Oxidative Polymerization of a Carbazole-Substituted Perfluorinated trans-A2B2 Porphyrin as a Potential Photodynamic Agent ^†

Abstract

Porphyrin-based photosensitizers have attracted considerable attention for photodynamic applications due to their ability to generate reactive oxygen species upon light irradiation. However, the development of stable and efficient photosensitizing systems remains an important challenge. In this work, a conjugated porphyrin polymer was developed through the oxidative polymerization of carbazolyl substituents incorporated into a 5,15-bis [4-(9-carbazolyl)phenyl]-10,20-bis(pentafluorophenyl)porphyrin (TPPF₁₀CBZ). The resulting polymer, P-TPPF₁₀CBZ, was characterized by spectroscopic and photophysical techniques to evaluate its structural and optical properties. Its ability to generate singlet oxygen and its photostability were also investigated, providing insights into its potential as a robust porphyrin-based photosensitizer for future photodynamic applications.

1. Introduction

Porphyrin-based photosensitizers (PSs) have attracted considerable attention in photodynamic inactivation (PDI) of pathogenic microorganisms owing to their intense absorption in the visible region, high singlet oxygen (O₂(¹Δ_g)) quantum yields, and structural versatility [1,2]. However, monomeric porphyrins often suffer from aggregation, photobleaching, and limited processability in solid or supported formats, which restricts their use in practical antimicrobial materials. Conjugated porphyrin polymers have emerged as an attractive strategy to overcome these limitations, since polymerization can preserve the intrinsic photophysical properties of the porphyrin macrocycle while improving structural stability, photostability, and potential for device integration [3,4].

Among the building blocks available for oxidative polymerization, carbazole-based units are particularly attractive due to their high redox stability, strong UV absorption, and efficient charge-transport properties, which have made them widely used in optoelectronic devices such as transistors, light-emitting diodes, biosensors, and photovoltaic systems [5]. Iron(III) chloride (FeCl₃) is commonly employed as an efficient oxidizing agent for the polymerization of carbazole derivatives, generating conjugated networks through oxidative C–C or C–N coupling between carbazolyl units [6]. Beyond their optoelectronic relevance, carbazole derivatives also display valuable biological properties, including antimicrobial, antitumoral, antioxidant, and anti-inflammatory activity [7,8].

The present work describes the synthesis and photophysical/photodynamic characterization of a new conjugated porphyrin polymer, P-TPPF₁₀CBZ, obtained through the oxidative polymerization of the carbazolyl substituents of 5,15-di(4-(9-carbazolylphenyl))-10,20-di(pentafluorophenyl)porphyrin (TPPF₁₀CBZ) using FeCl₃ as the oxidizing agent. The polymer was characterized by UV–visible absorption and steady-state and time-resolved fluorescence spectroscopy, and its ability to generate singlet oxygen and to withstand photobleaching was evaluated, to assess its potential as a robust porphyrin-based photosensitizer for PDI applications.

2. Materials and Methods

2.1. Equipment and Chemical Agents

All reagents were obtained from Sigma-Aldrich (Milwaukee, WI, USA) and used without further purification. Thin-layer chromatography (TLC) plates coated with silica gel (250 μm) were purchased from Analtech (Newark, DE, USA). Absorption spectra were carried out on a Shimadzu UV-2401PC spectrometer (Shimadzu Corporation, Tokyo, Ja-pan). Time-resolved experiments and the determination of fluorescence lifetimes (τ) were performed using a FluoTime 300 fluorescence spectrophotometer (PicoQuant, Berlin, Germany).

2.2. Synthesis

5,15-di(4-(9-carbazolylphenyl))-10,20-di(pentafluorophenyl)porphyrin (TPPF₁₀CBZ) was synthesized following a previously described methodology [9]. P-TPPF₁₀CBZ was synthesized by oxidative polymerization of TPPF₁₀CBZ with FeCl₃ using anhydrous chloroform (CHCl₃) as the solvent at room temperature under an Ar atmosphere for 8 h [6]. The progress of the reaction was monitored by TLC (silica gel, dichloromethane (DCM)/triethylamine (TEA) 99:1) until the complete disappearance of TPPF₁₀CBZ was observed. The polymer was purified by washing with methanol (MeOH) and subsequent centrifugation, yielding the final product P-TPPF₁₀CBZ [4].

2.3. Spectroscopic Studies

Absorption and fluorescence spectra were performed in a quartz cell of 1 cm path length using N,N-dimethylformamide (DMF) at room temperature. The fluorescence quantum yields (Φ_F) of the conjugates were calculated by comparison of the area below the corrected emission spectra with that of the reference [10]. Fluorescence lifetimes (τ) were acquired in TCSPC mode using a PicoHarp 300 module (minimum time resolution of 4 ps), and data were analyzed with EasyTau 2 software, considering the instrument response function (IRF) of a diluted Ludox scattering solution [11]. Photooxidation of 9,10-dime-thylanthracene (DMA) was used to determine singlet molecular oxygen production (Φ_Δ) in DMF [12].

3. Results and Discussion

3.1. Synthesis of P-TPPF₁₀CBZ

P-TPPF₁₀CBZ was synthesized by oxidative polymerization of the carbazolyl substituents of TPPF₁₀CBZ (Figure 1). The monomer was dissolved in anhydrous CHCl₃, and solid FeCl₃ was added in a 4:1 molar ratio relative to the porphyrin. The reaction was carried out at room temperature under an Ar atmosphere with constant stirring. Reaction progress was monitored by TLC (siliga cel, DCM/TEA 99:1), and completion of the coupling reaction was confirmed by the disappearance of the monomer spot. The solvent was removed under reduced pressure and the resulting solid was resuspended in MeOH. The polymer was isolated by centrifugation and the product was washed repeatedly with MeOH, affording P-TPPF₁₀CBZ in high yield. Oxidative coupling between carbazolyl units generated a conjugated polymeric network while preserving the integrity of the porphyrin macrocycles.

Figure 1. Synthesis and idealized chemical structure of P-TPPF₁₀CBZ.

3.2. Spectroscopic Properties

All the spectroscopic properties of the PSs are summarized in Table 1. The optical properties of P-TPPF₁₀CBZ were compared with those of the monomer TPPF₁₀CBZ using DMF as a solvent. The polymer displayed an intense Soret absorption band at 418 nm and four Q bands of lower energy between 500 and 700 nm (Figure 2A). These transitions are typical of meso-tetraphenyl free-base porphyrin derivatives with D₂h-symmetry [13]. Fluorescence emission spectra of TPPF₁₀CBZ, P-TPPF₁₀CBZ, and the reference porphyrin TPPF₂₀ are shown in Figure 2B. The polymer exhibited two emission bands centered at 645 and 710 nm, assigned to the Q_x(0-0) and Q_x(0-1) transitions characteristic of meso-substituted free-base porphyrins [10,12]. Both absorption and fluorescence emission spectra closely resembled that of the monomer, indicating that the spectroscopic identity of the porphyrin units is conserved within the polymer even though they form part of a highly conjugated system. Furthermore, P-TPPF₁₀CBZ showed a Φ_F slightly lower than that of its constitutive monomer (Table 1) in agreement with values reported for structurally related tetrapyrrolic macrocycles [14]. Finally, time-resolved fluorescence measurements demonstrated that TPPF₁₀CBZ displayed a monoexponential decay for both emission bands (τ ≈ 10.5–10.6 ns), typical of rigid meso-tetraarylporphyrins with negligible aggregation in DMF [15]. In contrast, the decay profiles of P-TPPF₁₀CBZ were best fitted by a biexponential function, with a major long-lived component (τ₁ ≈ 10.55–10.58 ns) essentially identical to the monomer lifetime, and a minor short-lived component (τ₂ ≈ 1.3–1.6 ns).

Table 1. Spectroscopic and photodynamic properties of TPPF₁₀CBZ and P-TPPF₁₀CBZ in DMF.

PS	λmáxAbs (nm)	λmáxEm (nm)	ΦF a	ΦΔ b
TPPF10CBZ	416	644	0.050 ± 0.005	0.93 ± 0.05
P-TPPF10CBZ	418	645	0.040 ± 0.004	0.87 ± 0.04

^a Fluorescence quantum yield. ^b Singlet molecular oxygen quantum yield.

Figure 2. (A) UV-visible absorption spectra of TPPF₁₀CBZ (magenta solid line) and P-TPPF₁₀CBZ (teal solid line) and (B) fluorescence emission spectra (λ_exc = 416 nm) of TPPF₂₀ (orange solid line), TPPF₁₀CBZ (magenta solid line), and P-TPPF₁₀CBZ (teal solid line) in DMF.

3.3. O₂(¹Δ_g) Production

The ability of P-TPPF₁₀CBZ to photosensitize the generation of O₂(¹Δ_g) was evaluated in DMF by monitoring the photooxidation of 9,10-dimethylanthracene (DMA), followed spectrophotometrically as the decrease in probe absorbance at 378 nm under irradiation at 416 nm. Both TPPF₁₀CBZ and P-TPPF₁₀CBZ showed a high capacity to generate O₂(¹Δ_g) in DMF. The singlet oxygen quantum yield of P-TPPF₁₀CBZ (Φ_Δ = 0.87 ± 0.04) was comparable to that of the monomeric porphyrin (Φ_Δ = 0.93 ± 0.05), indicating that polymerization does not significantly compromise the photodynamic efficiency of the porphyrin units. These results support that polycarbazole-containing PSs are efficient O₂(¹Δ_g) generators and promising candidates for PDI applications [16].

4. Conclusions

A new conjugated porphyrin polymer, P-TPPF₁₀CBZ, was successfully synthesized by oxidative polymerization of the carbazolyl substituents of TPPF₁₀CBZ using FeCl₃ as the oxidizing agent. UV–visible and fluorescence spectroscopy confirmed that the porphyrin units retain their characteristic absorption and emission profiles after polymerization, with only a slight decrease in Φ_F. Photodynamic assays showed that P-TPPF₁₀CBZ efficiently generates O₂(¹Δ_g), comparable to the parent monomer. These results suggest that P-TPPF₁₀CBZ possesses strong potential as a PS for photodynamic inactivation of pathogenic microorganisms.