The electronic effect of modulating R can be seen spectroscopically in the 31 P{ 1 H} NMR spectra and UV/Vis spectra of 1-R. The 31 P{ 1 H} chemical shift of 1-R becomes gradually more deshielded as the R group becomes more electron-withdrawing (Figure ), which demonstrates that although the R group is distant from the phosphorus centre it does still have a marked effect on its properties. This trend is evident in a plot of the 31 P NMR chemical shifts of 1-R against the corresponding σ-para Hammett constants of R, which shows a positive linear trend with an R 2 value of close to unity (Figure ). The colours of 1-R are various shades of orange and red. The UV/Vis spectra of each azophosphine shows a strong absorbance band in the UV region between 350 and 388 nm, and a much weaker absorbance band between 500 and 521 nm that is responsible for the colour of the compounds. The UV/Vis spectra of 1-R show only minor differences in absorption as the R group is varied. Overall, absorbance is red-shifted moving from 1-NMe2 to 1-CF3; however, 1-F is blue-shifted relative to 1-H due to the mesomeric effect of F. This is summarised in the plot of UV/Vis absorbance of 1-R against the corresponding σ-para Hammett constants of R (Figure ), which shows an overall linear trend with an R 2 of 0.82727.