Electronic and steric effects of platinum(ii) di-yne and poly-yne substituents on the photo-switching behaviour of stilbene: Experimental and theoretical insights

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Electronic and steric eﬀects of platinum(II) di-yne

and poly-yne substituents on the photo-switching

behaviour of stilbene: experimental and

theoretical insights†

Cite this: Dalton Trans., 2021, 50,

2555

a

b

a

Idris Juma Al-Busaidi,

Ashanul Haque,

John Husband,

Nawal K. Al Rasbi,

a

c

a

Osama K. Abou-Zied,

Rayya Al Balushi, Muhammad S. Khan * and

d

Paul R. Raithby

*

A series of mono-, di-, and poly(platina-ynes) incorporating stilbene spacer units with the formulae trans-

[

R–CuC–Pt(PBu

3

)

2

-CuC–R] (R

=

(E)-1,2-diphenylethene), trans-[(Ph)-(Et

3

P)

2

PtCuC–R–CuCPt

n

(

3

PEt )

2

(Ph)] (R = (E)-1,2-diphenylethene), and trans-[–(P Bu

3

)

2

PtCuC–R–CuC–]

n

(R = (E)-1,2-diphenyl-

ethene), respectively, have been synthesized and characterized to explore the eﬀects of ligand topology

on the photoisomerization and photophysical properties of these materials. The structural and photo-

physical properties of the complexes have been investigated and compared with those of the previously

reported mono-, di- and poly(platina-ynes) incorporating azobenzene spacers. We found that the

organometallic species 1M, 2M and 1P undergo topology-dependent reversible trans-to-cis photoisome-

Received 9th October 2020,

Accepted 24th January 2021

DOI: 10.1039/d0dt03502e

rsc.li/dalton

rization in CH Cl

2 2

solution. Computational modelling supported the experimental ﬁndings.

9

decades. A vast number of studies have been carried out to

shed light on the reactivity and excited-state properties of the

Introduction

The term molecular switch is used to define a class of mole- stilbene-based chromophores. For example, it has been

cules that can undergo reversible chemical transformation demonstrated that stilbene-based chromophores feature

between two or more states. This change in state, achieved by unique emission profiles, can undergo photoisomerization

1

0,11

the action of light (photochromism) or other means (viz. from the triplet manifold,

and also display electrocyclisa-

1

2

mechanochromism, electrochromism) has found a variety of tion. Another class of materials which have attracted the

1

–4

real-life applications.

A plethora of organic and organo- attention of researchers are π-conjugated “rigid-rod” poly-ynes

5

6

13–19

metallic systems with dithienylethene, spiropyran, azo- and poly(metalla-ynes).

Advantages of this class of

7

8

benzene, stilbene, etc. scaﬀolds have demonstrated reversible materials include, but are not limited to, their high conduc-

switchability in the solid state as well as in solution. Among tivity, tunable structural, photo-physical, photo-chemical and

these scaﬀolds, photoisomerization processes in π-conjugated electrochemical properties, and solubility in organic as well

stilbene motifs have been known for more than seven aqueous media, etc. Because of these features, these materials

have shown potential applications in optoelectronics, biologi-

1

4,20,21

cals, catalysis, sensing and other areas.

studies on poly-ynes and poly(metalla-ynes) by us and

have shown that the π-conjugation and the extent

of delocalization in such systems are highly dependent on the

geometry, the ligand environment around the metal centre

and the presence or absence of other metal ions.

instance, we have recently shown that the incorporation of a

second metal ion such Re(I) and Eu(III) is an eﬀective strat-

egy to fine tune their absorption and emission profiles. Prior

to this, we found that Pt(II) di-ynes and poly-ynes incorporating

azobenzene (I–IV, Chart 1) exhibit topology-dependent photo-

Comprehensive

a

Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al Khoud 123,

2

1–23

Sultanate of Oman. E-mail: msk@squ.edu.om

others

b

Department of Chemistry, College of Science, University of Hail, Ha’il 81451,

Kingdom of Saudi Arabia

c

Department of Basic Science, College of Applied and Health Sciences, A’Sharqiyah

1

0,24

For

University, Ibra 400, Sultanate of Oman

d

Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K..

2

5

26

E-mail: p.r.raithby@bath.ac.uk

†

Electronic supplementary information (ESI) available: Crystallographic data for

1M; absorption spectra of mono- and dibromo stilbene, absorption and emis-

sion spectra of azobenzene systems, photo-isomerization results, simulated, con-

voluted and line spectra of the complexes. CCDC 2035894 (1M). For ESI and crys-

tallographic data in CIF or other electronic format see DOI: 10.1039/d0dt03502e

2

7

physical properties. In general, the incorporation of the Pt

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21

Chart 1 Previously reported Pt(II) diynes (I & II) and poly-ynes (III & IV) incorporating azobenzene molecular switches.

(

II)-acetylide framework into the di-ynes and poly-ynes led to a

Results and discussion

Synthesis and spectroscopic characterization

reduction in the eﬃciency of the isomerization compared to

that observed in the related acetylide-functionalized azo-

benzene compounds.

A

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

Extending our study, we report herein the synthesis and between dibromo (1) and monobromostilbene (2) with tri-

characterization of a new series of mono, di- and poly-nuclear methylsilylethyne was performed in diisopropylamine/tetra-

i

platina-ynes incorporating stilbene ligands linking Pt(II) acety- hydrofuran ( Pr

2

NH/THF) to obtain 1,2-bis(4-((trimethylsilyl)

lide units in the dimeric and polymeric complexes. We demon- ethynyl)phenyl)ethene (1a) and 1-(trimethylsilyl)ethynyl(4-

2

8

strate how the substitution pattern aﬀects the photophysical styrylbenzene (2a) (Scheme 1). The protected di- and mono-

profiles. We also compare the photophysical property diﬀer- alkynes (1a–2a) were stable to air and light and were character-

1

13

ences between mono- and dinuclear Pt(II) acetylide complexes ized by IR and NMR ( H and C) spectroscopy, electrospray

incorporating stilbene and azobenzene systems. The experi- ionization mass spectrometry (ESI-MS), and elemental ana-

mental measurements are supported by quantum-mechanical lysis. Protodesilylation of 1a–2a was accomplished with dilute

modelling for a subset of materials, which is used to interpret aqueous KOH in methanol (MeOH)/THF to yield terminal di-

the spectroscopy and to investigate the energetics of the iso- and mono-alkynes 1,2-bis(4-ethynylphenyl)ethene (1b) and

merization process.

1-ethynyl-4-styrylbenzene (2b). The terminal alkynes 1b–2b

Scheme 1 Synthesis of ligand precursors 1a–1b/2a–2b, the Pt(II) di-ynes (1M/2M) and the poly-yne 1P.

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were purified by silica gel column chromatography and iso- calibrated against a polystyrene standard gave weight-average

−

1

lated as pale-orange powders in >80% yield. Due to the high molecular weight (M ) of 39 568 g mol for the poly-yne 1P,

w

reactivity of terminal alkynes, 1b–2b were freshly prepared corresponding to a degree of polymerization (DP) 48. The poly-

before reaction with the Pt(II) precursor complexes. The dehy- dispersity index (PDI) was found to be 1.7. The relatively

drohalogenation reaction between trans-[(Pt(Et P) PhCl] and narrow polydispersity (PDI < 2) in the molecular weight is con-

3

2

i

1

2 2 2

b in a 2 : 1 stoichiometry, in Pr NH/CH Cl , in the presence sistent with the proposed linear structure. A high DP in the

of CuI, at room temperature, aﬀorded the di-nuclear Pt(II) poly-yne was also supported by NMR spectroscopy, in which

diyne 1M. Similarly, the mono-nuclear Pt(II) diyne 2M was resonances corresponding to protons on the terminal end

n

obtained by reacting trans-[(P Bu

3

)

2

PtCl

2

] and 2b in a 1 : 2 ratio groups were too weak to be observed.

n

while polycondensation reaction between trans-[(P Bu

and 1b in a 1 : 1 ratio under similar reaction conditions

yielded the Pt(II) poly-yne 1P (Scheme 1). Purification of the Pt We were able to grow crystals of the Pt(II) diyne complexes 1M

3 2 2

) PtCl ]

Structural characterization

(

II) diynes and poly-yne was carried out using silica and and 2M, suitable for single-crystal X-ray diﬀraction. The crystal

1

0

alumina columns, respectively.

structure of 2M has been reported previously; and a prelimi-

The IR spectra of the Pt(II) diynes (1M–2M) and the poly-yne nary X-ray analysis confirms that the same polymorph was

1P) showed a single, sharp νCuC absorption at 2092–2093 cm⁻

1

,

obtained in this study. The structure of 1M is shown in Fig. 1

(

consistent with a trans configuration of the ethynylenic units and crystallographic parameters (Table ST1) and displacement

29

around Pt(II) centre. The νCuC values for the terminal alkynes parameters and bond parameters (Tables ST2–ST8) are pre-

−

1

(

1b–2b) were considerably lower (2101–2105 cm ) than corres- sented in the ESI.†

−1

ponding trimethylsilyl-protected alkynes 1a–2a (2152 cm ). The

1M crystallizes in the monoclinic space group P2 /c, with

1

Pt(II) diynes and poly-yne display lower νCuC values than those half the dimeric complex in the asymmetric unit and the mid-

in the corresponding protected or terminal dialkynes, an obser- point of the ethylene CvC bond corresponding to a crystallo-

vation that has been attributed to either metalla-yne π-back- graphic centre of symmetry. The X-ray structure of 1M con-

δ+ δ−

30

bonding or the M –C polarity.

firms the dinuclear Pt(II) complex in which the stilbene spacer

In all of the ligand precursors and the Pt(II) complexes, forms the central core of the complex, i.e., the Pt–CuC-p-C

6

4

H -

1

clear H NMR resonances from the protons of the aromatic CvC-p-C H CuC–Pt unit. The two arene rings are in a trans

6

4

ring systems were observed, indicating the formation of the arrangement about the ethylene CvC bond in the stilbene

3

1

expected products.

In general, the aromatic and vinyl spacer unit. The coordination geometry around each of the

protons in Z isomers are shifted to higher field than in their E two Pt(II) centres is square-planar (PtC P ) with a trans relation-

2

3

2,33

isomers.

We found that the commercially available starting ship between the two phosphine groups. All bond distances

materials, i.e. 1-bromo-4-styrylbenzene (δ = 6.94 ppm, d, J = about Pt(II) centre were found to be within the expected ranges

2

1,29,34–37

1

6.3 Hz, 1H and 7.01 ppm, d, J = 16.4 Hz, 1H) and 1,2-bis(4- for previously reported dimeric Pt(II) diyne complexes.

bromophenyl)ethane (δ = 7.02 ppm, d, J = 17.2 Hz, 2H) exhibi- The most interesting feature about this complex is the co-pla-

ted well defined olefinic peaks due to the trans configuration. narity of the stilbene spacer in which the two phenyl rings and

Similar resonances (between 6.7 and 7.0 ppm) of the stilbene the ethynylenic bond are in one plane as required by crystallo-

units were found in the monomer (2M), dimer (1M) and graphic symmetry. The measured dihedral angle between the

i

polymer (1P) in addition to the aromatic protons. Further con- two phenyl planes is 180.0°, identified by C7–C6⋯C6 –C7 .

firmation was made by irradiating the samples with UV light However, the terminal phenyl rings bonded to the Pt(II)

(

6

i.e. photo-isomerization). Upon irradiation, a new peak at centres are tilted from the planarity, with respect to the stil-

.4–6.5 ppm arises, attributed to the olefin protons in cis-stil- bene plane, with a dihedral angle of 17.97° identified by C11–

1

0

13

bene unit(s). The two distinct C NMR resonances of the C10–C8–C3. Moreover, the coordination plane of Pt(II) (PtC P )

2

ethynylic carbon atoms in the Pt(II) diynes and poly-yne were is almost perpendicular to the plane of stilbene spacer in

shifted downfield relative to the signals from the terminal which the measured dihedral angle is 95.66° (identified by

alkynes, in agreement with the expected structures. The reso- C4–C3–C1–P2).

nances due to the ethyl and butyl groups of the auxiliary

ligands in the Pt(II) diynes and poly-yne, respectively, could be

Optical spectroscopy

clearly identified.

Absorption spectra. Electronic spectra of stilbenes are gener-

The MS results confirm the molecular masses expected for ally sensitive to configuration and substituents attached to the

the ethynyl ligands and the Pt(II) diyne complexes. For arene rings. Peak positions red shift as the extension of

instance, the ESI-MS spectra of the complexes show molecular π-conjugation within the stilbene chromophore itself and

ion peaks at 372.70 (1a), 229.1 (1b), 276.5 (2a), 204.0 (2b), between the stilbene core and auxochromic groups

3

8,39

1

243.4 (1M) and 1006.0 (2M), confirming proposed formu- increases.

lations for the protected and deprotected ligands as well as the

Pt(II) di-yne complexes. H and C NMR, IR and ESI-MS 4,4′-dibromostilbene) show intense primary absorption bands

The spectra of the ligand precursors (4-bromostilbene and

1

13

spectra of the intermediate and final compounds can be found at ∼317–325 nm in dichloromethane (Fig. S31, ESI†), respect-

4

0

in ESI (Fig. S1–S30†). Gel permeation chromatography (GPC) ively, assigned to π → π* transitions. The room temperature

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Fig. 1 Crystal structure of 1M, with the unique atoms numbered as shown. Displacement ellipsoids are plotted at the 50% probability level and only

one orientation of the disordered ethyl groups is shown. Selected geometric data: Pt(1)–C(1) 2.019(8) Å, Pt(1)–C(10) 2.070(8) Å, Pt(1)–P(1) 2.281(2) Å,

Pt(1)–P(2) 2.295(2) Å, C(1)–C(2) 1.190(11) Å, C(2)–C(3) 1.446(12) Å, C(6)–C(9) 1.489(13) Å, C(1)–Pt(1)–C(10) 176.4(3)°, C(1)–Pt(1)–P(1) 86.4(2)°, C(10)–

Pt(1)–P(1) 91.8(2)°, C(1)–Pt(1)–P(2) 91.3(4)°, C(10)–Pt(1)–P(2) 90.3(4)°, P(1)–Pt(1)–P(2) 175.8(3)°, C(1)–C(2)–C(3) 177.3(12)°, C(4)–C(3)–C(8) 121.6(8)°,

i

C(9) –C(9)–C(6) 130.2.2(16)°, C(11)–C(10)–Pt(1) 121.7(6)°. Symmetry operator i: 1 − x + 2, −y + 2, −z + 1.

absorption spectra of trimethylsilyl-acetylide stilbene ligands

(

1a, 2a), Pt(II) di-ynes (1M, 2M) and the poly-yne (1P) in di-

chloromethane solution are shown in Fig. 2 and the spectral

data are provided in Table 1.

The acetylide-functionalised ligands show intense primary

5

−1

bands (2a: λ

=

= 331 nm, ε = 0.71 × 10 M cm ) (1a: at λ_m_a_x

max

336 nm, ε = 1.66 × 10 M⁻¹cm ) as expected. The introduc-

5

−1

tion of Pt(II) acetylide moieties modulates the absorption

profile of the stilbene units more eﬀectively. For example, a

significant red shift in the absorption maximum was noted for

the mononuclear Pt(II) di-yne complex 2M (λmax = 376 nm, ε =

5

−1

2

3

.34 × 10 M cm ) compared to the 4-bromostilbene (λ

=

max

5

−1

17 nm, ε = 2.69 × 10 M cm ). A more pronounced eﬀect

was noted in the dinuclear Pt(II) di-yne complex 1M (λmax

=

5

−1

3

80 nm, ε = 1.09 × 10 M cm ), which could be attributed to

2

7,28

the extended conjugation length.

In the case of the poly

(

platina-yne) 1P, the absorption maxima shifted to the visible

Fig. 2 Absorption spectra of stilbene-acetylide ligands (1a–2a), Pt(II) di-

5

−1

region (λ_m_a_x= 418 nm, ε = 2.59 × 10 M cm ). Overall, using ynes 1M, 2M and poly-yne 1P.

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Table 1 Comparative optical and ﬂuorescence spectral data of Pt(II) diynes and poly-yne incorporating stilbene and azobenzene spacers

Code

Photophysical data

1M

2M

1P

(I)

(II)

(III)

(IV)

λ

abs (nm)

ext (nm)

380

376

380

413

380

436, 460sh

2.47

2.77

441

350

³e ⁸⁰^,⁴⁰⁰

323

456

335

a

—

340

d

ems (nm)

423, 448, 477sh

1.10

2.82

409, 432, 478sh

1.09

2.96

b

e

τflu (ns)

—

3.39

—

2.33

op

c

Eg (eV)

2.35

2.97

−

5

a

b

c

All measurements were taken in 1 × 10 M CH

2

Cl

bandgaps (Eg ) were determined from onset of absorption in solutions. Very weak emission. Shorter than the instrument response function

IRF).

2

at room temperature._dExcitation wavelength in solution. Uncertainty is ±0.03 ns. Optical

op

e

(

this strategy, the absorption maxima of the stilbene core can

Emission spectra. The fluorescence spectra of 1M, 2M and

1P in dichloromethane solution at room temperature are

polymer

ligand

be shifted by up to 100 nm (λmax

− λmax = 99 nm).

Fig. 3 shows comparative absorption spectra of stilbene shown in Fig. 4. All compounds clearly show vibronic tran-

incorporated Pt(II) di-yne (1M) and poly-yne (1P) with their azo- sitions within the envelope of the S (π*) → S (π) fluorescence

1

0

2

7

benzene counterparts (I & III, Chart 1 and Table 1). The

absorption spectra of meta-azobenzene incorporated Pt(II) di-

yne (II) and poly-yne (IV) systems are shown in Fig. S32 (ESI†).

The absorption spectra of (I) show a red-shifted band (λ

=

max

5

−1

4

3

41 nm, ε = 0.91 × 10

80 nm, ε = 1.09 × 10 M cm ). The same behaviour was

M

cm ) compared to 1M (λmax

5

−1

noted for poly-ynes, (III) was red-shifted (λ

= 456 nm, ε =

max

5

−1

0

1

.95 × 10 M cm ) relative to 1P (λmax = 413 nm, ε = 2.59 ×

0 M cm ). Interestingly, azobenzene bearing platina-ynes

5

−1

exhibited absorption spectra that are broader than their stil-

bene counterparts are which may be due to more energy states

being accessible in the azobenzene systems. On the other

hand, the spectra of 1M and 1P show high ε values compared

to those of the azobenzene counterparts, which are consistent

with the enhanced electronic delocalization across stilbene

units. The enhanced absorption at visible wavelengths can be

attributed to metal-to-ligand charge-transfer (MLCT) exci-

4

1

tations. The enhanced ε of A band (e.g. the lower energy 383

and 398 nm for 1M) is considered to be the conjugation band

and is the most prominent feature in trans-stilbene complexes,

4

0

Fig. 4 Emission spectra of 1M, 2M and 1P in CH

was at 380 nm.

2

Cl

2

solution. Excitation

which are assigned to IL π → π* excitations.

Fig. 3 Absorption spectra of (a) Pt(II) di-ynes incorporating stilbene (1M) and azobenzene (I) cores. (b) Pt(II) poly-ynes incorpoarating stilbene (1P)

and azobenzene (III) cores.

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band (summarized in Table 1). Compared to 1M, a significant Photo-isomerization study

red shift is observed in the fluorescence spectrum of 1P,

Light/heat induced structural transformation (trans ↔ cis iso-

whereas 2M emits weakly with a blue shift as indicated in

Table 1. The diﬀerent linking of the ethynylstilbene unit(s) to

a central Pt(II) causes diﬀerent perturbations of the resulting

electronic characteristics especially for 1M. The measured

emission lifetimes for 1M, 2M and 1P for the main peaks are

all very short (1.09–2.47 ns) at room temperature, characteristic

of the spin-allowed singlet emission. These results together

with the small Stokes shift preclude the emitting state being a

merization and electrocyclization) and the mechanisms

45,46

involved in stilbene derivatives are well known.

For

example, the UV spectrum of stilbene exhibits two distinct fea-

47

tures: a lower energy A-band, and a higher energy B-band.

These two bands are blue shifted in cis isomers compared to

39,46

the trans.

This hypsochromism could be attributed to the

increase in interplanar angle θ (the angle through which the

plane of each phenyl group rotates out of the central plane of

the, essentially, double bond for the cis isomers) due to the

non-planarity of the most probable configuration of cis-stil-

4

2

triplet excited state. Moreover, for 1P, the excited-state life-

time is 2.47 times higher than 1M and 2M, implying that the

rigidity in the poly(platina-yne) plays a role in restricting the

torsional rotation of the arene rings around the central CuC

bond axis, which inhibits the barrier crossing and therefore

46

bene. In addition to the hypsochromism, the A-band in the

cis-stilbene is structureless, in contrast to the structured band

observed in trans-stilbene, and has a much lower molar

4

3,44

increases the excited state lifetime.

37,54

absorption coeﬃcient.

Contrarily the B-band of cis-stilbene

The fluorescence bands of 1M and 1P show bathochromic

shifts with respect to those of 2M owing to the more conju-

gated backbone. The emission study of 2M was repeated at two

(

also structureless) is more intense than that of trans-

52–54

stilbene.

Therefore, trans ↔ cis isomerization can easily be

monitored using these two bands. It has previously been

reported that the mono-substitutions in the para-position

reduce the activation energy and ease trans–cis isomerization

compared to pure stilbene. This is attributed to the high con-

tribution of the charge-separated resonance form in which the

−

6

−7

lower concentrations (1 × 10 and 1 × 10 M) in order to

investigate any possible self-quenching of emission due to

high concentration of monomer or dimer formation. However,

the emission spectra showed similar behaviour matching the

trend, which is a low emission intensity with decreasing con-

centration. This means that the low emission behaviour of 2M

is an inherent feature. The presence of two stilbene moieties

in the chemical structure of 2M is responsible for the observed

relatively large extinction coeﬃcient. However, the absorbance

band reflects a Franck–Condon (FC) envelope that is unstruc-

tured, compared to the absorption bands of 1M and 1P

48

central carbon–carbon bond is considered to be single.

The photo-isomerization behaviour of Pt(II) di- and poly-

ynes incorporating stilbene spacers were assessed at 254 and

365 nm, at ambient temperature, in dichloromethane for

diﬀerent time intervals. The absorption profile of non-irra-

diated (t = 0 min) and irradiated (t = 10, 20 and 25 min) poly-

yne 1P is depicted in Fig. 5. The absorption profiles of photo-

isomerization behaviour of stilbene-acetylide ligands (1a, 2a)

and Pt(II) di-ynes (1M, 2M) are shown in Fig. S34 and S35,

respectively (see the ESI†).

(Fig. 2), which indicates a large number of vibrational states

accessible in 2M. This stems from the ability of the stilbene

molecule to vibrate more freely in 2M due to its terminal

location, whereas in 1M and 1P the motion of stilbene is much

more restricted due to its central position and the more conju-

gated nature of the structures. On the other hand, high

vibrational levels are usually populated in the excited state by

FC excitation in which deactivation, in this case, is dominated

by the vibrational dynamics of stilbene on the excited state

potential energy surface. This is enhanced by the longer emis-

sion lifetime of the excited state of stilbene (nanoseconds, vide

supra) than vibrational relaxation (typically picoseconds),

which leads to population of a large number of vibrational

states before emission takes place. This is evidenced in the

unstructured emission band of 2M, compared to those of 1M

and 1P (see Fig. 4). Also, the presence of a large group attached

from one side to each stilbene molecule in 2M substantially

increases the density of states, giving a large number of

accessible vibrational states for the initially prepared excited

state to populate at the expense of radiative decay. All of

these factors lead to weaker emission from 2M, compared to

As can be seen, 1a, 2a started to photoisomerize after

10 min of irradiation, leading to a reduction in intensity of

band A and intensification of band B. On the other hand, for

1

M and 1P. The corresponding azobenzene incorporated Pt(II)

diynes and poly-ynes show either no or weak emission, relative

to 1M and 1P (Table 1). The whole azobenzene series shows no

emission or only weak emission in solution (Fig. S33, in the

ESI†).

Fig. 5 Absorption spectral proﬁle of 1P upon irradiation with light at

365 nm at diﬀerent exposure times.

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2

M and 1P, changes in the absorption profile were noted after

2

0 min of UV irradiation (Fig. S35a, see the ESI†). As is evident

from the spectra, irradiation of UV light converted trans

isomers to cis in a time dependent manner. In general, all

compounds exhibited following spectral features: band broad-

ening (unstructured), blue shift of band A and an increase in

molar absorption coeﬃcient of band B upon increasing UV

exposure time. Similar results were noted for both wavelengths

(

254 and 365 nm).

Schanze et al.

1

0

demonstrated that in Pt(II) acetylides

bearing two photoactive stilbene units, the irradiation of UV

light (365 nm) causes isomerization only in one unit. Linseis

4

0

et al. reported similar findings with vinyl ruthenium com-

plexes derived from diﬀerently configured stilbenes. We find

that upon radiation, at t = 0, the electronic spectra of 1M and

Fig. 6 Absorption spectral proﬁle of 1P prior to exposure (non-

exposed) and measured at diﬀerent intervals for the recovery process of

the initially exposed sample to 25 minutes irradiation at 365 nm.

2

M resemble those of the trans stilbenyl ruthenium complexes

while after 10–25 min of light exposure, the peaks shifted to

higher energy, more closely resembling the spectra for the cis-

configuration.

The photo-isomerization studies of azobenzene-based

materials in solution showed that acetylide ligands (Fig. S34c,

see the ESI†) and, to a limited extent, their Pt(II) complexes

On the other hand, the spectra of 2M started to resemble that

of the trans-configuration after 7 h and a full recovery was

achieved within 21 h. In the case of 1P, it took 24 h to attain

the absorption features similar to the original trans-configur-

ation absorption profile (Fig. 6). The 1P sample was further

monitored for an additional 6 h, however, no change was

observed in the absorption profile. This indicates that the

compounds remained stable even after exposure to UV

irradiation either at the low or high energy. The percentage

increase in the cis-isomer absorbance and the corresponding

decrease in the trans-isomer absorbance are included in

Tables ST9–13† for the diﬀerent periods of irradiation. It is

noteworthy that after 25 min of UV exposure, the absorption

spectra for all the compounds remain unchanged for almost

(

Fig. S35(c), see the ESI†) undergo reversible cis–trans photo-

2

7

isomerization under irradiation at UV wavelengths. This was

evident from a general decrease in the intensity of the absorp-

tion bands at longer wavelengths coupled with an increased

intensity of shorter-wavelength bands in a similar manner to

the stilbene analogues (1a, 1M and 1P). In both stilbene and

azobenzene systems, the extent of isomerization is less marked

in the Pt(II) di- and poly-ynes compared to their free acetylide-

containing ligands. The eﬀect of photo-irradiation or photo-

isomerization process in Pt(II) di-ynes (1M and 2M) and poly-

1

yne (1P) was also monitored by H-NMR spectroscopy and the

results are depicted in Fig. S18, S22, S26 and S28–S30.† Among

others, emergence of new signal between δ 6.46–6.55 ppm

clearly indicated the presence of cis-stilbene containing com-

plexes in the solution.

7

h, indicating the ability of the cis-configuration to retain the

energy eﬃciently for a relatively long period before starting the

recovery process to the trans-configuration.

Considering the complex kinetics of the photoisomeriza-

tion process that depend on several parameters (light energy

and intensity, time of irradiation, temperature), a more

detailed study on irradiation and recovery of the current com-

pounds will be the subject of future work.

Recovery time study

The determination of recovery time (time required to attain

the most stable configuration) is an important factor that

determines the application of a molecular switch. If the recov-

ery is complete (i.e. the absorption profile matches that at t = 0

prior to exposure), it can be concluded that the isomerization

Computational modelling

process is eﬃcient and no side reaction (photoaddition, trans-Stilbene is known to be 19.2 kJ mol⁻more stable than

photo-degradation or aggregation, etc.) is taking place. We the cis isomer. To underpin the eﬀect of metalation on the

1

9

determined the recovery time by monitoring spectral changes stability and isomerization of the stilbene isomers, cis–trans

after irradiating the complexes in solution at λmax = 254 or energy diﬀerences (ΔEcis–trans) of mono- (2M) and di-nuclear

3

65 nm for 25 min. Following irradiation, the samples were (1M) Pt(II) di-ynes were calculated computationally. Optimized

left in dark and the absorption spectra were collected after (DFT) gas-phase structures for cis- and trans-isomers of 1M

regular periods (i.e. one measurement per hour). Fig. 6 and and 2M are shown in Fig. 7. The predicted gas-phase struc-

Fig. S36 (see the ESI†) shows the most prominent changes tures are in close correspondence to those determined for the

noted during the recovery time study. As it is clear from the crystalline forms. Continuing with the numbering scheme pre-

figures, recovery time depends on the structure of the com- viously used for trans-1M, the dihedral C7–C6⋯C6′–C7′ equal

plexes. For 1M and 1P, the spectra started to resemble those of to 178.4° while C4–C3–C1–P2 equals 92.8°. In a notable diﬀer-

the trans-configuration after 19 h (Fig. 7 and Fig. S36, ESI†). ence between the gas-phase and crystalline structures, the

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Fig. 7 Optimized gas-phase structures of the cis and trans isomers of 1M and 2M.

C11–C10–C3–C8 dihedral is predicted to be 5.3°, suggesting indeed, the energy diﬀerence per cis configuration is less than

that the observed tilting of the terminal phenyl rings maybe that for 1M reflecting that the terminal stilbenes in 2M are

due to a packing eﬀect in the crystal. In the cis structures the closer in behaviour to free stilbene than is the bridging stil-

expected, large, interplanar angle θ, is clearly evident with C7– bene in 1M. The PCM results show no strong eﬀect on the cal-

C6⋯C6′–C7′ equal to 60.2° in cis-1M and the equivalent angles culated energy diﬀerences although ΔEcis–trans is slightly larger

equal to 61.5° and 61.6° for cis-2M. A summary of the calcu- for both 1M and 2M when the PCM is included compared to

lated energy diﬀerences is given in Table 2. It is clear, the cal- the gas-phase values. It should be noted that these ΔEcis–trans

culated energy diﬀerences for the metal-containing com- values are well below both the expected isomerization barrier

(the isomerization barrier is 202 kJ mol⁻in stilbene ) and the

1

9

pounds (2M & 1M) are close to those of stilbene itself.

The cis–trans value calculated for 1M is only slightly higher photon energies used in the photoisomerization studies

−

1

than that calculated for stilbene (20.9 kJ mol gas phase, reported here. Further, while the steric bulk of the Pt(II)

2

3.0 kJ mol⁻the polarizable continuum model, PCM); centres has been shown here not to strongly eﬀect the energy

1

suggesting that the metal centres are suﬃciently well separ- diﬀerence between isomers, this does not preclude a strong

ated in the cis isomer to not cause appreciable extra steric steric eﬀect on the barrier to internal rotation leading to iso-

2

7

crowding. The energy diﬀerence in 2M is greater than that for merization. For comparison, the calculated cis–trans energy

1

M: presumably because of the double-cis configuration; diﬀerence for the azobenzene incorporated platinum(II) di-yne

complex (I, Chart 1) is 72.21 kJ mol⁻. The much larger energy

1

diﬀerence for the azobenzene bearing complex reflects the

4

9

larger cis–trans energy diﬀerence for azobenzene itself (49 kJ

mol ) compared to stilbene.

Table 2 Calculated isomer energy diﬀerences for structures 1M and

−

1

2

M in the gas phase and PCM for CH

2

Cl

2

(ε = 8.93)

TD-DFT calculations at the ground-state optimized geome-

try were performed to calculate excitation energies. These verti-

cal energy diﬀerences together with the calculated oscillator

strengths can be used to provide comparison with the

measured electronic absorption spectra. The TD-DFT calcu-

lations were performed with, and without, the inclusion of the

PCM. Overlapped spectra resulting from the TD-DFT (gas

ΔEcis–trans (kJ mol⁻

1

)

a

Model

1M

2M

I

Gas phase

PCM

22.0

23.1

39.5

41.4

71.39

72.21

a

Ref. 27.

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Fig. 8 (a) Simulated absorption spectra of the cis and trans isomers of 1M and 2M using TD-DFT in the gas phase. (b) Simulated absorption spectra

of the cis and trans isomers of 1M and 2M using TD-DFT with inclusion of the PCM (CH Cl , ε = 8.93). The calculated transitions have been convo-

luted with Gaussian peaks with a FWHM of 3000 cm⁻to simulate the experimental spectra.

2

1

phase and PCM) calculations are shown in Fig. 8a/b and lated transitions indicate that the longest wavelength transition

Fig. S37–46 (ESI†). is overwhelmingly LUMO ← HOMO in character for both cis-

Individual spectra with line spectra indicating transitions and trans-isomers of 1M (99%) and that the largest contribution

from the ground state, are included in the ESI:† as shown in for cis- and trans-isomers of 2M is also LUMO ← HOMO (88%

Fig. 8a and b the spectrum of the trans isomer of 1M is domi- and 93%, respectively) with a minor contribution from

nated by an absorbance maximum at 416 nm (402 nm gas- LUMO+1 ← HOMO−1 (11% and 6% respectively). The predicted

phase) while that for trans-2M the peak is blue-shifted by 23 nm blue-shift in the absorbance of the cis isomers compared to

with a concurrent increase in absorptivity. These trends match those for the trans help to account for the spectral changes

well with the absorbance spectra shown in Fig. 2. Compared to observed in the photoisomerization experiments performed in

the trans spectra, the corresponding bands in the cis spectra CH

show a blue shift of 20 nm and a concurrent dramatic decrease vant frontier orbitals are shown in Fig. 9 and 10, respectively.

in absorptivity. These trends match well with the known absor- The HOMO/LUMO gaps of dinuclear Pt(II) complexes (1M)

bance spectra of cis- and trans-stilbene. Analysis of the calcu- are lower than those of mononuclear Pt(II) complexes (2M) at

2 2

Cl . The calculated electronic energy levels along with rele-

9

Fig. 9 Electronic energy levels. In each case the ﬁrst 50 levels above and below the LUMO and HOMO are shown.

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Fig. 10 Frontier orbitals of the cis- and trans-isomers of 1M and 2M.

3

.35/3.59 and 3.48/3.71 eV for the trans/cis stilbene geometries, energy transition in the predicted absorption spectra

respectively. And for both complexes the HOMO/LUMO gaps (Table ST14, ESI†). Similar observations were noted for dinuc-

are smaller for the trans isomers than the cis, thus the LUMO/ lear Pt(II) complexes of azobenezene. However, compound I

HOMO energy gaps correlate with the position of the lowest with para-substituted azobenezene, HOMO/LUMO gaps are

2564 | Dalton Trans., 2021, 50, 2555–2569

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significantly lower than 1M at 2.87/2.99 eV for the trans/cis azo systems, has been found to be aﬀected by the metalation. We

geometries (Table ST15, ESI†). This may explain the red shift found that the simulated spectra of the cis and trans isomers

observed in optical absorption for I in Fig. 3a.

of 1M and 2M show a blue shift of the most prominent band

Visual analysis of the orbitals suggests that LUMO ← with a concurrent reduction in molar absorptivity, while an

HOMO transition in both 1M and 2M, and the LUMO+1 ← enhancement of the molar absorptivity of the high energy

HOMO−1 transition in 2M represent transitions between peaks is also predicted. These results are consistent with con-

states of π symmetry, and result in locally excited (LE) states clusions drawn from the photoisomerization study.

with electron density spatially distributed in the excited state

very similarly to that in the ground state. Those features are

similar to those reported for Ru-based stilbene units with

dinuclear and mononuclear complexes. Further visual ana-

lysis of the frontier orbitals suggests that the degree of spatial

Experimental section

General procedures

4

0

overlap between the HOMO and LUMO orbitals is slightly All reactions were performed under a dry argon atmosphere

greater for the trans isomer of each complex than the cis which using standard Schlenk-line techniques. Solvents were pre-

correlates with the lower absorptivities, and oscillator dried and distilled before use according to standard pro-

5

0

strengths calculated for the cis complexes.

A Natural bond orbital analysis (Gaussian NBO Version 3.1) obtained from Sigma-Aldrich and used as received. The com-

cedures. All chemicals, except where stated otherwise, were

5

1

n

51

of trans-stilbene, based on the ground and excited-state popu- pounds trans-[Ph(Et

P)

lations, gives occupations of 1.98 and 1.87 respectively for the 2M were prepared following reported procedures.

σ- and π-components of the central bond in S , while only a σ-

Column chromatography was performed either with

bond is indicated in S1′ with an occupation of 1.98. This is Kieselgel 60 (230–400 mesh) silica gel or alumina (Brockman

entirely consistent with there being a large barrier for internal Grade-I). NMR spectra were recorded in CDCl using Bruker

PtCl],

trans-[(P Bu

)

PtCl

]

and

3

2

3

2

1

0

3

rotation about the central bond (and hence isomerization) in WM-250 and AM-400 spectrometers and a Bruker Avance III

the ground state, and that vibrational relaxation following exci- HD 700 MHz spectrometer equipped with 5 mm TCI H/C/N

1

13

tation to S

1

, at the ground-state equilibrium geometry, results cryoprobe. The H and C NMR spectra were referenced to

in rotation about the central bond and the subsequent for- solvent resonances. For labelled (numbered) chemical struc-

mation of ground state cis- and trans-isomers. NBO analysis for tures, please refer to Fig. S47 (ESI†). IR spectra were recorded

trans-1M gives a similar picture for S

.86 for the σ- and π-components) while in S₁the pertinent recorded with Shimadzu UV- 2450 spectrometer. Mass spectra

0

(occupations 1.98 and using a Cary 630 FTIR spectrometer. UV/vis spectra were

1

occupations are given as 1.98 and 1.70. Thus, the π-component were acquired using a Kratos MS 890 spectrometer using elec-

is seen to be only partially reduced (10%), and implying a trospray-ionization (ESI) techniques. Microanalysis was carried

barrier for internal rotation about the central bond remains in out at the Department of Chemistry, Sultan Qaboos University.

1 1

S . This would suggest that photoisomerization via S

requires UV–visible spectra were recorded on a Varian Cary 50 spectro-

light of suﬃcient energy to initially excite the complex to a photometer in 200–800 nm range. Quartz cuvettes of 1 cm

vibronic state above the barrier, and further, that rotation to a path length were used, and solvent background corrections

cis-like conformer must occur before vibrational relaxation were applied.

below the barrier height takes place. While further study is

Fluorescence spectra were recorded on a Shimadzu RF-5301

indicated, residual double-bond character in S is consistent spectrofluorophotometer and a PerkinElmer LS 55 fluo-

1

with the limited extent of photoisomerization observed for rescence spectrometer. The fluorescence spectra were cor-

these complexes.

rected for the diﬀerence in optical density at the excitation

wavelength and inner-filter eﬀect using the following

5

2,53

equation:

Conclusion

Fcorr ¼ Fobs ꢀ antilogððODex þ ODemÞ=2Þ

ð1Þ

A new series of Pt(II) di- and poly-ynes incorporating para-sub- where Fcorr and Fobs are the corrected and observed fluo-

stituted stilbene spacers have been successfully synthesized. rescence intensities, respectively, and ODex, ODem are the

The synthesized ligands and Pt(II) complexes have been fully optical density at both the excitation and emission wave-

characterized and the solid-state structure of a model di-yne lengths, respectively. Time-resolved fluorescence measure-

compound has also been determined. Photo-physical pro- ments were performed using a Time-Master fluorescence life-

perties of the materials in solution states have been deter- time spectrometer obtained from Photon Technology

mined. The structural and photo-physical features of the com- International. Excitation was at 380 nm using a light emitting

pounds were supplemented by computational studies at diode. The system response time, as measured from the scat-

TD-DFT level.

tered light, was estimated to be approximately 1.5 ns. The

We have demonstrated that all compounds undergo photo- measured transients were fitted to multi-exponential functions

induced reversible trans-to-cis isomerization in solution. The convoluted with the instrument response function (IRF). The

2

extent of isomerization in stilbene derivatives, like azobenzene fit was judged by the value of the reduced chi-squared (χ ). The

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experimental time resolution (after deconvolution) was using hexane/CH

2

Cl

2

(1 : 1 v/v) as the eluent, aﬀording 1b as a

−

1

approximately 100 ps, using stroboscopic detection.

The photoirradiation experiments were carried out in di- (CuC), 3261 (CuC–H). H NMR (700 MHz, CDCl

chloromethane (CH Cl under multiband UV lamp (q, 8Harom.), 7.10 (s, 2H , HCvCH), 3.14 (s, 2H

pale-orange powder (0.25 g, 91% yield). IR (cm ): ν 2158

1

3

) δ 7.49–7.46

1

3

2

)

a

b

, CuCH).

C

(

Mineralight Lamp UVGL-55; 230 V and 6 W). This type of NMR (176 MHz, CDCl ): δ 137.43, 132.50, 128.96 (aromatic

3

lamp emits radiation at both short (254 nm) and longer carbons), 126.37 (CvC), 121.38, 83.63 (CuC). ESI-MS m/z

+

(

365 nm) wavelengths; the shorter wavelength window was 229.10 [M + H]. Elemental Analysis: Calculated; C, 94.70; H,

used for these experiments. Spectra were recorded at diﬀerent 5.30, found: C, 94.58; H, 5.26.

intervals (0, 10, 20 and 25 min). Trimethyl((4-styrylphenyl)ethynyl)silane (2a). To a solution

Gel permeation chromatography (GPC) system was used to of (E)-1-bromo-4-styrylbenzene (1.00 g, 3.86 mmol) in Pr

characterize the synthesized polymer (organic poly-ynes). A THF (60 mL,1 : 4 v/v) under an argon atmosphere were added

Viscotek GPCmax instrument (VE2001) equipped with a TDA catalytic amounts of CuI (10 mg), Pd(OAc) (10 mg), and PPh

05 triple detector array consisting of refractive index (RI), (50 mg). The solution was stirred for 30 min at room tempera-

i

2

NH/

2

3

light scattering both at right and low angles (RALS, and LALS ture followed by the addition of (trimethylsilyl)ethyne

respectively), viscometer detectors, and as well, a UV detector (0.82 mL, 5.80 mmol) with vigorous stirring. The reaction

(

Viscotek 2600) was used. Three Polyanalytik SupeRes™ Series mixture was then refluxed overnight. Completion of the reac-

00 mm × 8 mm linear mixed bed columns having linear poly- tion was confirmed by silica thin-layer chromatography (TLC)

styrene molar mass ranges of 103 to 106 were used in the ana- and IR spectroscopy. The solvent mixture was removed under

lysis. The instrument was operated at a temperature of 35 °C reduced pressure. The solid residue was dissolved in CH Cl

and THF at a flow rate of 1 mL min served as eluent for the and purified by silica column chromatography, eluting with

analysis. The absolute number-average molecular weight (M ), hexane/CH Cl (1 : 1 v/v) to obtain the title compound as an

weight-average molecular weight (M ) and polydispersity index orange solid (0.83 g, 78% yield, m.p. = 117 °C). IR (cm ): ν

M /M ) of the synthesized polymers were obtained by analyz- 2152 (CuC). H NMR (700 MHz, CDCl ): δ 7.51–7.50 (d,

3

2

−1

n

2

−

1

w

1

(

w

n

3

ing the chromatograms with the OmniSEC 4.6.1 software 2Harom., J = 7 Hz), 7.45 (dd, 4H arom., J = 7 Hz), 7.36 (t, 1Harom.

,

package. An estimated value of (0.12) for refractive index incre- J = 14, 7 Hz), 7.28 (d, 2Harom., J = 7 Hz), 7.13 (d, 2H(a), J = 14

1

3

ment (dn/dc) was used in all cases.

Hz, HCvCH), 0.26 (s, 9 H , SiMe ). C NMR (176 MHz,

b 3

CDCl

3

): δ 137.46, 137.04, 132.30, 129.66, 128.73, 127.93,

Ligand synthesis

127.89, 126.60, 126.51 (aromatic carbons), 126.26 (CvC),

1

,2-Bis(4-((trimethylsilyl)ethynyl)phenyl)ethene (1a). To a 122.07, 105.18, 95.06 (CuC), 0.15 (SiMe ). ESI-MS: m/z 276.50

3

+

solution of (E)-1,2-bis(4-bromophenyl)ethane (0.70 g, [M ]. Elemental Analysis: calculated C, 82.55; H, 7.29; found;

i

2

.10 mmol) in Pr

atmosphere were added catalytic amounts of CuI (5 mg), Pd

OAc) (6 mg), and PPh (45 mg). The solution was stirred for protodesilylated in 4 : 1 (v/v) THF/MeOH (20 mL) using

0 min at room temperature followed by the addition of (tri- aqueous KOH (0.16 g, 2.85 mmol). The reaction mixture was

2

NH/THF (50 mL,1 : 4 v/v) under an argon C, 82.48; H, 7.27.

1-Ethynyl-4-styrylbenzene (2b). 2a (0.65 g, 2.35 mmol) was

(

3

2

3

methylsilyl)ethyne (0.67 mL, 4.79 mmol) with vigorous stir- stirred at room temperature for 45 min, after which time TLC

ring. The reaction mixture was then refluxed overnight. and IR revealed that all protected compound had been con-

Completion of the reaction was confirmed by silica thin-layer verted to the terminal dialkyne compound. The solvent

chromatography (TLC) and IR spectroscopy. The solvent mixture was then removed, and the residue dissolved in

mixture was removed in vacuo and the residue was dissolved in CH

CH Cl and purified by silica column chromatography, eluting using hexane/CH

with hexane/CH Cl (1 : 1 v/v) to obtain the title compound as pale-orange powder (0.40 g, 83% yield). IR (cm ): ν 2105

2

Cl

2

and purified by column chromatography on silica

2

Cl (1 : 1 v/v) as the eluent, aﬀording 2b as a

2

−

1

2

1

an orange solid (0.58 g, 74.29% yield, mp = 120.5 °C). IR (CuC), 3271 (CuC–H). H NMR (700 MHz, CDCl

3

) δ 7.52 (d,

−

1

(

cm ): ν 2152 (CuC). H NMR (700 MHz, CDCl

3

): δ 7.49–7.45 2Harom., J = 7 Hz), 7.48 (dd, 4Harom., J = 7 Hz), 7.37 (t, 1Harom.,

(

1

m, 4 H_a_r_o_m_.), 7.44 (m, 4H

), 7.07 (s, 2H , HCvCH), 0.26 (s, J = 14, 7 Hz), 7.27 (t, 2H_a_r_o_m_., J = 14, 7 Hz), 7.13 (d, 1H , J = 14

arom.

a

1

3

8 H

b

, SiMe

3

). C NMR (176 MHz, CDCl

3

): δ 137.17, 132.36, Hz, HCvCH), 7.08 (d, 1H

a

b

, J = 14 Hz, HCvCH), 3.13 (s, 1H ,

1

3

28.75 (aromatic carbons), 126.37, 122.46 (CvC), 105.11, 95.37 CuCH). C NMR (176 MHz, CDCl ): δ 137.83, 136.69, 132.46,

+

(

CuC), 0.14 (SiMe ). ESI-MS m/z 372.70 [M ]. Elemental 129.90, 128.74, 127.97, 127.80 (aromatic carbons), 126.64,

3

+

Analysis: calculated; C, 77.35; H, 7.57; found; C, 77.31; H, 7.48. 126.34 (CvC), 121.01, 83.74 (CuC). ESI-MS: m/z 204.00 [M ].

,2-Bis(4-ethynylphenyl)ethene (1b). 1a (0.45 g, 1.20 mmol) Elemental Analysis: calculated; C, 94.08; H, 5.92, found; C,

1

was protodesilylated in 4 : 1 (v/v) THF/MeOH (20 mL) using 93.99; H, 5.95.

aqueous KOH (0.15 g, 2.67 mmol). The reaction mixture was

Synthesis of Pt(II) di-ynes and poly-ynes

stirred at room temperature for 45 min, after which time TLC

and IR revealed that all protected compound had been con-

trans-[(Ph)-(Et P) PtCuC–R–CuCPt(PEt ) (Ph)] (1M) (R =

3

2

3 2

verted to the terminal dialkyne compound. The solvent (E)-1,2-diphenylethene). To a stirred mixture of 1b (0.15 g,

mixture was then removed, and the residue dissolved in 0.66 mmol) and trans-[Pt(PEt PhCl] (0.71 g, 1.31 mmol) in

CH Cl and purified by column chromatography on silica Pr NH (3 mL) and CH Cl (20 mL) was added CuI (0.5 mg).

3 2

)

i

2

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The solution was stirred at room temperature under an argon used for Pt. Geometry optimizations for the trans isomers where

atmosphere over a period of 18 h, after which all volatile com- initiated from structures extracted from the corresponding cif

ponents were removed under reduced pressure. The crude files, while initial structures for the cis isomers were created by

product was taken up in CH

2 2

Cl and passed through a silica manual rotation of the stilbene double bond(s) – in the case of

column with 1 : 1 (v/v) hexane/CH Cl as the eluent. The 2M, the cis isomer modelled was that with both of the stilbene

2

product was obtained as a yellow solid (0.19 g, 23% yield, mp = bonds having a cis configuration. In all cases geometry optimiz-

1

7

6

85 °C). IR (cm⁻): ν 2092 (CuC). H NMR (500 MHz, CDCl

.31–7.23 (m, 7H_a_r_o_m), 7.23–7.16 (m, 5H_a_r_o_m_.), 6.94–6.86 (m, nature of the stationary point. In addition to gas-phase calcu-

arom.), 6.74 (t, 2H ), 1.69 (qt, J = 7.5, 3.4 Hz, 24H ), 1.03 (p, lations, the eﬀect of the solvent (dichloromethane, ε = 8.93) was

3

) δ ation was followed by frequency calculations to confirm the

H

a

b

1

3

55

J = 7.7 Hz, 36H

1

c

). C NMR (176 MHz, CDCl

31.25, 128.66, 127.59, 127.43, 121.37 (aromatic carbons), input files and for visualization of results.

26.11 (CvC), 121.37 (CuC), 15.37, 13.24, 5.10, 8.18 (alkyl Excitation energies were calculated using the TD-DFT

carbons). ESI-MS m/z 1243.40 [M ]. Anal. Calcd: C, 52.17; H, method at the ground state optimized geometry, with, and

.49; P, 9.97; found; C, 52.16; H, 6.44; P, 9.94. without, the inclusion of the PCM. The 100 lowest-energy

trans-[R–CuC-Pt(PBu -CuC–R] (2M, R = (E)-1,2-diphenyl- single states were calculated, and the excitation energies and

ethene). A slightly modified literature procedure was fol- oscillator strengths used to simulate the electronic absorption

lowed using 2b (0.15 g, 0.73 mmol), trans-[(PBu ) PtCl ] spectra. GaussSum 3 was used to create the convoluted

0.25 g, 0.37 mmol), and CuI (1 mg). 2M was obtained as spectra using gaussian curves with a FWHM of 3000 cm , and

yellow crystals (0.27 g, 37% yield, m.p. = 242 °C). IR: ν cm

093 (–CuC–). H NMR (500 MHz, CDCl ) δ 7.51 (d, J = 7.5 Hz,

3

): δ 139.33, 134.09, approximated using the PCM. GaussView 5 was used to create

+

6

3 2

)

1

0

5

6

3

2

−

1

(

−

1

to extract the orbital contribution for each transition.

1

2

5

3

Single-crystal X-ray structure determination

H

arom.), 7.46–7.32 (m, 8Harom.), 7.26 (t, J = 6.1 Hz, 5Harom.),

7

.09 (s, 2H

a

), 7.07 (s, 2H

a

), 2.15 (td, J = 7.9, 3.8 Hz, 12H

b

), The single-crystal X-ray structure determination was performed

1

.72–1.57 (m, 12H ), 1.47 (h, J = 7.3 Hz, 12H ), 0.95 (t, J = 7.3 at room temperature on a Stoe IPS II diﬀractometer using mono-

c

d

1

3

Hz, 18H

e

). C NMR (176 MHz, CDCl

3

α

): δ 131.16, 128.80, chromatic Mo-K radiation (λ = 0.71073 Å). A multiscan absorp-

1

27.60, 127.47, 126.50 (aromatic carbons), 126.27 (CvC), tion correction was applied. The data reduction, including an

09.43 (CuC), 26.53, 26.51, 24.62, 24.56, 24.51, 24.07, 13.98 empirical absorption correction using spherical harmonics, was

+

(alkyl carbons). ESI-MS: m/z 1006 [M] . Anal. Calcd: C, 66.84; implemented in LANA. The crystal structures were solved by

57

H, 7.61; P, 6.16; found; C, 66.77; H, 7.60; P, 6.14.

direct methods using the online version of WinGX and then

trans-[–(P Bu ) PtCuC–R–CuC–] (1P, R = (E)-1,2-dipheny- refined by full-matrix least-squares (SHELXL-2014) on F .

n

2 58,59

3

2

n

lethene). To a stirred mixture of 1b (0.10 g, 0.44 mmol) and The non-hydrogen atoms were refined anisotropically. One of

n

i

trans-[(P Bu

3 2 2 2

) PtCl ] (0.29 g, 0.44 mmol) in Pr NH (10 mL) the triethylphosphine ligands was severely disordered and the

and CH Cl (10 mL) was added CuI (1 mg) under argon. After PEt₃fragment was refined over two sites with the total occu-

2

stirring at room temperature overnight, the solvent mixture pancy set at unity, and the C–C bond lengths and angles were

was removed under reduced pressure. The solid residue was restrained to be similar to those found in the ordered ethyl

dissolved in CH Cl₂and filtered through a short alumina groups. All of the hydrogen atoms were positioned geometrically

2

column to remove ionic impurities and residual catalysts. After in idealized positions and refined with the riding model approxi-

removal of the solvent, the crude product was purified by pre- mation, with Uiso(H) = 1.2 or 1.5Ueq(C). The molecular graphics

60

cipitation from MeOH in CH Cl . Subsequent washing with the program MERCURY from the CSD package was used.

2

hexane and drying under reduced pressure gave yellow

−

1

powders (0.38 g, mp = 285 °C). IR: ν cm 2093 (–CuC–). H

NMR (500 MHz, CDCl ) δ 7.34–7.22 (m, 6H_a_r_o_m_.), 7.21–7.10 (m, Data-access statement

3

2

8

1

H

arom.), 6.92 (s, 2H

a

), 2.22–2.00 (m, 12H

b

), 1.54 (ddp, J = 12.0,

), 0.86 (t, J = 7.0 Hz,

8H_e). C NMR (176 MHz, CDCl ): δ 131.17, 127.66 (aromatic

The crystal structure of 1M is available under the CCDC refer-

ence number 2035894.† Raw data from spectroscopic charac-

terization and computational modelling are available from the

authors on request.

.2, 3.8 Hz, 12H

c

), 1.38 (h, J = 7.4 Hz, 12H

d

1

3

carbons), 126.11 (CvC), 110.26 (CuC), 26.62, 26.51, 26.44,

4.61, 24.56, 24.50, 24.19, 24.05, 23.91, 13.93 (alkyl carbons).

2

−

1

GPC (THF): Mˉ = 22 632 g mol (n = 48); Mˉ_w= 39 568 g

mol ; PDI = 1.7. Anal. Calcd: C, 57.90; H, 8.30; P, 8.14; found;

C, 57.88; H, 8.27; P, 8.12.

nn

−

1

Conﬂicts of interest

The authors have no conflicts of interest to declare.

Computational modelling

Density Functional (DFT) calculations were carried out for cis-

and trans-isomers of 1M and 2M using the B3LYP hybrid func-

tional as implemented by Gaussian 09. The 6-31G(d,p) basis

Acknowledgements

54

set was used for main group elements, while the Def2SVP basis IJAB thanks the Ministry of Education (MOE), Sultanate of Oman

set, with corresponding eﬀective-core pseudopotential, was for a Ph.D. scholarship. MSK thanks The BP Oman Research

Dalton Trans., 2021, 50, 2555–2569 | 2567

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Grant (EG/SQU-BP/SCI/CHEM/19/01) and The Ministry of Higher 23 A. Haque, R. A. Al-Balushi and M. S. Khan, J. Organomet.

Education, Research and Innovation (MoHERI), Oman (Grant: Chem., 2019, 897, 95–106.

RC/RG-SCI/CHEM/20/01) for funding. PRR is grateful to the 24 Z. Li, E. Badaeva, D. Zhou, J. Bjorgaard, K. D. Glusac,

Engineering and Physical Sciences Research Council (EPSRC) for

continued funding (Grant no. EP/K004956/1).

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Electronic and steric effects of platinum(ii) di-yne and poly-yne substituents on the photo-switching behaviour of stilbene: Experimental and theoretical insights

DOI: 10.1039/d0dt03502e

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