Raman spectroscopic characteristics of phthalocyanine and naphthalocyanine in sandwich-type phthalocyaninato and porphyrinato rare earth complexes

Abstract

Raman spectroscopic data in the range of 400–1800 cm⁻¹ for a series of 15 mixed (octaethylporphyrinato)(naphthalocyaninato) double-decker complexes with tervalent rare earths M^III(OEP)(2,3-Nc) (M=Y, La, …, Lu except Ce and Pm) and intermediate valent cerium Ce(OEP)(2,3-Nc) have been collected using laser excitation sources emitting at 632.8 and 785 nm. Comparison with the Raman spectra of the corresponding bis(naphthalocyaninato) rare earths reveals that the Raman characteristics of mixed ring double-deckers M(OEP)(2,3-Nc) are dominated by the metallonaphthalocyanine M(2,3-Nc) fragment. Under excitation at 632.8 nm, which is far away from the main Q absorption band of the naphthalocyanine ligand, the most intense Raman band appears at 1598 cm⁻¹ for M^III(OEP)(2,3-Nc) (M≠Ce), and is assigned to the naphthalene stretching. On the other hand, for Ce(OEP)(2,3-Nc), this vibration appears as a strongly enhanced peak at 1550 cm⁻¹ together with a medium intense band at 1613 cm⁻¹. Typical Raman marker bands of the monoanion radical Nc⁻ were observed at 1499–1512 cm⁻¹ as a medium band and at 1521−1538 cm⁻¹ as a band whose intensity increases along with the rare earth contraction, resulting from the coupling of pyrrole CC and aza CN stretchings and aza CN-only stretchings, respectively. For Ce(OEP)(2,3-Nc), a weak and broad band centered at 1507 cm⁻¹ with contribution from both pyrrole CC and aza CN stretches together with isoindole stretches was the marker Raman band of Nc²⁻. When excited with laser radiation of 785 nm which is in close resonance with the main Q absorption band of naphthalocyanine ligand, the ring radial vibrations of the isoindole moieties at ca. 680 and 732 cm⁻¹ in the Raman spectra of both M^III(OEP)(2,3-Nc) and Ce(OEP)(2,3-Nc) are selectively intensified and appear as the most intense vibrations. The marker Raman bands of Nc⁻ in M^III(OEP)(2,3-Nc) and Nc²⁻ in Ce(OEP)(2,3-Nc) are observed in similar regions to those found for excitation at 632.8 nm. The scatterings with contribution from the Nc breathings, ring radial vibration, naphthalene stretchings, benzoisoindole stretchings, aza group stretchings, and the coupling of pyrrole CC and aza CN stretchings in M^III(OEP)(2,3-Nc) all show dependence on the rare earth radius, shifting to higher energy along with the rare earth contraction and showing the rare earth ionic size effect on the Raman characteristics of naphthalocyanine in the mixed ring double-decker compounds.

Introduction

Porphyrins and phthalocyanines are important classes of pigments that have fascinated chemists for many years due to their applications in various disciplines [1], [2]. Both series can form complexes with almost the complete Periodic Table of elements. In particular, sandwich-type complexes of phthalocyanines and porphyrins with large metal ions such as rare earth, actinide, early transition, and main group metals have been obtained, Fig. 1 [3], [4]. The rare earth sandwich complexes have attracted great attention due to their possible applications in molecular electronics, molecular optronics, and molecular iono-electronics [5], [6].

2,3-Naphthalocyanine (2,3-Nc), also see Fig. 1, has a more extended π-electron-delocalized system compared with phthalocyanine. As a result, its characteristic Q electronic absorption band is significantly red-shifted to the near-IR region. By the continuous efforts of Jiang and co-workers to develop new species of sandwich compounds [7], [8], several series of homoleptic and heteroleptic sandwich rare earth complexes containing naphthalocyanine have been reported, arising from their newly-developed methodology. Similar red-shifted characteristic Q bands have been recorded for these naphthalocyanine sandwich complexes compared with their phthalocyanine (Pc) counterparts [9], [10], [11], [12].

It is worth noting that starting from the first main group metal bis(phthalocyaninato) compound Sn(Pc)₂ in 1930s [13], the research in the field of rare earth sandwich-type porphyrinato and/or (na)phthalocyaninato metal complexes has progressively advanced from the study on bis(phthalocyaninato) complexes in the middle of 1960s [14] to mixed (porphyrinato)(naphthalocyaninato) and bis(naphthalocyaninato) complexes at the beginning of 2000 [7], [8] via bis(porphyrinato) complexes in 1980s [15] and mixed (porphyrinato)(phthalocyaninato) complexes in 1990s [9], [10], [11], [12], [16]. For the sandwich complexes with an unpaired electron in one of the tetrapyrrole ligands, i.e. the neutral double-deckers containing M^III ions, a fundamental question concerning the extent of hole delocalization has been raised [3], [4]. Among various spectroscopic approaches used to study the extent of unpaired electron delocalization in these sandwich molecules, vibrational techniques, namely IR and Raman spectrometry, have proved to be versatile methods [16], [17], [18], [19], [20], [21], [22], [23], [24]. A lot of work has been focused on the IR spectroscopic properties of porphyrinato and/or phthalocyaninato rare earth sandwich-type compounds. By contrast, the Raman technique has not been extensively applied for characterizing these complexes [3], [25]. Following the work of Aroca and Homborg on Raman spectroscopic characterization of rare earth bis(phthalocyaninato) compounds [22], [26], [27] and of Tran-Thi et al. on the FT-Raman spectra for a series of sandwich phthalocyaninato and porphyrinato gadolinium and cerium compounds [25], recently Jiang and Arnold have started to investigate systematically the Raman characteristics of phthalocyanine anions for a large number of phthalocyanine-containing homoleptic and heteroleptic sandwich complexes of rare earths, namely M(Pc^∗)₂, M(Por)(Pc′) (Pc′=Pc, Pc^∗), (Pc)M(Pc^∗)M(Pc^∗), M₂(Por)₂(Pc), and M₂(Por)(Pc)₂ ($\text{Pc}{}^{\mathrm{\ast }}\text{=}\text{substituted}$ Pc, Por=tetraaryl- or octaalkylporphyrin). Initially, we used a laser excitation source emitting at 632.8 nm because of the presence of the intense Q absorption band of phthalocyanine ligands around this wavelength in these complexes [28]. The dependence of the Raman characteristics of phthalocyanine in these sandwich complexes on the emitting laser source was investigated by using laser sources emitting at different wavelengths of 457.9, 488.0, 514.5, 647.1 or 780 nm [28], [29]. Our recent progress in designing a new synthesis route has led to isolation of novel naphthalocyanine-containing sandwich rare earth complexes [7], [8]. Herein, we describe the Raman spectroscopic characteristics of naphthalocyanine anions in the mixed (octaethylporphyrinato)(naphthalocyaninato) rare earth double-deckers M(OEP)(2,3-Nc) (M=Y, La, …, Lu except Pm) and the effects of ionic size and oxidation state of rare earth metal ion on the Raman characteristics of naphthalocyanine, by means of excitation with laser sources emitting at 632.8 and 785 nm.