## 1 Introduction  
Since squaric acid (3,4-dihydroxy-1,2-dioxocyclobut-3-ene) was first reported in 1959 by [PERSON] et al. [1], and later by other studies, as a rigid structure [2] with high chemical reactivity [3], its condensation with electron-rich precursors has been extensively studied [4; 5; 6] aiming to find products with potential applicability in several technological and biomedical areas [7; 8; 9]. Among the squaric acid derivatives, it is essential to emphasize the squaraine cyanine dyes, a core of functional dyes, first reported by [PERSON] and [PERSON] [10], known for its good photochemical stability, conductivity, sharp and intense Vis/NIR absorption and high absorption coefficients [8; 11; 12]. These photochemical and photophysical properties encourage the study of the suitability of these core of dyes to their application as photoconductive and nonlinear optical materials [13; 14], sensitizers for organic solar cells [15; 16], fluorescent probes for detection of functional groups [17], toxic substances [18; 19; 20] or biomolecules [21; 22; 23], and as photodynamic therapy (PDT) photosensizers [9; 24; 25].  
Cancer has become one of the biggest health problems and leading causes of death worldwide as a result of a combination of several factors, such as genetic predisposition, exposure to environmental contaminants and incorrect diet [26; 27]. Since conventional treatments reveal multiple side effects, such as being extremely debilitating and poorly effective in treating some types of tumors [28], substantial progress is being made in the areas of oncology, cellular and molecular biology and medicinal chemistry to improve chemotherapy and other treatments applied to cancer patients [29].  
PDT is a minimally invasive therapeutic strategy with reduced side effects that involve the death of target tissue cells by the combined action of adequate light, a photoactive molecule and molecular oxygen, which results in the production of reactive oxygen species (ROS) that promote cell structural and functional failure [30; 31]. This therapeutic modality has progressed over the last few years [32; 33], not only to study its potential use for the treatment of cancer, but also for dermatological [34] and ophthalmic conditions [35], such as psoriasis [36; 37] and age-related macular degeneration [38], respectively. Although some PDT photosensizers are already commercially available, mostly derived from porphyrins, these light-sensitive molecules have the disadvantage of being slowly eliminated from the body, causing skin photosensitivity [8; 39]. Thus, several research studies have recently reported the discovery of new compounds, such as cyanines [40], squaraines [24], chlorins [41] or phthalocyanines [42] which, by appropriate structural modifications, can ideally override the gaps of first-generation photosensizers [43].  
In this work, we report the synthesis and characterization of iodoquinoline- and benzothiazole-based unsymmetrical squaraine cyanine dyes with \(N\)-hexyl chains and the evaluation of their properties such as photostability and singlet oxygen production ability. The in vitro phototherapeutic effects of these squaraine-based photosensizers has been evaluated on human hepatocellular carcinoma (HepG2) and human colorectal adenocarcinoma (Caco-2) cell lines by analyzing the variation of cell viability after exposure to various irradiation and incubation conditions with the synthesized dyes. This study also aimed to understand the role of the introduction of iodine atom on photophysicochemical and photobiological properties, by comparing the results obtained in this study with those recently described by us [25].