The Nature of Interchain Excitations in Conjugated Polymers:  Spatially-Varying Interfacial Solvatochromism of Annealed MEH-PPV Films Studied by Near-Field Scanning Optical Microscopy (NSOM)

Abstract

The nature of interchain electronic species in conjugated polymers has been the subject of much debate. In this paper, we exploit a novel near-field scanning optical microscopy (NSOM)-based solvatochromism method to spatially image the difference in dipole moment, and hence the difference in degree of charge separation, between the ground and electronic excited states of the emissive interchain species in films of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). The method uses NSOM to collect emission from near the surface of solid samples that are placed into contact with liquids of varying polarity. The solvatochromic spectral shifts of the interfacial luminescence are measured as a function of solvent polarity; the results are analyzed with an interfacial dielectric continuum model to determine the dipole moment of emissive excited states. Experiments performed on films of the laser dye trans-4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) in poly(methyl methacrylate) (PMMA) demonstrate that our interfacial NSOM solvatochromic method and analysis can successfully reproduce the known dipole change of DCM upon photoexcitation. With the method calibrated, we then apply it to the interchain luminescence from the surface of thermally annealed MEH-PPV films. The interfacial solvatochromic analysis reveals that the dominant interchain species in annealed MEH-PPV films is "excimer-like", exhibiting an ∼4-7 D decrease in dipole moment upon optical excitation. In a few highly localized regions of the film (ca. 1-2 µm in diameter), however, the interchain excited state exhibits a large (∼9-13 D) increase in dipole moment upon excitation, indicative of minority interchain species with a large degree of charge separation, such as exciplexes or polaron pairs. The large variation in excited-state dipole moments observed throughout the film is suggestive of an entire family of interchain species, each characterized by a different degree of charge separation. The fact that the large-dipole interchain species are found in spatially segregated domains implies that interchain charge separation in conjugated polymer films is associated with the presence of defects. When the molecular weight of the polymer is lowered, the large excited-state dipole regions increase in spatial extent, suggesting that the defects that promote charge separation are intrinsic and may be associated with the chain ends.

Alternating and direct current characterization and photoinduced absorption studies of modified conjugated polymer thin films

“…It is however noted that some authors have also reported similar values 28,29 to the ones presented in this work. A possible contribution from a fluctuating permanent dipole associated with the presence of alkoxy side chains may account for the larger values, although a more detailed study is needed to establish the exact nature of any possible sidechain contribution.…”

Dispositivos poliméricos eletroluminescentes

“…O acoplamento vibrônico em polímeros orgânicos conjugados segue as mesmas regras de seleção que as de moléculas orgânicas luminescentes [104][105][106] . Isto quer dizer que serão acoplados os modos normais que apresentam simetria que leve a uma representação irredutível totalmente simétrica do integrando do operador no momento de dipolo de transição.…”

“…A relação observada de intensidades entre as bandas vibrônicas relaciona-se diretamente com os acoplamentos vibrônicos, mostrando que a probabilidade de transição entre os níveis ocorre de acordo com o Princípio de Franck-Condon [107][108][109] . O parâmetro que quantifica a probabilidade de transição entre os níveis vibrônicos e relaciona a energia de relaxação entre estados vibrônicos a uma dada temperatura com a freqüência de acoplamento elétron-fônon é o de Huang-Rhys [98][99][100][101][102][103][104][105][106][107][108] .…”

“…Utilizando-se os valores das intensidades das bandas e o nú-mero de onda obtido através do espectro vibracional, pode-se obter o parâmetro de Huang-Rhys (S) e a energia de relaxação de decaimento eletrônico (E Rel ), através da Equação 1 [102][103][104]109,110 . Quando polímeros conjugados são utilizados, além dos estados eletrônicos envolvidos, existem outras variáveis que controlam a região espectral e o perfil da banda.…”

“…A concentração do polímero em solução, portanto, é um importante fator de alteração do perfil espectral 104,132 . Soluções concentradas podem levar à agregação do polímero e, ao formar um dímero, o espectro deve sofre um desdobramento excitônico, que, de acordo com o mecanismo proposto por Kasha, produzirá efeitos espectrais diferentes dependendo da geometria dos mesmos.…”