Current research in the Harbron lab focuses on the synthesis and spectroscopic study of fluorescent polymers functionalized with photochromic sidechains. Photochromic molecules have two forms that can be interconverted by irradiating with different colors of light (often UV light for one form and blue or green light for the other). The two forms of a photochromic molecule have different absorption spectra and may also have very different properties (e.g. polarity, shape). The family of polymers we currently study has a conjugated backbone based on poly(phenylene vinylene) (aka PPV) and a photochromic sidechain based on azobenzene. Azobenzene’s photochromism is based on isomerization: the thermally stable trans form photoisomerizes to cis upon UV irradiation and back-isomerization to trans upon blue irradiation.

PPV derivatives are highly fluorescent and are being commercially developed into organic light-emitting diodes for display applications such as cell phones. Our goal is to control PPV fluorescence properties (e.g. intensity, color) by controlling the form of the photochromic sidechain with light.

 

Our initial investigations focused on fluorescence intensity. In 2004, we determined that the polymer fluorescence is much dimmer when the azobenzenes are in the cis form than when they are trans. The intensity changes are reversible, and we can cycle the fluorescence “off” and “on” many times. Figure A below shows the fluorescence spectrum of polymer MPA-10-PPV before irradiation (red), after UV irradiation to isomerizes the azobenzenes to cis (blue), and after blue irradiation to return the azobenzenes to trans (green). The blue line in Figure B shows the reversible changes in intensity over many cycles of UV and blue irradiation. We determined that these intensity changes are due to non-radiative energy transfer from the polymer backbone to the azobenzene sidechains. The energy transfer is more efficient when the azobenzenes are cis, so we are able to modulate the intensity of the polymer backbone by controlling the form of the sidechain with light.¹

We have also investigated reversible fluorescence color changes in these polymers. As shown in the figure above, a very subtle shift in the position of the fluorescence spectrum occurs upon transàcis isomerization (see normalized spectra in the inset to Figure A). We hypothesized that these shifts might be due to conformational changes in the PPV backbone because previous work by other investigators had demonstrated that inducing conformational change by changing the solvent yields similar changes in spectral position. We wanted a) to investigate this hypothesis and b) to determine whether we could find conditions under which the shifts in position could be made larger to the point of becoming visible fluorescence color changes. To accomplish these goals, we studied several polymer derivatives in co-solvent mixtures in which solvents that are known to induce polymer coiling were gradually introduced. When the polymer is on the verge of collapsing due to the co-solvent composition, large color changes are observed upon transàcis isomerization. The figure at left shows the polymer HPA-10-PPV in a 65:35 mixture of dichloromethane and methanol. The fluorescence spectra were recorded before irradiation, when the azobenzenes are trans (red), after UV irradiation when they are cis (blue), and after blue irradiation when they are again trans (green). The change in fluorescence spectral shape and position is dramatic and completely reversible over many cycles. In this example, the changes are large enough to be visible to the naked eye as a color change. The figure below shows two samples of the same solution. In A, the one on the left was not irradiated while the one on the right was irradiated with 30 s of UV light. Both samples were then irradiated with blue light. The color change goes from yellow-orange to green and back again and does not occur in the control sample.²

We are continuing our investigation of energy transfer processes in azobenzene-PPVs in collaboration with Prof. Doug English’s group at the University of Maryland, College Park. Fluorescence decays for several azobenzene polymers were measured using time-correlated single photon counting. The figure below shows the fluorescence decays of control polymer DM-10-PPV (black line), which lacks the azobenzene side chain, and MPA-10-PPV in its trans state (re line) and at the photostationary state concentration of cis azobenzenes (blue line). Comparison of the DM-10-PPV and trans MPA-10-PPV decays demonstrates that a high degree of quenching is observed upon addition of the trans azobenzene to the side chain. The quenching is further increased upon isomerization of the azobenzene side chains to cis. Analysis of the steady-state and time-resolved integrated intensities reveals several key features of this system: 1) Quenching of the PPV backbone by trans azobenzene involves both dynamic quenching and static quenching mechanisms. Static quenching by transazobenzene is facilitated by the formation of an essentially nonfluorescent transazobenzene-PPV complex that is likely the result of stacking interactions. 2) Quenching by cis azobenzenes occurs exclusively by a dynamic quenching mechanism with a higher efficiency than dynamic quenching by the trans isomer. 3) Dynamic quenching by both trans and cisazobenzenes is consistent with a FRET mechanism: the difference in trans and cis energy transfer efficiencies is accounted for by the different Förster radii of the PPV-trans azobenzene (15.0 Å) and PPV-cis azobenzene (18.6 Å) pairs. We used these results to improve the efficiency of fluorescence intensity modulation. Addition of a tert-butyl group to each azobenzene side chain substantially increases trans-state brightness by blocking undesirable static quenching while cis tert-butyl azobenzene side chains retain the powerful quenching efficiency of ordinary cisazobenzene.³