2.1 Tetranuclear copper(I) and silver(I) chalcogenides and pnictogenides

A series of tetranuclear copper(I) and silver(I) complexes with μ₄-bridging chalcogenides and pnictogenides, [M₄(μ-P^∧P)₄(μ₄-E)]²⁺ [M = Cu, E = S, Se, PPh; M = Ag, E = S, Se, Te; P^∧P = bis(diphenylphosphino)methane (dppm), bis[di(4-methylphenyl)phosphino]methane], have been prepared in our laboratory (Fig. 1) [20–25]. In general, the four metal atoms in these complexes are arranged in a distorted rectangular array and are bridged by a chalcogenido or pnictogenido ligand atop of it. The apices of the bridging diphosphine ligands are not pointing to the same direction but are in an alternating up and down saddle-like fashion.

These complexes are found to be soluble in common organic solvents and possess rich photophysical properties. The electronic absorption spectra of this class of complexes generally showed a high-energy absorption shoulder at ca. 240−260 nm with an extinction coefficient in the order of 10⁴ dm³ mol⁻¹ cm⁻¹, and a lower energy absorption tail in the region of 350−450 nm. The emission energies of these complexes are found to be dependent on both the chalcogenido or pnictogenido ligand and the metal centre. Red shifts in the solid-state emission energies are observed when changing only the chalcogenido or pnictogenido ligand from S (P^P = dppm; M = Cu, 579 nm; M = Ag, 516 nm) to Se (P^P = dppm; M = Cu, 595 nm; M = Ag, 527 nm) to Te (P^P = dppm; M = Ag, 574 nm) to PPh (P^P = dppm; M = Cu, 718 nm), as well as changing only the metal centres from Ag₄ (P^∧P = dppm; E = S, 516 nm; E = Se, 527 nm) to Cu₄ (P^∧P = dppm; E = S, 579 nm; E = Se, 595 nm). Thus, the emission of these copper(I) and silver(I) clusters with bridging chalcogenido or pnictogenido ligands are assigned to originate predominantly from a ligand-to-metal charge transfer LMCT (E²⁻ → M₄) triplet state, probably mixed with a Cu(I) or Ag(I) metal-centred MC (d−s or d−p) state, modified by weak metal···metal interactions.

The phosphorescent states of these complexes are found to be quenched by a series of pyridinium acceptors via an outersphere electron transfer process, which is indicative of their highly reducing nature in the excited states. For example, an excited-state reduction potential, E°([Cu₄]³⁺/[Cu₄]²⁺*) of −1.55(10) V vs. S.S.C.E. has been estimated for [Cu₄(μ-dppm)₄(μ₄-Se)]²⁺ with a reorganisation energy λ of 1.12(10) eV. Transient absorption difference spectrum recorded for [Cu₄(μ-dppm)₄(μ₄-Se)]²⁺ in the presence of a pyridinium acceptor showed absorption bands at ca. 390, 484 and 693 nm which is characteristic of the pyridinyl radical, and inter-valence charge transfer (IVCT) bands for [Cu^ICu^ICu^ICu^II], respectively [26].

2.2 High nuclearity gold(I) chalcogenides

In addition to the tetranuclear copper(I) and silver(I) chalcogenide complexes, a series of high nuclearity gold(I) chalcogenide clusters have been isolated in our laboratory via the reactions of the dinuclear gold(I) complexes, [Au₂(μ-P^∧P)Cl₂] (P^∧P = dppm and Ph₂N(ⁿPr)PPh), with H₂S in a mixture of ethanol and pyridine (Fig. 2) [27–29]. The resulting multinuclear complexes with chalcogenido ligands, [Au₁₀{μ-Ph₂PN(ⁿPr)PPh₂}₄(μ₃-S)₄]²⁺, [Au₆{μ-Ph₂PN(p-CH₃C₆H₄)PPh₂}₃(μ₃-S)₂]²⁺ and [Au₁₂(μ-dppm)₆(μ₃-S)₄]⁴⁺, are found to be strongly emissive both in the solid-state and in fluid solutions. The lowest-energy excited state of this class of complexes are assigned as derived from an admixture of ligand-to-metal charge transfer LMCT[E²⁻ → Au(I)] and metal-centred MC(d → s/d → p) states, modified by weak Au···Au interactions [27–29].

3.1 Polynuclear copper(I) and silver(I) alkynyl complexes

Besides the copper(I) and silver(I) complexes with chalcogenido and pnictogenido ligands, a series of alkynyl complexes with copper(I) and silver(I) metal centres have also been successfully synthesised by us (Fig. 3) [30–35]. The electronic absorption spectra of the triangulo-M₃ complexes, [M₃(μ-P^∧P)₃(μ₃-η¹-C≡CR)₂]⁺ and [M₃(μ-P^∧P)₃(μ₃-η¹-C≡CR)]²⁺ [M = Cu(I), Ag(I); R = aryl, alkyl; P^∧P = dppm; Ph₂N(R′)PPh₂, R′ = Ph, ⁿPr, C₆H₄CH₃-4, C₆H₄F-4], are dominated by the intraligand absorption bands of the diphosphine and alkynyl ligands but with a low-energy shoulder tailing to ca. 400 nm.

Long-lived and intense luminescence is observed upon photo-excitation of the complexes in the solid-state or in fluid solutions. Similar to the copper(I) and silver(I) chalcogenido and pnictogenido complexes, the emission energies of these complexes are also found to be dependent on the nature of the ligand and the metal core. For instance, the emission energies of the monoynyl complexes [Cu₃(μ-dppm)₃(μ₃-η¹-C≡CR)]²⁺ in acetone solution show an order of: R = C₆H₄–OCH₃-4 (483 nm) ≈ Ph (499 nm) > C₆H₄–NH₂-4 (504 nm) > ^tBu (640 nm) ≈ ⁿC₆H₁₃ (650 nm), which is in line with the increasing electron-donating ability of the alkynyl ligand. Red shifts of emission energies are also noted when going from Ag(I) to Cu(I) complexes with identical diphosphine and alkynyl ligands. These observations are indicative of substantial involvement of a ligand-to-metal charge transfer LMCT[C≡C → Cu₃] character in their emissive state. However, in view of the presence of short M···M distances in these complexes, especially for the bicapped system, mixing of a certain degree of metal-centred MC[d–s] character is also possible. Therefore, the lowest-lying emissive state of these complexes is best described as an admixture of ³LMCT[C≡C → Cu₃]/³MC[d–s] character modified by weak M···M interactions [30–35]. As an extention of this work, a series of trinuclear Cu(I) oligoynyl complexes, [Cu₃(μ-dppm)₃{μ₃-η¹-(C≡C)_nR}₂]⁺ (n = 1, 2; R = H, Ph, C₆H₄–CH₃-4, C₆H₄–OCH₃-4) have recently been studied by us [36]. The emission spectra of the complexes show emission bands with vibrational progressional spacing of ca. 1811−2058 cm⁻¹, which are characteristic of ν(C≡C), and the emission energy is dependent on the nature of the substituents on the diynyl ligand (Fig. 4). Thus, the low-lying ³LMCT emissive state for these oligoynyl complexes are likely to be mixed with the ³IL[π–π*] of the oligoynyl moieties [36].

In addition to the trinuclear Cu(I) and Ag(I) alkynyl complexes, a series of tetranuclear Cu(I) and hexanuclear Cu(I) and Ag(I) alkynyl complexes have been synthesised in our laboratory. The tetranuclear Cu(I) alkynyl complex, [Cu₄(μ-dppm)₄(μ₄-η¹,η²-C≡C–)]²⁺, was synthesised by the reaction of [Cu₂(μ-dppm)₂(CH₃CN)₂]²⁺ with Me₃SiC≡CH in the presence of ⁿBuLi in THF under an inert atmosphere of nitrogen [37]. Interestingly, the C≡C²⁻ moiety in this tetranuclear complex is found to oscillate inside the distorted rectangular array of four copper atoms with a ΔG^‡ value of ca. 38 kJ mol⁻¹ [38]. On the other hand, reaction of the dinuclear complex [M₂(μ-dppm)₂(CH₃CN)₂]²⁺ (M = Cu, Ag) with 1,4-diethynylbenzene in the presence of ⁿBuLi in THF afforded the respective diynyl-bridged hexanuclear Cu(I) and Ag(I) complexes, [M₃(μ-dppm)₃(μ₃-η¹-C≡CC₆H₄C≡C–4)M₃(μ-dppm)₃](BF₄)₄ [39]. The electronic absorption spectra of these complexes are dominated by the high-energy absorption of the diphosphine and alkynyl intra-ligand π–π* transitions. Low-energy absorptions at ca. 324−416 nm are noted which are assigned as predominantly LMCT [acetylide → Cu₄/M₃] character. Mixing of MC d–s state and IL π–π* character is also possible, particularly in the case of the hexanuclear species.

These d¹⁰ alkynyl complexes are found to be strong triplet emitters in the green to orange region upon photo-excitation. Blue shift in emission energies is observed on going from the hexanuclear Cu(I) complex to the Ag(I) analogues, supportive of the assignment of a mixed LMCT[C≡C → M₃]/MC[d–s] character in their lowest-lying emissive state. In view of the unusually long lifetime of the emissive state and the low-lying π* orbital energy of the diynyl unit, possible involvement of a ³IL [π–π*] character could not be completely excluded.

3.2 Rhenium alkynyl complexes

In addition to d¹⁰ metal complexes, our group has also been interested in the studies of other transition metal alkynyl complexes. In particular, a series of neutral rhenium(I) alkynyl complexes, [Re(diimine)(CO)₃C≡CR] (R = aryl, alkyl) have been synthesised in our laboratory (Fig. 5) [14]. The electronic absorption spectra of these rhenium(I) alkynyl complexes show low-energy absorption bands at ca. 391−440 nm, attributed to a MLCT [d_π(Re) → π*(diimine)] transition. Upon photo-excitation, these complexes show long-lived intense luminescence in the solid-state and in fluid solutions at room temperature. Their emission energies are found to be easily tuned by the systematic variations of the substituents on the alkynyl ligand and the diimine ligand (Table 1) [14]. In general, a more electron-donating alkynyl ligand would give a lower energy MLCT emission and this is consistent with the σ- and π-donating ability of the alkynyl moieties. Thus, the long-lived emission for this class of complexes is assigned as originating from a d_π(Re) → π*(diimine) MLCT triplet state.

In view of the synthetic versatility of this class of complexes towards the use of different alkynyl ligands and the growing interest in the study of metal-containing rod-like complexes, a series of mono- and di-rhenium rod-like complexes with conjugated oligoynyl ligands, [Re(diimine)(CO)₃(C≡C)_nR] and [(CO)₃(diimine)Re(C≡C−C≡C)_nRe-(diimine)(CO)₃], have been synthesised (Fig. 5) [14,40–42]. The luminescence properties have been found to be influenced by the number of C≡C units in addition to the substituents on the diimine and oligoynyl ligands (Table 2) [40,41]. In general, the triynyl complexes are found to emit at higher energies than their respective diynyl and monoynyl analogues. An assignment of a ³MLCT[d_π(Re) → π*(C≡CR)] or a metal-perturbed ³IL[π → π*{(C≡C)_nR}] origin is not compatible with such a trend. Rather a ³MLCT[dp(Re) → π*(diimine)] assignment is suggested. Such anomalous energy trend in the ³MLCT emission bands is suggested to arise from the decreased overlap integral between the d_π(Re) and the triynyl π(RC≡C−C≡C−C≡C) orbitals than that of d_π(Re) and the π(RC≡C−C≡C) and π(RC≡C) orbitals resulting from the delocalisation of electron density across the (C≡C)_n unit. As a result, the d_π(Re) is stabilised to a larger extent for complexes with more C≡C units giving rise to a higher energy ³MLCT emission. Thus, an assignment of a ³MLCT [d_π(Re) → π*(diimine)] origin for the lowest lying emissive state for these complexes is favoured.

3.3 Mixed-metal alkynyl complexes

With the homo-metallic alkynyl systems discussed in previous sections in hand, we are interested in the exploration of luminescent mixed-metal molecular rods and materials.

3.3.1 Rhenium(I)–copper(I) and –silver(I) mixed-metal alkynyl complexes

The rhenium(I)–copper(I) mixed-metal alkynyl complexes, [M₃(μ-P^∧P)₃{μ₃-η¹-C≡C−C₆H₄−C≡C-p-Re(N^∧N)(CO)₃}₂]⁺ and [M₃(μ-P^∧P)₃{μ₃-η¹-C≡C−C≡C-p-Re(N^∧N)(CO)₃}₂]⁺ [43,44], and decanuclear rhenium(I)–silver(I) alkynyl complexes, [Ag₆(μ-dppm)₄{μ₃-C≡CC≡C−Re(N^∧N)(CO)₃}₄](PF₆)₂ [45], were synthesised using a metalloligand approach (Fig. 6).

The complex cations of the pentanuclear Re(I)–Cu(I) and Re(I)–Ag(I) mixed-metal complexes consist of an isosceles triangle of the d¹⁰ metal atoms with a dppm ligand bridging each edge to form a roughly planar {M₃P₆} core, with two Re(I) butadiynyl metalloligands capping the M₃ core in a μ₃-η¹ fashion. In contrast, the complex cations of the decanuclear complexes show a Ag₆ core decorated with four Re(I) butadiynyl metalloligands, with each of the silver atom linked to two bridging diphosphine ligands and each butadiynyl group has one end coordinated to three silver atoms in a μ₃-η¹ bonding mode and the other end coordinated to a rhenium atom. The Ag₆ core consists of two Ag₃ triangular planes that are arranged in a nearly perpendicular fashion.

The electronic absorption spectra of these mixed-metal complexes show high-energy absorption bands typical of intraligand π–π* transitions of the diphosphines, alkynyl and diimine ligands. In addition, low-energy absorption bands at ca. 434−482 nm are also noted and the absorption energies of these bands are found to be sensitive to the nature of the diimine ligands, with the trend in line with the π-acceptor ability of the diimine ligands: Br₂phen < phen < Me₂bpy ≤ ^tBu₂bpy, and the low-energy absorptions are assigned as MLCT [d_π(Re) → π*(N^∧N)] transitions (Br₂phen = 4,7-dibromo-1,10-phenanthroline, phen = 1,10-phenanthroline, Me₂bpy = 4,4′-dimethyl-2,2′bipyridine, ^tBu₂bpy = 4,4′-di-tert-butyl-2,2′-bipyridine). In general, slight red shifts are noted on going from the pentanuclear Re(I)–Ag(I) complexes to the corresponding Re(I)–Cu(I) analogues, in line with the poorer electron-donating ability of the Ag^I₃ core than the Cu^I₃ counterpart, which would render the Re(I) centre less electron-rich, leading to a larger cluster-perturbed MLCT energy gap [43–45].

Upon photo-excitation, these mixed-metal complexes are found to exhibit intense orange to red luminescence in the solid-state and in fluid solutions, which is at a lower energy region when compared to their corresponding precursor complexes. Similar to the electronic absorption spectroscopy, the emission energies are found to be dependent on the nature of the diimine ligand, diphosphine ligand, and the metal core. Degassed dichloromethane solutions of the complexes with different diimine ligands show emission energy dependence on the π* orbital energies of the diimine ligands. As an example, the trinuclear complex [Cu₃(μ-dppm)₃{μ₃-η¹-C≡CC≡C−Re(bpy)(CO)₃}₂]⁺ emits at 706 nm, while its ^tBu₂bpy analogue emits at 678 nm. With reference to previous spectroscopic work on rhenium(I) diimine systems, the intense low-energy emissions are assigned as emanated from ³MLCT [d_π(Re) → π*(diimine)] excited state, probably mixed with some ³LLCT [π(C≡CC≡C) → π*(diimine)] character. A slight blue shift in emission energies is observed when changing the diphosphine ligand in [Cu₃(μ-P^∧P)₃{μ₃-η¹-C≡CC≡C−Re(bpy)(CO)₃}₂]⁺ from P^∧P = dppm (706 nm) to dppa (701 nm) and in [Cu₃(μ-P^∧P)₃{μ₃-η¹-C≡CC≡C−Re(Me₂bpy)(CO)₃}₂]⁺ from P^∧P = dppm (688 nm) to dppa (677 nm). This is in accordance with the stronger electron-donating ability of dppm than dppa, which would render the Cu(I) alkynyl moieties more electron-rich and in turn render the cluster-modified Re(I) centre more electron-rich, raising the energy of the d_π(Re) orbitals and finally resulted in a lower energy ³MLCT emission. In view of the perturbation of emission energies by changing the diphosphine ligand and the d¹⁰ metal centre, the lowest-lying emissive state in these mixed-metal complexes are best described as having predominantly ³MLCT [d_π(Re) → π*(diimine)] character, probably mixed with some ³LLCT[π(C≡CC≡C) → π*(diimine)] character that has been perturbed by the d¹⁰ metal core.

In view of the versatile bonding motif exhibited by the alkynyl group, we are also interested in the construction of mixed-metal alkynyl complexes via the π-bonding mode of the alkynyl group. A series of mixed-metal η²-alkynyl-coordinated complexes of rhenium(I)–copper(I) and –silver(I), [{η²-Re(CO)₃(bpy)(C≡CPh)}₂M]⁺ (M = Cu, Ag), have been successfully isolated in our laboratory (Table 3) [46]. The metalloligand [Re(CO)₃(bpy)C≡CPh] react with the corresponding d¹⁰ metal complexes [M(MeCN)₄]PF₆ (M = Cu, Ag) in THF at room temperature in an inert atmosphere of nitrogen to give the π-bonded species as yellow crystals after recrystallisation. Low-energy absorptions at ca. 396 nm are noted in the electronic absorption spectra of the complexes, which are assignable as a MLCT [d_π(Re) → π*(bpy)] transition. The blue shift of the lowest-energy absorption band of the π-bonded species [M = Ag (600 nm), Cu (590 nm)] compared to their precursor complex [Re(CO)₃(bpy)C≡CPh] (654 nm) is a consequence of the weaker π-donating ability of the alkynyl ligand upon π-coordination to the d¹⁰ metal ions which would stabilise the d_π(Re) orbitals to a larger extent. Similar trends have also been observed in the emission of the mixed-metal complexes, and the emission is assigned accordingly as derived from states of a ³MLCT [d_π(Re) → π*(bpy)] character [46].

Table 3

Electronic absorption and emission data for mixed-metal rhenium(I)–copper(I) and –silver(I) alkynyl complexes

Complex	Absorptiona λ/nm (ɛ/dm3 mol−1 cm−1)	Medium (T/K)	Emission λem/nm (τo/μs)
[Re(bpy)(CO)3–C≡C–Ph]	286 (37,250)	Solid (298)	585 (0.19)
298 sh (30,160)	Solid (77)	570
326 sh (10,430), 420 (3240)	CH2Cl2 (298)	654 (<0.1)
[{η2-Re(CO)3(bpy)(C≡CPh)}2Ag]+	250 (26,890), 292 (28,240)	Solid (298)	535 (<0.1)
396 (5930)	Solid (77)	533
CH2Cl2 (298)	600 (0.16)
[{η2-Re(CO)3(bpy)(C≡CPh)}2Cu]+	246 (28,160), 288 (29,000)	Solid (298)	550 (0.35)
396 (6050)	Solid (77)	540
CH2Cl2 (298)	590 (0.18)

^a In CH₂Cl₂ at 298 K.

3.4 Molecular platinum rods

The development of luminescent materials with ³MLCT emission has aroused much attention in recent years. Apart from the rhenium(I) alkynyl systems, our group has designed and synthesised a series of platinum-containing carbon-rich materials, [Pt(trpy)C≡CR]⁺, which also emit from a ³MLCT manifold [48]. With the on-going interest in the chemistry of carbon-rich metal-containing systems, in particular those of metal-containing polynes, we have directed one of our goals towards the study of luminescent platinum(II) terpyridyl-capped carbon-rich molecular rods. Recently, a series of luminescent molecular platinum rods, [Pt(^tBu₃trpy)(C≡C)_nPt(^tBu₃trpy)](X)₂ (n = 1, 2, X = OTf; n = 4, X = PF₆) have been successfully synthesised [49].

The platinum atoms of the complex cations of these molecular platinum rods adopt an essentially square-planar geometry with slight distortion due to the coordination constraints imposed by the terpyridyl ligand. The two platinum(II) terpyridyl ends are connected by the alkynyl chain in a linear [Pt](C≡C)_n[Pt] fashion with n = 1, 2 and 4. The dimension of the carbon-rich platinum rods extends from molecular scale [n = 1 (5.16 Å), 2 (7.71 Å)] to nanoscale [n = 3 (12.83 Å)].

The electronic absorption spectra of these platinum-containing carbon-rich materials show intense absorption bands at about 308−342 and 428−522 nm in acetonitrile at 298 K. With reference to previous spectroscopic studies on the related mononuclear [Pt(^tBu₃trpy)C≡CC≡CH](OTf) [50] and related complexes [48], the high-energy structured absorption bands at ca. 308−342 nm are assigned as intra-ligand (IL) π → π*[(C≡C)_n] and π → π*(^tBu₃trpy) transitions of the alkynyl bridge and the terpyridine ligand, while the low-energy band at ca. 428−522 nm is ascribed to a MLCT[d_π(Pt) → π*(^tBu₃trpy)] transition, probably mixed with some ligand-to-ligand charge transfer LLCT[π(C≡C)_n → π*(^tBu₃trpy)] character. A blue shift of the lowest-energy absorption band in these complexes is observed with an increasing number of C≡C units [n = 1 (484−522 nm), 2 (436−466 nm), 4 (428−452 nm)]. This anomalous observation is likely to be a consequence of the greater stabilisation of the d_π(Pt) orbital upon increasing the length of the conjugated carbon chain. This can be accounted by the π-donor ability exhibited by the alkynyl chain, which gives rise to a filled–filled pπ–dπ overlap, and the π-acceptor ability of the alkynyl chain, which results in a dπ–π* interaction. A similar argument has also been suggested to rationalise the trend observed in the related rhenium(I) alkynyl system [14].

Photo-excitation of the platinum molecular rods in the solid-state and in fluid solutions give intense luminescence at ca. 550−625 nm. The occurrence of the emission at similar energies to that found in the related mononuclear complex [Pt(^tBu₃trpy)C≡CC≡CH](OTf) [50] suggested that the origin of the luminescence is derived from states of predominantly ³MLCT[d_π(Pt) → π*(^tBu₃trpy)] character, probably mixed with some intra-ligand ³IL[π → π*(C≡C)_n], and ³LLCT[π(C≡C)_n → π*(^tBu₃trpy)] character. The emission spectra of the monoynyl and diynyl complexes in glass at 77 K exhibit vibronically structured bands with vibrational progressional spacings of about 1250 cm⁻¹, which is typical of the ν(C···C) and ν(C···N) stretching modes of the terpyridine ligand, while the emission spectrum of the tetraynyl complex in glass at 77 K show vibronically structured bands with vibrational progressional spacings of ca. 2100 cm⁻¹, typical of the ν(C≡C) stretch in the ground state. These observations are indicative of the involvement of the terpyridine ligand and alkynyl bridge in the excited state for these complexes. For reasons discussed for the electronic absorption spectroscopy, a blue shift of the emission energies was also observed on going from the monoynyl complex to the diynyl complex in butyronitrile glass at 77 K. It is likely that the emission of the tetraynyl complex would involve an increased contribution from the ³IL[π → π*(C≡C)_n] excited states, which account for the observed lowest emission energy and vibrational progressional spacings of ca. 2100 cm⁻¹. Therefore, the emission for these complexes could best be described as an admixture of ³MLCT[d_π(Pt) → π*(^tBu₃trpy)], ³LLCT[π(C≡C)_n → π*(^tBu₃trpy)], and ³IL[π(C≡C) → π*(^tBu₃trpy)] character, with an increase in the ³IL contribution upon increasing the π-conjugation of the alkynyl bridge.