Síntese de novos compostos fotocrómicos, termicamente reversíveis, derivados de benzopiranos, naftopiranos e naftofuranos capazes de gerar uma única espécie corada
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2016-02-02
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Algumas moléculas quando irradiadas com luz alteram a sua cor de modo reversível
devido a uma reação fotoquímica que conduz à formação de espécies químicas coradas que
revertem à forma inicial térmica- ou fotoquimicamente. Este fenómeno, designado por
fotocromismo, foi descoberto em meados do seculo XIX mas só nas últimas décadas despertou
o interesse alargado da comunidade científica.
Nos últimos anos, têm sido desenvolvidas diversas aplicações para estas moléculas, em
particular, envolvendo compostos fotocrómicos do tipo T, que revertem termicamente à forma
inicial. A sua incorporação em lentes plásticas oftálmicas permitiu obter lentes que escurecem
em função da intensidade solar e que alcançaram um elevado sucesso comercial.
De entre as várias classes de compostos fotocrómicos, os naftopiranos são uma das
classes mais estudadas devido à facilidade de síntese, elevada sensibilidade à luz solar,
possibilidade de gerar diferentes cores, cinética controlável e à sua elevada resistência à
fotodegradação. A principal limitação destes compostos reside na lenta velocidade de
descoloração: sob irradiação UV ocorre a abertura do ciclo pirânico originando um fotoisómero
com uma configuração transoid-cis que facilmente isomeriza dando origem a um segundo
fotoisómero de maior tempo de vida, transoid-trans, responsável por uma cor residual que pode
persistir durante minutos ou mesmo horas.
O principal objetivo deste trabalho foi sintetizar uma nova classe de compostos
fotocrómicos, benzo- e naftopiranos fundidos, com um anel extra entre o ciclo pirânico e o anel
naftalénico. Estes compostos, sob irradiação UV (solar), geram apenas uma única espécie
corada, dado que a alteração estrutural efetuada impede a isomerização da dupla ligação
formando-se apenas o fotoisómero transoid-cis, eliminando assim o problema da cor residual.
Este trabalho abrangeu numa primeira fase a síntese de benzopiranos fundidos, a sua
caracterização estrutural e o estudo das suas propriedades fotocrómicas por fotólise de
relâmpago.
Numa segunda fase foram exploradas diversas metodologias para a síntese de
naftopiranos fundidos que conduziram a alguns compostos com estruturas e propriedades
inesperadas. Adicionalmente foram desenvolvidas duas novas vias sintéticas que permitiram
obter naftopiranos fundidos partindo em ambos os casos de naftóis. A partir do 1-naftol foi
possível obter naftopiranos fundidos com um ciclo extra entre o anel pirânico e o anel naftalénico que apresentam excelentes propriedades fotocrómicas. A partir da 5-hidroxi-7H benzo[c]fluoren-7-ona e através de uma síntese bastante expedita desenvolveu-se uma nova
classe de compostos, os indeno-naftopiranos fundidos.
As propriedades fotocrómicas destes novos compostos foram avaliadas em solução e
em matrizes híbridas orgânicas-inorgânicas, à temperatura ambiente, observando-se o
desenvolvimento de colorações intensas sob irradiação UV. Estudos de RMN comprovaram a
formação de uma única espécie corada que reverte ao estado inicial, incolor, em poucos
segundos/minutos. Este comportamento representa uma vantagem significativa face aos
naftopiranos comuns.
Enquanto os naftopiranos fundidos apresentam uma cinética de interconversão entre a
forma fechada e a forma aberta adequada a aplicações em lentes, os indeno-naftopiranos são
mais rápidos e apresentam um elevado potencial pois permitem, facilmente, modificações na
sua estrutura que permitirão controlar as suas propriedades.
No decorrer deste trabalho foi ainda descoberta uma nova classe de compostos
fotocrómicos com uma estrutura pouco usual, os 1-vinilideno-naftofuranos. Estes compostos
exibem fotocromismo à temperatura ambiente quando misturados com sílica gel. A irradiação
com luz solar de sílica dopada com estas moléculas gera em poucos segundos uma coloração
rosa/violeta ou verde que se desvanece com uma velocidade que depende dos substituintes
presentes na sua estrutura. Os 1-vinilideno-naftofuranos apresentam ainda propriedades
acidocrómicas: a adição de ácido trifluoroacético leva ao desenvolvimento de uma coloração
enquanto a adição de trietilamina permite voltar ao estado incolor.
Todos os compostos sintetizados foram caracterizados por diversas técnicas
espectroscópicas (IV, RMN de 1H e 13C, massa de alta e baixa resolução) de forma a determinar
a sua estrutura e as propriedades fotocrómicas foram determinadas por fotólise de relâmpago
ou por irradiação continua.
Some molecules when irradiated with light change their colour reversibly due to a photochemical reaction leading to the formation of colored chemical species which then returns to the original form termally and/or photochemically. This phenomenon, called photochromism, was discovered in the mid nineteenth century, but only in the last decades arouse attention of the scientific community. In recent years, several applications for these molecules have been developed, particularly involving photochromic compounds that return thermally to the initial form (type- T). Their incorporation in ophthalmic plastic lenses made it possible to obtain lenses that can adapt to the solar intensity and have achieved a high commercial success. Among several classes of photochromic compounds, naphthopyrans are one of the most studied classes due to their easy synthesis, high sensitivity to sun light, possibility of generating different colours, tuneable kinetics and a high resistance to photodegradation. The main limitation of these compounds lies in the slow bleaching rate: under UV irradiation the opening of the pyran ring occurs resulting in a photoisomer with a transoid-cis configuration that can easily isomerize originating a second photoisomer with a higher life-time, the transoid-trans, responsible for a residual colour that can persist for minutes or even hours. The main objective of this work was to synthetize new classes of photochromic compounds, fused benzo- and naphthopyrans, with an extra ring between the pyran cycle and the naphthalene ring. These compounds, under irradiation, are able to generate a single coloured species, since the structural alteration prevents the isomerization of the double bond forming only the photoisomer transoid-cis, therefore eliminating the problem of residual colour. This work started with the synthesis of fused benzopyrans, their structural characterization and the study of their photochromic properties by flash photolysis. In the second part several methodologies, were explored to synthesize fusednaphthopyrans that led to some compounds with unexpected structures and properties. Additionally, two new synthetic routes were developed that produced fused naphthopyrans, starting in both cases from naphthols. Starting from 1-naphtol it was possible to obtain fusednaphthopyrans with an extra cycle between the pyran and naphthalene rings that exhibit excellent photochromic properties. Starting from 5-hydroxi-7H-benzo[c]fluoren-7-one and through a very expeditious synthesis a new class of photochromic compounds, fused indenenaphthopyrans has been obtained. The photochromic properties of these new compounds were evaluated in solution and organic-inorganic hybrid matrices, at room temperature, showing the development of intense coloration under UV irradiation. NMR studies showed the formation of a single coloured species that return to the initial colourless state in a few seconds/minutes. This behaviour represents a significant advantage compared with common naphthopyrans. While the fused-naphthopyrans show an interconversion kinetics between the closed and open forms suitable for lens applications, the indene-naphthopyrans are faster and have a high potential because their structure can be easily modified, allowing the control of their properties. In the course of this work a new class of photochromic compounds, 1-vinylidenenaphthofurans, with an unusual structure was discovered. These compounds exhibit photochromism at room temperature only when mixed with silica gel. The sunlight irradiation of doped silica-gel generates, in a few seconds, pink / purple or green colours, that shows colour decay conditioned to the substituents present in their structure. The 1-vinylidene-naphthofurans also exhibit acidocromic properties: adding trifluoroacetic acid leads to the development of a coloration while adding triethylamine returns them to a colourless state. All synthesized compounds were characterized using spectroscopic techniques (IR, 1H and 13C NMR, mass of high and low resolution) to determine their structure and their photochromic properties were determined by flash photolysis or continuous irradiation.
Some molecules when irradiated with light change their colour reversibly due to a photochemical reaction leading to the formation of colored chemical species which then returns to the original form termally and/or photochemically. This phenomenon, called photochromism, was discovered in the mid nineteenth century, but only in the last decades arouse attention of the scientific community. In recent years, several applications for these molecules have been developed, particularly involving photochromic compounds that return thermally to the initial form (type- T). Their incorporation in ophthalmic plastic lenses made it possible to obtain lenses that can adapt to the solar intensity and have achieved a high commercial success. Among several classes of photochromic compounds, naphthopyrans are one of the most studied classes due to their easy synthesis, high sensitivity to sun light, possibility of generating different colours, tuneable kinetics and a high resistance to photodegradation. The main limitation of these compounds lies in the slow bleaching rate: under UV irradiation the opening of the pyran ring occurs resulting in a photoisomer with a transoid-cis configuration that can easily isomerize originating a second photoisomer with a higher life-time, the transoid-trans, responsible for a residual colour that can persist for minutes or even hours. The main objective of this work was to synthetize new classes of photochromic compounds, fused benzo- and naphthopyrans, with an extra ring between the pyran cycle and the naphthalene ring. These compounds, under irradiation, are able to generate a single coloured species, since the structural alteration prevents the isomerization of the double bond forming only the photoisomer transoid-cis, therefore eliminating the problem of residual colour. This work started with the synthesis of fused benzopyrans, their structural characterization and the study of their photochromic properties by flash photolysis. In the second part several methodologies, were explored to synthesize fusednaphthopyrans that led to some compounds with unexpected structures and properties. Additionally, two new synthetic routes were developed that produced fused naphthopyrans, starting in both cases from naphthols. Starting from 1-naphtol it was possible to obtain fusednaphthopyrans with an extra cycle between the pyran and naphthalene rings that exhibit excellent photochromic properties. Starting from 5-hydroxi-7H-benzo[c]fluoren-7-one and through a very expeditious synthesis a new class of photochromic compounds, fused indenenaphthopyrans has been obtained. The photochromic properties of these new compounds were evaluated in solution and organic-inorganic hybrid matrices, at room temperature, showing the development of intense coloration under UV irradiation. NMR studies showed the formation of a single coloured species that return to the initial colourless state in a few seconds/minutes. This behaviour represents a significant advantage compared with common naphthopyrans. While the fused-naphthopyrans show an interconversion kinetics between the closed and open forms suitable for lens applications, the indene-naphthopyrans are faster and have a high potential because their structure can be easily modified, allowing the control of their properties. In the course of this work a new class of photochromic compounds, 1-vinylidenenaphthofurans, with an unusual structure was discovered. These compounds exhibit photochromism at room temperature only when mixed with silica gel. The sunlight irradiation of doped silica-gel generates, in a few seconds, pink / purple or green colours, that shows colour decay conditioned to the substituents present in their structure. The 1-vinylidene-naphthofurans also exhibit acidocromic properties: adding trifluoroacetic acid leads to the development of a coloration while adding triethylamine returns them to a colourless state. All synthesized compounds were characterized using spectroscopic techniques (IR, 1H and 13C NMR, mass of high and low resolution) to determine their structure and their photochromic properties were determined by flash photolysis or continuous irradiation.
Descrição
Tese de Doutoramento em Ciências Químicas e Biológicas
Palavras-chave
Fotocromismo , Benzopiranos , Cor , Naftopiranos , Naftofuranos , Irradiação UV-Vis