The absorption spectra of pyrene and OTPh overlap. The absorption spectrum for OTPh/NaDC/pyrene corresponds to the sum of the two individual spectra of pyrene and OTPh, as the subtraction of the OTPh/NaDC spectrum from the OTPh/NaDC/pyrene spectrum is the same as for the NaDC/pyrene spectrum (Figure ). At an excitation wavelength of 335 nm, pyrene is mostly excited and the emission intensity for the NaDC/pyrene gel is much higher than the emission for the OTPh/NaDC gel (Figure ), reflecting the large difference in the molar absorption coefficients at this excitation wavelength and in the singlet excited state lifetimes for OTPh and pyrene. The pyrene emission intensity of the OTPh/NaDC/pyrene gel is slightly lower (ca. 8%) than that of the NaDC/pyrene gel. When the samples were excited at 280 nm, the absorption of OTPh was higher than for pyrene, the OTPh emission was observed at wavelengths shorter than 360 nm (inset Figure ), and the pyrene emission for the gel containing OTPh is ca. 12% lower than in the absence of OTPh. The pyrene emission intensity of the gel containing OTPh should have been higher than for the NaDC/pyrene gel if energy transfer would have occurred from excited OTPh to ground state pyrene. The lack of energy transfer indicates that OTPh and pyrene can be considered as independent chromophores in the gel. The small decrease in the pyrene fluorescence intensities in the presence of OTPh is likely a consequence of a small decrease in the number of photons absorbed by pyrene due to the absorption by OTPh at both excitation wavelengths of 335 and 280 nm (0.07-0.12). A significant decrease of the pyrene emission would have been observed if OTPh had quenched the singlet excited state of pyrene. The fact that pyrene and OTPh are in separate environments makes it possible to use the fluorescence of pyrene as a probe to interrogate if the structure of the primary aggregates in NaDC gels is significantly changed at the molecular level when OTPh was added to the gel. The intensity ratio for the pyrene emission peaks close to 371 nm (I) and 383 nm (III) is sensitive to the polarity of the environment around pyrene. This I/III ratio is high in water (1.92 ± 0.01 average two independent experiments, Figure ) and low (0.5-0.6) in non-polar solvents. The I/III ratio was determined to be 0.77 ± 0.01 for pyrene bound to NaDC in solution or in the hydrogel. This value is similar to those observed previously for pyrene in NaDC solutions and gels (0.7-0.8). The I/III ratio for the emission of pyrene did not change when OTPh was added to NaDC in solution or in the gel (Table ). In addition, no excimer emission was observed for solution samples. The ratio of the emission intensities at 470 and 380 nm, where the excimer and monomer predominantly emit, was very low and this ratio was the same in the presence or absence of OTPh (Table ). In the case of the gels, the values for this intensity ratio are higher than the ratio in solution, suggesting that a small amount of excimer could be present in these samples.