The triplet-triplet absorption spectra of three newly synthesized N-substituted 4,5,6,7-tetrachlorophthalimides (TCP) were measured experimentally and calculated with density functional theory. The heavy atom effect increases the intersystem crossing rate, and the transient triplet absorbance could be measured. Fluorescence emission was not observed. The transient absorption spectra show two peaks in the region of 385-410 nm and 770-830 nm having a red shift with increasing size of N substituent. The singlet and triplet states and their vertical transitions were investigated theoretically by NEVPT2/CASSCF(12,9) and CAM-B3LYP methods for a benchmark molecule N-Phenyl-maleimide, and by CAM-B3LYP for N-Phenyl-TCP. It is shown that the singlet excited S1 state and the pi-pi* triplet state have similar geometries, thus intersystem crossing is most likely for N-Phenyl-TCP. No crossing between the singlet ground state and the singlet excited states could be found on the linear synchronous transit paths of S0-S1-T1-S0 states using CAM-B3LYP method. The computed singlet ground state absorption and triple-triplet absorption spectra agree well with the experimental ones, in both spectra the first state is a pi-pi* state while the second state is the n-pi* state with an energetic difference of 0.1 eV.