Poly(2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[def]phenanthrene)) (PCPP) is a stable blue light emitting conjugated polymer even after annealing at 150 degrees C or operation of the device in air. The spiro form of PCPP, poly(2,6-(3',6'-bis(2-ethylhexyloxy)-4,4'-spirobi(4H-cyclopenta[def]phen anthrene))) (spiro-PUP), has been synthesized by Yamamoto polymerization. The PL emission spectrum of the polymer in THF solution shows a maximum peak at 398 nm, nearly identical with that of PCPP. The PL spectrum of the polymer in the solid state exhibits a maximum peak at 451 nm, which is red-shifted over 50 nm from that of the solution, due to the inter-chain interaction of the polymers. The PL spectra of spiro-PCPP in the mixture of THF and MeOH from 9:1 to 3:7 ratios showed increasing peaks at 458 and 484 nm. With an increased ratio of the hydrophilic solvent (MeOH), the inter-chain interaction of the hydrophobic polymers was enhanced to show peaks at 458 and 484 nm. This phenomenon has the same tendency as compared to the PL spectrum in solid thin film state. The polymer LED with the configuration of ITO/PEDOT/spiro-PCPP/Ca:Al emitted light with maximum peaks at around 463 nm. The emission color of the spiro-PCPP device is sky blue with the CIE coordinates of (0.14, 0.15), which are close to those of the standard blue (0.14, 0.08). Crown Copyright (c) 2008 Published by Elsevier Ltd. All rights reserved.