A Very Long Engagement

Figure 1. Phosphorescence. (Taken from Ref. [1])

The topic of the week concerns a photolumiscent phenomenon known as ‘phosphorescence’ (Figure 1). While it is related to fluorescence, it has a major difference that it does not re-emit the radiation it absorbs immediately. The reason behind this delay is down to quantum mecahincs, and that is associated with the ‘forbidden’ energy state transitions. The key is through a process called ‘intersystem crossing’.*


Figure 2. The theories behind the work. (Taken from Ref. [1])

The diagram in the publication explains the phenomenon in a succinct manner (Figure 2) [1]. It is clear that there is a ‘competition’ between the emissions either as fluorescence or phosphorescence, and they correspond to 2 different energy pathways. The aromatic compound, Br6A, contains a bromo and an aldehyde functional group, which are both triplet-producing. In a disordered, solution state (Figure 2, part a), fluorescence is emitted due to a singlet (S) decay. While some triplets (T) can be generated, they will be lost as vibrations, and no phosphorescence can be emitted. The situation in a crystalline form is different (Figure 2, part b). The singlet decay can be suppressed (no fluorescence emission), and a singlet-to-triplet conversion can lead to the emission of phosphorescence. This can be rationalized by the O…Br interactions among the network of the aromatic molecules – electron density in the oxygen atom on the carbonyl group is delocalized to the outer orbitals of the bromine atom, and this promotes a spin-orbit coupling and leads to intersystem crossing. Unfortunately, while phosphorescence can be achieved, the quantum yield is only modest because of the fast excimer formation, vibrational loss is still prevalent (Figure 2, part c). It is only by dilution of the aromatic compounds for which we can suppress completely the vibrational loss, and leads to intense phosphorescence and achieves a promising quantum yield (Figure 2, part d).


Figure 3. Br6A. (Taken from Ref. [1])

Indeed , phosphorescence is an extremely rare phenomenon in organic compounds.
The research group has shown us that it is possible to incorporate the character of phosphorescence into pure organic compounds by adopting the reasoning describef above. Thus they have shown That Br6A is capable of the emission of phosphorescence with a modest quantum yield. (Figure 3) In order to enhance phosphorescence, they use the dilution strategy to achieve this aim. When the compound is diluted with a reasonably similar compound, dibromobenzene, the quantum yield is enhanced. The similarity of dibromobenzene means that it will not affect the overall crystal stacking of the compoundS. The best quantum yield achieved is close to 55%. [1]


Figure 4. Phosphorescence with different colours. (Taken from Ref. [1])

Another focus of the work is the generation of phosphorescence leading to the emission of different colours.** (Figure 4) This nicely illustrates the relationships between chemical structure and properties. The default Br6A leads to the emission of a green colour. They create 3 more model compounds, with gives rise to 3 different colours. This is rationalized through different electron densities present in the aromatic nuclei.

a. BrC6A (blue) – the original methoxy group is replaced by a less electron-donating alkyl group, which reduces the electron density of the aromatic nucleus.
b. BrS6A (yellow) - a thiol ether is included. This leads to a ‘red shift’.
c. Np6A (orange)– A Naphthyl nucleus is present of a single benzene nucleus, which obviously increases both the electron density and the conjugation length.

Thus by modifications of the chemical structures, one can significantly fine-tune the chemical properties of the compounds, and these changes undoubtly provide insights for useful applications.

Notes:

A good starting point is the Wikipedia page for 'phosphorescence'
http://en.wikipedia.org/wiki/Phosphorescence

*I have to admit that, even in a qualitative manner, this topic is a complex one. The rationale behind all these phenomena warrants a complete article. I do not want to divert the focus from the more intriguing aspects of the research work in this publication. Thus the rationalizations of all the theories behind this work will be based on the perspective of the research workers. Further discussions are welcome.

**Each colour corresponds to a particular wavelength on the electromagnetic spectrum. By figuring out the wavelength of a particular colour, and using the relation E = hf, this can eventually provide information about the electron density of a conjugated organic compound.

Reference:

1. Activating efficient phosphorescence from purely organic materials by crystal design.
Onas Bolton1, Kangwon Lee, Hyong-Jun Kim1, Kevin Y. Lin and Jinsang Kim
Nature Chemistry
Published Online 13 February 2011
DOI: 10.1038/NCHEM.984