Two-Dimensional Conjugated Polymer Assemblies: Interchain Spacing for Control of Photophysics

Abstract

The viability of conjugated polymer thin film electrooptical devices and sensors depends strongly on high quantum efficiency thin films with excellent charge and exciton mobilities.1,2 The brightness of electroluminescent devices depends on the charge transport and emission efficiency, and energy migration to low energy sites can provide the necessary wavelength shifts for lasing.3 A highly sensitive and specific sensory device requires that a recognition event be coupled to an amplification mechanism. We are particularly interested in exploiting the collective transport properties of electrons, electron holes, or excitons traveling along a conjugated polymer backbone to amplify analyte binding events.1 Movement of these excited states strongly depends on the interaction and orientation of an individual polymer relative to its neighbors.4 Interchain distance has strong impact on the performance of electrooptical devices based on conjugated polymers5 and only very recently has a theoretical model describing the interaction between neighboring chains become available.6 Despite the accepted fact that interchain interactions alter photoluminescent properties of conjugated polymers, no reports of a general method for controlling the interchain spacing have appeared. In both small molecules7 and polymers,4d,e close association of π-systems often causes a substantial decrease in the PL quantum yield relative to isolated chromophores. The diminished PL quantum yields make conjugated polymer thin films containing close cofacial π-stacking unattractive for use as transducing elements in chemosensors.8 However, minimizing interpolymer distance is necessary for optimal energy transfer of excitons between polymer chains.4d,e Interrogation of the influence that interchain distance has on thin film photophysics requires precise control over the packing and order of the individual polymer chains with respect to one another.9 By restricting the polymer chains to a 2-dimensional liquid interface, a Langmuir monolayer provides the requisite polymer ordering and dynamics necessary to probe interchain effects. Scheme 1 depicts poly(p-phenylene ethynylenes) (PPEs)10 containing a substitution pattern (regiorandom) where the hydrophilic and hydrophobic groups are para to each other producing an “edge-on” organization at the air-water interface.12 Our previous studies indicated that the edge-on orientation forces the phenyl rings (barring any steric repulsion) on the polymer chains to pack in a close cofacial manner causing fluorescence quenching. Despite the low quantum yields, the edge-on polymers are attractive because Langmuir-Blodgett or Langmuir-Schaefer13 films of these polymers provide high surface density and orient polar recognition elements toward the air-film interface. Herein, we report a systematic study of the relationship between cofacial interpolymer distance and solid-state photophysics. We predict that increasing the interpolymer distance will provide improved photophysical properties. To investigate our hypothesis, polymers (1-3) with differing degrees of side chain bulk were synthesized.14 As expected, the side chain bulk influences the packing of the polymers at the air-water interface. Table 1 includes the area per repeating unit for polymers 1-3, derived from extrapolated onset of the pressure-area isotherm. The area per repeating unit increases as the bulk of the side chain increases from dimethyl (1) to diisopentyl (3) and the area per repeat unit suggests that both subunits in each polymer orient edge-on. We calculate interchain distances of 4.0, 4.4, and 4.9 A for polymers 1, 2, and 3 respectively.12 X-ray diffraction data obtained from drop cast films of polymers 1-3 on aluminum corroborate the interchain distance trend observed on the Langmuir trough. From the X-ray data, interchain spacings of 3.6, 3.9, and 4.3 A were observed for polymers 1-3 respectively, providing further indication that increased side chain bulk provides greater polymerpolymer spacing.15