**Spectroscopic Behaviour of Two Novel Azobenzene Fluorescent Dyes and Their Polymeric Blends**

**Rosita Diana <sup>1</sup> , Ugo Caruso 2,\* , Stefano Piotto <sup>3</sup> , Simona Concilio <sup>4</sup> , Rafi Shikler <sup>5</sup> and Barbara Panunzi <sup>1</sup>**


Academic Editor: Radosław Podsiadły

Received: 14 February 2020; Accepted: 16 March 2020; Published: 17 March 2020

**Abstract:** Two novel symmetrical bis-azobenzene red dyes ending with electron-withdrawing or donor groups were synthesized. Both chromophores display good solubility, excellent chemical, and thermal stability. The two dyes are fluorescent in solution and in the solid-state. The spectroscopic properties of the neat crystalline solids were compared with those of doped blends of different amorphous matrixes. Blends of non-conductive and of emissive and conductive host polymers were formed to evaluate the potential of the azo dyes as pigments and as fluorophores. Both in absorbance and emission, the doped thin layers have CIE coordinates in the spectral region from yellow to red. The fluorescence quantum yield measured for the brightest emissive blend reaches 57%, a remarkable performance for a steadily fluorescent azo dye. A DFT approach was employed to examine the frontier orbitals of the two dyes.

**Keywords:** azobenzene; dye; fluorophore; colorant; polymeric blend

#### **1. Introduction**

Azobenzene derivatives are π-conjugated molecules that have aroused enormous research interest owing to their fascinating characteristics. First, the unique broad UV/Vis absorbance spectra related to the considerable number of vibronic states in each energy level. The color is due to the presence of the N=N chromophore chemical group absorbing light in the visible spectrum. Suitable substituents can achieve the tuning of the color to cover the entire visible spectrum.

For this reason, from the past until today, azo compounds have been widely used as dyes and pigments. A significant amount of azo dyes is used in manufacturing processes of textile, paper, packaging pharmaceutical, and even food industries [1]. Excellent compatibility with the matrix and chemical and thermal stability are the main requirements. Despite this, photo-catalyzed degradation through illumination by solar light was found in most of the azo pigments, with the resulting discoloration of the dyed substrate.

On the other hand, the ability of azobenzene compounds to reversibly change from the stable *trans*-isomer to *cis* form upon photoirradiation (photoisomerization) causes nonradiative deactivation and very small/negligible fluorescence. The rapid photoisomerization of the azo bridge finds applications in on-off photoswitching techniques, such as in optical data storage, dye-sensitized solar cells, pharmaceuticals, and non-linear optics [2–8]. Azo-containing molecules can play the

role of energy dissipator (quencher) associated with other emitting chromophores, through rapid nonradiative pathways, while the donor is excited for the occurrence of a Förster resonance energy transfer (FRET) [9]. Unlike many chromophores commonly utilized as fluorescent materials, there are a limited number of examples of steadily fluorescent azobenzene compounds.

The structural versatility and the tunable spectroscopic properties of the azobenzene derivatives could be exploited if they are themselves fluorescent. Attempts to increase and to modulate the fluorescence performance of azobenzene compounds are quite recent [10–15] and based on preventing fast photoisomerization. This is possible with bulky substituents close to the N=N group [16–21] or intramolecular hydrogen bonding preventing the rotation around the nitrogen–carbon bond [19]. The influence of hydrogen-bond interactions on the excited-state dynamics of azo dyes has been examined [22,23]. It appeared that by restricting rotation and isomerization, the excited-state intramolecular proton-transfer (ESIPT) [21,24] lead to emissive azo dyes in solution [10,16,24–29]. Conversely, examples of solid-state azo fluorophores are rare in the scientific literature. Those few are typically red emitters [30–33], particularly advantageous in the context of biological and medical measurements [34–37].

In previous contributions [10,13], we studied azobenzene scaffolds for their unique structural pattern related to their photophysical properties. In this work, two novel azobenzene dyes, A1 and A2 in Scheme 1, were explored. To fulfill the criteria as fluorescent dyes, we have sought stable, processable, and sterically encumbered structures. Based on the same symmetrical bis-azobenzene skeleton (AB in Scheme 1), the two chromophores differ for the terminal substituents, both electron-donor (NEt2) or electron-acceptor (NO2) groups. DFT computational study was employed to get information on HOMO-LUMO localization at the different conjugated patterns. To impede photoisomerization, we added methoxy substituents on the AB moiety and two terminal Schiff bases ESIPT undergoing sites to restrict molecular rotations.

**Scheme 1.** Synthesis and structure of the dyes A1 and A2.

Under sunlight, the dyes are stable over three months both in solution and in the solid-state, retaining their orange-red color (see CIE: coordinates, International Commission on Illumination, in Tables 1 and 2). As fluorophores, a significant emission in solution and the crystalline phase was

**<sup>−</sup> <sup>−</sup> λ λ**

**λ ε**

recorded. Different host matrixes were employed to produce polymeric blends from A1 and A2. In addition to classical non-conductive hosts such as polyvinyl chloride (PVC) and poly (styrene) (PS), we also checked conductive, poly (vinylcarbazole) (PVK), and even emissive, poly (9,9-dioctylfluorene) (PFO), polymers. Their coloring ability was examined dissolved in PVC, a white matrix used in many industrial processes. The emission in the solid-state was evaluated in PS, PVK, and PF blends. The last one recently emerged as a useful polymeric matrix for optoelectronic devices [38]. In PFO, the photoluminescence quantum yield (PLQY) of the A1 orange-red blend increases up to an outstanding 57% value.


**Table 1.** Optical data for A1 and A2 in solution and for polyvinyl chloride (PVC) blends.

<sup>a</sup> Wavelength of UV-Visible absorbance maxima in THF solution. <sup>b</sup> Molar absorption coefficient. <sup>c</sup> Wavelength of emission maxima in THF solution. <sup>d</sup> PLQYs in THF solution. <sup>e</sup> Wavelength of UV-Visible absorbance maxima measured on the spin-coated film. <sup>f</sup> Absorption CIE coordinates on the spin-coated film.


**Table 2.** Optical data for A1 and A2 as neat solid samples and in 10 wt%. poly (styrene) (PS), poly (vinylcarbazole) (PVK), and poly (9,9-dioctylfluorene) (PFO) film blends.

<sup>a</sup> Wavelength of UV-Visible absorbance maxima; <sup>b</sup> Wavelength of emission maxima; <sup>c</sup> Photoluminescent quantum yield; <sup>d</sup> Emission CIE coordinates; <sup>e</sup> Optical data for films of PS, PVK, and PFO, obtained in the same conditions as the blends.

#### **2. Results and Discussion**

#### *2.1. Synthesis and Optical Behavior of the Dyes and Their PVC Blends*

As summarized in Scheme 1, the dyes A1 and A2 were obtained by condensation of the diamino derivative AB-NH<sup>2</sup> [39] with 4-(diethylamino)-2-hydroxybenzaldehyde and 2-hydroxy-4-nitrobenzaldehyde, respectively. Though structurally similar, a different conjugation pattern is recognizable, D-π-D-π-D respectively for A1 and A-π-D-π-A for A2 (where D = electron donor moiety, A = electron acceptor moiety, and π = conjugated system).

Two ESIPT undergoing sites are generated by the condensation of the amino-terminal groups of the precursor AB-NH<sup>2</sup> with the salicylic aldehydes, guaranteeing emission in solution. According to a recent approach [26,39,40], it was found that the fast proton transfer in ESIPT sterically encumbered probes are impeded due to restriction of intramolecular rotation (RIR effect), also providing solid-state emission. The central methoxy groups cause steric hindrance on the conjugated skeleton without too much lowering solubility.

Identification and purity degree evaluation were assessed by mass spectrometry and <sup>1</sup>H NMR. Phase behavior was examined by optical observation and DSC/TGA analysis. All materials are thermally stable up to 330 ◦C under nitrogen flow with high melting points. The two dyes are soluble in most organic solvents, such as acetone, tetrahydrofuran (THF), methylene chloride, tetrachloroethane (TCE), dimetilformammide (DMF), 1-metil-2-pirrolidone (NMP), and dimethyl sulfoxide (DMSO). Their absorption and emission maxima, in solution and the solid films, are reported in Table 1.

In Figure 1, absorption and emission curves of A1 and A2 in THF solution and a picture of the related samples (inset) in natural light and under UV lamp at 365 nm are shown. The yellow THF solutions emit in the lime-yellow and the red region, respectively, with a negligible solvatochromic effect depending on the solvent polarity. PLQYs (see Table 1) have been measured in THF solution by relative methods using as standard quinine sulfate for A1 [41] and zinc phthalocyanine for A2 [42]. The solutions are stable and retain their optical characteristics up to three months under natural light at room temperature.

**Figure 1.** Absorption (above) and emission (below) curves of A1 (black curves) and A2 (red curves) in THF solution. The same samples in natural light (above) and under UV lamp at 365 nm (below) in the insets.

The spin-coated thin films (obtained as described in the Materials and Methods section) of crystalline A1 and A2 have a red and yellow-orange color, respectively (see CIE in Table 1). Images of the spin-coated crystalline films of neat A1 and A2 under polarized microscope are reported in the Supplementary Materials, in Figure S3. The absorption spectra are reported in Figure 2, compared with the spectra of PVC blends at different dopant percentage. PVC as an economical and versatile thermoplastic colorable polymer is an excellent candidate to test the two compounds as dyes. It is the world's third largest thermoplastic material widely used in construction, packaging, devices, and the textile industry. The demand for stable dyes for this white material is still high. The doped PVC homogeneous amorphous blends are an example of stable dyed blends with pigments at 10% and 30% by weight. In both cases, the dyes are soluble up to 30%. The diethylamino terminal groups make compound A1 more soluble than the dinitro derivative; hence A1 has a higher solubility limit (40%) in PVC. The PVC films have CIE coordinates in the orange-red region (see Figure 2), more red-shifted for A2-PVC, the same behavior recorded for THF solutions. The PVC films of blended A1 and A2 kept over three months in the air under natural light at room temperature perfectly retain transparency and optical characteristics. No swelling nor release of the same films were detected on samples kept in distilled water for 30 days at room temperature.

**Figure 2.** On the left: absorption curves of A1 (black curves), A1-PVC 10% (red curve) and A1-PVC 40% films (blue curve) above; absorption curves of A2 (black curves), A2-PVC 10% (red curve) and A2-PVC 30% films (blue curve) below. In the insets: 30% A1-PVC (above) and 30% A2-PVC (below) thin films. On the right: CIE diagram of A1 (triangle) and both its PVC blends (square) above; CIE diagram of A2 (triangle) and both its PVC blends (square) below.

In Figure 3, an SEM image of 30% A1-PVC film deposed onto quartz slide is reported. The SEM analysis confirms that there is no visible structuring in the spin-coated film, also after three months under natural light at room temperature. All PVC blended samples of A1 and A2 exhibit similar morphological characteristics.

As for PL performance of neat A1 and A2, the crystalline dyes emit in the orange-red region with very similar maxima, see Table 2. The Stokes Shifts are about 140 nm. Appreciable PLQYs (see Table 2) measured on the crystalline samples spin-coated on quartz slides are a good result for azobenzene dyes. The PL response for A1 is higher than the nitro derivative A2 as a result of the different conjugation patterns, as discussed in the DFT analysis section. In Figure 4a, the emission spectra of the crystalline dyes are reported, and their PL behavior will be discussed in the next section.

**Figure 4.** Emission spectra of the neat A1 (black curves) and A2 (red curves) samples (**a**), of their PS blends (**b**), PVK blends (**c**), and PFO blends (**d**). In the inset, the same spin-coated samples used for the measurements (A1 on the left and A2 on the right).
