Introduction and New Aspects
2
Steen Brøndsted Nielsen
Abstract
This chapter provides a brief introduction to biochromophores encountered in
nature, and how their π-conjugated structures determine their excitation
energies, i.e., their colour. Perturbations of electronic structure by a microenvironment such as water or charge sites are discussed. These may lead to a colour
change (or modulation), depending on the character of the electronic transition.
As detailed results for particular chromophores are presented in subsequent
chapters, future challenges and new aspects within the research field are instead
considered as the author sees them.
2.1
Introduction
Chromophores are ubiquitous in nature: They account for the green colour of
leaves, the red colour of blood, the orange colour of carrots, the yellow-green
emission from fireflies, and the green or red emission from fluorescent proteins,
just to mention a few relevant occurrences. In these examples the chromophore
either acts alone, or is one constituent of a complicated network of light absorbers
that play together like an orchestra where all have an important role. To qualify as a
chromophore, the molecule should be able to absorb visible light. Here, however,
we will extend this definition to include also molecules that absorb ultraviolet light.
This requirement for absorbing either visible or ultraviolet light implies that the
relevant molecules or molecular ions are highly π-conjugated. In other words, the
electronic wavefunctions are delocalised over several sp
2 -hybridised atoms. A
physicist will use an argument based on an electron-in-a-box with infinite walls
to conclude that the larger the length of the box (the region the photoactive electron
can take up), the closer is the separation between the ground-state and first-excited
S.B. Nielsen (*)
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: sbn@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_2,
# Springer-Verlag Berlin Heidelberg 2013
11
2
Steen Brøndsted Nielsen
Abstract
This chapter provides a brief introduction to biochromophores encountered in
nature, and how their π-conjugated structures determine their excitation
energies, i.e., their colour. Perturbations of electronic structure by a microenvironment such as water or charge sites are discussed. These may lead to a colour
change (or modulation), depending on the character of the electronic transition.
As detailed results for particular chromophores are presented in subsequent
chapters, future challenges and new aspects within the research field are instead
considered as the author sees them.
2.1
Introduction
Chromophores are ubiquitous in nature: They account for the green colour of
leaves, the red colour of blood, the orange colour of carrots, the yellow-green
emission from fireflies, and the green or red emission from fluorescent proteins,
just to mention a few relevant occurrences. In these examples the chromophore
either acts alone, or is one constituent of a complicated network of light absorbers
that play together like an orchestra where all have an important role. To qualify as a
chromophore, the molecule should be able to absorb visible light. Here, however,
we will extend this definition to include also molecules that absorb ultraviolet light.
This requirement for absorbing either visible or ultraviolet light implies that the
relevant molecules or molecular ions are highly π-conjugated. In other words, the
electronic wavefunctions are delocalised over several sp
2 -hybridised atoms. A
physicist will use an argument based on an electron-in-a-box with infinite walls
to conclude that the larger the length of the box (the region the photoactive electron
can take up), the closer is the separation between the ground-state and first-excited
S.B. Nielsen (*)
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: sbn@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_2,
# Springer-Verlag Berlin Heidelberg 2013
11