Temperature dependence of fluorescence quantum yields of some side-substituted anthracene sulphonates

Of the four anthracene sulphonates (Na-salt), anthracene-1-sulphonate (1-AS), anthracene-2-sulphonate (2-AS) and anthracene-1, 5-disulphonate (1, 5-AS) show temperature dependence of φ_f, whereas for anthracene-1, 8-disulphonate (1, 8-AS) φ_f is independent of temperature. E_a for 1-AS, 2-AS and 1, 5-AS are 939 cm^?1, 1144 cm^?1 and 445 cm^?1, respectively. A low energy T_nπ¹ level supplied by ?SO^?₃ group could be responsible for efficient ISC in 1, 8-AS of symmetry C_2v. The possibility of tunnelling due to molecular distortion i.e. “geometry factor” cannot be ruled out also.