Samuel W. Thomas, PhD, Tufts University
As described in our original proposal, we are interested in investigating the effect of covalently-bound fluorescence quenchers on the properties of conjugated oligomers and polymers. Our original hypothesis was that the fluorescence quenchers, through rapid deactivation of photoexcited states, would reduce the observed rate of photooxidative degradation of the conjugated systems. While we were working on the synthesis of systems similar to those proposed in our original proposal, we became aware of a study published in early 2012 that effectively performed a substantial number of the experiments we had proposed—correlating the properties of small-molecule electron acceptors and conjugated polymeric electron donors to the photochemical stability of their blended solid-state films.1 We have therefore focused on work on one specific aspect of the original proposal: photochemical modification of key properties of organic semiconductors using photolabile o-nitrobenzyl ester (NBE) substitutents.
Our previous work in this area, published before start of support by this PRF award, was focused on five-ring phenylene-ethynylene/phenylene-vinylene (PEPV) conjugated oligomers.2 Derivatives of these PEPV oligomers that had NBE groups attached to the conjugated backbone had low quantum yields of fluorescence (~ 0.1) in polar solvents such as acetonitrile because of photoinduced electron transfer quenching. UV-induced photolysis of the NBE group at 365 nm, however, broke the covalent bond between the electron-accepting nitroaromatic ring and the conjugated system, restoring the fluorescence to efficiencies (0.7) similar to those in control oligomers with no quencher. As the photoproduct was a carboxylic acid, the photolysis also yielded a change in solubility, with exposure of the photoproduct to aqueous base yielded precipitation of the deprotonated carboxylates. This work was the first example, to our knowledge, of organic semiconductors with phototunable properties using photolabile groups.
In an effort to extend this approach of photomodulated solubility to materials that could be easily processed in devices that use organic semiconductors, we examined oligo- and polythiophenes with photolabile solubilizing chains.3 Polythiophenes are more photochemically stable than materials with exocyclic main chain double bonds such as poly(phenylene-vinylene)s or the PEPV oligomers discussed previously. In this PRF-supported project, we altered the design used so that the only solubilizing alkyl chains on the oligomers or polymers were connected to the thiophene-based backbone through photolabile NBE linkers. For nearly all conjugated polymers, long alkyl substituents are necessary dissolve the polymers in organic solvents, and therefore make them processable by techniques such as spin coating or printing. These chains, however, pose a number of problems, such as occupying film volume with optoelectronically inert groups and participating in photochemical degradation processes.4 Our design, therefore, was for NBE photolysis to remove all solubilizing alkyl chains from the conjugated system, yielding oligomers and polymers with reduced solubility.
Similar to the behavior observed with PEPV oligomers, quaterthiophene conjugated oligomers with octyloxy chains attached to photolabile NBE groups showed an increase in fluorescence quantum yield upon UV photolysis. In contrast, however, polymeric analogs showed different showed different behavior, consistent with aggregation of polymers upon cleavage of the solubilizing chains. Upon UV photolysis the polymer absorbance red-shifted in solvents of low polarity, and the polymer fluorescence was strongly quenched. These observations are consistent with the aggregation of conjugated polymers, and demonstrate the efficacy of our approach to photomodulation of solubility of organic semiconductors with light.
In addition to our investigations of photoresponsive polythiophenes in solution, we irradiated thin films of this polymer that were spun-cast on glass slides. As shown in the TOC figure, upon irradiation at 365 nm for 30 minutes, the absorbance of the film decreased only 8–14% decrease upon rinsing with toluene. Areas not irradiated with UV light remained freely soluble in toluene. These results highlight the potential application of these materials in the construction of multilayer organic devices using primarily solution-based deposition techniques by rendering deposited layers insoluble by UV irradiation before deposition of additional layers. This approach would minimize more expensive and time-consuming vapor-phase deposition steps.
The impact of PRF funded research has had an important impact on the research progress, my career as a PI, and the two students involved. Because the primary author of the PRF-supported paper was supported through a research assistantship, he was able to focus entirely on executing his research progress. The result of this was second-year graduate student Zach Smith being first author on a paper in ACS Macro Letters. The high quality of the paper is highlighted by it being the top paper read in the journal in July 2012, as well as being highlighted by Synfacts. The publication of the paper enabled by PRF support and accompanying accolades has an obvious positive impact on my career, and enabled the student to advance past oral examinations and research progress meetings with his committee with relative ease because of the large body of work he had already accomplished. Finally, a chance observation while executing this research has opened up an entirely new project based on unexpected structural effects of non-conjugated side-chains on the optical properties of conjugated polymers. Finally, as part of the highly related research to the PRF-supported project, we demonstrated need for an X-Ray diffractometer , which in part led to funding of an NSF MRI grant for this instrument.
1. Tromholt, T.; Madsen, M. V.; Carle, J. E.; Helgesen, M.; Krebs, F. C. J Mater Chem 2012, 22, 7592-7601.
2. Pawle, R. H.; Eastman, V.; Thomas, S. W. J Mater Chem 2011, 21, 14041.
3. Smith, Z. C.; Pawle, R. H.; Thomas Iii, S. W. ACS Macro Letters 2012, 825-829.
4. Manceau, M.; Bundgaard, E.; Carle, J. E.; Hagemann, O.; Helgesen, M.; Sondergaard, R.; Jorgensen, M.; Krebs, F. C. J Mater Chem 2011, 21, 4132-4141.