Two Is Better than One? Investigating the Effect of Incorporating Re(CO)3Cl Side Chains into Pt(II) Diynes and Polyynes

Re(CO) Cl Side Chains into Pt(II) Diynes and Polyynes

Article

Two Is Better than One? Investigating the Eﬀect of Incorporating

Ashanul Haque, Rayya Al-Balushi, Idris Juma Al-Busaidi, Nawal K. Al-Rasbi, Sumayya Al-Bahri,

Mohammed K. Al-Suti, Muhammad S. Khan,* Osama K. Abou-Zied,* Jonathan M. Skelton,*

and Paul R. Raithby*

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sı Supporting Information

ABSTRACT: Pt(II) diynes and polyynes incorporating 5,5′- and 6,6′-

disubstituted 2,2′-bipyridines were prepared following conventional Sonogashira

and Hagihara dehydrohalogenation reaction protocols. Using Pt(II) dimers and

polymers as a rigid-rod backbone, four new heterobimetallic compounds

incorporating Re(CO) Cl as a pendant functionality in the 2,2′-bipyridine core

were obtained. The new heterobimetallic Pt−Re compounds were characterized by

analytical and spectroscopic techniques. The solid-state structures of a Re(I)-

coordinated diterminal alkynyl ligand and a representative model compound were

determined by single-crystal X-ray diﬀraction. Detailed photophysical character-

ization of the heterobimetallic Pt(II) diynes and polyynes was carried out. We ﬁnd

that the incorporation of the Re(CO) Cl pendant functionality in the 2,2′-bipyridine-containing main-chain Pt(II) diynes and

polyynes has a synergistic eﬀect on the optical properties, red shifting the absorption proﬁle and introducing strong long-wavelength

absorptions. The Re(I) moiety also introduces strong emission into the monomeric Pt(II) diyne compounds, whereas this is

suppressed in the polyynes. The extent of the synergy depends on the topology of the ligands. Computational modeling was

performed to compare the energetic stabilities of the positional isomers and to understand the microscopic nature of the major

optical transitions. We ﬁnd that 5,5′-disubstituted 2,2′-bipyridine systems are better candidates in terms of yield, photophysical

properties, and stability than their 6,6′-substituted counterparts. Overall, this work provides an additional synthetic route to control

the photophysical properties of metallaynes for a variety of optoelectronic applications.

INTRODUCTION

materials, molecular electronics, bioimaging, and the

■

capture of organic and inorganic pollutants, among others.

It is now well established that the optical properties of the

poly(metallaynes) can be tuned by informed selection of the

Polyynes and poly(metallaynes) are widely studied materials

with a diverse range of uses. Enormous progress has been

made in understanding the chemistry and photophysical

properties of these materials, leading to a number of

,19

metals, spacers, and auxiliary ligands.

Several strategies are

−7

available to improve the properties of metallaynes, such as

introducing donor−acceptor (D−A) ligands, increasing the

eﬀective conjugation length, changing the position of ligand

attachment, shielding the polymer backbone, forming supra-

molecular assemblies, etc. We recently demonstrated that

Pt(II) diynes and polyynes bearing 2,7- and 3,6-carbazole

spacers have diﬀering levels of conjugation and frontier

orbitals, leading to substantial variation in optical properties.

We have also shown that the topology around the metal

centers has an impact on the isomerization of photoactive

cores such as azobenzenes.

applications.

It is now well established that “rigid-rod”

metalated systems oﬀer improved properties and performance

over their organic counterparts.

,7,8

The inclusion of a

transition metal ion such as Cr(III), Mo(II), W(III), Mn(I),

Fe(II), Ru(II), Co(III), Rh(III), Ni(II), Pd(II), Pt(II), Au(I),

etc. into the organic backbone through σ-linkages drastically

modulates and often improves the structural, photophysical,

and redox properties. Insertion of a heavy metal ion into an

organic backbone assists in bypassing spin-forbidden electronic

transitions via intersystem crossing, leading to a substantial

increase in the population of the emissive states and improved

luminescence. Based on this concept, several successful

Received: September 16, 2020

Published: December 31, 2020

attempts have been made to combine the semiconducting

properties of conjugated polyynes with the electronic eﬀects

induced by the presence of the heavy metal.

,10−13

Many of

these metallaynes have proven to be good candidates for

applications in the areas of photovoltaics, light-emitting

diodes, magnetic materials, catalysts, nonlinear optical

2020 American Chemical Society

Inorg. Chem. 2021, 60, 745−759

Inorganic Chemistry

Article

Scheme 1. Synthesis of the Precursors 1a−1c/2a−2c, the Pt(II) Diynes M1−M4, and the Pt(II) Polyynes P1−P4

Reaction conditions: (i) Pd(OAc) , CuI, PPh , Pr NH/THF; (ii) KOH, MeOH/THF; (iii) Re(CO) Cl, toluene, 60 °C; (iv) 1.0 equiv of trans-

[

(P Bu ) PtCl ], CuI, Pr NH, CH Cl ; (v) 2.0 equiv of trans-[Pt(PEt ) PhCl], CuI, Pr NH, CH Cl .

An additional and promising strategy to optimize the

optoelectronic properties is to incorporate a second (hetero)

Pr NH/THF to obtain 5,5′-bis(trimethylsilylethynyl)-2,2′-

bipyridine 1a and 6,6′-bis(trimethylsilylethynyl)-2,2′-bipyri-

22−25

metal ion into the main backbone or as a side chain.

The

dine 2a (Scheme 1). The diterminal alkynes 5,5′-bis-

introduction of a second metal into a metallayne induces

(

ethynyl)-2,2′-bipyridine 1b and 6,6′-bis(ethynyl)-2,2′-bipyr-

donor−acceptor interactions and inﬂuences the energy levels

idine 2b were then obtained by removal of the trimethylsilyl

protecting groups with aqueous KOH in MeOH/THF. The

diterminal alkynes were puriﬁed by silica gel column

chromatography giving 1b and 2b in 60−76% yield. The

protected and diterminal alkynes were fully characterized by

infrared (IR) spectroscopy, multinuclear NMR spectroscopy,

of the frontier orbitals and hence the absorption/emission

24,25

wavelength, conductivity, and redox behavior.

particular, introduction of an ion such as Re(I) facilitates the

movement of charges, narrows the band gap E , and produces

long-lived luminescence. It is reported that Re(I)-2,2′-

diimine complexes show environment-sensitive metal-to-ligand

charge-transfer (MLCT) excited states, which can be exploited

30,31

and electrospray ionization (ESI) mass spectrometry.

The diterminal alkynes 1b and 2b were reacted with

rhenium(I) pentacarbonyl chloride in toluene at 60 °C

overnight under an argon atmosphere to obtain the Re-

(

cation by alumina column chromatography, the chelated

diterminal alkynyl ligands were obtained in overall 49−62%

yield and were again characterized using IR, multinuclear

NMR, and mass spectrometry. The room temperature reaction

of the Re(CO) Cl-incorporated diterminal alkynyl ligands 1c

and 2c with 2 equiv of trans-[Pt(PEt ) (Ph)Cl] in Pr NH/

to develop luminescent and redox-active materials. Li and co-

workers found that the incorporation of Re(I) as a pendant

side chain into a bithiazole-containing poly(platinayne)

signiﬁcantly modiﬁed the photophysical properties of the

polymer. Such Pt(II)−Re(I) heterobimetallic systems are

unique in allowing the eﬀects of two diﬀerent metal ions to

be synergistically combined, potentially leading to new and

useful physical properties.

Prompted by the exemplary features of Pt(II) diynes and

polyynes and the interesting luminescence properties of the

Re(I) fragment, we have synthesized and characterized two

CO) Cl-chelated 5,5′-bis(ethynyl)-2,2′-bipyridine 1c and

,6′-bis(ethynyl)-2,2′-bipyridine 2c (Scheme 1). After puriﬁ-

CH Cl under an argon atmosphere in the presence of a CuI

Re(I)-coordinated diterminal alkynyl ligands, viz., (CO) ClRe-

catalyst aﬀords the Pt(II) diynes trans-[(Et P) (Ph)Pt−C

(

Ι)-5,5′-bis(ethynyl)-2,2′-bipyridine (1c) and (CO) ClRe(Ι)-

C−R−CC−Pt(Et P) (Ph)], with R = (CO) ClRe-2,2′-

,6′-bis(ethynyl)-2,2′-bipyridine (2c), together with the

bipyridine-5,5′-diyl (M3) and R = (CO) ClRe-2,2′-bipyr-

corresponding Pt(II) diynes M3/M4 and polyynes P3/P4.

Based on structural characterization, photophysical measure-

ments, and computational modeling, we show how the

incorporation of Re(I) inﬂuences the optical properties of

the systems and thereby provides an additional dimension for

modifying the properties of these conjugated metallopolymers.

idine-6,6′-diyl (M4), as yellow solids in 40−45% yield

(

Scheme 1). Similarly, the CuI catalyzed dehydrohalogenation

polycondensation reaction between trans-[(P Bu ) PtCl ] and

readily aﬀords the corresponding polyynes P3/P4 (Scheme 1).

c and 2c in a 1:1 ratio under similar reaction conditions

All compounds were readily soluble in CH Cl . The IR

spectra of the Pt(II) diynes and polyynes show a single sharp

RESULTS AND DISCUSSION

−1

■

absorption around 2066−2089 cm , consistent with a

CC

Synthesis and Spectroscopic Characterization. A

modiﬁed Pd(II)/Cu(I)-catalyzed cross-coupling reaction

between 5,5′- and 6,6′-dibromo-2,2′-bipyridine (Scheme 1)

and ethynyltrimethylsilane (TMSA) was performed in

trans conﬁguration of the alkynyl bridging ligands around the

Pt(II) center. The carbonyl groups in the (bipy)Re(I)(CO) Cl

chromophore give rise to three clearly resolved IR bands in the

−

range 1853−2018 cm in a CH Cl solution. The presence

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Article

of the intact Re(I) chromophore in CH Cl solutions of the

polyynes P3 and P4 was conﬁrmed from the presence of CO

−

stretching bands between 1897 and 2018 cm and 1871−

−

012 cm , respectively. The Pt(II) di- and polyynes showed

lower v frequencies than the corresponding chelated

CC

diterminal alkynyl ligands 1c and 2c, which we attributed to

charge transfer between the metal and the 2,2′-bipyridine

moiety.

The H and ¹³C NMR spectra of all compounds exhibit the

expected signals including those from the acetylenic carbons

(

see the Experimental Section). The P NMR spectra of the

Pt(II) diynes and polyynes conﬁrm the trans arrangement of

the phosphine ligands, and the P NMR spectra of M3 and

M4 show larger J

coupling constants than the polyynes P3

Pt−P

and P4 by about 300 Hz.

Gel-permeation chromatography (GPC) using a polystyrene

calibration gave molecular weights in the range of 90,000−100

00 g/mol for the polyynes P1 and P2, corresponding to a

degree of polymerization (DP) between 75 and 86 repeat units

and a polydispersity index (PDI) between 1.3 and 1.7. The

weight-average molecular weights of P3 and P4 are in the

range of 77,000−83 000 g/mol, corresponding to a DP

between 50 and 55 and a PDI of 1.2−1.5. These molecular

weights should, however, be treated with caution in view of the

diﬃculties inherent in characterizing rigid-rod polymers with

GPC. GPC does not give absolute molecular weights but

provides a measure of the hydrodynamic volume, and rodlike

polymers in a solution possess very diﬀerent hydrodynamic

properties to more ﬂexible systems. Calibration of the GPC

with a polystyrene standard is thus likely to overestimate the

molecular weights of the polyynes. However, the lack of

discernible resonances from end groups in the NMR spectra

nonetheless points to a high degree of polymerization in these

polyynes. ESI mass spectrometry conﬁrmed the molecular

structures of the alkynyl ligands and the dinuclear Pt(II)−

Re(I) acetylide complexes.

Figure 1. Crystal structures of (a) 1c and (b) M4 showing the atom-

numbering scheme used in Table 1. The thermal ellipsoids are shown

at 50% probability.

lecular interactions, with horizontal π···Cl contacts (2.877 Å)

between adjacent molecules (Figure 2a).

Attempts were also made to grow crystals of the

Re(CO) Cl-incorporated model Pt(II) diyne compounds M3

and M4. Single crystals of M4 suitable for X-ray diﬀraction

were grown by slow diﬀusion of hexane to a concentrated

solution of the complex in CH Cl . Key crystallographic

parameters for this structure are summarized in Table S1.

Despite several attempts at doing so, we were not able to

obtain crystals of M3 that were suitable for single-crystal

diﬀraction studies. Moreover, the crystals of M4 were, in

general, of poor quality and weakly diﬀracting, and thus while

our structure is suﬃcient to conﬁrm the overall molecular

geometry, the bond parameters should be treated with caution.

M4 crystallizes in the triclinic space group P-1. The

molecular structure comprises discrete trimetallic moieties

X-ray Diﬀraction. To complement the spectroscopic

characterization of the newly synthesized materials, we

attempted to determine the crystal structures of the reported

complexes by single-crystal X-ray diﬀraction.

deﬁned by a central Re(2,2′-bipyridine-6,6′-diyl)(CO) Cl unit

Single crystals of the mononuclear Re complex 1c were

grown by slow diﬀusion of hexane into a solution of the

complex in CH Cl . Crystallographic parameters for this

attached to a pair of trans-[(Ph)(PEt ) Pt−CC−] units

(

Figure 1b). The six-coordinate Re(I) metal center adopts a

distorted octahedral geometry. In general, 2,2′-bipyridine

structure are summarized in Table S1. 1c crystallizes in the

derivatives of the fac-Re(CO) Cl complex adopt an almost

monoclinic space group P2 /n. Figure 1a shows the molecular

planar geometry with respect to the basal OC−Re−CO

structure, and selected bond lengths and bond angles are given

in Table 1. The crystal structure shows that the Re(I) center

adopts a distorted octahedral coordination environment with

three carbonyl groups, a chelating bipyridine ligand and one

chlorido ligand. The structure also conﬁrms the successful

attachment of the acetylene R−CCH groups to the

bipyridine ligand. The bipyridine ligands adopt a cis

conﬁguration, as deﬁned by the N atoms, with an average

Re−N(Py) bond distance of 2.192(9) Å. The three carbonyl

ligands are in a facial (fac) conﬁguration with C−Re−C bond

angles in the range of 85.3(6)−90.5(5)°, and the average Re−

CO bond length is 1.90(2) Å. The Re−Cl bond is longer at

plane. However, deviation from planarity can occur due to

steric hindrance between functional groups on the 2,2′-

bipyridine core and the carbonyl (CO) ligands on the metal,

leading to changes in the properties of the spacer group. The

two rings in the bipyridine spacer are slightly twisted with

respect to one another, with a torsion angle of 13° deﬁned by

the N(1), C(6), C(7), and N(12) atoms. This hints at a degree

of strain in the bipyridine ligand, and we anticipate that the

deviation from planarity would lead to some disruption of the

conjugation in the ligand π system compared to a more planar

geometry.

Selected bond lengths and angles from the M4 structure are

summarized in Table 1. The Re−CO bond distances are in the

range of 1.895(19)−1.911(18) Å, the average Re−N bond

distance is 2.190(15) Å, and the Re−Cl bond distance is

2.514(4) Å. The N(1)−Re−N(12) bond angle is 76.4(6)°.

The two Pt(II) centers adopt a distorted square planar

.468(3) Å, and the Cl −Re−C bond angle is 174.6(4)°.

These parameters are comparable to those of previously

published Re(I) bipyridine systems. The crystal structure

consists of vertical columns of stacked complexes stabilized by

C−H···Cl (2.902 Å) and C−H···O−C (2.512 Å) intermo-

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Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) from the Crystal Structures of 1c and M4

distance

[Å]

angle

[deg]

Re(1)−N(1)

Re(1)−N(2)

Re(1)−Cl(1)

C(15)−Re(1)−N(1)

C(17)−Re(1)−N(1)

C(16)−Re(1)−C(17)

C(15)−Re(1)−C(17)

C(17)−Re(1)−Cl(1)

C(16)−Re(1)−N(1)

2.189(8)

2.194(9)

2.468(3)

90.5(4)

172.9(4)

85.3(6)

90.5(5)

94.9(4)

101.8(5)

Re(1)−C(16)

Re(1)−C(15)

Re(1)−C(17)

C(16)−Re(1)−N(2)

C(15)−Re(1)−N(2)

C(17)−Re(1)−N(2)

N(1)−Re(1)−N(2)

C(15)−Re(1)−Cl(1)

C(16)−Re(1)−Cl(1)

1.877(19)

1.908(17)

1.923(13)

173.7(5)

94.9(4)

98.4(4)

74.5(4)

174.6(4)

90.2(5)

Re(1)−C(3C)

Re(1)−C(1C)

Re(1)−C(2C)

Re(1)−N(1)

Re(1)−N(12)

Re(1)−Cl(1)

Pt(1)−C(14)

Pt(1)−C(17)

Pt(1)−P(2)

1.895(19)

1.91(2)

C(3C)−Re(1)−C(1C)

C(3C)−Re(1)−C(2C)

C(1C)−Re(1)−C(2C)

C(3C)−Re(1)−N(1)

C(1C)−Re(1)−N(1)

C(2C)−Re(1)−N(1)

C(14)−Pt(1)−C(17)

C(14)−Pt(1)−P(2)

C(17)−Pt(1)−P(2)

C(14)−Pt(1)−P(1)

P(2)−Pt(1)−P(1)

92.7(8)

91.2(8)

88.9(8)

92.6(7)

169.8(8)

99.7(7)

176.6(10)

88.5(6)

90.2(6)

93.3(6)

175.8(3)

177.6(8)

87.4(5)

93.0(5)

1.911(18)

2.176(17)

2.204(13)

2.514(4)

2.008(17)

2.04(4)

2.280(7)

2.293(6)

2.003(18)

2.06(2)

Pt(1)−P(1)

Pt(2)−C(16)

Pt(2)−C(23)

Pt(2)−P(4)

C(16)−Pt(2)−C(23)

C(16)−Pt(2)−P(4)

C(23)−Pt(2)−P(4)

2.281(5)

2.292(4)

Pt(2)−P(3)

Figure 2. Crystal packing of 1c (a), highlighting the intermolecular C−H···Cl, π···Cl, and C−H···O−C interactions, and of M4 (b), highlighting

the π−π stacking and C−H···π/C−H···Cl interactions between adjacent pairs of molecules.

geometry. The Pt−CC bond distances are 2.008(17) and

2b, Re(I)(CO) Cl-chelated ligands 1c and 2c, and the

.003(18) Å for the Pt(1) and Pt(2) centers, respectively,

Re(CO) Cl incorporated Pt(II) di- and polyynes M3/M4

31,34

−5

which are consistent with related Pt(II) diyne complexes.

and P3/P4 were collected in 10 M CH Cl solutions (Figure

The Pt−P bond distances range from 2.280(7)−2.293(6) Å.

3). Table 2 compares the absorption maxima of Pt(II) diynes

and polyynes with and without the pendant Re(I) moieties, the

The C−Pt−C angle is close to linear (176.6(10) and

77.6(8)° for Pt1 and Pt2, respectively). The presence of

latter taken from our previous work. The absorption spectra

of the ligands 1b/1c and 2b/2c are compared to those of the

model compounds M1/M2 and M3/M4 in Figure S1 .

The absorption spectra of the bis(ethynyl)-5,5′-bipyridine

the Re(I) center and the cis conﬁguration of the bipyridine

rings of the spacer results in a Pt···Pt distance of 10.850 Å.

Finally, the packing diagram of the complex (Figure 2b) shows

a π−π stacking interaction between the pyridine rings on

adjacent pairs of molecules (3.317 Å separation between ring

planes), together with evidence of C−H···π and C−H···Cl

interactions.

ligand 1b, the Re(CO) Cl-chelated ligand 1c, and the

corresponding Pt(II) diyne M3 and polyyne P3 are compared

in Figure 3a. The spectrum of 1b displays intense absorption

bands at λ_m_a_x≈ 315 and 328 nm and a shoulder feature around

348 nm. The spectrum of 1c displays a noticeable shift in the

Absorption Spectroscopy. Room temperature optical

absorption spectra of the bis(ethynyl)bipyridine ligands 1b and

absorption edge relative to 1b, with absorption bands at λ_m_a_x

≈

Inorg. Chem. 2021, 60, 745−759

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Figure 3. Comparison of the absorption spectra of the 5,5′-bis(ethynyl) bipyridine compounds 1b, 1c, M3, and P3 (a) and the 6,6′-bis(ethynyl)

−

bipyridine compounds 2b, 2c, M4, and P4 (b). Both sets of spectra were measured in 10 M CH Cl solutions at room temperature.

Table 2. Absorption Maxima of the Pt(II) Diynes and Polyynes with and without Pendant Re(I) Moieties Measured at Room

Temperature

without pendant Re(I) (ref 31)

with pendant Re(I) (this work)

λ_m_a_x[nm] (ε × 10 dm mol cm⁻¹

−1

compd

λ_m_a_x[nm]

compd

)

269, 297, 359, 381

258, 282, 298, 334, 349

271, 295, 362, 398

339 (1.58), 424 (2.92)

271 (9.94), 300 (9.03), 322 (7.28), 395 (5.25)

343 (1.76), 419 (3.98), 448 (6.57)

233, 264, 299, 337, 352

276 (6.97), 300 (7.50), 324 (6.15), 388 (4.94), 402 (5.21)

ε was not measured in the work in ref 31. M1/M2, P1/P2 - thin ﬁlms; M3/M4, P3/P4 - 10⁻⁵M CH Cl solution.

25, 332, and 350 nm. Absorption bands at ∼270 and 300−

50 nm can be assigned to π → π* transitions associated with

bipyridine π → π* transition that is highly red-shifted

compared to 1b/1c due to a destabilized highest occupied

molecular orbital (HOMO, see below). While somewhat

surprising, this does account for the large enhancement of the

extinction coeﬃcient compared to the long-wavelength MLCT

band in 1c.

the bipy/Ph and CC moieties, respectively, whereas broader

bands at ∼400−450 nm can be attributed to MLCT

transitions. This is in line with previous studies that have

identiﬁed low-lying MLCT excited states involving a Re dπ

donor orbital and a ligand π* acceptor,

the metal serving to make the ligand a better electron acceptor

and possibly also forcing it into a more planar conformation

and increasing the eﬀective conjugation in the ligand orbitals.

The MLCT state has been shown to be long-lived and to

luminesce at longer wavelengths in the visible spectrum.

The extended tail of this MLCT band is a signature of the

extended electronic delocalization in the alkynyl bipyridine

derivatives. Clear eﬀects of incorporating the Re(I) core into

the Pt(II) diyne can be seen, for example, in the extended

absorption of 1c and M3 relative to 1b and M1 (Figure 3,

Figure S1). The steric hindrance due to the Pt(II) fragments is

expected to be higher in systems based on 6,6′-bipyridine than

in the 5,5′-counterparts, leading to diﬀerent levels of

conjugation. This is evidenced by the optical properties and

computational modeling (vide infra), although ideally this

should be conﬁrmed by further structural characterization.

The Pt(II) diyne and polyyne systems M3 and P3 both

display strong long-wavelength absorptions, with a broad band

centered around λ_m_a_x≈ 424 in M3 and an asymmetric feature

in P3 comprising a primary peak at λ_m_a_x≈ 448 nm and a

secondary feature around 419 nm. Both also show weaker

secondary maxima, which occur at ∼339 and 343 nm in M3

and P3, respectively. There is thus a notable red shift in both

bands and an enhancement of the extinction coeﬃcient of the

longer-wavelength bands on going from the diyne to the

polyyne. Interestingly, calculations show that the long-

wavelength transition in M3 is not an MLCT band but is a

36,37

with chelation to

The spectra of the 6,6′-bis(ethynyl)bipyridine complexes in

Figure 3b illustrate that, as for the 5,5′-functionalized systems,

chelation of the ligand to Re(CO) Cl leads to a general red

shift in the absorption proﬁle and introduces an MLCT band,

again at ∼410 nm. Incorporation of the chelated spacer unit

into the Pt(II) diyne and polyyne results in a further red shift

and an increase in extinction coeﬃcient (λ_m_a_x≈ 395 and 388/

402 nm, respectively). Comparison of the spectra of the

heterometallic diynes and polyynes to the corresponding

homometallic Pt(II) species (M1/P1 and M2/P2) shows that

adding a second metal ion leads to signiﬁcant changes (Table

2), for example, a large bathochromic shift of the lower energy

bands from 398 nm in P1 to 448 nm in P3, which we ascribe

to stabilization of the LUMO. The 6,6′-bipyridine species

generally show blue-shifted absorption maxima compared to

the corresponding 5,5′-bipyridine species, which we account

for by the diﬀerent positions of the alkynyl groups and the

steric strain in the Pt(II) systems hindering the conjugation.

In principle, we might also expect the hindered conjugation

in the 6,6′-bipyridine species to lead to lower extinction

coeﬃcients compared to the 5,5′-bipyridine analogues. While

this is borne out for the 1b/2b and 1c/2c pairs, the M3/M4

pair, and to some extent also P3 and P4, shows the opposite

trend. As shown in Figure 2b, the model complex M4 exhibits

molecular stacking in the solid state, and it is possible that

aggregation in a solution through a similar mechanism may

36,37,39

alter the extinction coeﬃcient. In particular, it has been

reported that “V-shaped” conjugated molecules, such as M4,

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Figure 4. Room temperature emission spectra of (a) the 5,5′-bipyridine species 1c, M1, and M3 and (b) the corresponding 6,6′-bipyridine species

−5

c, M2, and M3, measured in 3 × 10 M CH Cl solutions.

2 2

Table 3. Excitation and Emission Wavelengths of the Bis(ethynyl)bipyridine Ligands 1c/2c and Model Compounds M1/M2

and M3/M4 Measured in 3 × 10⁻⁵M CH Cl Solutions and in the Solid State, Together with the Photoluminescence

Quantum Yields Φ Measured in Solution

solution (CH Cl )

solid state

λ_e_x[nm]

λ_e_m[nm]

382, 422

480, 514

414, 426, 565

381, 466

434, 462, 492, 568, 603

576

λ_e_x[nm]

λ_e_m[nm]

350

360

390

340

400

415

0.080

0.13

320

390

340

380

410

389, 695

388, 619

423, 561, 608

371, 389, 693

610, 656

591

not detected

0.094

0.16

The quantum yields are relative to coumarin 460, and the quantum yield of Re(bpy)Cl(CO) is 0.0031 in CH Cl .

form aggregates in halogenated solvents, leading to enhance-

ment of the absorption cross section. Measurement of the

strategy for improving the PL properties of neutral Re(I)

complexes.

absorption spectra of 1c/2c, M1/M2, and M3/M4 at solution

Room temperature emission spectra of the Re(I)-coordi-

nated ligands 1c/2c, the homometallic Pt(II) diynes M1/M2,

and the heterobimetallic Pt(II)−Re(I) diynes M3/M4 were

concentrations from 1 × 10⁻to 3 × 10 M (Figure S2)

^s^h^o^w^s^t^h^a^t^a^l^t^h^o^u^g^h^t^h^e^b^aⁿ^d^s^h^a^p^e^aⁿ^d^λmax remain the same,

the molar extinction coeﬃcient shows some sensitivity to the

concentration. This suggests that the stacking and/or C−

H···π/C−H···Cl interactions visible in the X-ray structure

might inﬂuence the optical properties in solution. We also

measured absorption and ﬂ uorescence emission spectra at

varying concentrations (Figure S2) and found that increasing

the concentration by up to 3× did not produce any major

changes in spectral features, which is also in line with the

results of the picosecond-nanosecond dynamics measurements

presented below.

−5

−

collected in 3 × 10 M CH Cl solutions (Figure 4, Table 3).

We also collected room-temperature solid-state emission

spectra of the compounds (Figure S3 ), together with

ﬂuorescence excitation and emission spectra in CH Cl

solutions (Figure S4). The spectra are somewhat complex

and composed of multiple maxima and/or shoulder features.

The ﬂuorescence excitation spectra of 2c, M1, M2, and M4 are

similar to the absorption spectra, providing evidence that the

measured emission is from the metal complexes, whereas those

of 1c and M3 show a signiﬁcant red shift, which we tentatively

ascribe to solution e ﬀects such as aggregation or ligand

dissociation (Figure S4 ).

As is commonly observed in Re(I) acetylide complexes, the

Re(I)-chelated bipyridine ligands 1c/2c and the bimetallic

diyne model complexes M3/M4 were all found to be emissive

at room temperature both in a solution and in the solid state.

The solution spectra of all four compounds show a strong

primary luminescence feature and several weaker bands at

longer wavelengths. Upon excitation at 390 and 340 nm, the

Pt(II) diynes M1 and M2 also show emission in solution from

∼380−565 nm. The emission proﬁles were found to be red-

shifted in the solid state (Table 3).

Photoluminescence Spectroscopy. Re(I) complexes are

well-known for their intense, unstructured emission in the

orange region of the visible spectrum, which originates from

MLCT excited states. The energy, the intensity, and the

lifetime of the emission are highly sensitive to the nature of the

diimine and ancillary ligands, and a large range of

photoluminescence quantum yields (PLQYs) has been

reported for Re(I) complexes. The linkage of two metal

atoms to the same chromophore has been shown to increase

the metal d character in the frontier molecular orbitals, thereby

enhancing the spin−orbit coupling between the emissive triplet

state and the singlet manifold. Linking two heavy metals to a

single heterocyclic ligand is thus an interesting potential

Inorg. Chem. 2021, 60, 745−759

Inorganic Chemistry

Article

Table 4. Parameters for the Fits in Figure 5 Listing the Excited-State Lifetime Components τ (n = 2−3) and Their

Corresponding Contributions α to the Overall Decay Transients

λ_e_x[nm]

λ_d_e_t[nm]

τ₁[ps]

α₁

τ₂[ns]

α₂

τ₃[ns]

α₃

350

380

420

500

580

684 ± 51

597 ± 55

200 ± 30

359 ± 51

0.75

0.61

0.79

0.38

2.43 ± 0.09

2.74 ± 0.16

3.14 ± 0.22

5.66 ± 0.38

0.25

0.34

0.20

0.21

20−23

0.05

0.01

0.41

22.2 ± 0.7

Large uncertainty due to small contribution.

At room temperature, the photoluminescence quantum yield

Φ (PLQY; Table 3) of M1 and M2 was very low and could not

be reliably measured. From computational modeling studies

(

see below), the lowest-lying excited states of these

compounds are π → π* transitions, which are usually

nonemissive at ambient temperature. On the other hand,

excitation of heterobimetallic complexes M3 and M4 at 400

and 415 nm in a solution, and 380 and 410 nm in the solid

state, led to strong emission in the yellow-orange region of the

spectrum as expected for diimine Re(I) tricarbonyl species. We

ascribe the shorter-wavelength emission to the reverse of the

Re dπ → bipyridine π* MLCT transition as reported for other

Re(I) chromophores. While we were not able to assign the

longer-wavelength transitions deﬁnitively, quantum-chemical

calculations (see below) suggest that they may be associated

with emission from formally spin-forbidden triplet states. The

PLQY measured for the heterobimetallic complexes is also

Figure 5. Room temperature ﬂuorescence decay transients of 1c/2c

−5

and M3/M4 in CH Cl (1 × 10 M). The instrument response

function (IRF) is shown as a dashed black line. The black solid lines

show ﬁts of the measured transients to multiexponential functions.

The excitation and detection wavelengths and the ﬁt parameters are

given in Table 4. Note that the artifact at ∼4 ns in the 1c decay

transient is due to “afterpulsing” in the photomultiplier tube detector

and is excluded from the ﬁtting, so it does not aﬀect the values

reported in Table 4.

−

higher than for Re(bpy)Cl(CO) (3.1 × 10 in CH Cl ),

which indicates that the incorporation of a second metal limits

nonradiative decay pathways. In general, our data indicates that

incorporation of the Re(I) fragment into the heterobimetallic

diynes improves the PLQY and leads to a red shift of emission

maxima, although the emission intensity from the hetero-

bimetallic complexes is lower than that from the homometallic

Re(I) complexes at shorter wavelengths. As expected, the

emission properties of the 5,5′- and 6,6′-systems diﬀer

signiﬁcantly, which can be attributed to geometric constraints

limiting the conjugation between the bipyridine and Pt(II)

cores separated by the ethynyl units.

Two components with lifetimes τ = 684 ps and 2.43 ns were

obtained for 1c. The shorter-lived component can be assigned

to the relaxation of the MLCT state, whereas the nanosecond

lifetime of the other component is typical of π → π* decay and

can thus be ascribed to the bipyridine ligand. In 2c, the 6,6′

substitution pattern reduces the lifetime of the MLCT state to

97 ps compared to the 5,5′-substituted 1c but raises the

In contrast to the solution spectra, the position of the band

centers in the solid-state spectra of 1c and 2c is very similar

lifetime of the π* decay to 2.74 ns and introduces a new, long-

lived state with a lifetime of 20−23 ns. The latter are indicative

of improved conjugation increasing the local heterogeneity of

the system.

(Figure S3). The emission proﬁles of the model compounds

M3 and M4 are both red-shifted compared to the chelated

spacers, and the shift in emission wavelength of M3 is more

prominent than that of M4. Finally, in contrast to M3 and M4,

the polymers P3 and P4 were found to exhibit only weak

emission both in solution and in the solid state at room

temperature (Figure S5). The same was found for P1 and

The presence of the Pt(II) fragments in M3 further reduces

the lifetime of the MLCT state to 200 ps but slightly increases

the π* lifetime to 3.14 ns. This can be accounted for through

increased stability of the π* state due to the more extended

conjugation in the planar structure. M3 also exhibits a very

small contribution from the longer-lived component (20−23

ns) seen in 2c. In M4, the steric eﬀects due to the Pt(II)

fragments in the 6,6′ positions lead to an increased

contribution from the long-lived state (20−23 ns) at the

expense of the MLCT state, as indicated by the ﬁtting weights

in Table 4. These eﬀects also increase the π* lifetime to 5.66

ns.

P2. The solution emission proﬁles of P3 and P4 are very

similar to those of 5,5′-dibromobipyridine and 6,6′-dibromoi-

pyridine, respectively, suggesting a dominant bipyridine-

centered emission and suppression of the reverse MLCT

transition seen in the chelated spacers 1c/2c and the model

compounds M3/M4.

Picosecond-Nanosecond Dynamics. Time-resolved ﬂu-

orescence spectra of 1c, 2c, M3, and M4 in CH Cl were

Increasing the concentration of the four species from 1 ×

−

−5

measured at room temperature close to the emission maxima

in Table 3. The transients are shown in Figure 5, and

parameters from a series of multiexponential ﬁts are

summarized in Table 4. Decay transients measured over a

longer observation window, in order to accurately characterize

longer-lived components, are shown in Figure S6 .

to 3 × 10 M did not produce any notable changes to

the measured ﬂuorescence decay transients (Figure S7). This

indicates that increasing the concentration does not lead to

excimer formation, as this would typically cause a buildup of

signal (rise time) in the transients similar to that found in other

systems such as pyrenes. The increase in concentration may,

Inorg. Chem. 2021, 60, 745−759

Article

Figure 6. Simulated optical absorption spectra of the bis(ethynyl) bipyridine ligands 1b and 2b (a, b), Re(CO) Cl-chelated ligands 1c and 2c (c,

d), and Pt(II) diynes M3 and M4 (e, f) obtained from time-dependent density-functional theory (TD-DFT) with an implicit CH Cl solvent. In

each subplot, the simulated spectra (blue/red shaded regions) are generated from the singlet (spin-allowed) excitations as a sum of Gaussian

functions with a nominal line width σ of 0.1 eV. The wavelengths and relative oscillator strengths of individual transitions are marked by solid

vertical lines of the same color as the spectrum. The wavelengths of triplet (forbidden) transitions are shown as dashed vertical lines and gold stars.

however, still lead to aggregation in the ground state, which

explains the increase in the extinction coeﬃcient observed in

the absorption spectroscopy measurements.

Computational Modeling. To better understand the

changes in optical properties on chelating the bipyridine units

to the Re(I) centers and subsequently incorporating the

chelated spacers into the Pt(II) model complexes, we carried

out molecular quantum-chemical calculations using density-

functional theory (DFT) on the bipyridine-based diterminal

alkynyl ligands 1b and 2b, chelated diterminal alkynyl ligands

bands in the simulated spectra are a reasonable qualitative

match to the measured absorption proﬁles.

The calculations also predict that all six compounds possess

low-lying triplet (spin-forbidden) excited states 100−150 nm

below the onset of the absorption from the lowest-energy

singlet states (marked by stars and dashed lines in Figure 6).

The density of these triplet states generally increases on going

from the spacer ligands 1b/2b to the chelated spacers 1c/2c to

the model compounds M3/M4. As noted in the previous

section, these states may be associated with the weaker long-

wavelength features in the solution emission spectra in Figure

1c and 2c, and the Pt(II) diynes M3 and M4. To match the

. Although it is, in principle, possible to model emission

conditions of the solution measurements as closely as possible,

the calculations were performed with an implicit solvent of

CH Cl .

processes using TD-DFT, the procedure is considerably more

involved than calculating the transition wavelengths and

oscillator strengths to model absorption spectra, and we do

not consider it feasible to do so for the six compounds being

examined in this work.

The brightest singlet (spin-allowed) transitions and the 2−3

lowest-lying triplet (spin-forbidden) transitions in the simu-

lated spectra in Figure 6 were analyzed by inspecting the

molecular orbitals involved and, for transitions comprising

more than one signiﬁcant excitation between occupied and

virtual states, by using the method of natural transition orbitals

Images of the optimized structures are given in Figures S8 −

S13 , and the Cartesian coordinates are provided in Listings

S1 −S6 . For all three pairs, we found that the 5,5′-

bis(acetylide) bipyridine compounds were more energetically

favorable (in CH Cl ) than the 6,6′-analogues, with 1b, 1c,

−

and M3 calculated to be 7.44, 30.3, and 7.97 kJ mol lower in

energy than the corresponding 2b, 2c, and M4 compounds,

respectively.

Simulated optical absorption spectra obtained using time-

dependent DFT (TD-DFT; Figure 6) reproduce the key

trends in the measured spectra in Figure 3. For both bipyridine

ligands, the calculations predict a red shift in the absorption

proﬁle and the appearance of new longer-wavelength

(

NTOs) to visualize the composite occupied “particle” and

unoccupied “hole” states. Table 5 lists the calculated

wavelengths, oscillator strengths, and our assignments of the

brightest singlet (spin-allowed) transitions in each of the six

complexes, while Table 6 lists the wavelengths and the

assignments of the low-lying triplet (spin-forbidden) states. A

full breakdown of the states listed in Tables 5 and 6 into

transitions between pairs of occupied and virtual orbitals, and

isosurfaces showing the NTOs, is given in Tables S2 −S13 and

Figures S14 −S27/S31 −S47, respectively.

absorption bands upon chelation with Re(CO) Cl, followed

by a further red shift and enhancement of the oscillator

strengths of the long-wavelength bands on incorporation of the

chelated spacer units into Pt(II) diynes. Bearing in mind the

use of a uniform line width when simulating the spectra and

the fact that we only calculated the lowest-lying excited states

of the larger complexes, the positions and intensities of the

A comparison of the oscillator strengths of the spin-allowed

transitions in the 5,5′-bis(acetylide) bipyridine compounds to

Inorg. Chem. 2021, 60, 745−759

Inorganic Chemistry

Article

Table 5. Transition Wavelengths λ, Oscillator Strengths f, and Assignments of the Brightest Singlet (Spin-Allowed)

Transitions in the Bipyridine Spacers 1b/2b, Re(CO) Cl-Chelated Bipyridine Spacers 1c/2c, and Model Pt-Incorporated

Diyne Compounds M3/M4

λ [nm]

assignment

π → π*

λ [nm]

assignment

π → π*

1.294

262

0.566

0.078

0.937

MLCT

π → π*

345

295

0.066

0.256

0.073

0.090

MLCT

dominant π → π*

dominant MLCT

363

335

327

1.635

0.356

0.063

π → π* + MLCT (Pt)

MLCT (Re)

331

284

281

0.595

0.368

0.063

π → π* + MLCT (Pt)

mixed

lower oscillator strength of the transition compared to the 5,5′-

bipyridine analogue 1b.

Chelation of 1b results in new frontier Re-based orbitals, of

which the HOMO - 1 orbital has the best spatial overlap with

the bipyridine-based LUMO and gives rise to a weak MLCT

band at 355 nm (Figure 6c). The HOMO - 3 orbital remains

similar in form to the HOMO in 1b, and the much stronger π

Table 6. Transition Wavelengths λ and Assignments of the

Low-Lying Triplet (Spin-Forbidden) Transitions in the

Bipyridine Spacers 1b/2b, Re(CO) Cl-Chelated Bipyridine

Spacers 1c/2c, and Model Pt-Incorporated Diyne

Compounds M3/M4

→

π* transition occurs at 305 nm. The particle and hole states

λ [nm]

assignment

λ [nm]

assignment

obtained from the NTO analysis of the two transitions are

shown in Figure 8. The spectrum of 2c also shows a weak

MLCT band with a comparable, if slightly lower, oscillator

strength, which again occurs between a Re-based HOMO - 1

and the bipyridine ligand-based LUMO (Figure 6d, Figure

S18 ). As for 1b/2b, this transition is blue-shifted compared to

that in 1c due to the higher-energy LUMO (Figure S29 ). The

strong shorter-wavelength feature in Figure 6d is a

combination of three bands at 295, 284, and 280 nm. The

longer-wavelength band at 295 nm was assigned as

predominantly a π → π* transition based on the NTOs but

with some MLCT character (Figure S19 ), the latter of which

may explain its ∼3× smaller oscillator strength than the

corresponding π → π* transition in 1c, relative to the smaller

π → π*

mixed

dominant MLCT

π → π*

426

374

320

492

391

377

π → π*

dominant MLCT

MLCT

π → π*

443

402

374

466

423

dominant π → π*

π → π*

∼

2× diﬀerence in oscillator strengths of the electronic

transitions in 1b/2b (cf. Table 5). This reduction in oscillator

strength can also be seen in the measured spectra in Figure 3.

The two higher-energy transitions at 284 and 280 nm were

assigned as predominantly MLCT, which is consistent with

their weak oscillator strengths (Figures S20/S21).

those of the 6,6′-bipyridine analogues shows that the latter are,

in general, signiﬁcantly weaker. The simulated spectrum of the

(

bisethynyl)bipyridine ligand 1b (Figure 6a) shows a single

prominent peak at 290 nm corresponding to the π → π*

electronic transition between the highest-occupied and lowest-

unoccupied molecular orbitals (HOMO/LUMO; Figure 7a).

The lowest-energy transition in the 6,6′-bis(ethynyl)bipyridine

analogue 2b occurs at a shorter wavelength of 262 nm and has

a much lower oscillator strength of f = 0.57 vs 1.29 (Figure

The simulated spectrum of M3 (Figure 6e) shows a major

peak comprising a single bright state at 363 nm and a shoulder

feature composed of two transitions at 335 and 327 nm. The

long-wavelength transition mostly comprises a HOMO →

LUMO excitation and was assigned as a π → π* with some

MLCT character from participation of the Pt d orbitals in the

HOMO (Figure 9a). The ligand-based HOMO is therefore

destabilized and raised above the equivalent Re-based orbitals

that form the HOMO in 1c. The extended conjugation in the

HOMO due to incorporation into the diyne may also explain

the enhanced oscillator strength of the transition relative to 1b

and 1c. The two shoulder features can both be assigned, based

on the NTOs, as MLCT transitions from the Re moiety

(Figure 9b, Figures S23/24). The two prominent peaks in the

spectrum of M4 (Figure 6f) arise from three transitions at 331,

284, and 281 nm which, as for M3, can again be assigned as a

long-wavelength π → π* and two shorter-wavelength MLCT

bands (Figures S25 −27). All three are blue-shifted with respect

b). As in 1b, the major component of the transition is the

HOMO → LUMO excitation, but there is a substantial minor

component associated with the HOMO → LUMO+2

excitation (Figure 7b). Comparison of the LUMOs of 1b

and 2b shows that the diﬀerent substitution pattern in 2b

lowers the degree of conjugation in the LUMO, and

comparison of the orbital energies in 1b and 2b (Figure

S28 ) indicates that the higher energy (shorter wavelength)

transition in 2b is most likely due to destabilization of the

LUMO. The lower degree of conjugation also leads to poorer

spatial overlap with the HOMO, which explains the mixing of

the LUMO+2 orbital into the π → π* excited state. This would

further raise the transition energy, and the poorer spatial

overlap between the HOMO and LUMO may explain the

Inorg. Chem. 2021, 60, 745−759

orbitals to the electronic transition as marked. The isosurfaces are drawn to a contour value of 2.5 × 10 e bohr .

Article

Figure 7. (a) Images of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO/LUMO) in 1b, which account for the bright π →

π* electronic excitation. (b) Images of the HOMO, LUMO, and LUMO+2 orbital in 2b with the percentage contribution of the latter virtual

−

−3

Figure 8. Occupied particle and virtual hole states obtained from natural transition orbitals (NTOs) analyses of the dominant optical absorptions

−

−3

of 1c at 355 (a) and 305 nm (b) identiﬁed in Figure 6/Table 5. The isosurfaces are drawn to a contour value of 2.5 × 10 e bohr .

to the corresponding transitions in M3, which can be ascribed

to the destabilization of the LUMO as seen in the 1b/2b and

443 and 402 nm are of mixed MLCT and π → π* character

(Figures S40/S41), while the higher-lying state at 374 nm is

predominantly an MLCT excitation (Figure S42). The

simulated spectrum of M3 in Figure 6e shows two triplet

states below the bright, long-wavelength singlet excitation,

which were both characterized as a π → π* transitions (Figures

S43/S44). In M4, there are three triplet states at notably

longer wavelengths than the absorption onset, and these were

again characterized as π → π* transitions (Figures S45 −S47).

The change in the nature of the triplet states on going from the

c/2c pairs (Figures S28 −S30 ).

The low-lying triplet states in 1b and 2b were assigned based

on the NTOs as π → π* excitations (Table 6; Figures S31 −

S36 ). The lowest-lying triplet excitation in 1c is predicted to

occur at 492 nm and was also characterized as a π → π*

Figure S37 ), whereas the next-highest triplet excitations at

91 and 377 nm are primarily MLCT bands (Figures S38/

S39 ). In 2c, on the other hand, the lowest-lying triplet states at

Inorg. Chem. 2021, 60, 745−759

Article

HOMO and red shift of the π → π* transition represents an

interesting eﬀect of incorporating the spacer into the diyne.

The higher energy and lower oscillator strength of the π → π*

transition in the 6,6′-bipyridine ligand 2b can be ascribed to

destabilization of the LUMO and poorer spatial overlap with

the HOMO resulting in higher-energy virtual orbitals being

involved in the transition. This highlights the sensitivity of the

optical properties to the topology of the bipyridine spacer

group, as observed in the absorption and emission measure-

ments. As for 1b, chelation of 2b with Re(CO) Cl introduces a

weak low-lying MLCT band but also introduces some MLCT

character into the higher-energy π → π* transition, which may

again serve to reduce its oscillator strength. The Pt(II) diyne

M4 shows similar spectral features to M3, with the

destabilization of the bipyridine-based LUMO in the former

resulting in a general predicted blue shift of the electronic

transitions. Finally, analysis of the spin-forbidden triplet

excitations also shows that the spacer topology, the chelation

of the spacer with Re(I), and the incorporation of the chelated

spacers into Pt(II) diynes have a substantial eﬀect on the

nature of the emissive states.

CONCLUSIONS

■

We have synthesized and characterized two Re(I)-coordinated

diterminal alkynyl ligands, viz., Re(CO) Cl-5,5′-diethynyl-2,2′-

bipyridine and Re(CO) Cl-6,6′-diethynyl-2,2′-bipyridine,

which we have subsequently incorporated as spacer groups

into Pt(II) diynes and polyynes. Structural characterization of a

Pt(II) diyne model complex reveals π stacking and a variety of

weak intermolecular/interchain interactions in the solid state.

As evidenced by optical absorption and emission spectroscopy

and computational modeling, the photophysical properties of

the Pt(II) diynes and polyynes show a high sensitivity to the

topology of the bipyridine spacer group. Optical spectroscopy

shows that chelation of the bipyridine ligands produces a red

shift in the π → π* transition and introduces weak, long-

wavelength MLCT bands, both of which are further red-shifted

in the Pt(II) diynes and polyynes. The metalated species

appear to show aggregation in a solution, leading to substantial

enhancement of the extinction coeﬃcient with concentration.

Emission spectroscopy reveals prominent blue/green emission

associated with the Re(I) chromophore in the chelated

bipyridines and Pt(II) diynes, whereas the Pt(II) polyynes

only show shorter-wavelength bipyridine-centered emission.

Picosecond-nanosecond dynamics measurements identify

short-lived MLCT excited states and 1−2 long-lived π* states

in the chelated spacers, and incorporation of the spacers into

the model Pt(II) diynes suppresses the MLCT state and

signiﬁcantly shortens its lifetime. Computational modeling

conﬁrmed the higher energetic stability of the 5,5′-bipyridine

species compared to the 6,6′-bipyrdine analogues and allowed

us to establish the nature of the brightest electronic transitions

and low-lying triplet states in the optical absorption spectra.

These calculations also reveal a remarkable destabilization of

the bipyridine-based π orbital in the model diynes due to

interaction with the Pt d orbitals, which results in the optically

bright π → π* transitions being lowered in energy compared to

the long-wavelength Re dπ → π* MLCT transition in the

chelated spacer precursors. In summary, our systematic studies

have elucidated the eﬀect of introducing a second metal ion

into rigid-rod Pt(II) polyynes by chelation of the main-chain

spacer groups and thus highlighted the potential utility of this

Figure 9. Occupied particle and virtual hole states obtained from

natural transition orbitals (NTOs) analyses of the optical

absorptions of M3 at 363 (a) and 335 nm (b) identiﬁed in Figure

/Table 5. The NTOs associated with the second MLCT transition at

27 nm are substantially similar to those in (b) and are therefore not

shown (cf. Figure S24 ). The isosurfaces are drawn to a contour value

of 2.5 × 10⁻e bohr .

−3

chelated spacers to the heterobimetallic diynes is consistent

with the shortening of the MLCT state lifetime and

lengthening of the π* lifetime observed in the picosecond-

nanosecond dynamics measurements (cf. Table 4). The mixed

character of the triplet states in the 6,6′-bipyridine species may

also explain the larger contribution of the longer-lived π → π*

states to the transients in Figure 5.

In summary, for the 5,5′-bipyridine series, these electronic-

structure calculations show that chelation of the bipyridine

spacer ligand 1b with Re(CO) Cl in 1c leads to a red shift of

the optically bright π → π* electronic transition in 1b and

introduces a weaker low-lying MLCT band. Subsequent

incorporation of the chelated bipyridine into the Pt(II) diyne

M3 destabilizes the highest-lying bipyridine-based orbital, red

shifts the π → π* transition to lower energy than the Re-

bipyridine MLCT state in 1c and enhances its oscillator

strength, and also red shifts and enhances the MLCT state in

the chelated spacer precursor. The destabilization of the

Inorg. Chem. 2021, 60, 745−759

Inorganic Chemistry

ESI-MS: m/z 516 [M] . Anal. calc. for C17

Article

strategy for ﬁne-tuning the optoelectronic properties of these

materials for future technological applications.

ClRe: C - 38.81; H

2.16; N - 5.28%, found: C - 38.79; H - 2.14; N - 5.26%.

trans-[(Ph)(Et P) Pt−CC−R−CC−Pt-(PEt ) (Ph)] (R =

3 2

(

CO) ClRe(I)-2,2′-bipyridine-5,5′-diyl) (M3). To a stirred solution

EXPERIMENTAL SECTION

of trans-[(PEt ) (Ph)PtCl] (0.85 g, 0.16 mmol) and 1c (0.40 g, 0. 78

■

3 2

mmol) in CH Cl / Pr NH (50 mL 1:1 v/v) under argon was added a

General Procedures. All reactions were performed in a dry argon

atmosphere using standard Schlenk techniques. Solvents were distilled

catalytic amount (∼5 mg, 0.0026 mmol) of CuI. The yellow solution

was stirred at room temperature for 15 h, after which all volatile

components were removed under reduced pressure. The residue was

and predried before being used according to standard procedures.

Unless stated otherwise, all chemicals were obtained from Sigma-

dissolved in CH Cl and passed through a silica column eluting with

Aldrich and used without further puriﬁcation. trans-[Pt(Ph)Cl-

hexane/CH Cl (1:1, v/v). Removal of the solvents under vacuum

(

PEt ) ] and trans-[(P Bu ) PtCl ] were prepared following reported

2 2

43,44

gave the title complex as a yellow/orange solid (0.044 g, 0.029 mmol,

0% yield, decomposition temperature 198.5 °C). IR (CH Cl ): ν/

procedures.

Column chromatography was performed using either

Kieselgel 60 (70−230) silica gel or Brockman grade ΙΙ−ΙΙΙ alumina.

−

cm 1885, 1909, 2016 (CO), 2085(CC). H NMR (250 MHz,

NMR spectra were recorded on Bruker MM-250 and WM-400

CDCl ): δ/ppm 8.82 (s, 2H, H ), 7.76 (dd, J = 6.9, 1.6 Hz, 2H,

spectrometers in CDCl . H and C NMR spectra were referenced to

6,6

H ), 7.41 (s, 2H, H ), 7.26 (d, J = 7.2, 4H, Hortho^P^h⁾^,⁶^.⁹⁴⁽^t^,^J⁼⁷^.⁶

solvent resonances and P NMR spectra were referenced to an

external phosphoric acid standard (85% H PO ). Mass spectra were

3,3

4,4

^H^z^,⁴^H^,^Hmeta Ph), 6.83 (t, J = 7.5 Hz, 2H, H Ph), 1.75 (m, 24H,

para

PCH ), 1.12 (t, 36H, J = 6.4 Hz, P(CH CH )). C NMR (100 MHz,

acquired using a Kratos MS 890 spectrometer using electrospray

ionization (ESI). CH Cl solutions of the ligand precursors, diynes,

CDCl ): δ/ppm 150.47 (C ), 144.22 (C ), 138.86(C ), 127.1 (C

2,2

6,6

3,3

−

−4

Ph), 121.2 (C ), 115.3 (C ), 110.4 (CC), 15.09 (P(CH CH )),

and polyynes were prepared at concentrations of 10 −10 M. IR

spectra of 10⁻M solutions were recorded using a Cary 630 FT-IR

spectrometer. Absorption spectra were recorded at solution

4,4

5,5

.05 (CH ). P{ H} NMR (162 MHz, CDCl ): δ 10.02 ( J

Pt−P

649.47 Hz). ESI-MS: m/z 1525 [M] . Anal. calc. for

−

−5

C H N O P Pt ReCl: C - 41.74; H - 5.02; N - 1.84%, found: C -

concentrations of 1 × 10 −3 × 10 M on a Varian-Cary 50 UV−

visible spectrophotometer in a 1 cm quartz cuvette. Solution emission

spectra were recorded from 10⁻M solutions using a Shimadzu RF-

53 76

1.23; H - 5.27; N - 1.98%.

trans-[(Ph)(Et P) Pt−CC−R−CC−Pt−(PEt ) (Ph)](R =

3 2

(

CO) ClRe-6,6′-bis(ethynyl)-2,2′-bipyridine) (M4). M4 was pre-

301 PC spectroﬂuorophotometer. Solid-state emission spectra were

pared following a similar procedure as for M3 starting from trans-

(PEt ) (Ph)PtCl] (0.079 g, 0.15 mmol) and 2c (0.037 g, 0.073

recorded using a PerkinElmer LS 55 ﬂuorescence spectrometer.

[

Lifetime measurements were performed using the time-correlated

single photon counting (TCSPC) setup described elsewhere.

Microanalyses were performed using a PerkinElmer 2400 Series II

CHNS/O elemental analyzer. Molar masses of the Pt(II) polyynes

were determined by gel-permeation chromatography/light-scattering

3 2

mmol). A yellow solid was obtained after puriﬁcation (0.050 g, 0.033

mmol, 45%; decomposition temp. 255.5 °C). IR (CH Cl ): ν/cm

−

875, 1910, 2013 (CO), 2066 (CC). H NMR (250 MHz,

CDCl ): δ/ppm 7.73 (d, J = 7.7 Hz, 2H, H ), 7.63 (t, J = 7.9 Hz, 2H,

5,5

H ), 7.42 (dd, J = 7.9, 2.0 Hz, 2H, H ), 7.32 (d, J = 7.5 Hz, 4H,

(

GPC/LS) analysis. GPC was carried out using two PL Gel 30 cm, 5

3,3

4,4

−

^Hortho^P^h⁾^,⁷^.⁰⁵⁽^t^,^J⁼⁷^.⁷^H^z^,⁴^H^,^Hmeta Ph), 6.84 (t, J = 7.3 Hz, 2H,

μm mixed C columns at 30 °C running in THF at 1 mL min with a

Roth Model 200 high-precision pump. This was coupled to a DAWN

DSP Wyatt Technology multiangle laser light-scattering (MALLS)

apparatus with 18 detectors and auxiliary Viscotek Model 200

diﬀerential refractometer/viscometers, which were used to calculate

the molecular weights.

Synthesis and Characterization of Precursors, Dimers, and

Polymers. The precursors (1a, 1b, 2a, and 2b), dimers (M1 and

M2), and polymers (P1 and P2) were prepared and characterized

^Hpara Ph), 1.75 (m, 24H, PCH ), 1.10 (m, 36H, P(CH CH ).

NMR (100 MHz, CDCl ): δ/ppm 156.88 (C ), 151.15 (C ),

2,2

6,6

36.07 (C ), 130.89−127.80 (C Ph), 121.43 (C ), 117.75 (C ),

3,3

4,4

5,5

31 1

12.66 (CC), 15.09 (PCH CH ), 7.97 (CH ). P{ H} NMR (162

MHz, CDCl ): δ 9.92 ( J

= 2650.25 Hz). ESI-MS: m/z 1528 [M +

Pt−P

4] . Anal. calc. for C H N O P Pt ReCl: C - 41.74; H - 5.02; N -

53 76 2 3 4 2

1.84%, found: C - 41.89; H - 5.19; N - 1.90%.

trans-[( Bu

Pt−CC−R−CC−]

(R = Re(CO) Cl-2,2′-

n 3

0,31

bipyridine-5,5′-diyl) (P3). CuI (0.015 g, 0.079 mmol) was added

according to previously reported procedures.

Rhenium(I) Tricarbonyl Chloride-5,5′-bis(ethynyl)-2,2′-bi-

pyridine (1c). A mixture of 1b (0.050 g, 0.24 mmol) with

rhenium(I) pentacarbonyl chloride (0.18 g, 0.50 mmol) was dissolved

in toluene (20 mL). The solution was stirred at 60 °C overnight. After

cooling to room temperature, the solvent was removed under reduced

pressure. The mixture was puriﬁed by passing it through an alumina

column using hexane/CH Cl (1:2) as the eluent, and further

to a mixture of trans-[Pt(P Bu

3 2 2

) Cl ] (0.040 g, 0.0060 mmol) and 1c

(0.030 g, 0.0060 mmol) in Pr

NH/CH Cl (50 mL, 1:1 v/v). The

solution was stirred at room temperature for 15 h, after which all

volatile components were removed under reduced pressure. The

residue was dissolved in CH Cl and puriﬁed through a short alumina

column. After removal of the solvent under reduced pressure, a yellow

ﬁlm was obtained and then washed with methanol to give P3 (0.0054

g, 82% yield). Further puriﬁcation could be performed by

puriﬁcation by preparative alumina thin layer chromatography yielded

c as an orange solid (0.064 g, 0.13 mmol, 49% yield, decomposition

−

precipitation from CH Cl in MeOH. IR (CH Cl ): ν/cm 1897,

2 2 2 2

−

temp. 250 °C). IR (CH Cl ): ν/cm 1896, 1905, 2015 (CO),

1920, 2018 (CO), 2089 (CC). H NMR (250 MHz, CDCl ): δ/

120 (CC), 3296 (CCH). H NMR (250 MHz, CDCl ): δ/ppm

ppm 7.94 (d, 2H, J = 7.0 Hz, H ), 7.17 (t, 2H, J = 7.0 Hz, H ),

3,3′

4,4′

.11 (dd, J = 2.0, 0.78 Hz, 2H, H ), 8.13 (d, J = 7.9 Hz, 2H, H ),

6.86 (d, 2H, J = 7.6 Hz, H₅_,₅_′), 2.11 (m, 12H, PCH (CH ) (CH )),

1.49 (m, 12H, PCH (CH ) (CH )), 1.41 (m, 12H,

2 2 2 3

,6′

3,3′

2 2 2 3

.11 (dd, J = 8.1, 2.0 Hz, 2H, H₄_,₄_′), 3.50 (s, 2H, CC−H).

NMR (100 MHz, CDCl ): δ/ppm 155.32 (C ), 149.64 (C₆_,₆_′),

PCH (CH ) (CH )), 0.92 (t, J = 7.2 Hz, 18H, P(CH ) CH )). C

2,2′

2 2 2 3 2 3 3

37.75 (C₃_,₃_′), 124.66 (C₄_,₄_′)₊, 119.30 (C ), 88.46, 79.90 (CC).

NMR (100 MHz, CDCl ): δ/ppm 156.72 (C₂_,₂_′), 145.28 (C₆_,₆_′),

5,5′

ESI-MS: m/z 511 [M + 2] . Anal. calc. for C H N O ClRe: C -

136.27 (C₃_,₃_′), 126.15 (C₄_,₄_′), 122.10 (C₅_,₅_′), 115.98, 109.32 (CC),

38.81; H - 2.16; N - 5.28%, found: C - 38.78; H - 2.15; N - 5.25%.

29.72−23.18 (PCH CH CH CH ), 14.35 (CH ). P{ H} NMR

Rhenium(I) Tricarbonyl Chloride-6,6′-bis(ethynyl)-2,2′-bi-

(162 MHz, CDCl ): δ/ppm 3.29 ( J

= 2340.25 Hz). Anal. calc. for

Pt−P

pyridine (2c). Similar procedures to those used to obtain 1c were

followed starting from 2b (0.13 g, 0.63 mmol). 2c was obtained as an

orange solid after puriﬁcation (0.080 g, 0.39 mmol, 62% yield;

(

C H N P O ClRePt) : C - 44.47; H - 5.46; N - 2.53%, found: C -

41 60 2 2 3 n

−

4.61; H - 5.51; N - 2.59%. GPC (THF): M = 61,000 g mol (n =

−

5), M = 77,000 g mol , polydispersity index = 1.26.

−

decomposition temp. 149 °C). IR (CH Cl ): ν/cm 1853, 1907,

trans-[( Bu P) Pt−CC−R−CC−] (R = Re(CO) Cl-2,2′-

017 (CO), 2115(CC), 3291 (CCH). H NMR (250 MHz

bipyridine-6,6′-diyl) (P4). P4 was prepared as described above

CDCl ): δ/ppm 8.11 (d, 2H, J = 7.6 Hz, H ), 7.82 (t, J = 7.9 Hz,

for P3 starting from trans-[Pt(P Bu ) Cl ] (0.040 g, 0.0060 mmol)

3,3′

H, H₄_,₄_′), 7.54 (d, J = 7.7 Hz, 2H, H₅_,₅_′), 3.9 (s, 2H, CNC−H).

and 2c (0.030 g, 0.0060 mmol) in CH Cl / Pr NH (40 mL, 1:1, v/v)

2 2 2

C NMR (100 MHz, CDCl ): δ/ppm 157.41 (C ), 146.12 (C ),

with catalytic CuI (0.011 g, 0.0058 mmol). After puriﬁcation, the

2,2′

6,6′

138.53 (C₃_,₃_′), 130.71 (C₄_,₄_′), 122.68 (C₅_,₅_′), 89.75, 81.70 (CC).

product was obtained as a bright yellow solid (0.0046 g, 70% yield,

Inorg. Chem. 2021, 60, 745−759

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

−

decomposition temp. 256 °C). IR (CH Cl ): ν/cm 1871, 1911,

ASSOCIATED CONTENT

sı Supporting Information

−

−1

■

012 cm (CO) 2068 cm (CC). H NMR (250 MHz,

CDCl ): δ/ppm 8.10 (m, 2H, H ), 7.32 (t, 2H, J = 7.2 Hz, H₄_,₄_′),

3,3′

.24 (d, 2H, J = 7.0 Hz, H₅_,₅_′), 1.56 (m, 14H, PCH (CH ) (CH )),

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02747.

.47−1.29 (m, 22H, PCH (CH ) (CH )), 0.88 (t, J = 7.0 Hz, 18H,

P(CH ) CH ). C NMR (100 MHz, CDCl ): δ/ppm 156.23 (C ),

2,2′

Crystallographic data for 1c and M4; additional

spectroscopic data; images and Cartesian coordinates

of the optimized structures of 1b, 2b, 1c, 2c, M3, and

M4; and characterization of optical transitions identiﬁed

from TD-DFT calculations on 1b, 2b, 1c, 2c, M3, and

M4 including breakdowns into orbital components,

images of NTOs, and orbital energy-level spectra (PDF)

45.21 (C₆_,₆_′), 136.32 (C₃_,₃_′), 126.08 (C₄_,₄_′), 120.79 (C₅_,₅_′), 115.32,

12.54 (CC), 30.13−23.68 (PCH CH CH CH ), 14.37 (CH ).

P{ H} NMR (162 MHz, CDCl ): δ/ppm 3.32 ( J

= 2339.18).

Pt−P

Anal. calc. for (C H N P O ClRePt) : C - 44.47; H - 5.46; N -

2.53%, found: C - 44.55; H - 5.50; N - 2.56%. GPC (THF): M =

−1

55,000 g mol (n = 50), M = 83,000 g mol , PDI = 1.51.

X-ray Crystallography. Single-crystal X-ray diﬀraction experi-

ments were performed at 150 K on a STOE IPDS (II) diﬀractometer

using monochromatic Mo−K radiation (λ = 0.71073 Å) with the

Accession Codes

sample temperature controlled using an Oxford Diﬀraction Cryojet.

CCDC 1964156 and 2009579 contain the supplementary

crystallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, U.K. (fax: +44 1223 336033).

The X-Area software was used for data collection and indexing. The

structure was solved and reﬁned using full-matrix least-squares on F

in SHELX2014 from the WinGX suite. A multiscan absorption

correction was applied. There was extensive disorder in the alkyl

groups of the phosphine ligands, which were modeled over two or

three sites using partial occupancies, constrained to sum to unity, and

with additional constraints placed on the bond parameters to maintain

reasonable bond lengths and angles. With the exception of some of

the disordered carbon atoms in the alkyl chains of the phosphine

ligands, all non-hydrogen atoms were reﬁned anisotropically.

Hydrogen atoms were included using rigid methyl groups or a riding

model, with partial occupancies used as appropriate. Reﬁnement was

continued until convergence, and in the ﬁnal cycles of reﬁnement, a

weighting scheme was used that gave a relatively ﬂat analysis of

variance.

AUTHOR INFORMATION

■

Corresponding Authors

Muhammad S. Khan − Department of Chemistry, Sultan

Qaboos University, Al Khod 123, Sultanate of Oman;

Email: msk@squ.edu.om

Paul R. Raithby − Department of Chemistry, University of

Bath, Bath BA2 7AY, U.K.; orcid.org/0000-0002-2944-

Time-Resolved Fluorescence. Lifetime measurements were

0662 ; Email: p.r.raithby@bath.ac.uk

performed using time-correlated single photon counting (TCSPC).

Jonathan M. Skelton − Department of Chemistry, University

of Bath, Bath BA2 7AY, U.K.; Department of Chemistry,

University of Manchester, Manchester M13 9PL, U.K.;

orcid.org/0000-0002-0395-1202 ;

Email: jonathan.skelton@manchester.ac.uk

Osama K. Abou-Zied − Department of Chemistry, Sultan

Qaboos University, Al Khod 123, Sultanate of Oman;

orcid.org/0000-0003-0497-8412; Email: abouzied@

squ.edu.om

The TCSPC setup, described elsewhere, is part of an ultrafast

spectrometer (Halcyone, Ultrafast Systems, LLC) used to measure

femtosecond ﬂuorescence upconversion. Brieﬂy, an excitation is

obtained using a regenerative ampliﬁed Ti:sapphire laser (Libra,

Coherent), which generates compressed laser pulses centered on 800

nm with a 70 fs fwhm, 4.26 W power, and a 5 kHz repetition rate.

0% of the output pulse is used to pump a Coherent OPerA Solo

optical parametric ampliﬁer (Light Conversion Ltd.) to generate

spectrally tunable light from 240−2600 nm. For the current

measurements, the OperA was adjusted at 350 and 380 nm (∼20

nJ) and used as the excitation beam after passing through a

depolarizer to cancel any contributions from rotational dynamics

Authors

Ashanul Haque − Department of Chemistry, College of

Science, University of Ha’il, Ha’il 81451, Kingdom of Saudi

Arabia; Department of Chemistry, Sultan Qaboos University,

Al Khod 123, Sultanate of Oman; orcid.org/0000-0002-

6780-632X

Rayya Al-Balushi − Department of Basic Sciences, College of

Applied and Health Sciences, A ’Sharqiyah University, Ibra

(DPU-25, Thorlabs). A photomultiplier tube with an instrument

response function (IRF) of ∼250 ps, measured from scattered

excitation light, was used as the detector. Fluorescence was attenuated

and directed to the detector, and a monochromator was used to adjust

the detection wavelength. Decays were recorded to ∼10,000 counts in

the peak channel. The decay transients were ﬁtted to multiexponential

functions convolved with the IRF.

Computational Modeling. Molecular quantum-chemical calcu-

00, Sultanate of Oman; orcid.org/0000-0002-5665-

175

lations were carried out using the density-functional theory (DFT)

formalism as implemented in the Gaussian09 software. The CAM-

Idris Juma Al-Busaidi − Department of Chemistry, Sultan

Qaboos University, Al Khod 123, Sultanate of Oman

Nawal K. Al-Rasbi − Department of Chemistry, Sultan

Qaboos University, Al Khod 123, Sultanate of Oman

Sumayya Al-Bahri − Department of Chemistry, Sultan Qaboos

University, Al Khod 123, Sultanate of Oman

B3LYP hybrid functional was used in conjunction with Pople split-

valence basis sets of 6-31g and 6-31g** quality for the H and non-H

atoms, respectively. The LANL2DZ pseudopotential and corre-

sponding double-ζ basis sets were used to describe the Pt and Re

atoms. Initial models of 1b/2b, 1c/2c, and M3/M4 were prepared

from X-ray structures or using the Avogadro software. The

molecular structures were optimized in the gas phase, and the

minima were conﬁrmed to be stationary points from the absence of

imaginary modes in the vibrational Hessian matrix. Time-dependent

DFT (TD-DFT) calculations were carried out on the optimized

models using adiabatic B3LYP to identify the 50 lowest-energy singlet

and triplet states, a subset of which were characterized using natural

Mohammed K. Al-Suti − Department of Chemistry, Sultan

Qaboos University, Al Khod 123, Sultanate of Oman

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02747

Notes

transition orbitals. Visualization of the frontier orbitals was

performed using VESTA.

The authors declare no competing ﬁnancial interest.

Inorg. Chem. 2021, 60, 745−759

Inorganic Chemistry

Article

15) Yam, V. W.; Au, V. K.; Leung, S. Y. Light-emitting self-

ACKNOWLEDGMENTS

■

assembled materials based on d(8) and d(10) transition metal

M.S.K. thanks The BP Oman (Grant: EG/SQU-BP/SCI/

CHEM/19/01) and The Ministry of Higher Education

MoHE), Oman (Grant: RC/RG-SCI/CHEM/20/01) for

funding. P.R.R. is grateful to the Engineering and Physical

Sciences Research Council (UK) for support (EP/K004956/

). R.A.A. gratefully acknowledges The Research Council

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(

activation of platinum-acetylide complex for olefin hydrosilylation.

ACS Macro Lett. 2017, 6, 1146−1150.

(17) Green, K. A.; Cifuentes, M. P.; Samoc, M.; Humphrey, M. G.

Syntheses and NLO properties of metal alkynyl dendrimers. Coord.

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(

TRC), Oman (Project No. BFP/RGP/EI/18/076) for

(18) Lima, J. C.; Rodriguez, L. Applications of gold(I) alkynyl

funding and A’Sharqiyah University, Oman for a research

grant (ASU-FSFR/CAS/FSHN-01/2019). J.M.S. is currently

supported by a UK Research and Innovation Future Leaders

Fellowship (MR/T043121/1) and is also grateful to the UK

Engineering and Physical Sciences Research Council for

funding (EP/K004956/1 and EP/P007821/1) and to the

University of Manchester for the award of a Presidential

Fellowship. The authors acknowledge Dr. Rashid Ilmi,

Department of Chemistry, Sultan Qaboos University, Oman

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level of conjugation in carbazole-based precursors and their mono-,

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