The results correlate with singlet oxygen generationboth series 4 and parent compounds 1 and 2 have high ΦΔ and, as a consequence, good antiviral activity. In contrast, compounds from the series 3 produced very low amounts of singlet oxygen and, accordingly, are not active against viruses. The exception is compound 4a, which displayed low antiviral activity. This effect might be due to its specific structure, as 3a also demonstrated the weakest antiviral properties in series 3. The presence of two strong acceptor polar groups may negatively impact the dye's incorpotation into the viral membrane, reducing its efficacy.