Photopolymerization is nowadays used in a number of technologies and its application scope is constantly expanding. In particular, UV curing is a very popular technique used for the assembly of a variety of products and materials. However, UV light has low penetration depth, high energy consumption, and potential to damage materials from overexposure. Carrying out photopolymerization using safer and lower energy (i.e. visible light) photons has significant advantages as visible light has greater penetration power which is more suitable for highly pigmented polymers. Many organic and inorganic compounds are also sensitive to UV irradiation, and thus curing with visible light avoids the occurrence of unwanted side reactions arising from degradation of the materials. The development of efficient visible light driven photoinitiating systems is the focus of intense research effort. Such systems are usually based on a combination of organic dyes known as photosensitizers (PSs) and various additives (e.g., iodonium salt, amines, silane, etc.) which act as co-initiators. A photosensitizer component undergoes intersystem crossing (ISC) and forms longliving triplet excited states which are able to initiate chemical transformation leading to free radical formation and thus cause polymerization. ISC is a spin-forbidden process and so chemical structures of potential PSs must be rationally designed to enhance triplet state yields (ΦT) and lifetimes (τT). ISC can be promoted in organic dyes by the introduction of heavy atoms, such as iodine and bromine or transition metals which are known to enhance spin-orbit coupling. However, this approach has significant drawbacks such as higher costs and unwanted side effects, including negative environmental impact, increased toxicity and shortened triplet-state lifetimes. Spin-orbit charge transfer intersystem crossing (SOCT-ISC) represents an attractive alternative approach for generating long living triplet excited states in heavy-atom-free molecules. This ISC mechanism has attracted much attention in recent years and corresponding dyes found some unique niches in applications such as photodynamic therapy (PDT) 8 and triplet-triplet annihilation upconversion (TTA-UC). Boron dipyrromethene (BODIPY) dyes are so-far the most well-studied class of compounds undergoing SOCT-ISC. Due to their excellent spectroscopic characteristics and convenient synthesis, introducing substituents in almost any position in the core structure is possible thus allowing to tune molecular geometries and excited state energies, and as a result the SOCT-ISC in BODIPYs can be effectively optimized. Heavy-atom-free BODIPY photosensitizers were recently demonstrated by us as efficient photoinitiators for free-radical polymerization of acrylate monomers. Notably these PSs showed comparable or even higher photopolymerization rates than the reference heavy-atom-containing diiodo-substituted BODIPY which forms triplet excited states due to enhanced spin-orbit coupling. However, developing BODIPY-based photoinitiating systems that efficiently initiating the free radical polymerization for a wide range of monomers under mild irradiation conditions (low light intensity and under air) is still a challenge. One of the complications is a strong dependence of triplet state yields on solvent polarity, which is a known characteristic of SOCT-ISC. Recently Uddin et al. demonstrated this by using several BODIPY donor-acceptor dyads as photoinitiators for the polymerization of monomers of varying polarity. A general decrease in polymerization rate was observed as monomer polarity increased, however the magnitude of the polarity effect varied between dyes. This was attributed to differences in the CT state stabilization pathways of the BODIPYs. Dyes capable of undergoing efficient SOCT-ISC in low polarity solvents are beneficial, because monomers and formulations used for polymerization are relatively non-polar. The polarity of the matrix also decreases as polymerization occurs due to the conversion of more polar double bonds into nonpolar single bonds. Efficient SOCT-ISC in low and moderate polarity solvents can be achieved by introducing electron-accepting substituents in the dipyrromethene core. 11 However, this often leads to a decrease in triplet state yields due to enhancement of ground state recombination rate (CT→S0), due to stabilization of the CT state by electron-withdrawing groups and narrowing CT-S0 energy gap (Figure ). It is also difficult to design PSs efficiently operating in a broad range of polarities or predict the triplet state yield for different solvents based on the structure. Although recent efforts have shown that it is possible to accurately predict singlet oxygen generation quantum yields (Φ) for BODIPYs by using quantitative structure-property relationship (QSPR) and machine-learning techniques, such computational methods are still being optimized. In this work we investigated BODIPYs containing electronwithdrawing ethoxycarbonyl substituents in positions 2 and 6, which promote electron transfer and allow for efficient SOCT-ISC in both polar and non-polar solvents, such as toluene. Different electron donor substituents, such as anthracenyl, pyrenyl and methoxyphenyl groups, were employed in the meso position to modulate the donor-acceptor capability of the BODIPYs and in turn optimize formation of long-living triplet excited states via SOCT-ISC. Highly efficient charge transfer and triplet states formation in non-polar solvent was confirmed by short and long delay transient absorption measurements and singlet oxygen ( 1 O2) quantum yield (ΦΔ) measurements which showed values of up to 94% in nonpolar solvent (toluene). Further, the photosensitizing ability of diethoxycarbonyl-BODIPY dyes was evaluated by employing them as photoinitiators in free-radical polymerization of acrylamide monomers and recording of holographic structures. The photopolymer used in this work consists of a cellulose acetate-polyethylene glycol (CA-PEG, Figure ) matrix, containing acrylamide as a monomer and N,N-methylenebisacrylamide as a cross-linker which provides increased stability of the hologram in water. Diethoxycarbonyl-BODIPYs were employed as photosensitizers in a two-component photoinitiating system containing N-phenylglycine (NPG) as a co-initiator. NPG was chosen as a co-initiator as its efficiency for the free-radical polymerization has been demonstrated in both a photosensitive resin for 3D printing and holographic recording materials. NPG was shown to react with triplet excited states of photosensitizers, undergoing electron transfer and forming corresponding radical cation (NPG• + ) which further undergoes deprotonation/decarboxylation to give anilinomethyl radical (PhNHCH2•) that initiates polymerization of methacrylate and acrylamide monomers (Figure ). Cellulose acetate was selected as a binder for the following key reasons: 1) the layers produced in this binder are not sensitive to humidity and the optical devices recorded in the layers have unchanged diffraction efficiency after being immersed in water ; 2) the obtained solid layers are of high optical quality and very low scattering ; 3) it is possible to control the porosity of the binder by varying the PEG concentration, which is needed for the development of holographic sensors. Holographic recording in photopolymers uses the wave interference of light waves to create the refractive index modulation and, as a result, form a diffraction grating. In this work, transmission gratings with the spatial frequency of 800 lines/mm were recorded in the photopolymer layers sensitized with BODIPYs. Performance of these new photosensitizers was studied through real-time recording of diffraction efficiency growth, which was compared to the reference heavy-atomcontaining dye, meso-phenyl-2,6-diiodoBODIPY (BDPI2). The obtained results prove that ethoxycarbonyl BODIPYs are efficient photosensitizers for polymerization of acrylate monomers in the CA-PEG matrix. The resulting photopolymers displayed higher exposure sensitivity achieving diffraction efficiency of up to 56%, compared to 27% for BDPI2. These results expand the arsenal of sustainable, environment-friendly, stable and highly efficient photopolymerizable materials for holographic recording under green light excitation.