## 1 Introduction  
Chlorins, which are \(\beta\),\(\beta\)-reduced porphyrins, are macrocyclic molecules resembling porphyrins, which occur naturally and constitute the main ingredients of chlorophylls. A reduced double bond in one of the four pyrrole macrocycle rings distinguishes them from porphyrins (Figure 1). Chlorin is the core chromophore of chlorophyll, a macrocycle of dihydroporphyrin containing one pyrrole ring and three pyrrole rings. They can be formed via prototropic tautomerization in suitable tetrapyrroles where the structural properties do not inhibit tautomerization. Due to its extraordinary photophysical properties, several new chlorins have been isolated from natural sources, such as marine microbes. In addition, several new synthetic analogs have been prepared as a result of the diligent work of chemists to explore structure-function relationships [1, 2]. Chlorins, unlike porphyrins, exhibit intense absorption in both the blue and red regions, which can be attributed to their relatively lower symmetry. They exhibit characteristic optical features, which are altered from those of porphyrins, such as fluorescence, as well as a weaker Soret band and red-shifted, intensified Q-bands. These spectral properties are considered ideal for photosynthetic functions. In the spectra of chlorins, a strong absorption band appears in the violet-blue region (\(\sim\)380-450 nm, known as the B band or Soret band) and in the red region (\(\sim\)600-700 nm, known as the Q band). The absorption of light in both regions produces a green color typical of chlorin, which can be observed in solution or thin films (for example, a leaf) upon illumination with white light [1]. The absorption properties of chlorins can be altered by peripheral substituents and metal ions chelated in the center of the macrocycle. The solubility of most chlorins in aqueous solutions is poor due to their strongly hydrophobic nature [2]. Chlorins with NIR fluorescence emission have been applied in many disciplines of medicine, ranging from photodynamic therapy (PDT) to diagnostics, including flow cytometry and optical imaging [2]. PDT involves the localized application of the photosensitizer (PS) accompanied by its illumination in the tissue with a light beam of a specific wavelength. The light beam of required intensity should be absorbed by PS previously supplied to the treated tissue. Absorption of the light's quantum energy by PS molecules triggers a photodynamic reaction, which results in the generation of singlet oxygen and other reactive oxygen species (ROS) [3; 4]. Since ROS are highly cytotoxic entities, this phenomenon has been applied to treatments of various tumorous tissues [4]. The ROS species also reveal an antimicrobial solid effect, which can be used against antibiotic-resistant microbes in the form of so-called photodynamic antimicrobial chemotherapy (PACT) [5; 6; 7]. In recent years, one of the most commonly used chlorins has been chlorin e6 (Ce6) (Figure 2). Other studies on novel chlorin-based PSs are ongoing. Chemical modifications of chlorin gave access to modified macrocycles of improved physicochemical properties, including higher water solubilities and more efficient singlet oxygen-generation abilities. Many modified chlorins reveal light absorption in the so-called phototherapeutic window, which is the range of light with sufficient penetration and energy. In addition, improved pharmacokinetics and pharmacodynamics make chlorin derivatives perfect PSs for PDT [1]. Figure 1: Differences and aromatic paths of chlorin and porphyrin. Figure 2: The chemical structure of chlorin e6 (Ce6). Different approaches have been considered to improve the physicochemical and pharmacological parameters of chlorins. [PERSON] et al. conjugated chlorin molecules with glucose chains to improve their selectivity against gastrointestinal cancers, while [PERSON] et al. modified the structure of the original chlorin molecule to increase its water solubility [2; 8].