Photomodulated PPV emission in a photochromic polymer film

We present studies of fluorescence intensity modulation in neat films of a photochromic poly(p-phenylenevinylene) (PPV) derivative. Poly(2-hexyloxy-5-((10-(4-(phenylazo)phenoxy)decyl)oxy)-1,4-phenylenevinylene) (HPA-10-PPV), a PPV derivative functionalized with photoaddressable azobenzene side chains, is known to undergo modulated changes in fluorescence intensity upon azobenzene isomerization in solution. These changes are caused by nonradiative transfer of excitation energy from the PPV backbone to the azobenzene moiety. The efficiency of energy transfer is higher for the cis azobenzene isomer than for trans, a difference that enables fluorescence intensity modulation. Upon film formation, the fluorescence spectra of neat HPA-10-PPV films undergo a red shift of ca. 30 nm compared to those measured in dilute solution. Application of ultraviolet irradiation to induce trans -> cis azobenzene isomerization yields a reduction in fluorescence intensity in HPA-10-PPV films that is reversed upon azobenzene cis -> trans back isomerization. These reversible intensity changes can be cycled many times in the film with a modulation efficiency that is virtually identical to that measured in dilute solution. The isomeric state of the azobenzene side chains at the time of film casting has a negligible impact on film fluorescence properties and subsequent isomerization behavior in the film. This result implies that the azobenzene has ample free volume for isomerization in the film and that its isomeric state does not substantially impact film morphology in a way that affects the fluorescence. Spatial control of fluorescence intensity in HPA-10-PPV films is demonstrated by inscribing an image with increased fluorescence intensity on a lower intensity background. The successful photomodulation of fluorescence intensity in polymer films further demonstrates the potential for photochromic conjugated polymers as a class to find applications in optical data storage.