Egyptian Blue as nIR emitter

So far, there was one novel nIR fluorophore overseen in most publications: Egyptian Blue (λex

= 625 nm,λem = 930 nm, Fig. 15a). The absorption and the color properties of this pigment are well-known, but the emission in the near-infrared was only recently described by Accorsi et al. [215]. The nIR emission first became useful when one had to investigate artifacts where the handling should be kept to a minimum. With the conversion of visible light into the nIR emission, Egyptian Blue is ideal for non-invasive investigations. It was used to detect traces of Egyptian Blue in old monuments, such as the Greek Parthenon [216]. The longevity of the pigment itself and its PL is evident. Ancient decorations have survived for several thousand years in both dry and damp surroundings, as well as in various museums, and still exhibit bright luminescence without photobleaching. An example of the nIR photoluminescence collected from an antique sample can be seen in Figure 15b. Aside from the nIR emission, the Stock-shift of ≈ 300 nm, PL lifetimes in µs-range, its quantum yield is the highest for all known nIR fluorophores: QY(Egyptian Blue) = 10.5%, QY(ssDNA/SWCNTs) ∼ 0.1 -1% [215], [96].

Geometric structure of Egyptian Blue

Egyptian Blue (CaCuSi4O10) is a blue pigment known since 3600 BC [217]. It was the earliest artificial pigment that was synthesized by mankind. It was widely used in murals and decoration, and its application spread out to the Mediterranean areas, Mesopotamia and the ancient Roman Empire. With the fall of the Roman Empire, the knowledge of its synthesis was lost. Despite its rich history, Egyptian Blue is also a defined compound with the chemical formula: CaCuSi4O10 (Fig. 15a) [218]. It can be synthesized by mixing the following minerals together: lime (CaCO3), sand (SiO2), a mineral containing copper (e.g.

malachite (CuO2(COO3)(OH)O2 or metallic copper), as well as a small amount of alkaline base, such as potassium carbonate (K2CO3) or soda (Na2CO3). The mixture is then heated up to temperatures around 800−900^◦C [219]. It is important that the mixture is exposed to oxygen to prevent the formation of red cuprite (Cu2O).

CuO₂(COO₃)(OH)O₂ + 8SiO₂ + CaCO₃ alkaline base

−−−−−−−→

800 - 900^◦C 2CaCuSi₄O₁₀ + 3CO₂ + H₂O Egyptian Blue is a copper silicate of the cuprorivaite class MCuSi₄O₁₀, where M is an alkaline earth metal (M = Ca, Sr, Ba). Two alkaline earth metals Calcium and Barium result in two blue pigments: the aforementioned Egyptian Blue (CaCuSi₄O₁₀) and the Han Blue (BaCuSi₄O₁₀). Latter was synthesized during the Han dynasty (208 B.C.) in China [220].

Both compounds are isostructural, have the same Cu-O distances and space group (P4/ncc)

[221]. A similar compound with two less equivalents of quartz than Han Blue is Han Purple (BaCuSi₄O₆). The crystal structure of MCuSi₄O₁₀ is tetragonal with SiO₄ silicate pyramids forming the structural framework. Four of those silicate pyramids form a Si₈O₂₀sheet, which is responsible for the major percentage of the height of one unit cell [222]. The heart of the complex is the square-planar CuO⁶⁻₄ chromophore. In the complex the copper ion has aD_4h symmetry and is centered between four oxygen atoms in a very stable coordination. The fluorescence originates from Cu²⁺ ions linked by the corners of 4 tetrahedral silicate moieties (SiO₄) into a three-dimensional crystal structure. The CuSi₄O₁₀ layers alternate with Ca²⁺

or other alkaline earth metals. The stability of the structure does not allow to remove the copper ion by light exposure, high temperature, acids, etc. [215].

Figure 15: Optical and geometric properties of Egyptian Blue. (a) Photography image of the Egyptian Blue powder and the schematics of its unit cell with Cu²⁺ ions in blue, Ca²⁺in turquoise, O²⁻in red, and SiO⁴⁻₄ tetrahedra in green, (b)Hunting in the marshes, mural fragment from the tomb chapel of Nebamun (1400−1300 BC). Visible (top) vs. nIR photoluminescence (bottom) photography. White areas in the second image correspond to Egyptian Blue pigment. Adapted with permission from [215], all rights lie byThe Trustees of the British Museum, British Museum, London, UK.

On the contrary, the alkaline earth metals act as independent counterions and are positioned between the layers. Due to its layered structure, Egyptian Blue easily forms multilayer nanosheets [223].The structure of a unit cell is shown in Figure 15b. The nature of alkaline earth metals has a direct influence on the luminescence of Cu²⁺. The ion radius of alkaline earth metals are decreasing as following: Ba²⁺ >Sr²⁺ >Ca²⁺. The same gradual trend can be observed in the red-shift of the emission maxima (930, 936, and 964 nm, respectively) [224]. As the ion radius gets larger, the lattice expanses and the crystal field experienced by Cu²⁺ gets weaker. Thus, the lattice expansion of the unit cell results in a red-shift both in absorption and in emission spectra [221].

Photophysics of Egyptian Blue

The optical properties of transition-metals are generally determined by (1) the nature of the ligands, the metal cation and its coordination number, (2) the geometrical arrangement of these ligands around the metal cation, and (3) the distances between the metal cation and the ligands [216]. To understand the photophysical properties of Egyptian Blue it is helpful to reconstruct the energy diagram of Cu²⁺ under consideration of these influences (Fig. 16). We start with a free Cu²⁺ ion with the outer electron configuration of 3d⁹. First, it translates into the octahedral ligand field of a CuO⁶⁻₄ complex, accompanied by splitting of d⁹ into 5 energy levels (e_g and t_2g). The higher levels consists of two orbitals: x² −y² and z². The three orbitals with lower energies are xy, xz and yz. The next transition is into a square-planar CuO⁶⁻₄ complex with lower symmetry O_h −→ D_4h. Here, the x²−y² and thez² orbitals experience two different influences due to the Jahn-Teller effect and the lattice constraint. The ligands are positioned in the plane around copper, thus making the x²−y² orbital the most unfavorable. Its energy is now the highest. At the same time, the z-direction is no longer energetically unfavorable, and the energy of thez² orbital is now the lowest. From the 3 degeneratedt_2g orbitals, the xy-orbital has a slightly higher energy than xz and yz. In the final step, the CuO⁶⁻₄ unit is embedded into the CaCuSi₄O₁₀ structure.

Here, the z-direction gets slightly less favorable, as the Cu²⁺ ion experiences ligands and counterions from the the various layers of the whole complex.

Figure 16: Optical properties of Cu²⁺ in Egyptian Blue. The energy diagram for the conversion from the free Cu²⁺ ions into the CaCuSi₄O₁₀.

The excitation spectrum of Egyptian Blue covers a wide range of 500 −700 nm, which matches the three optical transitions from the ground state ²B_1g into ²B_2g, ²E_g and ²A_1g (650, 620, and 540 nm, respectively) [221]. There is a difference of -0.4 eV between the highest absorption energy of Egyptian Blue (540 nm = 2.3 eV) and the highest absorption energy of the CuO⁶⁻₄ complex (460 nm = 2.7 eV). This difference is attributed to the anisotropic E_R(r) field which is created by the whole crystal and affects all electrons in the complex.

The anisotropic E_R(r) field of the Egyptian Blue does not occur in the more symmetrical CuO⁶⁻₄ complexes [216]. Thus, not all CuO⁶⁻₄ exhibit bright blue color, which results from the lack of absorption in the blue region in case of CaCuSi₄O₁₀.

The emission of CaCuSi₄O₁₀is centered around 930 nm and results from the energy transition from the lowest excited state to the ground state ²B_2g(xy) −→² B_1g(x² −y²) [216]. The emission should be formally the reverse of the corresponding absorption transition. Yet the energies differ:

Emission: ²B2g −→² B1g = 10 500 cm⁻¹ Absorption: ²B1g −→² B2g = 12 500 cm⁻¹

When one considers only the unit cell and the local environment of Cu²⁺ ions, Egyptian Blue has an inversion center. Thus, the intraconfigurational electronic (d-d) transitions are parity-forbidden. The observed emission intensity can be measured due to the unsymmetrical vibrational modes coupled with the electronic transitions, which disrupts the symmetry [225].

Non-radiative relaxation from the highest energy level²A_1gto²B_2g is efficient. The relaxation from²B_2g to the ground level ²B_1g is less populated and thus less frequent [225]. Therefore, the emission exhibits very long luminescence decay times. The reported lifetimes range from 107 to 130 µs [215], [226] and was calculated to τ = 124 µs in our measurements (see Sec.

4.3.2 for details).

3 Experimental Procedures

3.1 Materials