Two-photon pumped lasing stilbene-type chromophores containing various terminal donor groups: relationship between lasing efficiency and intramolecular charge transfer

A series of new two-photon pumped (TPP) up-conversion lasing dyes, named as CSPI, DPASPI, PSPI, DEASPI and HEASPI respectively, have been synthesized. These dyes are stilbene-type chromophores end-capped with the same accepto r group but with varied donor groups, whose different electron-donating abi lities can be ordered by H-1 NMR chemical shift data. PM3 calculations show that attaching different donors changes the bond length alternation (BLA) and charge density distribution of the styrylpyridine skeleton. These chrom ophores exhibit different two-photon up-conversion emission behavior. Pumpe d by a 1064 nm, similar to 40 ps mode-locked Nd:YAG laser pulses, all chrom ophores showed two-photon pumped (TPP) up-conversion laser emission, but th e efficiencies showed a difference of about two orders of magnitude, rangin g from an up-conversion efficiency of 10.7% for DEASPI to 0.13% for CSPI un der the same experimental conditions. The results show that the efficiency of two-photon pumped emission is related to the intramolecular charge trans fer character of the chromophore. The two-photon pumped lasing efficiency i ncreases both with decreasing BLA and with increasing intramolecular charge transfer character, measured by the parameter Delta rho (1,2).