Fluorescence behavior of cis-methyl orange stabilized in cationic premicelles

•Fluorescence of cis-MO stabilized in cationic premicelles has been reported.•cis-MO is stabilized in cationic premicelles to minimize conformational strain.•S1 fluorescence dominates over S2 in MO unlike that of usual azobenzenes.•The MOcis · EA is stable over the MOtrans · EA complex by ≈2.94 kJ mole−1.•The fluorescence of cis MO originated through initial absorption around 375 nm.

The cis-isomer of methyl orange (MO), stabilized in cationic premicelles, has been found to be fluorescence active when excited at wavelength ⩽270 nm. An intense fluorescence band with maximum at 575 nm along with a broad moderate intensity band in the range of 370–530 nm and a low intensity band at 361 nm have been observed. The major band at 575 nm has been attributed to S1 → S0 (n–π*) fluorescence unlike the other azobenzenes where the S2 → S0 (π–π*) fluorescence is usually reported. UV–Vis spectral and surface tension study indicate that the dye exists in the trans form in dye-surfactant ionpair (DSIP) at very dilute concentrations of the surfactant. But the polar cis-isomer is stabilized by micellization of the DSIPs as the concentration of the surfactant is increased. The fluorescence and hence the cis-isomer again disappear when normal micelles are formed above the normal CMC of the surfactant. It has been suggested that the symmetry forbidden S1 → S0 (n–π*) transition of MO becomes allowed due to the formation of the twisted cis-form. TD-DFT calculations have been used as an auxiliary tool to identify the possible structures and electronic transitions responsible for the specific absorption and fluorescence properties of MO observed in presence of premicellar cationic surfactants.

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