A new family of cyclometalated ruthenium(II) complexes [Ru(N boolean AND N)(2)(C boolean AND N)](+) derived from the pi-extended benzo[h]imidazo[4,51]quinolone ligand appended with thienyl groups (n = 1-4, compounds 1-4) was prepared and its members were characterized for their chemical, photophysical, and photobiological properties. The lipophilicities of 1-4, determined as octanol water partition coefficients (log P-o/w), were positive and increased with the number of thienyl units. The absorption and emission bands of the C boolean AND N compounds were red-shifted by up to 200 nm relative to the analogous Ru(II) diimine systems. All of the complexes exhibited dual emission with the intraligand fluorescence ((IL)-I-1, C boolean AND N -based) shifting to lower energies with increasing n and the metal-to-ligand charge transfer phosphorescence ((MLCT)-M-3, N boolean AND N-based) remaining unchanged. Compounds 1-3 exhibited excited state absorption (ESA) profiles consistent with lowest-lying (MLCT)-M-3 states when probed by nanosecond transient absorption (TA) spectroscopy with 532 nm excitation and had contributions from (IL)-I-1(C boolean AND N) states with 355 nm excitation. These assignments were supported by the lifetimes observed (< 10 ns for the (IL)-I-1 states and around 20 ns for the (MLCT)-M-3 states) as well as a noticeable ESA for 3 with 355 nm excitation that did not occur with 532 nm excitation. Compound 4 was the only member of the family with two (MLCT)-M-3 emissive lifetimes (15, 110 ns), and the TA spectra collected with both 355 and 532 nm excitation was assigned to the (IL)-I-3 state, which was corroborated by its 4-6 mu s lifetime. The ESA for 4 had a rise time of approximately 10 ns and an initial decay of 110 ns, which suggests a possible (MLCT)-M-3-(IL)-I-3 excited state equilibrium that results in delayed emission from the (MLCT)-M-3 state. Compound 4 was nontoxic toward human skin melanoma cells (SKMEL28) in the dark (EC50 = > 300 mu M); 1-3 were cytotoxic and yielded EC50 values between 1 and 20 mu M. The photocytotoxicites with visible light ranged from 87 nM with a phototherapeutic index (PI) of 13 for 1 to approximately 1 mu M (PI = > 267) for 4. With red light, EC50 values varied from 270 nM (PI = 21) for 3 to 12 mu M for 4 (PI = > 25). The larger PIs for 4, especially with visible light, were attributed to the much lower dark cytotoxicity for this compound. Because the dark cytotoxicity contributes substantially to the observed photocytotoxicity for 1-3, it was not possible to assess whether the (IL)-I-3 state of 4 led to a much more potent phototoxic mechanism in the absence of dark toxicity. There was no stark contrast in cellular uptake and accumulation by laser scanning confocal and differential interference contrast microscopy to explain the large differences in dark toxicities between 1-3 and 4. Nevertheless, the study highlights a new family of Ru(II) C boolean AND N complexes where pi-conjugation beyond a certain point results in low dark cytotoxicity with high photocytotoxicity, opposing the notion that cyclometalated Ru(II) systems are too toxic to be phototherapeutic agents.