Enhancing exciton diffusion in porphyrin thin films using peripheral carboalkoxy groups to influence molecular assembly

The effects of molecular arrangement on the singlet exciton diffusion properties of free-base carboalkoxyphenylporphyrins containing a variety of alkyl substituents were investigated in solution-cast thin films. Exciton diffusivity was calculated using relative quenching efficiencies (Q) obtained by measuring the singlet photoluminescent decay lifetime PL(t) of pristine porphyrin films and films doped with 0.06-0.2% volume fraction of [6,6]-phenyl-C61-butryic acid methyl ester (PCBM). The quenching efficiency and PL lifetime decay data was used in a 3D exciton diffusion Monte Carlo simulation model to extract the exciton diffusion coefficient (D) and diffusion length (LD). Five carboalkoxyphenyl porphyrins (TCAPPs) were synthesized and analyzed in solution-cast thin films, tetra(4-carbomethoxyphenyl) porphyrin (TCM4PP), tetra(4-carbobutoxyphenyl) porphyrin (TCB4PP), tetra(4-carbohexoxyphenyl)porphyrin (TCH4PP), tetra(4-carbo-2-ethylhexoxyphenyl) porphyrin (TCEH4PP), and tetra(4-carbooctoxyphenyl)porphyrin (TCO4PP). Longer alkyl chain derivatives yielded increased PL decay lifetimes and extended the exciton diffusion lengths (L-D) for octyl (TCO4PP) and hexyl (TCH4PP) porphyrin derivatives. We observed an increase in the L-D from 15 nm for TCM4PP to 25 nm for TCH4PP, while a branched alkyl chain derivative (TCEH4PP) showed the lowest L-D of 14 nm. UV-Vis and XRD data indicate that molecular organization is strongly dependent upon the peripheral carboalkoxy chain, and that nematic molecular organization resulted in an increase in the exciton diffusion length. Our findings are an important step toward a deeper understanding of the exciton diffusivity and molecular packing relationship of simple free-base porphyrins.