An ‘imperial radiation’: Experimental and theoretical investigations of the photo-induced luminescence properties of 6,6′-dibromoindigo (Tyrian purple)

Abstract

6,6′-dibromoindigo (DBI) is the main component of Tyrian purple, or Imperial purple, one of the most controversial, sought-after and expensive colouring matters of antiquity. Evidence of its use, both as a pigment and as a dye, is found in the archaeological record, especially in the countries surrounding the Mediterranean basin. The photo-induced luminescence properties of a synthetic sample of DBI were studied in this paper. Time-Dependent Density Functional theory (TD-DFT) was used to predict and rationalise the optical absorption and emission properties of DBI in DMSO and compared with the experimental data. The emission properties of the solid-state sample were characterised for the first time. DBI shows an emission maximum in the infrared range, at about 870 nm, and lifetime in the picosecond range. The spatial distribution of the photo-induced luminescence emission could also be recorded using a commercial infrared-sensitive camera, opening up the possibility of non-invasively investigating the use of DBI on historical artefacts.

Introduction

Tyrian purple, characterised by its main marker compound 6,6′-dibromoindigo (DBI), is a naturally occurring compound that has been the focus of fascinating research in fields ranging from archaeology, art history, socio-political and religious studies, to chemistry, material science and the pharmaceutical industry. Few pigments have sparked the imagination of the wider audience and combined the interests of such diverse scholars. Tyrian purple (also known as Imperial purple or Royal purple) possesses all the elements of prestige: its mythical origins (associated with the Nymph Tyros and the god Melqart in Phoenicia and Hercules in Greece [1, 2]), its popularity in antiquity, rarity, cost, association with high social status and its production methods shrouded in secrecy, until concerted studies and direct experiments shed light on the lost knowledge of ancient craftsmen [[3], [4], [5]].

Its history originated in the countries surrounding the Mediterranean basin with the exploitation of shellfish such as Hexaplex trunculus, Bolinus brandaris and Stramonita haemastoma (previously known as Murex trunculus, Murex brandaris and Purpura haemastoma, respectively). Other pigment-producing molluscs can be found worldwide, including the coastlines of northern-western Europe (Nucella Lapilus), South America (Plicopurpura pansa) and Japan (Rapana venosa) [6, 7]. Tyrian purple is derived from the fluid contained in the hypobranchial glands of the mollusc. The various chemical pathways, involving the enzymatic hydrolysis of precursors in the secretions followed by the oxidation and photochemical conversion of intermediate chromogens, have been reviewed previously [6, 8, 9].

Tyrian purple was used as a dye for textiles and as a pigment for polychrome surfaces, often extended with/stained onto inorganic white/transparent pigments [10]. Textiles can be dyed with Tyrian purple through a direct process, in which organic fibres are first soaked in the translucent liquid (precursors) secreted by the mollusc and then exposed to air and light until the final purple colour is formed. However, it is the indirect vat dyeing process, involving the establishment of reducing (alkaline) aqueous conditions to solubilise the pigment (leuco form), enzyme deactivation, precursor stabilisation and photochemical control, which led to the optimisation and widespread diffusion of the dyeing technique [8].

The discovery of the pigment is attributed to the Minoans in the Agean Bronze Age, and its earliest identification on a wall painting in Akrotiri (Santorini) is dated to the 17th century BCE [11, 12]. Earlier archaeological evidence (crushed shells) in eastern Crete [13] and Italy [14] may suggest established production methods as early as the late Middle Bronze Age (c. 18th century BCE) [15]. Its first confirmed use as a dye for textiles comes from the analysis of fibres found in the tombs of Qatna (Syria), a site destroyed by the Hittites in the 14th century BCE [16]. Nonetheless, it is the Phoenicians who developed a large-scale textile dyeing industry with a major centre located in the city of Tyre (Lebanon), whose name remained associated with the purple pigment [4]. The earliest archaeological evidence of this dyeing industry, a 14th/13th century BCE pot sherd with purple deposits resulting from vat dyeing, was found in Sarepta (Lebanon) [[17], [18], [19]]. The complex history of Tyrian purple and its socio-political and religious significance have been reviewed by several authors [2, 5, 6, 15, [20], [21], [22]]. Its identification on objects or artefacts, through scientific analysis, is summarised in Table 1. While this review does not claim to be exhaustive, it provides a means of analysing historical and geographical trends in the evolution of the use of the pigment and the dye in the countries surrounding the Mediterranean basin. Although future discoveries and analysis may alter or refine our views, the current archaeological record appears to indicate that Tyrian purple was predominantly used as a pigment in Greece between the 17th and 1st centuries BCE. In the same period, archaeological evidence reveals a thriving dyeing industry, although textiles dyed with Tyrian purple have been found mainly in the Levant and regions associated with the Persian Empire (Israel, Lebanon, Turkey, Cyprus, Syria and Iraq). However, as textiles are not commonly preserved in the archaeological record in countries with wet climates, this perspective may be artificially skewed towards a predominant use of the pigment in Greece and the West. As discussed below, there is much evidence in primary literary sources that the dye was used in the Western countries bordering the Mediterranean.

Textual sources are essential to understand the fascination that textiles dyed with Tyrian purple exerted on the Greeks and Romans. From a scientific perspective, as early as the 4th century BCE, no less a scientist than Aristotle was the first scholar to describe the anatomy of muricid shellfish and their ability to produce a purple pigment [63]. In the 1st century CE, the Roman writer Pliny the Elder gave the first extensive description of the dyeing process and of the production of the pigment purpurissum [64]. The royal, ritual, social, moral associations related to the colour purple are well described by Herodotus and Plutarch, who describe emissaries dressed in purple who were mistaken for priests, and dyed textiles as symbols of mistrust and abuse of power, often associated with the East and the Persian Empire [65]. In particular, the association between royal figures and Tyrian purple was already established during the Achaemenid dynasty in Persia (559-330 BCE) [66], and can be traced back to the Assyrian and Babylonian kings [67]. Alexander the Great, during his conquest of Persian territories, seized large quantities of purple textiles [68], which he wore during official ceremonies. Sumptuary laws appeared as early as the 7th century BCE in Greece, forbidding women from wearing garments with purple borders [69]. In Republican Rome (509-27 BCE), the success of generals could be measured by the amount of purple clothing they wore [70]. Further sumptuary laws restricted the use of the colour to the Emperor during the reign of Nero (37–68 CE) [2]. Prices were considerably raised under Diocletian (284–305 CE) [71] and threat of death or exorbitant fines for its use continued to be promulgated under Justinian (527–565 CE). Nonetheless, recent analyses of textiles found in Roman and Coptic Egypt (1st– 7th centuries CE) indicate that middle and lower classes had some access to the elite dyestuff [50]. The predominance of textiles found in Egypt is noticeable and can be related, as mentioned, to the delicate nature of organic fibres, which survive particularly well in desert climates compared to other archaeological contexts.

After the decline of the Western Roman Empire in the 5th century CE, the dye was primarily used for textiles made in or imported from the Eastern half of the Roman Empire [70]. In Western Europe, it was also used on a smaller scale as a pigment for manuscripts. By the 15th century, Tyrian purple had fallen into disuse and the fall of Constantinople in 1453 or the 1464 decree of Pope Paul II, stipulating the use of kermes to dye cardinals’ robes, are cited as terminus ante quem for the use of Tyrian purple [72]. Although the status symbol of purple colour is well documented in antiquity in countries spanning modern Europe and the Middle East [2], the colour is appreciated less in Asia [73] and Tyrian purple has only been identified in a few instances on ancient textiles from Japan [74] and China [6]. In South America, purple dyeing was known from early Peruvian civilisations (6th century CE) [52] and is still a living tradition in Mexico [6].

In 1832, the Venetian chemist Bizio re-discovered the marine origin of the pigment and identified its indigoid nature [75]. The structure of DBI and its identification as the major coloured compound of Tyrian purple was elucidated in 1909 by Friedländer [76], who reported a large number of shellfish (12,000 Bolinus Brandaris) producing as little as 1.4 g of dry pigment. Recent investigations and experiments indicate that 10,000 shellfish (Hexaplex Trunculus) provide 100 L of coloured solution, which can in turn dye approximately 1 kg of wool [77]. Other related compounds− indigo (IND), 6-bromoindigo (MBI), indirubin (INR), 6-bromoindirubin (6MBIR), 6′-bromoindirubin (6′MBIR), 6,6′-dibromoindirubin (DBIR), isatin (IS), 4-bromoisatin (4BIS) and 6-bromoisatin (6BIS)− have been identified with HPLC, although in much smaller amounts compared to DBI and in varying combinations depending on the type of shellfish used and production methods [9, 37, [78], [79], [80]].

The chemical synthesis of DBI (C₁₆H₈Br₂N₂O₂) (Fig. 1) has been the subject of much research during the 20th and early 21st century [9], and industrial and pharmaceutical applications are still driving the search for a productive and cost-efficient pathway [8]. Investigations of the optical characteristics of DBI have been mainly focused on UV–visible absorption, reported for the solid state (λ_max 640 nm or 540 nm depending on light geometry) [81], and for solutions in various solvents, such as water (calculated λ_max 592 nm) [82], DMF (experimental λ_max 600 nm) [83], and DMSO (experimental λ_max 598 nm) [84]. The photoluminescence (PL) of DBI has also been characterised in DMF solution with an emission maximum reported at 523 nm, following excitation at 355 nm [85].

The research presented here aims further to investigate the optical properties of synthetic DBI in DMSO solution and in the solid state (powder). A sample was analysed with FTIR (ATR, diffuse and specular reflectance) and SEM-EDX to characterise its molecular and elemental composition and assess the purity of the reference sample. The FTIR spectra of DBI acquired in diffuse and specular reflectance are reported here to provide references for further non-invasive analysis. The absorbance (total reflection), excitation and emission profiles, as well as the lifetime decay and photoluminescence quantum yield (PLQY) were characterised for the solvent solution. The emission profile and the lifetime decay of the solid-state sample were also investigated, and are reported here for the first time. A theoretical study at the DFT level of theory was conducted to provide greater insight into the spectroscopic properties of 6,6′-DBI in DMSO solution. The spatial distribution of the photoluminescence emission in the infrared (IR) range of the solid-state sample was also characterised using an infrared-sensitive camera.

Non-invasive analytical equipment, such as portable XRF and reflectance FTIR spectroscopy, is sometimes used for in-situ analysis of Tyrian purple [24, 31, 35, 42, 48, 60, 86], but invasive chromatographic techniques are required to identify the compounds present in historic Tyrian purple samples at a molecular level, and to ascertain the mollusc species used to produce the dye/pigment. The characterisation of DBI with both PL imaging and spectroscopy presents the first steps in the evaluation of these techniques for the non-invasive identification and mapping of Tyrian purple in historic samples.