Spectroscopic and electrochemical properties of Pt(II) complexes with aromatic terdendate (C^N^C) cyclometallating ligands

J. Photochem.

Photobiol. A:

Chem,

173

67 (1992) 173-179

Spectroscopic and electrochemical prop+ies of Pt(I1) complexes with aromatic terdendate (C N C) cyclometallating ligands Mauro

Maestri

Dipartimento Chimico

“G.

Ciamician ‘:

Christine Deuschel-Cornioley

Univenitd di Bologna, 40126 Bologna (Italy)

and Alex von Zelewsky

Institute of Inorganic Chemistry, Univekty

of Fribourg, P&o&s

(Received February 11, 1992; accepted March

CH-1700

(Switzerland,l

5, 1992)

Abstract Absorption spectra, emission spectra, emission lifetime and electrochemical behavior of the complexes c-Pt(Hdpbhq)2, t-Pt(dpbhq)S(Et),, t-Pt(dpbhq)py, and t-Pt(dpbhq)pyr (where dpbhq’is the Cdeprotonated form of 5,6-dihydro-2,4-diphenylbenzo(h)quinoline (H,dpbhq) and py and pyr are pyridine and pyrazine) have been studied. The structured luminescence spectra observed at 77 K have been assigned to transitions having: (i) metalto-ligand charge transfer (MLCT) character in the case of Pt(H-dpbhq), and mainly MLCT character with some ligand-centered (LC) contribution for the three terdentate tran,s Pt(II) complexes. The luminescence emission in all cases involves the dpbhq ligand even though the electrochemical results suggest a lowest unoccupied molecular orbital (LUMO) centered on the pyrazine ligand in the Pt(dpbhq)pyr complex.

1. Introduction The study of the photochemical and photophysical properties of transition metal complexes is of great interest for theoretical and practical applications [l-4]. In the past 20 years, most of the attention in this field has been focused on complexes of the polypycidine-type family [5-a]. A few years ago, we (and others) [g-11] began a systematic study of the excited state behavior of cyclometallated complexes owing to the structural similarity between cyclometallating and polypyridine ligands and the higher ligand field strength of Ccompared with N. Among the cyclometallated complexes studied so far, many are square planar bis-cyclometallated Pt(II) and Pd(I1) complexes [ll]. All of them have cis configuration [ 12-141. The very strong tendency to form the cis compounds is probably due to the strong trans effect of the C--1igand. C,C-trans-bis-cyclometallated complexes can be prepared [15] with terdentate (C-N C) ligands such as 2,6_diphenylpyridine (H,dppy) and 5,6-dihydro-2,4_diphenylbenzo(h)quinoline (Hadpbhq), ligands analogous to terpyridine, but with two carbon donor atoms. We have prepared a series of trans complexes with these ligands with the aim of comparing the spectroscopic behavior of cis and trans cyclometallated planar complexes. We report here the results concerning one cis and three trans complexes with dpbhq*- (C- deprotonated form of Hzdpbhq) (Fig. 1).

lOlO-6030/92/$5.00

0 1992- Ebevier

Sequoia. All rights reserved

c-Pt(Hdpbhq),

t-Pt(dpbhq)L Fig.

1. Structural

2. Experimental

formulae

of the

complexes

studied

(L=S(Et),,

py,

pyr).

details

The preparation, purification, and characterization of c-Pt(Hdpbhq),, tPt(dpbhq)S(Et),, f-Pt(dpbhq)py and &Pt(dpbhq)pyr have been reported previously [15]. Polarographic grade acetonitrile (AN) and spectroscopic grade CHZC12, MeOH and EtOH were used as solvents. Low temperature measurements (77 K) were performed in 15 v/v MeOH/EtOH. When necessary the solutions were deaerated by repeated freeze-pump-thaw cycles. The absorption spectra were recorded with a Perkin-Elmer A6 spectrophotometer. Emission spectra (uncorrected) were obtained with a Perkin-Elmer US0 spectrofluorimeter equipped with a Hamamatsu R 928 phototube. Emission lifetimes were measured either with an Edinburgh 199 DS single photon counting equipment or with the PerkinElmer LS50 spectrofluorimeter working in the phosphorescence mode. Electrochemical measurements were carried out in AN solution with 0.1 M TEAP as supporting electrolyte by means of an Amel 448 oscillographic polarograph. The working electrodes were a hanging Hg electrode and a Pt electrode for cathodic and anodic processes, respectively_ The potential values reported are vs. standard calomel electrode (SCE).

3. Results The complexes studied are thermally stable in the solvents used, during the time periods of the experiments. They show intense absorption bands in the UV region (Fig. 2). The wavelength of the lowest energy absorption maximum in CH2C12 are reported in Table I. All the complexes show a reversible reduction wave and an irreversible (or quasiirreversible) oxidation wave; the peak potentials are reported in Table 1. All the complexes show structured luminescence emission at 77 K in the MeOH/ EtOH mixture (Fig. 3). The highest energy features of the phosphorescence emissions are given in Table 1 together with the corresponding emission lifetimes. t-Pt(dpbhq)S(Et), and t-Pt(dpbhq)py also exhibit a luminescence band (Amax= 495 run in bath cases) at room temperature in deaerated CHZCIZ solutions with lifetimes of 77 and 150 ns respectively.

175

E 24000

18B00

6000

0 250

308

Fig. 2. Absorption TABLE

358

spectra of: (a) l&dpbhq;

488

nm

450

(b) t-Pt(dpbhq)pyr;

(c) c-Pt(Hdpbhqjz.

1

Absorption, free ligand

emission and electrochemical

data of the complexes

Emissionb

Absorptiona h (nm)d

e

A (nnV

H,dpbW

322

12800

447

c-Pt(Hdpbhq)z t-Pt(dpbhq)S(Et), t-PNdpbhq)py t-Pt(dpbhq)pyr

410 359 359 356

3800 18500 20800 15800

499 490 490 491

XxI~Cl~, 293 K. bMeOH/EtOH 1:5 v/v, 77 K ‘Peak potentials in AN, TEAP 0.1 dWavelength of the lowest energy ‘Wavelength of the highest energy fPhosphorescence emission lifetime Qreversible wave.

studied and of the H,dpbhq

E, 7- @If 1.9x106 9 30 35 14

M, VS. SCE. absorption maximum. feature of the phosphorescence values, estimated error f 10%.

Absorption and emission data of the H,dpbhq Table 1 and Figs. 2-3 for comparison purposes.

(V)’

Of-

0/t

-

-

-

1.85 1.84 1.86 1.50

+ -t + +

1.029 1.02s 0.988 1.109

emission.

free ligand are also reported

in

4. Discus&on

For the sake of simplicity, we discuss the spectroscopic properties of the complexes examined in the frame of the “localized molecular’orbital configuration” approach 116, 173 (metal-centered (MC), ligand-centered (LC), and charge. transfer (CT) either metal-to-ligand (ML) or ligand-to-metal (L&E) transitions) even though this simplified picture is not filly satisfactory for the following reasons: (i) the cyclometallated

176

100

Iem

50

0 450

550

650

Fig. 3. Emission spectra of: (a) l&,dpbhq; (b) t-Pt(dpbhq)pyr; (c) c-Pt(Hdpbhq),. complexes exhibit a large degree of covalency of the metal-C bonds in the ground state; (ii) excited state configurations of different nature (LC and CT excited configurations) are sufficiently close in energy to be intermixed. 4.1. Absorption spectra The absorption properties of t-Pt(dpbhq)S(Et),, t-Pt(dpbhq)py, t-Pt(dpbhq)pyr have been discussed in a previous paper [18]. The bands below 340 nm have been assigned to LC transition, while the bands-above 340 nm have been assigned to MLCT transitions, with some contribution from strongly perturbed LC transitions. c-Pt(Hdpbhq), shows intense absorption bands in the near UV spectral region (below 340 nm) (Fig. 2). These bands can be related to LC transition since the free cyclometallating ligand (Hadpbhq) exhibits similar bands in the same spectral region. The bands at 362 and 410 nm (E= 13800 and 3800 respectively) can be related neither to MC transitions owing to their relatively high intensities nor to LMCT transitions, owing to their relatively low energy. Thus, these bands can only be a result of LC or MLCT transitions_ The band at 362 nm can be assigned, as for the three trans complexes [18], to MLCT transitions with some possible contribution from strongly perturbed LC transitions. The band at 410 nm can be related with more confidence to a pure MLCT transition because of its high energy separation from the lowest energy absorption maxima of the free ligand. This assignment is also confirmed by the disappearance of these bands upon photochemical oxidative addition with CHaI or CHzClz [ll]_ 4.2. Electrochemistry Cyclometallated Pt(I1) complexes usually show irreversible oxidation and reversible one-electron reduction processes 1111. The complexes studied in this paper follow this

177 general trend and thus, as for the previously studied Pt(II) complexes, we can associate the reduction process with the ligands and the oxidation process with the metal. cand l-Pt(dpbhq)py show the same value of the first Pt(Hdpbhq)z, t-Pt(dpbhq)S(Etz), reduction potential, which means that the ligand involved in the reduction process is the common dpbhq ligand. On the contrary, the first reduction potential of Pt(dpbhq)pyr (see Table 1) clearly indicates that the lowest unoccupied molecular orbital (LUMO) of the complex is a fl orbital of the pyr ligand. 4.3. Emission spectra In complexes of metals in low oxidation states, like Pt(II), LMCT transitions cannot be responsible for the observed luminescence. The involvement of an MC excited state can also be excluded since in the very rare cases of emission they exhibit large and unstructured bands [19]. The structured bands exhibited by the complexes examined (Fig. 3) can only be due to emission from LC or MLCI’ excited states. Distinction between these types of transitions will be made for each specific complex on the basis of the characteristics of the corresponding emission spectrum. The cti-Pt(Hdpbhq)2 complex shows an emission spectrum at 77 K with the following characteristics: (i) a structured band; (ii) the highest energy feature is redshifted from that of the free ligand of about 2300 cm-l; (iii) the emission lifetime is 8 ps. This behavior resembles that of the cti-Pt(I1) complexes previously investigated [ZO-223 and points to an MLCI assignment of the emitting excited state. It has to be noted, however, that the dpbhq ligand has a r system more expanded than that of phenylpyridine type ligands; for this reason LC excited states are lower in energy and closer to MLCT states. Consequently, the MLCT character of the emitting excited state is presumably less pure in this complex than in the previously studied cis complexes. t-Pt(dpbhq)S(Et)2, t-Pt(dpbhq)py, and t-Pt(dpbhq)pyr exhibit identical emission spectra at 77 K with the following characteristics: (i) a structured band; (ii) the highest energy feature (at the same energy in all cases) is red-shifted from that of the free m th e emission lifetimes are 30,35 and 14 F respectively ligand of about 2000 cm-l; (“‘) (see Table 1). This behavior suggests that the emitting excited state is again MLCI in nature, with some LC contribution. It is evident from red-shift and lifetime values (see Table 1) that in these complexes the emitting excited states possess a greater degree of LC character than does the cls-Pt(Hdpbhq)2 complex. As noted before [23] this could be due to the different geometry of the complexes (trans vs. cis), but we believe that this is a result of a lower ligand field strength of the trans compared with the cis complexes, caused by the rigidity of the terdentate ligand, which is not able to coordinate the metal in the proper way. An X-ray structure determination of Pt(dPPY)Py PSI has shown that the Pt-C bond distances are longer than in the cis complexes, owing to the rigidity of the dppy ligand. Since dpbhq ligand is even more rigid than the dppy ligand similar bond distances should be assumed in the case of the dpbhq complexes. These distances prevent a good interaction between metal and dpbhq ligand with two consequences: (i) decrease of the ligand field strength; (ii) decrease of the energy of the highest occupied molecular orbital (HOMO), the d orbital (which causes the observed blue-shift of the emission energy of the trans compared with the cis complexes). The identical emission spectra of the three complexes indicate that the ligand involved in the transition is the terdendate dpbhq ligand. This is obvious for the Pt(dpbhq)S(Et) z since only the dpbhq ligand can be involved in MLCI’ transitions. For the Pt(dpbhq)py complex the emission from an MLCT excited state involving the dpbhq ligand has a simple explanation; the LUMO of the dpbhq has to be somewhat lower in energy with respect to the LUMO of the pyridine, due

178

Fig_ 4. Schematic

representation

of the crystal structure of t-Pt(dppy)py.

to the more

extended v-system in the former Iigand. In the case of Pt(dpbhq)pyr the emission from an MLCT excited state involving dpbhq is surprising because of the cyclic voltammetry of the complex (see Table l), which clearly indicates that the LUMO of the complex is a r* orbital of the pyr Iigand. One explanation for this unexpected luminescence behavior could be based on geometric factors. X-ray structure determination [18] of the similar.t-Pt(dppy)py complex (Fig. 4) has shown that while the two phenyl and the pyridine rings of the dppy ligand lie in the same (coordination) plane; the pyridine ring is perpendicular to this plane. It seems reasonable that Pt(dpbhq)pyr (and also Pt(dpbhq)py) exhibits the same structure as Pt(dppy)py. Comparative nuclear magnetic resonance (NMR) spectroscopy on Pt(dppy)py, Pt(dpbhq)pyr Pt(dpbhq)py supports this assumption [18]. In this geometry the rr interaction between Pt and the two ligands is such that the d-metal orbital involved in the MLCT (Pt -pyr) transition is not the same as that involved in the MLCT (Pt -dpbhq) transition. If the d( -pyr) orbital is so low in energy with respect to the d( +dpbhq) orbital to counterbalance the lower energy of the pyrazine v-system compared with the r-system of dpbhq, the MLCT (Pt --+pyr) may not be the lowest energy transition. In other words, if the energy difference between the d-metal orbitals involved in the two different MLCT transitions is higher than the energy difference between the r-systems of the pyr and dpbhq ligands, the MLCI’ (Pt +dpbhq) may be the lowest energy transition even though the LUMO is localized on the pyr ligand.

Acknowledgment This work was supported by the Minister0 della Pubblica Istruzione, the Consiglio Nazionale delle Ricerche and by the Swiss National Science Foundation.

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