching from charge

achieved in several diﬀerent ways, such as by increasing the

extent of conjugation in the system, or adding substituents to

build-up within the device.³⁵As a result, ﬁne-tuning of the CT

state is essential for the development and evaluation of new

TADF molecules. The energy of the CT state in solution and in

modify the electron density or the conformation within the π-

system.^10,11When a molecule comprising donor (D) and

thin ﬁlms can be modulated in various ways, notably by placing

acceptor (A) subunits exhibits charge-transfer (CT) emission,

the emitter into diﬀerent polarity solvents or solid matrices.³⁶

extension of conjugation and/or adding substituents drastically

The ¹CT energy can also be shifted by the addition of

shifts the emission wavelength,¹²thereby providing a versatile

means for the rational tuning of emission color.

substituents onto an unsubstituted D−A system.³⁷

The motivation for the present work was to explore how

selective functionalization of known D−A−D and D−A

molecules 1a,³¹2a³⁰and 3a³⁷with methoxy groups can be

used to ﬁne-tune the emission properties, particularly in the

context of TADF and room-temperature phosphorescence

(RTP). The methoxy groups are in “remote” positions, in

contrast to our previous study on the eﬀect of methyl

substituents in sterically locking positions adjacent to the D−

Thermally activated delayed ﬂuorescence (TADF) has

emerged as an eﬃcient method for triplet harvesting via the

upconversion of triplet excited states to singlet states.^5,13−15

TADF leads to interesting fundamental photophysical proper-

ties and in particular to high-eﬃciency OLEDs¹⁶⁻²⁸by

overcoming the 25% internal quantum eﬃciency limit imposed

by the spin statistics of exciton formation.²⁹Although the

detailed mechanism of TADF is still not fully understood, these

types of molecules typically emit from a singlet charge-transfer

(¹CT) state, and recent experimental and computational

research shows that having this state in resonance with a local

Received: November 7, 2018

A

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A bonds (Figure 1).³⁷For this study, phenothiazine donors in

combination with dibenzothiophene-S,S-dioxide as the acceptor

methoxy group on the RTP emission of these molecules with

respect to 3a. This systematic study signiﬁcantly contributes to

our understanding of how substituents aﬀect the multifaceted

photophysics of TADF and RTP molecules.

RESULTS AND DISCUSSION

■

Synthesis and Characterization. Detailed synthetic

procedures for the compounds 1b, 1c, 2b, 3b, and 3c are

reported in the General Experimental Details section. Scheme 1

shows the route to 1c as a representative example of the

procedures used to prepare all the new methoxy-substituted

systems. Compound 4 was synthesized via a Buchwald−Hartwig

coupling of commercially available m-anisidine with 3-

bromoanisole. Diarylamine 4 was then cyclized by heating

with sulfur and iodine in 1,2-dichlorobenzene (DCB) to give the

unusual asymmetrically substituted 2,6-dimethoxyphenothia-

zine product 5. The isolated yield of 5 was only 10% because of

diﬃculties in separation from a complex mixture of products. 5

was then coupled via an S_NArreaction with 2,8-diﬂuorodibenzo-

thiophene-S,S-dioxide (6),³⁷to give the D−A−D compound 1c

in 19% yield.

The new molecules were characterized by ¹H and ¹³C NMR

spectroscopy (General Experimental Details section and

Supporting Information S1), elemental analyses, and mass

spectrometry. The NMR data for 1c, 2b, and 3c revealed peaks

from an averaged mixture of conformers where all conformers

are freely interconverting in solutions, like their corresponding

parent systems 1a, 2a, and 3a. For asymmetrically substituted

phenothiazine compounds (1c and 3b), two distinct diaster-

eomers, RR/SS or RS (meso) should be observed if the ﬂipping

at phenothiazine N is prevented. Only one set of peaks is present

Figure 1. D−A−D and D−A molecules investigated in this study.

have been utilized, as the parent (unfunctionalized) heterocycles

are established building blocks in well-characterized TADF

systems.^{24,30,31,38−46}In previous work, methoxy substituents

attached at selected sites on a bis(carbazolyl)diphenylsulfone

have been demonstrated to shorten emission lifetimes.⁴⁷

Here, compounds 1b and 1c were initially synthesized to

probe the eﬀect of methoxy groups at diﬀerent positions on the

phenothiazine donors, an eﬀect that has yet to be systematically

explored in these D−A−D TADF systems. The methoxy groups

at para positions to the N atom in 1b have a signiﬁcant eﬀect on

the emission properties compared to 1a, whereas the methoxy

groups at meta positions in 1c have less eﬀect with respect to 1a.

This can be explained by the diﬀering inductive and mesomeric

electronic eﬀects of methoxy groups in varying ring positions.⁴⁸

With methoxy groups in 1b signiﬁcantly inﬂuencing the

luminescence, compounds 2b, 3b and 3c were also synthesized

with para-methoxy substitution. Molecule 3a was previously

shown to exhibit strong RTP without TADF because of the

methyl groups in sterically demanding positions.³⁷This eﬀect

has recently also been investigated in phenoxazine analogs.⁴⁹

3b−c were speciﬁcally synthesized to assess the eﬀect of the

1

in the H NMR spectrum of 1c, indicating that diastereomers

easily interconvert in solution. In contrast, for 3b, two sets of

peaks were present in the NMR spectrum corresponding to two

diastereomers in the ca. 1:1 ratio as the barrier is too high for

inversion at nitrogen. The diastereomers in 3b could not be

separated by chromatography or crystallization because of their

extreme similarity; however, they were distinguished using

highly specialized NMR techniques (see below).

Two independent crystallizations of 3b, from 2,2,4-

trimethylpentane/THF and n-hexane/PhCl, yielded isomor-

phous solvated crystals of 3b·2THF and 3b·1.6PhCl with similar

molecular conformations and packing motifs of the host

molecules. Both structures contain two independent 3b

molecules, shown in Figure 2. The dibenzothiophene (DBT)

moiety is practically planar, with cis ax−ax⁵⁰orientation of

Scheme 1. Synthetic Route to 1c

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proton multiplicity using pure shift techniques^51,52(Figure 3b)

alongside variable temperature studies (Figure 4).^53,54Most of

the carbon signals are also duplicated, although a few appear not

to be. However, acquisition of an ultrahigh resolution ¹H−¹³C

HSQC spectrum (Figure 5) using recent compressive sampling

techniques^53,54conﬁrmed that the duplicated signals have

almost the same chemical shift.

To conﬁrm that diastereomers of 3b were not interconverting,

a series of ¹H NMR spectra were acquired at diﬀerent

temperatures (Figure 4). The duplicated signals do not coalesce

and merge into a single signal as would be expected if there was

interconversion between diastereomers. Further ¹H ROESY and

¹H COSY experiments also indicate that no exchange is

occurring (Figure S2).

Figure 2. One of the independent molecules (Mol-1) of 3b in the

crystal structure of 3b·2THF, showing the disorder of the methoxy

groups (thermal ellipsoids at 50% probability level). Details of each

independent molecule within the structure are shown in Figure S6a −d.

The distribution of methoxy groups between positions i−iv is given in

Table 1.

The ¹H−¹³C HSQC, acquired using compressive techniques

to produce very high resolution along the carbon dimension, was

combined with the pure shift ¹H NMR spectrum using

generalized indirect covariance, to produce a pure shift HSQC

spectrum where peaks are simpliﬁed into singlets. The resulting

ultrahigh resolution spectrum facilitates the identiﬁcation of the

phenothiazine (PTZ) substituents. PTZ moieties are folded

along their N···S vectors, the dihedral angle between the arene

rings being 131−139° (mean 136°). The lone pair of the N atom

is conjugated with the DBT π-system, as allowed by the eclipsed

conformation about the N−C(DBT) bond (within 1−2° in 3b·

2THF and 1−7° in 3b·1.6PhCl). This bond is therefore much

shorter than the N−C(PTZ) bonds, averaging respectively

1.403(4) and 1.438(4) Å in 3b·2THF, 1.404(5) and 1.440(5) Å

in 3b·1.6PhCl. Excluding the methoxy substituents, molecule 3b

has an approximate (noncrystallographic) mirror plane passing

through the SO₂group. It is noteworthy that in both structures

methoxy groups are disordered unequally between alternative

positions, that is, diﬀerent isomers can occupy the same

crystallographic site, explaining the impossibility to separate

them by crystallization. Moreover, the crystals are centrosym-

metric, that is, opposite enantiomers must be present in equal

amounts. Crystallographic occupancies of the four possible

methoxy group positions (i−iv) are shown in Figure 2 and Table

1; however, this does not reveal how the disorder between

1

diastereomers. Figure 5 shows that every H environment is

bonded to a very similar carbon, and in many cases, the carbons

have the same chemical shift. However, with the use of

compressive NMR techniques, high-resolution characterization

of this compound was achieved.

Electrochemical Properties. Cyclic voltammetry (CV)

measurements on molecules in Figure 1 were performed in N,N-

dimethylformamide (DMF) and dichloromethane (DCM)

(Figures 6 and S3a −f, Tables S1 and S2). DMF is a more

suitable solvent for these systems because of the wider

electrochemical solvent window enabling observation of both

oxidation and reduction waves. However, the oxidation waves

were not reversible for 1c, 3a, 3b, and 3c in DMF. The oxidation

waves in DCM were reversible for 1c, but not for 3a, 3b, and 3c

even though the reverse cathodic waves are improved in DCM

compared to in DMF.

Comparison of the CV traces of 1a, 1b, and 1c in Figure 6

shows that the methoxy group facilitates oxidation (by 0.12 V) at

the donor group in 1b and 1c compared to 1a. A diﬀerent

methoxy group eﬀect is seen in the reduction of the acceptor unit

where negligible change is observed between the reduction

waves of 1a and 1c, but in 1b, the reduction is more diﬃcult with

a diﬀerence of ca. 0.12 V. The diﬀerences in the reduction

potentials for 1b and 1c indicate that the methoxy groups para-

to the N promote electronic interactions between the donor and

acceptor units more than when they are in the meta position.

Inductive eﬀects dominate when methoxy groups are in the

meta-position in these systems.

Table 1. Distribution of Disordered MeO Groups for Each

Molecule in the Unit Cell (See Figure 2 for Notation)

3b·2THF

3b·1.6PhCl

methoxy position Mol-1 (%) Mol-2 (%) Mol-1 (%) Mol-2 (%)

I

II

III

IV

87

13

82

18

80

20

40

60

80

20

80

20

58

42

37

63

The oxidation waves in 1a and 1c represent two simultaneous

one-electron oxidations of both donor groups. This observation

implies an absence of any electronic interaction from one

oxidized phenothiazine unit on the oxidation of the second

phenothiazine unit; therefore, these donors are eﬀectively

electronically isolated from each other. Nevertheless, the

apparent oxidation wave in 1b in DMF is resolved into two

distinct one-electron oxidation waves (Figure 7). In this case, the

oxidation of one phenothiazine unit makes the second donor

oxidation more diﬃcult. The oxidized phenothiazine unit in 1b

must have a higher degree of electronic conjugation with the

acceptor unit for the observation of both oxidation waves. The

square wave traces show a potential diﬀerence of 110 mV

between the two oxidation waves, where a mixed valence cation

radical of 1b is present. Two, one-electron oxidation waves are

positions i−ii correlates with that in iii−iv. Positions ii and iii are

not likely to be occupied in a single molecule, as this would result

in a very short intramolecular contact. Nevertheless, in either

solvate, one independent molecule shows a preponderance

(≥80%) of i,iii-substitution (RR/SS), while in the other

independent molecule, methoxy groups are more evenly

distributed, albeit with some prevalence of positions i and iv

(meso-conﬁguration). In 3b·1.6PhCl, the methoxy group

disorder is accompanied by minor orientational disorder of

the PTZ cores (see Figure S6c).

Detailed NMR Discussion. Further evidence for the rigidity

of 3b and the presence of diastereomers was obtained using

several NMR techniques. First, ¹H NMR suggests that all signals

1

are duplicated. This is not clear using standard H NMR

techniques, as in Figure 3a, but it is revealed when removing the

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Figure 3. (a) 600 MHz ¹H NMR spectrum of 3b in DMSO-d₆and (b) pure shift ¹H NMR spectrum of 3b in DMSO-d₆at 298 K. (c,d) Expansions of

regions of (a,b) to show how removing multiplicity reveals the presence of multiple species. Removing the proton−proton multiplicity shows that all

signals are duplicated. In many cases, equivalent peaks from the same proton in each diastereomer are <0.005 ppm apart.

Figure 4. 500 MHz ¹H NMR spectra of 3b in DMSO-d₆acquired from 298 to 373 K.

also evident in the CV traces of 1b and 3c in DCM with

approximate potential diﬀerences of 90 and 70 mV (Figure S3g).

The frontier orbital energies listed in Table 2 were estimated

from CV measurements in DMF. The eﬀect of the methoxy

groups at the para positions of the parent D−A−D and D−A

systems is that both HOMO and LUMO levels are higher in

energies compared to the unfunctionalized derivatives.

For the tetramethyl molecules, addition of two methoxy

groups (3b) does not induce a signiﬁcant change in the

HOMO−LUMO levels. With four methoxy groups (3c), the

HOMO level increases in energy by 0.10 eV compared to 3a, but

it should be noted that the oxidation waves are not reversible in

DMF for these molecules (Table S1). Interestingly, the LUMO

energies are not shifted by the methoxy substitutions in 3b−c.

The conjugational separation between D and A in 3a−c is

extensive, and as a result, even addition of methoxy groups does

not alter how the DBT unit is reduced.

conformations of phenothiazine are named as previously

identiﬁed.⁵⁰All possible conformers of the molecules shown

in Figure 1 were optimized and located as minima (Figure S4a −

h). The axial conformers were the lowest energy minima in all

cases. The experimental and computed geometric parameters

for 1a, 3a, and 3b are in very good agreement (Table S3) which

gives conﬁdence in B3LYP/6-31G(d) as an appropriate

chemistry model for D−A−D systems in the ground state.

The most stable ax−eq conformer of 1a is only 1 meV higher in

energy than the most stable ax−ax conformer of 1a (Figure 8).

However, when methoxy groups are present, as in 1b and 1c, the

ax−ax conformers are clearly preferred energetically (Table 3).

Similar ﬁndings apply for the simpler D−A systems 2a and 2b,

with the ax conformer being most prevalent. The calculated

optimized geometries and their energies are based on gas-phase

states without external eﬀects such as intermolecular inter-

actions present in the zeonex ﬁlms. The small energy diﬀerences

in the conformers may be subtly diﬀerent in zeonex, and thus,

the ax−eq conformer ratios for 1a−c and 2a,b in Table 3 are not

considered here to be quantitative in zeonex ﬁlms. The ax−ax

forms are much lower in energy than the ax−eq and eq−eq for

Computations. Hybrid density functional theory (DFT)

calculations were performed at the B3LYP/6-31G(d) level to

examine which conformers are likely to be present in solutions

and in zeonex. Axial (co-planar) and equatorial (perpendicular)

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Figure 5. High-resolution pure shift ¹H−¹³C HSQC NMR spectrum of 3b produced combining compressive and covariance techniques.

Figure 6. Cyclic voltammograms of 1a, 1b, and 1c in 0.1 M TBAPF₆in

DMF.

3a, 3b, and 3c. Consequently, ax−ax conformers would

represent at least 99% of the distribution in solution or zeonex

for 3a−c at room temperature.

The rotation barriers between ax−ax and eq−ax conformers

in D−A−D systems (ax and eq conformers in the case of D−A

systems) were estimated by ﬁxing the C(donor)−N(donor)−

C(acceptor)−C(acceptor) torsion angle between one donor

Figure 7. (a) CV and (b) square wave voltammetry (SWV) traces for

1b in DMF.

group and the acceptor moiety. Each geometry was optimized

with the CNCC torsion angle as the only constraint. The torsion

angle was adjusted in 5° intervals to give relative rotation energy

proﬁles (Figure S4i −k). The rotation barrier is only 0.12 eV for

1a, suggesting that ax−ax and eq−ax conformers interconvert in

solution at room temperature along with many other con-

formers. On fabrication of a solid zeonex ﬁlm with evaporation

of the solvent, the conformers are locked in the zeonex matrix,

and interconversion is negligible in the solid matrix as signiﬁcant

geometrical distortions would have to occur for conformers to

interconvert (e.g., ax−ax to ax−eq).

By constraining the “NC₄S” ring to be planar and optimizing

the geometries, the inversion barriers at nitrogen were

estimated. For 1a, 1b, 1c, 2a, and 2b, the inversion barrier

energies are similar to the low rotation barrier energies. As a

result, these conformers, including diastereomers, interconvert

Table 2. Estimated HOMO, LUMO, and HOMO−LUMO

Gap (HLG) energies Obtained from CV Experiments in DMF

HOMO (eV)

LUMO (eV)

HLG (eV)

1a

1b

1c

2a

2b

3a

3b

3c

−5.37

−5.26

−5.30

−5.46

−5.29

−5.91

−5.89

−5.81

−3.09

−2.93

−3.06

−3.00

−2.91

−2.69

−2.67

−2.70

2.28

2.33

2.24

2.46

2.38

3.22

3.11

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from the acceptor unit to the LUMO and HOMO of 1b

compared to 1c (Figure 9). This supports the increased

Figure 8. Most stable ax−ax and eq−ax conformers of 1a with frontier

orbital energies from DFT calculations at the B3LYP/6-31G(d) level.

in solutions at room temperature. The ¹H peaks observed in the

NMR spectra of these compounds (Section S1) are sharp for

each environment, suggesting that the peaks are averaged

because of rapid interconversions of many diﬀerent conformers

in solutions. For 3a, 3b, and 3c, the rotation barrier energies are

0.29, 0.32, and 0.33 eV, respectively, which mean that

interconversion between conformers via rotations would take

place in solutions at room temperature. Conversely, the

inversion barrier energies are much higher in 3a, 3b, and 3c at

1.26, 1.31, and 1.35 eV, respectively. Such values indicate that no

inversions take place at room temperature, thus stereoisomers

would not interconvert. Because there are distinct diaster-

eoisomers, (RR/SS) and meso, in 3b, separated peaks

corresponding to these isomers are observed in the solution-

state NMR spectra of 3b (Figures 3−5 and S2). All these studies

together highlight the rigidity of molecules 3a−c, and this

suggests why the excited states can emit at such long lifetimes

(ms timescale; see below).

The LUMO and HOMO energies obtained from electronic

structure calculations on the two static “gas-phase” minima of

1a, with essentially identical energies at B3LYP/6-31G(d) level,

show notable diﬀerences between ax−ax and ax−eq conformers

of −0.41 and +0.37 eV for LUMO and HOMO, respectively

(Figure 8). By contrast, the frontier orbital energies diﬀer by

only +0.17 eV for the LUMO and +0.20 eV for the HOMO from

3a to 3c with methoxy groups (Table 3). The conformations in

these D−A−D and D−A systems therefore inﬂuence the

frontier orbital energies, in addition to the electronic eﬀects of

the methoxy groups.

Figure 9. Frontier orbital energies of the lowest energy minima of 1b

and 1c calculated at the B3LYP/6-31G(d) level. The percentages

represent the orbital contribution of the acceptor unit (dibenzothio-

phene-S,S-dioxide) to the frontier orbitals.

electronic conjugation between the donor and acceptor units

in 1b resulting in the observation of a mixed valence radical

cation in the CV data of 1b (Figure 7).

The computations demonstrate how several diﬀerent ax−ax

and ax−eq conformers exist in solutions for 1a, 1b, and 1c and

diﬀerent ax and eq conformers are present in 2a and 2b. The ax−

ax conformers are essentially the only conformers present in 3a,

3b, and 3c solutions. Detailed emission measurements were

performed on these systems in zeonex in order to identify the

diﬀerent emissions and their percentage contributions arising

from axial and equatorial conformers.

Photophysical Properties. Absorption spectra in dichloro-

methane solution showed very similar proﬁles for 1b, 2b, 3b, and

3c, but a diﬀerent proﬁle was observed for 1c with a more

intense band at 260 nm (Figure S5a). This diﬀerence is

attributed to the methoxy groups at meta-positions to N for 1c,

in contrast to para for the other molecules. The lowest energy

band cut oﬀs are at 410 nm for 1b and 1c, 400 nm for 2b, and

385 nm for 3b and 3c. This trend suggests that the highest

occupied molecular orbital (HOMO)−lowest unoccupied

molecular orbital (LUMO) gap (HLG) energy increases in

The frontier orbitals of 1b and 1c with diﬀerent methoxy

substituent positions were examined in detail in order to explain

the diﬀering CV data. There are increased orbital contributions

Table 3. Computed Energies in eV at B3LYP/6-31G(d)

a

b

c

d

e

energy diﬀerence

conformer ratio in solution

rotation barrier

inversion barrier

HOMO

LUMO

HLG

1a

1b

1c

2a

2b

3a

3b

3c

0.00

0.11

0.04

0.03

0.12

0.26

0.31

0.32

51:49

98:2

83:17

75:25

99:1

100:0

0.12

0.11

0.15

0.14

0.17

0.29

0.32

0.33

0.09

0.21

0.11

0.14

0.23

1.26

1.31

1.35

−5.51

−5.30

−5.35

−5.70

−5.49

−5.60

−5.49

−5.40

−1.39

−1.20

−1.29

−1.57

−1.46

−1.35

−1.25

−1.18

4.12

4.10

4.06

4.13

4.03

4.25

4.24

4.22

a

The energy diﬀerence between the lowest energy axial and equatorial conformers. For 1a−1c and 3a−3c, ax−ax vs ax−eq. For 2a and 2b, ax vs eq.

b

c

The relative ratio of axial and equatorial conformers in solution at 290 K, assuming that only two conformers are present. The rotation barrier

d

e

energy about the N−C bond between donor and acceptor units. The inversion barrier energy at one nitrogen atom. For lowest energy axial

conformers only.

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the sequence 1b/1c < 2b < 3b/3c. Photophysical measurements

on the ﬁve methoxy-substituted molecules allow comparison

with the reported emission data of the parent D−A−D and D−A

structures 1a,³¹2a,³⁰and 3a³⁷to probe structure/property

relationships for achieving TADF or RTP, or both. Figure 10

ratio of 6.50 in the zeonex matrix, whereas 1b and 1c have lower

ratios of 1.66 and 2.52, respectively. The data for methoxy-

substituted 1b−c compared with 1a (no methoxy groups)

indicate that the methoxy groups decrease the contribution of

triplet excited states to the emission. DF and phosphorescence

are dependent on oxygen as triplet excited states are involved in

these emission pathways. A similar trend is observed with the

D−A series, where the I_{no O}/I_Oratio drops from 5.09 in 2a to

2

2.04 in 2b. Both 2a and 2b show high energy shoulders which

can be attributed to ﬂuorescence from the local singlet state

(¹LE).

The eﬀect of methoxy groups on the I_{no O}/I_Ointensity ratios

2

is quite diﬀerent for the tetramethyl derivatives, with values of

2.25, 8.69, and 2.31 for 3a, 3b, and 3c, respectively. The

emission spectra for 3a−c are clearly diﬀerent from other D−A−

D and D−A systems here, with well-structured emission in the

absence of oxygen. Subtractions of the emission spectra of 3a−c

in the absence of oxygen (black lines in Figure 11) by the

corresponding emission spectra of 3a−c in the presence of

oxygen (red lines in Figure 11) give the emission spectra of 3a−c

from the triplet states only (Figure 12 left panel). The virtually

identical structured emission spectra of 3a−c in Figure 12 (left

panel) are identiﬁed as RTP, presumably from combinations of

local donor and acceptor contributions. The phosphorescence

spectra in Figure 12 generated from subtractions match very

closely with the ms delay time-jump data for 3b−3c (Figure

S5e).

These data highlight how deconvolution can be eﬀectively

utilized to identify overlapping emissions bands. Closer

inspection of the oxygen-independent emission spectra for

3a−c reveals diﬀerences where the structured high energy band

for 3c is at considerably lower energy than the corresponding

band for 3a or 3b (Figure 12 right panel). Such a shift indicates

the eﬀect of the four methoxy groups in 3c. In the series 3a−c,

the conjugational separation between donor and acceptor units

is extensive. Four methoxy groups are required to induce a red

shift in the emission of 3c. The data suggest that two methoxy

groups in 3b are insuﬃcient for signiﬁcant donor−acceptor

communication and a red shift is not observed.

Figure 10. Steady-state emission spectra of 1a, 1b, and 1c in the zeonex

matrix at 290 K in the absence of oxygen. The onset of the main

emission band is shown with corresponding energy for the 0−0

transition. The 0−0 transition allows accurate comparison of the

emission.

shows the steady-state emission spectra of 1a−c in the zeonex

matrix in the absence of oxygen: the emission bands of 1b and 1c

broaden and red-shift with respect to 1a. The order of emission

maximum energies is 1b < 1c < 1a which does not agree with the

order of estimated HLG energies from CV measurements (1c <

1a < 1b; Table 2). Several diﬀerent emissions must be present

that complicate interpretations of these spectra. This is not

surprising as many diﬀerent static conformers in varying ratios

would be present in zeonex. Unlike 1a, the steady-state spectra

of both 1b and 1c show high energy band shoulders where the

shoulder is much more pronounced in 1b than in 1c. These

shoulders may be attributed to local ﬂuorescence (¹LE), but may

contain some contributions from CT ﬂuorescence from the axial

conformer ¹CT(ax). Phosphorescence spectra for 1b and 1c at

80 K in zeonex show poorly structured bands with similar high

energy onsets at 2.71 and 2.74 eV, respectively (Figure S5b).

The larger eﬀect of the methoxy groups at para positions, over

methoxy groups at meta positions, is clear from the comparison

of emission spectra of 1a, 1b, and 1c (Figure 10).

The structured high energy band is not dependent on oxygen

and is therefore assigned as local ﬂuorescence (¹LE). Broad

lower-energy ﬂuorescence bands are also present in these

tetramethyl systems, which are assigned as CT ﬂuorescence

[¹CT(ax)] from axial conformers. The assignments of ¹LE and

¹CT(ax) emission peaks are based on the fact that ¹LE emission

appears at much higher energy, and has ﬁne structure unlike the

broad CT(ax) emissions. The CT emission peaks are at

Oxygen dependence experiments can be used to deconvolute

emission processes where the triplet excited state is involved.⁴⁹

Pure ﬂuorescence emissions from singlet states have fast decay

times, so oxygen quenching of the emissions is unlikely to occur

in the solid zeonex matrix. By contrast, if emission originates

from excited-state molecules via a triplet excited state, the

emission is long-lived, and thus is likely to be quenched by

oxygen molecules. The radiative processes that are expected to

be quenched by oxygen include the direct phosphorescence of a

local triplet excited state (³LE) and the delayed ﬂuorescence

(DF) appearing as a result of the triplet state returning to the

singlet state [¹CT(eq)] by RISC (i.e., TADF).³¹Oxygen-

dependent emission intensity ratios (I_{no O}/I_O) are determined

1

wavelengths >500 nm, whereas the LE emission peaks are

below 450 nm. Therefore, the ¹LE emission is clearly

distinguished from the other emissions despite some overlap

of the spectra. The sequential increase in the PLQY from 3a

(7%) to 3b (13%) to 3c (17%) can be attributed to an increase

of the local phosphorescence induced by the two methoxy

groups in 3b, and to an increase in the ﬂuorescence intensity due

to the eﬀect of the four methoxy groups in 3c. PLQY

measurements are summarized in Figure S5f, highlighting the

eﬀect of the methoxy groups in the diﬀerent positions in these

systems. Curiously, methoxy groups in the meta-positions on 1c

do not reduce the PLQY of the D−A−D system compared to 1a.

Time-dependent emission measurements reveal DF emis-

sions for 1b, 1c, and 2b by comparison with data previously

reported for 1a and 2a (Figure 13).^30,31The time period of

2

by taking the integral of emission with and without oxygen, and

the values reported are independent of emission wavelength.

Figure 11 shows steady-state emission spectra in the absence

of oxygen (black lines) and in the presence of oxygen (red lines)

for all the compounds studied here. Molecule 1a has a I_{no O}/I_O

2

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Figure 11. Steady-state emission spectra showing the oxygen dependence of the emissions in the zeonex matrix at room temperature for all systems

studied. I_{no O}/I_O= emission intensity (no O₂)/emission intensity (with O₂) over the whole spectrum. Photoluminescence quantum yields (PLQY)

2

(Φ) are quoted with an error of 3%. (No O₂= Black. With O₂= Red).

Figure 12. Normalized phosphorescence (left) and ﬂuorescence (right) spectra in zeonex at 290 K for 3a, 3b, and 3c. The normalized spectra in the left

panel were obtained by subtractions of the emission curves in the absence of oxygen (black lines) with the corresponding oxygen-independent

emission curves (red lines) from Figure 11. The normalized spectra in the right panel are taken from the oxygen-independent emission curves (red

lines) from Figure 11.

around 1 μs between the occurrence of prompt ﬂuorescence

(PF) (≈10 ns) and phosphorescence (Ph) (≈1 ms)

corresponds to the decay of DF. The DF spectra are shown

for 1b and 1c in Figure S5b. Variable-temperature time-

dependent plots conﬁrm that 1a,³¹1b, and 1c have delayed

emission components in the 125 ns to 0.1 ms range where these

molecules emit via a TADF pathway (Figure S5c,d). No

signiﬁcant increase in the delayed emission intensity for

molecule 1c is observed by raising the temperature above 180

K, compared to 250 K for 1a,³¹with clear increases observed up

to 320 K for 1b. The variable-temperature and time-dependent

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Figure 13. Time-dependent emission plots for all the molecules in the zeonex matrix at 290 K. The labels indicate the main type of emission observed

at the given positions in the decays. Extremely weak ¹LE emission is detected in 1a at early ns delays times³¹but does not signiﬁcantly contribute to the

steady-state emission (Figure 11). Extremely weak RTP is also detected in 1b but does not signiﬁcantly contribute to the steady-state emission as no

blue shift of the emission is observed in the absence of oxygen as exactly the same spectral shape and λ_maxare observed in the presence and absence of

oxygen (Figure 11).

Figure 14. Assignments of emission bands through a combination of deconvoluting oxygen dependence spectra (Figure 11) by subtractions, and

ﬁtting time-jump data within the steady-state emission spectra (“all emissions”) for 1a, 1b, 1c, 2a, and 2b. The time-jump data (Figure S5b,d) and

deconvoluted spectra generated from Figure 11 are summed in appropriate ratios to suitably ﬁt under the “all emission” spectra. The ratio of CT(eq)

PF to DF was estimated using the change in intensity of the CT(eq) emission band on removal of oxygen (Figure 11). The time-jump emission

components when added together ﬁt very well under the “all emissions” spectra, showing that all emissions have been identiﬁed and no major emission

contributions were missed. See the literature for previous use of these methods.⁴⁹Assignments for 3a−c are detailed in Figure 12.

plots for 2a³⁰and 2b also show TADF emission in the 150 ns to

3 ms range.

Prompt ﬂuorescence (PF) results from the direct decay of the

singlet state [¹LE or ¹CT(eq) or ¹CT(ax)]. Only the equatorial

conformation of the studied molecules exhibits RISC. Only

CT(eq) emission is observed in the DF in the microsecond

region, with the exception of the slow CT(ax) emission from 3c

due to conjugational separation.

In molecules 1a−c and 2a−b, a mixed conformational

distribution is observed (Table 3); however, no CT(ax)

emission for 1a−c, 2a−b is observed. Literature reports show

that phenothiazine derivatives that are exclusively in the ax−ax

conformation show a signiﬁcantly blue-shifted CT band.⁴⁹

CT(eq) emission is therefore observed for 1b−c and 2a−b and

is assigned as equatorial based on the position of the CT spectra

with respect to data for 1a reported here and in previous

literature,⁴⁹and data for other reported derivatives.⁴⁹The

CT(eq) assignment matches well with the reported PLQY

values (Figure S5f) and with the general trends observed in the

DFT calculations (Table 3). Molecules showing essentially

exclusive ax−ax (>90%) conformation in DFT calculations

(Table 3) have lower PLQYs. The ax−ax conformers of 1a−c,

2a−b are, therefore, much less emissive relative to the ax−eq

conformers.

Only ¹CT(eq) emission is observed for molecules 1a−c and

2a−b as the CT(eq) emission in the μs delayed region matches

very closely with the CT emission observed in the steady-state

spectra. Any blue shift observed in the steady-state emission

spectra of 1a−c and 2a−b in the absence of oxygen (Figure 11)

has been clearly assigned to phosphorescence using the time-

dependent emission and time-jump data (Figure S5b,d). In

complete contrast, for 3a−c, the molecules are 100% in the axial

conformation. Because of the high rigidity of 3a−c (as shown by

the rotational and ﬂipping barriers in Table 3), the axial−axial

conformation becomes more emissive compared to the same

conformation in 1a−c and 2a−b.

Time-dependent emission plots for 3a and 3b reveal no

emissions in the range of 0.1 μs and 0.2 ms between PF and RTP.

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This happens in general for signals that decay at a very slow rate.

Table 4. Emission Photophysical Parameters for Molecules

Studied in the Zeonex Matrix

1

The LE and CT(ax) emissions in 3a−b have completely

decayed before the early microsecond region as they have fast

decay rates. The phosphorescence emission decays at an

extremely slow rate, and so no photons are detected in the

microsecond region where the integration times used are still

reasonably short. Therefore, a time gap with no emission is

observed, even with a highly sensitive iCCD camera. This

implies no DF emissions from these compounds in zeonex.

Remarkably for 3c, the time range where no emissions were

observed is much smaller, between 6 μs and 0.2 ms. There is

emission with slow times in the range of 0.1−6 μs. This μs

emission from 3c is extremely weak, and it is detected by the

sensitive iCCD camera. The emission from 3c at μs times is so

weak (<0.1% of the total emission) that this minor feature was

not pursued any further. Such μs emissions are not detected in

3a−b.

From all the emission spectra (oxygen, deoxygenated, and

time-dependent) with spectral subtractions, the diﬀerent

emissions in the TADF molecules 1a, 1b, 1c, 2a, and 2b are

assigned as shown in Figure 14. In the ¹CT(eq) (CT

ﬂuorescence from the equatorial conformer) transitions, the

intensity of TADF (DF) with respect to PF decreases in the

order 1a > 1c > 1b. It is interesting that the decrease of the RTP

intensity is in the order 1a to 1c then 1b and is coupled with the

increase of the local ﬂuorescence intensity. In 1a−c, the methoxy

groups facilitate local ﬂuorescence at the expense of RTP due to

increased ax−ax emission contributions.

λ_{max em}

E

¹CT

T₁

eV (nm)

ΔE_ST(eq)

c

^{m onset}a

^(eq)b

b

entry (nm) Φ_em3% eV (nm)

eV (nm)

(eV)

1a

1b

1c

2a

2b

3a

3b

3c

515

553

538

506

560

503

504

506

38

20

42

24

7

2.76

(450)

2.63

(471)

2.70

(458)

2.72

(456)

2.64

(469)

2.76

(450)

2.71

(458)

2.74

(453)

2.64

2.69

0.05

0.15

0.08

0.02

0.25

(470)

(461)

2.56

(484)

2.62

(473)

2.75

(451)

2.54

(488)

2.71

(458)

2.74

(453)

2.77

(447)

2.79

(444)

2.71

(457)

2.71

(458)

2.70

(459)

13

17

a

b

For the main emission band in the spectrum. From onsets of either

time-jump data (Figure S5b.d,e) or deconvoluted data in Figures 12

c

1

and 14. Diﬀerence between CT_(eq)and T₁energy.

Figure 15 highlights the processes that occur with the TADF and

RTP analogs following excitation from S₀.

The situation is diﬀerent with the D−A system 2a which

appears to exhibit intense TADF and little, if any, RTP in steady-

state emissions (Figure 14). The time-dependent emission plot

for 2a suggests some weak RTP between 10 and 1 ms (Figure

13). While it is diﬃcult to conﬁrm any signiﬁcant RTP within the

¹CT band in 2a, the emission does not show any blue shift in the

absence of oxygen (Figure 11). This is in contrast to 1a, where

the emission signiﬁcantly blue-shifts in the absence of oxygen

because of the higher energy local phosphorescence. The D−A

system 2b exhibits a similar blue shift without oxygen as in 1a

and this is also attributed to RTP. The phosphorescence

spectrum of 2b generated by deconvolution matches very closely

with the time-jump data (Figure S5d). Therefore, it is suggested

that the methoxy groups promote RTP with respect to the

parent system 2a. All of the information from the emission data

is summarized in Table 4.

Figure 15. Jablonski diagrams to represent the photophysical processes

that occur in (a) TADF materials (molecules 1a−c, 2a−b) and (b) for

phosphorescent (Ph) materials (molecules 3a−c).

1

3

Figure 15a summarizes how alignment of the CT and LE

excited states in the TADF molecules 1a−c and 2a−b allows for

harvesting of triplet states via the RISC process. The presence of

the RISC results in PF and DF occurring on the ns and μs

timescales, respectively. For molecules 3a−c (Figure 15b), the

¹CT and ³LE states are too widely separated for RISC to occur.

The 100% axial conformation of 3a−c results in excited states

being trapped in the ³LE state, and because of the rigidity of the

molecules, long-lived RTP can occur.

Table 4 summarizes the eﬀect of the methoxy groups on the

emission of the molecules studied herein. The key points are as

follows: (i) methoxy groups red-shift the emission in 1b−c and

2b with respect to the parent systems 1a−2a. (ii) The para-

methoxy groups lower the PLQY of 1b and 2b, but PLQY

remains essentially the same when meta-methoxy substituents

are used (1c). (iii) The methoxy substituents on 1b−c and 2b

The barrier for the interconversion of conformers is expected

to be very high in the solid state. There is strong evidence for this

in the spectra of 1a, 1c, and 2b where the ax−ax conformer

clearly exists in the zeonex ﬁlms. For these molecules, RTP is

seen at long times (into the ms region, see Figure 13). For the

molecules studied, the axial−axial conformation is required to

observe RTP, as in the other conformations (ax−eq) and (eq−

eq), RTP is not seen because of fast ns or early μs scale relaxation

processes (CT emission and TADF). Observing RTP in these

molecules on the ms timescale strongly suggests the ax−ax

conformation cannot convert when locked in the rigid zeonex

matrix as interconversion would result in fast CT emission

instead. The gas phase calculations are performed in the ground

state as the conformational distribution in the solid state is

expected to be the average distribution in solution. The

1

widen the ΔE_ST(eq)gap as a result of the red shift in the CT

energy where the triplet energy remains eﬀectively unchanged.

The widening of the ΔE_ST(eq)gap in combination with the

increased axial contributions explains why 1b−c and 2b have

reduced TADF contributions. (iv) It is, however, interesting that

the PLQY is not reduced in 1c, suggesting that meta-methoxy

substitution could be useful in future TADF analogs for ﬁne

tuning ΔE_ST. (v) In 3a−c, the methoxy groups do not

signiﬁcantly shift the phosphorescence emission but appear to

promote RTP contributions and also raise the PLQY. These

factors are all important for designing future TADF and RTP

analogs. Combining all the data presented in this manuscript,

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arbitrary units and adjusted to compare visually with 1a, 2a, and 3a,

respectively, for convenience. The SWV measurements for 1b in Figure

7 were recorded with a scan rate of 25 mV s⁻¹for improved resolution.

The HOMO and LUMO levels in Table 2 were obtained from the

onsets of the redox waves using the equations HOMO ≈ ionisation

potential (IP) = |e|(E_ox(onset) + 5.1) eV and LUMO ≈ electron aﬃnity

(EA) = −|e|(E_red(onset) + 5.1) eV. The +5.1 V value is the ferrocenium/

ferrocene couple potential.

calculations broadly follow the trends observed experimentally

and support the assignments given.

CONCLUSIONS

■

Methoxy groups at sites remote from the D−A bonds have been

shown to modulate the emission properties of a systematic series

of D−A−D and D−A organic molecules in a sophisticated

fashion through a balance of electronic and conformational

eﬀects. The energetic preference for axial over equatorial

conformers of the phenothiazine units upon addition of

methoxy groups has been established by hybrid-DFT

computations. This conformational preference results in

reduced TADF combined with an increase in local luminescence

(¹LE or ³LE in the form of RTP or both). Using methoxy groups

to increase the contribution of RTP to the overall emission of

D−A−D and D−A molecules is a new strategy that could be

useful for designing rigid axial molecules with favorable RTP.

These and other subtle substituent eﬀects are being investigated

in the search for optimal TADF and RTP molecules.

All calculations were carried out with the Gaussian 09 package.⁵⁶

Ground-state (S₀) geometries were fully optimized from diﬀerent

starting geometries using the popular B3LYP^57,58functional with the 6-

31(d) basis set.^59,60Optimized geometries at the more computationally

demanding model chemistry B3LYP/6-311G(d,p) did not have better

agreement with experimental geometries than those at B3LYP/6-

31G(d) level. All fully optimized S₀geometries were found to be true

minima based on no imaginary frequencies found from frequency

calculations. There are more minima/conformers located for 1b, 1c, 2b,

3b, and 3c where the methoxy group orientations are diﬀerent to those

shown in Supporting Information Section S4. These higher energy

minima are not included in the ﬁgures as these are considered to be of

little additional signiﬁcance to the study. The rotation energy proﬁles

were constructed from partially optimized geometries where one

C(donor)−N−C(acceptor)−C(acceptor) torsion angle is ﬁxed at 5°

intervals and each geometry optimization was carried out with the opt =

modredundant command. The inversion energy barrier for each

molecule was estimated from the energy of a planar N geometry, by

constraining the NC₄S ring to be planar and the geometry was partially

optimized, and the energy of the most stable fully optimized geometry

was determined. Electronic structure calculations were carried out on

selected minima to obtain the frontier orbital energies (HOMO/

LUMO) for comparison with experimental values from CV measure-

ments.

GENERAL EXPERIMENTAL DETAILS

■

All reactions were carried out under an argon atmosphere unless

otherwise stated. Starting materials were purchased commercially and

were used as received. Solvents were dried using an Innovative

Technology solvent puriﬁcation system and were stored in ampoules

under argon, with the exception of anhydrous DCB, which was

purchased from Sigma-Aldrich and used as received.

Thin-layer chromatography (TLC) analysis was carried out using

Merck silica gel 60 F₂₅₄TLC plates and spots were visualized using a

TLC lamp emitting at 365, 312, or 254 nm. Silica gel column

chromatography was performed using silica gel 60 purchased from

Fluorochem.

Thin ﬁlms in zeonex were prepared by spin-coating with a guest/

zeonex ratio of (1:20 w/w) from toluene solutions. Absorption and

emission spectra were collected using a UV-3600 double beam

spectrophotometer (Shimadzu) and a Fluorolog ﬂuorescence spec-

trometer (Jobin Yvon). The extinction coeﬃcient determination was

performed in CH₂Cl₂solution.

¹H and ¹³C NMR spectroscopy was carried out on Bruker AV400,

Varian VNMRS 600 and 700, and Varian Inova 500 NMR

spectrometers. Residual solvent peaks were referenced as described in

the literature,⁵⁵and all NMR data were processed in MestReNova V11.

Melting points were carried out on a Stuart SMP40 machine with a

ramping rate of 4 °C min⁻¹. Videos were replayed manually to

determine the melting point.

Phosphorescence, PF, and DF spectra and decays were recorded

using nanosecond gated luminescence and lifetime measurements

(from 800 ps to 1 s) with either a high energy pulsed Nd:YAG laser

emitting at 355 nm (EKSPLA) or a N₂laser emitting at 337 nm with

pulse width 170 ps. Emission was focused onto a spectrograph and

detected on a sensitive gated iCCD camera (Stanford Computer

Optics) between 350 and 700 nm with sub-nanosecond resolution.

Time-dependent and time-jump measurements were performed by

exponentially increasing the gate and delay times. The curve obtained

directly from this process does not represent the real luminescence

decay. However, this is easily corrected for by integrating the measured

spectra and dividing the integral by the corresponding integration time.

In this way, each experimental point represents a snapshot of the

number of photons emitted per second at a time t = delay + (integration

time)/2. The luminescence decay is then obtained by plotting each

experimental point against time and ﬁtting with the sum of

exponentials. When required, in this way, we are able to collect

luminescence decaying over 8 decades in time and in a single

experiment, providing true kinetic data. The kinetic behavior in the

molecules studied in the present work is highly complex in the solid

state, with multiple exponential decays/overlapping transitions that

cannot be accurately ﬁtted. For this reason, assignment of emission

peaks is given at positions/ranges in time rather than as ﬁtted decays.

For initial development of these methods; see a previously published

literature study.⁶¹PLQY measurements were performed using a

Quantaurus-QY Absolute PL quantum yield spectrometer using zeonex

ﬁlms prepared as described above.

High-resolution mass spectrometry was carried out on a Waters LCT

Premier XE using ASAP ionization with TOF detection. Samples were

analyzed directly as solids.

Elemental analysis was performed on an Exeter Analytical E-440

machine.

Any stated use of hexane refers to a mix isomers grade with the

exception of crystal growth for X-ray crystallography, where n-hexane

was used.

2,8-Diﬂuorodibenzothiophene-S,S-dioxide (6) and 2-bromodiben-

zothiophene-S,S-dioxide (13) were synthesized as reported in the

literature.³⁷Compound 13 was made using commercial 2-bromodi-

benzothiophene (TCI supplier).³⁷

Molecules 1a,³¹2a,³⁰and 3a³⁷were analyzed using a combination of

literature samples and literature data. Assignments of the photophysical

characteristics of 1a, 2a, and 3a are taken from the literature.

CV measurements were recorded at a scan rate of 100 mV s⁻¹at

room temperature using an air-tight single-compartment three-

electrode cell equipped with a glassy carbon working electrode (3

mm surface diameter), Pt wire counter electrode, and Pt wire pseudo-

reference electrode. The cell was connected to a computer-controlled

Autolab PG-STAT 30 potentiostat. The solutions contained the

compound (1 mg/mL) and n-Bu₄NPF₆(0.1 M) as the supporting

electrolyte in dichloromethane (DCM) or DMF. All potentials were

determined with the decamethylferrocene/decamethylferrocenium

couple as an internal reference in DCM at −0.534 V or in DMF at

−0.485 V for the usual reference standard of the ferrocene/ferrocenium

couple (FcH/FcH⁺) at 0.0 V. The current values in CV traces for 1b, 1c,

2b, 3b, and 3c in the Supporting Information (Section 3) are in

X-ray Crystallography. Crystals of X-ray quality were obtained by

layering n-hexane or 2,2,4-trimethylpentane on top of a solution of 3b in

THF or PhCl, which gave isomorphous solvates. Diﬀraction experi-

ments were carried out on a D8 Venture 3-circle diﬀractometer (Bruker

AXS) with a PHOTON 100 CMOS area detector, using Cu Kα

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The Journal of Organic Chemistry

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radiation (λ = 1.54184 Å) from an Incoatec IμS microsource with

focussing mirrors and a Cryostream (Oxford Cryosystems) open-ﬂow

N₂gas cryostat. The intensities were corrected for absorption by

numerical integration based on crystal face-indexing, using SADABS

program.⁶²The structures were solved by dual-space intrinsic phasing

method using the SHELXT 2018/2 program⁶³and reﬁned by full-

matrix least squares using SHELXL 2018/3 software⁶⁴on the OLEX2

platform.⁶⁵Crystal data are reported in Table S4 in Supporting

Information. The asymmetric unit of 3b·2THF contains two molecules

of 3b, one ordered THF molecule, two disordered THF molecules

which could not be reﬁned satisfactorily and were masked using the

PLATON SQUEEZE program,⁶⁶and some part-occupied THF

positions statistically mixed with part-occupied methoxy groups,

tentatively estimated as adding up to one more THF molecule per

asymmetric unit. The asymmetric unit of 3b·1.6PhCl contains two

molecules of 3b, one ordered and two disordered chlorobenzene

molecules in general positions, and 20% of a chlorobenzene molecule

disordered around an inversion center.

CIF ﬁles have been deposited with the Cambridge Structural

Database: CCDC-1866643 (3b·2THF) and 1866361 (3b·1.6PhCl).

Synthesis and Characterization. 2,8-Bis(3,7-dimethoxypheno-

thiazin-10-yl)dibenzothiophene-S,S-dioxide (1b). To a stirred

solution of 3,7-dimethoxyphenothiazine (10) (300 mg, 1.16 mmol, 2

equiv) in dry DMF (10 mL) under argon was added Cs₂CO₃(0.94 g,

2.89 mmol, 5 equiv). The reaction mixture was stirred at ambient

temperature for 30 min after which 2,8-diﬂuorodibenzothiophene-S,S-

dioxide (6) (146 mg, 0.58 mmol, 1 equiv) was added in one portion.

The reaction mixture was then heated with vigorous stirring at 155 °C

for 16 h. The reaction mixture was allowed to cool to ambient

temperature, and then water (50 mL) was added. The aqueous mixture

was then extracted with EtOAc (3 × 50 mL). The combined organic

extracts were washed with 1 M HCl_(aq)(2 × 100 mL), dried with

MgSO₄, and ﬁltered. The solvent was removed under reduced pressure

to give a crude residue. The residue was puriﬁed by silica gel column

chromatography, eluting initially with hexane followed by 5, 20, and

30% EtOAc/hexane (v/v). The solvent was removed under reduced

pressure to give an orange solid which was recrystallized from boiling

hexane with dropwise addition of ethanol until dissolution was

achieved. Cooling to −18 °C resulted in product crystallization. The

crystals were collected by ﬁltration and washed with cold hexane to give

the title product as a dark yellow solid (50 mg, 12%).

¹H NMR (400 MHz, CD₂Cl₂): δ 7.70 (d, J = 8.5 Hz, 2H), 7.29 (d, J =

2.1 Hz, 2H), 7.36−7.20 (m, 4H), 7.14 (t, J = 8.2 Hz, 2H), 6.72 (dd, J =

8.4, 1.0 Hz, 2H), 6.69 (dd, J = 8.6, 2.6 Hz, 2H), 6.60 (dd, J = 8.2, 1.0 Hz,

2H), 6.54 (d, J = 2.6 Hz, 2H), 3.89 (s, 6H), 3.72 (s, 6H); ¹³C{¹H} NMR

(101 MHz, CD₂Cl₂): δ 159.9, 156.8, 149.8, 143.0, 142.4, 133.9, 133.1,

129.3, 127.6, 123.7, 122.7, 120.3, 118.9, 116.5, 113.4, 111.2, 110.2,

107.6, 56.6, 56.0; HRMS-ASAP-TOF⁺m/z: calcd for C₄₀H₃₁N₂O₆S₃

[M + H]⁺, 731.1339; found, 731.1331; Anal. Calcd for C₄₀H₃₀N₂O₆S₃:

C, 65.74; H, 4.14; N, 3.83. Found: C, 65.35; H, 4.23; N, 3.69; mp

decomp. >350 °C.

2-(3,7-Dimethoxyphenothiazin-10-yl)dibenzothiophene-S,S-di-

oxide (2b). 2-Bromodibenzothiophene-S,S-dioxide (13) (590 mg, 2

mmol, 1 equiv) and 3,7-dimethoxyphenothiazine (10) (545 mg, 2.1

mmol, 1.05 equiv) were dissolved in anhydrous toluene (100 mL) and

degassed by bubbling with argon for 15 min. Pd₂(dba)₃(37 mg, 40

μmol, 0.02 equiv) and Xphos (95 mg, 200 μmol, 0.1 equiv) were added

and the solution was bubbled for a further 15 min. NaO^tBu (288 mg, 3

mmol, 1.5 equiv) was added and the mixture was heated at 100 °C with

stirring under argon for 18 h. After cooling to ambient temperature,

water (150 mL) was added and the organic products were extracted

into CH₂Cl₂and washed with water and brine. The organic layer was

dried over MgSO₄and the solvent was removed under vacuum to give

the crude product. The crude product was puriﬁed by silica gel column

chromatography eluting with 50% CH₂Cl₂/petroleum ether (v/v).

Removal of the solvent under reduced pressure gave a crude solid which

was recrystallized from acetone to give a pure product as a white solid

(240 mg, 25% yield).

¹H NMR (400 MHz, acetone-d₆): δ 7.81 (d, J = 7.0 Hz, 1H), 7.77 (d,

J = 7.5 Hz, 1H), 7.69 (td, J = 7.5, 1.2 Hz, 1H), 7.65−7.60 (m, 2H), 7.58

(d, J = 8.7 Hz, 2H), 7.41 (d, J = 2.3 Hz, 1H), 7.17 (d, J = 2.8 Hz, 2H),

7.07 (dd, J = 8.7, 2.8 Hz, 2H), 7.02 (dd, J = 8.7, 2.3 Hz, 1H), 3.88 (s,

6H); ¹³C{¹H} NMR (101 MHz, DMSO-d₆): δ 157.4, 152.0, 138.2,

135.0, 134.1, 133.5, 132.6, 131.0, 130.4, 128.3, 127.8, 123.4, 122.2,

121.7, 115.5, 114.1, 113.5, 106.3, 55.7; HRMS-ASAP-TOF⁺m/z: calcd

for C₂₆H₁₉NO₄S₂[M]⁺, 473.0750; found, 473.0750; Anal. Calcd for

C₂₆H₁₉NO₄S₂: C, 65.94; H, 4.04; N, 2.96. Found: C, 65.79; H, 4.01; N,

2.91; mp 256−258 °C.

2,8-Bis(1,9-dimethyl-3-methoxyphenothiazin-10-yl)-

dibenzothiophene-S,S-dioxide (3b). To a two-necked round-bottom

ﬂask equipped with a stirrer bar were added KO^tBu (380 mg, 3.40

mmol, 2.2 equiv) and 1,9-dimethyl-3-methoxyphenothiazine (11) (797

mg, 3.1 mmol, 2 equiv). The solids were dried under vacuum for 30 min

and backﬁlled with argon and dry DMF (40 mL) was added via syringe.

The mixture was stirred for 30 min then degassed by bubbling argon

through the solution for 30 min. 2,8-Diﬂuorodibenzothiophene-S,S-

dioxide (6) (390 mg, 1.6 mmol, 1 equiv) was added under a high ﬂow of

argon, and the mixture was heated to 110 °C for 16 h. The mixture was

allowed to cool to ambient temperature, water (50 mL) was added, and

the aqueous layer was extracted using EtOAc (4 × 50 mL). The organic

layers were combined, washed with 1 M HCl (2 × 50 mL), and dried

using MgSO₄. The organic layer was ﬁltered and the solvent was

removed to give the crude product as a brown solid. The crude mixture

was puriﬁed using silica gel column chromatography eluting with 5−

30% (v/v) EtOAc/hexane increasing EtOAc in 5% increments.

Removal of solvent under reduced pressure gave the product as an

oﬀ-white solid. Recrystallization in boiling hexane with the dropwise

addition of ethanol followed by cooling to −18 °C gave the title

compound as a pale orange solid (133 mg, 12%). Note that this

molecule shows two diastereomers that do not interconvert in solution

and many of the carbon peaks in the ¹³C{¹H} NMR spectrum are split

as a result of this; therefore, carbon spectra are reported to two decimal

places to show the separation between these peaks.

¹H NMR (400 MHz, DMSO-d₆): δ 7.65 (d, J = 8.7 Hz, 2H), 7.52 (d,

J = 8.8 Hz, 4H), 7.21 (d, J = 2.8 Hz, 4H), 7.02 (dd, J = 8.8, 2.8 Hz, 4H),

6.92 (d, J = 2.3 Hz, 2H), 6.85 (dd, J = 8.7, 2.3 Hz, 2H), 3.85 (s, 12H);

¹³C{¹H} NMR (151 MHz, CD₂Cl₂): δ 158.4, 152.3, 136.4, 134.3,

133.5, 130.0, 128.7, 122.8, 115.4, 114.0, 113.7, 106.1, 56.2; HRMS-

ASAP-TOF⁺m/z: calcd for C₄₀H₃₁N₂O₆S₃[M + H]⁺, 731.1339; found,

731.1344; Anal. Calcd for C₄₀H₃₀N₂O₆S₃: C, 65.74; H, 4.14; N, 3.83.

Found: C, 65.78; H, 4.15; N, 3.75; mp 273−275 °C.

2,8-Bis(2,6-dimethoxyphenothiazin-10-yl)dibenzothiophene-S,S-

dioxide (1c). To a stirred solution of 2,6-dimethoxyphenothiazine (5)

(300 mg, 1.16 mmol, 2 equiv) in dry DMF (15 mL) under argon was

added Cs₂CO₃(0.94 g, 2.89 mmol, 5 equiv). The reaction mixture was

stirred at ambient temperature for 30 min after which 2,8-

diﬂuorodibenzothiophene-S,S-dioxide (6) (146 mg, 0.58 mmol, 1

equiv) was added in one portion. The reaction mixture was then heated

with vigorous stirring at 155 °C for 16 h. The reaction mixture was

allowed to cool to ambient temperature, and then water (50 mL) was

added. The aqueous mixture was then extracted with EtOAc (3 × 50

mL). The combined organic extracts were dried with MgSO₄and were

ﬁltered. The solvent was removed under reduced pressure to give a

crude residue. The residue was puriﬁed by silica gel column

chromatography with EtOAc/hexane solvent and gradient elution

from 30 to 50% (v/v) at 10% intervals. The solvent was removed under

reduced pressure to give an orange solid which was recrystallized from

boiling hexane with dropwise addition of ethanol. Hot ﬁltration through

cotton wool resulted in precipitation of the product in the cotton wool.

Dissolution of the precipitate with CH₂Cl₂followed by evaporation

under reduced pressure gave the product as a dark orange powder (80

mg, 19% yield).

¹H NMR (600 MHz, DMSO-d₆): δ 7.65 (dd, J = 8.7, 0.9 Hz, 2H),

7.50 (d, J = 7.7 Hz, 2H), 7.45 (t, J = 7.1 Hz, 2H), 7.37 (t, J = 7.7 Hz,

1H), 7.34 (t, J = 7.7 Hz, 1H), 7.10 (t, J = 2.9 Hz, 2H), 7.03 (dd, J = 7.7,

2.3 Hz, 2H), 6.38 (ddd, J = 8.7, 2.4, 1.0 Hz, 2H), 6.23−6.19 (m, 2H),

3.86 (d, J = 9.8 Hz, 6H), 2.37 (d, J = 5.0 Hz, 6H), 2.35 (s, 6H); ¹³C{¹H}

NMR (151 MHz, DMSO-d₆): δ 157.73, 157.72, 149.56, 149.54, 139.31,

139.26, 137.61, 136.71, 136.69, 136.65, 136.60, 135.36, 135.32, 132.03,

132.00, 131.80, 131.75, 129.43, 129.39, 128.59, 127.40, 127.27, 126.65,

L

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J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Article

123.51, 115.37, 114.04, 111.16, 103.51, 103.48, 55.71, 55.67, 17.66,

17.62, 17.44; HRMS-ASAP-TOF⁺m/z: calcd for C₄₂H₃₅N₂O₄S₃[M +

H]⁺, 727.1753; found, 727.1770; Anal. Calcd for C₄₂H₃₄N₂O₄S₃: 69.40;

H, 4.71; N, 3.85. Found: C, 68.99; H, 4.69; N, 3.80; mp decomp. 349

°C.

solvent under reduced pressure gave the title compound as a pink solid

(6.59 g, 95%).

¹H NMR (400 MHz, acetone-d₆): δ 7.09 (d, J = 7.4 Hz, 1H), 6.98−

6.93 (m, 2H), 6.84 (d, J = 2.8 Hz, 1H), 6.74 (dd, J = 8.7, 2.8 Hz, 1H),

6.67 (td, J = 7.4, 1.1 Hz, 1H), 6.41 (dd, J = 8.1, 0.9, 1H), 5.91 (s, 1H),

3.77 (s, 3H), 2.26 (s, 3H), 2.16 (s, 3H); ¹³C{¹H} NMR (101 MHz,

acetone-d₆): δ 157.4, 146.0, 135.4, 135.2, 131.2, 127.3, 126.6, 125.0,

119.3, 116.9, 114.7, 112.6, 55.6, 18.3, 18.0; HRMS-ASAP-TOF⁺m/z:

calcd for C₁₅H₁₈NO [M + H]⁺, 228.1383; found, 228.1378.

Bis(4-methoxyphenyl)amine (8). This molecule was synthesized by

a new route, and data are consistent with the literature.⁶⁹

2,8-Bis(1,9-dimethyl-3,7-dimethoxyphenothiazin-10-yl)-

dibenzothiophene-S,S-dioxide (3c). To a dry two-necked round-

bottom ﬂask equipped with a stirrer bar were added Cs₂CO₃(710 mg,

2.17 mmol, 5 equiv) and 1,9-dimethyl-3,7-dimethoxyphenothiazine

(12) (247 mg, 0.86 mmol, 2 equiv). The mixture was dried under

vacuum for 30 min and backﬁlled with argon and dry DMF (13 mL)

was added via a syringe. The solution was stirred for 30 min and then

deoxygenated by bubbling argon through the solution for 30 min. 2,8-

Diﬂuorodibenzothiophene-S,S-dioxide (6) (0.110 g, 0.43 mmol, 1

equiv) was added under a high ﬂow of argon, and the solution was

heated to 155 °C for 16 h. The reaction mixture was allowed to cool to

ambient temperature, water (25 mL) was added, and the aqueous layer

was extracted using EtOAc (4 × 50 mL). The organic layers were

combined, washed with 1 M HCl_(aq)(2 × 50 mL), separated, dried with

MgSO₄, and ﬁltered. The solvent was removed under reduced pressure

to give the crude product as a brown solid. The crude product was

puriﬁed using silica gel column chromatography eluting with 5−40%

EtOAc/hexane (v/v) increasing EtOAc in 5% increments. Removal of

the solvent under reduced pressure gave the product as an oﬀ-white

solid. Recrystallization in boiling hexane with the dropwise addition of

ethanol followed by cooling to −18 °C giving a white solid which was

ﬁltered and washed with cold Et₂O to give the product as a white solid

(87 mg, 26%).

Using general procedure A, p-anisidine (4.17 g, 33.90 mmol, 1.13

equiv), NaO^tBu (5.77 g, 60.00 mmol, 2 equiv), Pd(dppf)Cl₂·CH₂Cl₂

(0.74 g, 0.90 mmol, 0.03 equiv), toluene (100 mL), and 4-bromoanisole

(5.61 g/3.76 mL, 30 mmol, 1 equiv) were used to make the title

compound with heating at 100 °C for 4 h. The crude mixture was

extracted as described for 4-methoxy-2-methyl-N-(o-tolyl)aniline (7).

Column chromatography was performed as detailed in general

procedure A. Removal of solvent under reduced pressure gave the

title compound as a red solid (5.08 g, 74%).

¹H NMR (400 MHz, acetone-d₆): δ 6.99−6.94 (m, 4H), 6.84−6.79

(m, 4H), 6.77 (s, 1H), 3.73 (s, 6H); ¹³C{¹H} NMR (101 MHz,

acetone-d₆): δ 154.7, 139.3, 119.6, 115.4, 55.7; HRMS-ASAP-TOF⁺m/

z: calcd for C₁₄H₁₆NO₂[M + H]⁺, 230.1176; found, 230.1185.

Bis(2-methyl-4-methoxyphenyl)amine (9). This molecule has been

previously synthesized, and data are consistent with the literature.⁷⁰

Using general procedure A, 4-methoxy-2-methylaniline (4.65 g,

33.90 mmol, 1.13 equiv), NaO^tBu (5.77 g, 60.00 mmol, 2 equiv),

Pd(dppf)Cl₂·CH₂Cl₂(0.74 g, 0.90 mmol, 0.03 equiv), toluene (100

mL), and 4-bromo-3-methylanisole (6.04 g/4.24 mL, 30 mmol, 1

equiv) were used with heating at 100 °C for 4 h. The reaction mixture

was extracted and puriﬁed by column chromatography as in the

procedure for 4-methoxy-2-methyl-N-(o-tolyl)aniline (7). Column

chromatography was performed as detailed in general procedure A.

Removal of solvent under reduced pressure gave the title compound as

a red solid (6.06 g, 78%).

¹H NMR (400 MHz, CD₂Cl₂): δ 7.42 (d, J = 9.2 Hz, 2H), 6.93 (d, J =

2.5 Hz, 4H), 6.85 (d, J = 2.5 Hz, 4H), 6.38−6.43 (m, 4H), 3.86 (s,

12H), 2.36 (s, 12H); ¹³C{¹H} NMR (176 MHz, CD₂Cl₂): δ 158.3,

151.0, 138.2, 137.9, 133.58, 133.56, 129.4, 122.9, 115.6, 114.4, 111.3,

105.3, 56.0, 18.4; HRMS-ASAP-TOF⁺m/z: calcd for C₄₄H₃₉N₂O₆S₃

[M + H]⁺, 787.1965; found, 787.1962; Anal. Calcd for C₄₄H₃₈N₂O₆S₃:

C, 67.15; H, 4.87; N, 3.56. Found: C, 66.85; H, 5.10; N, 3.40; mp

decomp. 303 °C.

¹H NMR (400 MHz, acetone-d₆): δ 6.78 (d, J = 2.5 Hz, 2H), 6.67−

6.60 (m, 4H), 5.64 (s, 1H), 3.73 (s, 6H), 2.19 (s, 6H); ¹³C{¹H} NMR

(101 MHz, acetone-d₆): δ 155.4, 138.1, 130.9, 121.1, 117.2, 112.3, 55.6,

18.3; HRMS-ASAP-TOF⁺m/z: calcd for C₁₆H₂₀NO₂[M + H]⁺,

258.1489; found, 258.1485.

2,6-Dimethoxyphenothiazine (5): General Procedure B. This

molecule has not been reported previously. The synthesis was based

on a modiﬁed literature procedure.⁷¹

To a dry 50 mL two-neck round-bottomed ﬂask equipped with a

stirrer bar were added bis-N-(3-methoxyphenyl)amine (4) (3.50 g, 15.3

mmol, 1 equiv), sulfur (1.47 g, 45.9 mmol, 3 equiv), and I₂(697 mg,

2.74 mmol, 0.18 equiv). Under a ﬂow of argon was added dry DCB (20

mL). The reaction mixture was then deoxygenated by bubbling with

argon for 30 min and was then heated to 180 °C for 4 h. After cooling

the reaction mixture to room temperature, the mixture was puriﬁed by

loading the entire reaction mixture onto a silica column packed with

hexane. After removing DCB solvent by eluting with hexane (750 mL),

the product was eluted with 20−60% (v/v) CH₂Cl₂/hexane in 500 mL

increments at 10% intervals, followed by 60, 70, and 100% CH₂Cl₂/

hexane (v/v) with 0.5% Et₃N in 500 mL increments each. Removal of

the solvent under reduced pressure gave a yellow oil, which solidiﬁed on

drying under high vacuum (384 mg, 10%).

Bis(3-methoxyphenyl)amine (4): General Procedure A. This

molecule has been synthesized by a new route, and data are consistent

with the literature data.⁶⁷

To a 250 mL round-bottomed ﬂask equipped with a stirrer bar were

added NaO^tBu (7.09 g, 73.8 mmol, 2 equiv) and Pd(dppf)Cl₂·CH₂Cl₂

(904 mg, 1.11 mmol, 0.03 equiv), and the ﬂask was ﬂushed with argon.

Dry toluene (120 mL) was then added via cannula, followed by m-

anisidine (5.00 g/5.5 mL, 40.6 mmol, 1.1 equiv) and 3-bromoanisole

(6.90 g/4.7 mL, 36.9 mmol, 1 equiv) via a syringe. The solution was

bubbled with argon for 15 min, and the mixture was heated to 100 °C

with stirring for 18 h. On cooling the reaction mixture to ambient

temperature, the solvent was removed under reduced pressure to give a

crude oil. The crude oil was puriﬁed by silica gel column

chromatography eluting with 20% EtOAc/hexane with 0.5% Et₃N

(v/v). Removal of solvent yielded the title product as a red oil (6.05 g,

72%).

¹H NMR (400 MHz, acetone-d₆): δ 7.41 (s, 1H), 7.14 (t, J = 8.1 Hz,

2H), 6.75−6.66 (m, 4H), 6.45 (ddd, J = 8.2, 2.4, 0.9 Hz, 2H), 3.75 (s,

6H); ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 160.8, 144.4, 130.2, 110.8,

106.7, 104.0, 55.4; HRMS-ASAP-TOF⁺m/z: calcd for C₁₄H₁₆NO₂[M

+ H]⁺, 230.1176; found, 230.1179.

4-Methoxy-2-methyl-N-(o-tolyl)aniline (7). This molecule has been

synthesized by a new route, and data are consistent with the literature

data.⁶⁸

Using general procedure A, 4-methoxy-2-methylaniline (4.66 g, 33.9

mmol, 1.13 equiv), NaO^tBu (5.77 g, 60.0 mmol, 2 equiv), Pd(dppf)Cl₂·

CH₂Cl₂(0.74 g, 0.9 mmol, 0.03 equiv), toluene (100 mL), and 2-

bromotoluene (5.11 g/3.60 mL, 30.0 mmol, 1 equiv) were used to make

the title compound with heating at 100 °C for 4 h. The solvent was

removed under reduced pressure at 55 °C to prevent the mixture

solidifying on solvent evaporation. 1 M HCl_(aq)was added (100 mL),

and the mixture was extracted with CH₂Cl₂(4 × 100 mL). The solvent

was removed under reduced pressure to yield a brown solid. Column

chromatography was performed as in general procedure A. Removal of

¹H NMR (400 MHz, acetone-d₆): δ 7.65 (s, 1H), 6.89 (t, J = 8.1 Hz,

1H), 6.79 (d, J = 8.5 Hz, 1H), 6.44 (dd, J = 8.3, 1.0 Hz, 1H), 6.36 (dd, J

= 8.5, 2.6 Hz, 1H), 6.28 (dd, J = 8.0, 1.0 Hz, 1H), 6.26 (d, J = 2.6 Hz,

1H), 3.79 (s, 3H), 3.70 (s, 3H); ¹³C{¹H} NMR (151 MHz, DMSO-d₆):

δ 159.1, 154.6, 142.6, 141.8, 127.2, 126.8, 107.5, 106.9, 104.7, 103.9,

100.5, 55.7, 55.0; HRMS-ASAP-TOF⁺m/z: calcd for C₁₄H₁₄NO₂S [M

+ H]⁺, 260.0740; found, 260.0730; mp 113−115 °C.

3,7-Dimethoxyphenothiazine (10). This molecule has been

synthesized by a new route, and data are consistent with the literature

data.⁷²

Using general procedure B, bis-N-(4-methoxyphenyl)amine(8)

(2.11 g, 9.2 mmol, 1 equiv), sulfur (885 mg, 27.6 mmol, 3 equiv), I₂

M

DOI: 10.1021/acs.joc.8b02848

J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry

Article

(420 mg, 1.66 mmol, 0.18 equiv), and DCB (12 mL) were used to make

the title compound. The compound was puriﬁed by silica gel column

chromatography initially eluting with hexane, followed by EtOAc/

hexane 5−30% (v/v) increasing in 5% increments. The column was

repeated because of diﬃculties removing DCB in the ﬁrst column.

Removal of the solvent under reduced pressure gave the product as a

yellow solid (405 mg, 17%).

13-7). J.S.W. and M.R.B. thank EU Horizon 2020 grant

agreement no. 732103 (HyperOLED) for funding.

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Trans. 2011, 40, 1028−1030.

1,9-Dimethyl-3-methoxyphenothiazine (11). Using general pro-

cedure B, 4-methoxy-2-methyl-N-(o-tolyl)aniline (7) (3.50 g, 15.4

mmol, 1 equiv), sulfur (1.48 g, 46.2 mmol, 3 equiv), I₂(0.51 g, 2.0

mmol, 0.13 equiv), and DCB (20 mL) were used to make the title

compound. The compound was puriﬁed by silica gel column

chromatography initially eluting with hexane, followed by EtOAc/

hexane 5−30% (v/v) increasing in 5% increments. Removal of solvent

gave a yellow solid (1.63 g, 41%).

¹H NMR (400 MHz, CD₂Cl₂): δ 6.92 (d, J = 7.4 Hz, 1H), 6.87 (d, J =

7.4 Hz, 1H), 6.79−6.71 (m, 1H), 6.51 (s, 1H), 6.48 (d, J = 2.7 Hz, 1H),

5.69 (s, 1H), 3.70 (s, 3H), 2.24 (s, 3H), 2.23 (s, 3H); ¹³C NMR (101

MHz, acetone d₆): δ 156.1, 141.8, 134.6, 129.8, 125.1, 124.5, 123.2,

122.5, 120.3, 118.6, 115.7, 110.3, 55.8, 17.2, 17.0; HRMS-ASAP-TOF⁺

m/z: calcd for C₁₅H₁₆NOS [M + H]⁺, 258.0947; found, 258.0952; mp

112−114 °C.

1,9-Dimethyl-3,7-dimethoxyphenothiazine (12). Using general

procedure B, bis(2-methyl-4-methoxyphenyl)amine (9) (3.50 g, 13.6

mmol, 1 equiv), sulfur (1.48 g, 40.8 mmol, 3 equiv), I₂(0.45 g, 1.8

mmol, 0.13 equiv), and DCB (20 mL) were used to make the title

compound. The compound was puriﬁed by silica gel column

chromatography initially eluting with hexane, followed by EtOAc/

hexane 5−30% (v/v) increasing in 5% increments. Removal of the

solvent gave a yellow solid (280 mg, 7%).

¹H NMR (400 MHz, DMSO-d₆): δ 6.58 (d, J = 2.8 Hz, 2H), 6.50 (d,

J = 2.8 Hz, 2H), 6.31 (s, 1H), 3.65 (s, 6H), 2.23 (s, 6H); ¹³C{¹H} NMR

(176 MHz, CD₂Cl₂): δ 155.2, 134.8, 123.3, 119.1, 115.3, 110.0, 56.0,

17.4; HRMS-ASAP-TOF⁺m/z: calcd for C₁₆H₁₈NO₂S [M + H]⁺,

288.1053; found, 288.1050; mp decomp. 179 °C.

ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.joc.8b02848.

■

S

¹H and ¹³C{¹H} NMR spectra, absorption spectra, CV

traces and data, additional emission data, and computa-

tional data (PDF)

Crystallographic information ﬁle for 3b·2THF (CIF)

Crystallographic information ﬁle for 3b·1.6PhCl (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: m.r.bryce@durham.ac.uk.

(13) Im, Y.; Kim, M.; Cho, Y. J.; Seo, J.-A.; Yook, K. S.; Lee, J. Y.

Molecular Design Strategy of Organic Thermally Activated Delayed

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(15) Wong, M. Y.; Zysman-Colman, E. Purely Organic Thermally

Activated Delayed Fluorescence Materials for Organic Light-Emitting

Diodes. Adv. Mater. 2017, 29, 1605444.

■

ORCID

Mark A. Fox: 0000-0002-0075-2769

Andrei S. Batsanov: 0000-0002-4912-0981

Zhongjie Ren: 0000-0002-7981-4431

Fernando B. Dias: 0000-0001-9841-863X

Martin R. Bryce: 0000-0003-2097-7823

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Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The authors thank EPRSC for funding grant code EP/

L02621X/1. R.S.N. thanks the ﬁnancial support from CAPES

Foundation, Ministry of Education-Brazil (grant no. BEX9474-

N

DOI: 10.1021/acs.joc.8b02848

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