Overall, the method could be applied to a large scope of peptides and Cz(I)PN derivatives. Nevertheless, the moderate incorporation efficiency for longer peptide sequences, the need for excess amounts of peptides, and the generation of 1:1 diastereomeric mixtures in most cases can be a drawback for certain applications. Inspired by the reported synthesis of 4CzIPN derivatives via an SNAr reaction of different carbazoles on 3CzFIPN (5), we attempted to use Cys as a nucleophile in this transformation (Figure ). To our delight, the SNAr reaction could be achieved in an open flask with DMSO and a Tris buffer as reaction solvents (see Table in the Supporting Information for reaction optimization). From the amino acid cysteine to pentameric peptides, the introduction of 3CzIPN occurred in 67-88% yield with exclusive formation of the desired products 6aa-6ea. Cys within a cationic cell penetrating peptide TAT can also be selectively modified, affording 6fa. Additionally, 3CzIPN was successfully introduced onto a mitochondria localization sequence (MLS) and a nucleus localization sequence (NLS) to give 6ga and 6ha. In the case of cyclic peptide 5l, a potent v3 integrin binder, Cys at the C-terminus was selectively modified leading to 6ia in 65% yield. Furthermore, Substance P yielded peptide conjugate 6ka. Two -helical peptides binding to the protein -catenin and MDM2 gave conjugates 6na and 6ma, respectively. With various Cz(I)PN-peptide conjugates in hand, we determined their photophysical properties to evaluate their potential as biocompatible photocatalysts. The absorption spectra feature several bands located below 400 nm, which are attributed to the donor-acceptor * transition of the carbazole-cyanobenzene. An additional band extending to 450 nm is observed for all 3/4 carbazolyl conjugates, in DMSO or in water, matching with the excitation spectra (Figure and Figure in Supporting Information). In DMSO, the excitation spectra of 4CzlPN and its peptide conjugates show three distinct peaks at 290, 318, and 330 nm, along with a broad band extending up to 350 nm. The spectra of 8Cl-4CzlPN and its conjugates are blue-shifted by approximately 1150-1450 cm⁻¹ (Figure ). Additionally, the emission maximum of 8Cl-4CzlPN is blue-shifted by around 334 cm⁻¹ compared to 4CzlPN. The emission of 6ca is only slightly shifted (368 cm⁻¹) compared to its parent dye 4CzlPN, while 3ya shows a more pronounced blue shift of 1884 cm⁻¹. Similarly, the shift observed between 3yb and 8Cl-4CzlPN is 1833 cm⁻¹. A slight red shift was noted for the 3yb and 6ca conjugates when the solvent was changed from DMSO to water (Figure in Supporting Information), likely due to intramolecular charge transfer (ICT) influenced by the hydrophobic Cz(I)PN cores. Water, known to quench luminescence due to competition with non-radiative deactivation pathways, caused a decrease in both emission and excitation intensities, although they remained relatively strong. The lifetimes of the different compounds were studied at 330 nm and 380 nm to broaden applicability for biological studies. for exact values.