Dear Sanjida Yeasmin , I am not sure if you can calculate conjugation length with FTIR analysis, you can compare samples and explain that something is occurring due to the conjugation length, but not quantify.

However, in order to check the presence of the conjugation sites, you can use UV-spectroscopy (UV-VIS). If you want to quantify them you can use 1H NMR as well. If you want to check the group positions, I would recommend J calculation from Nuclear Overhause Effect (NOE) in NMR technique.

Regards,

Matheus Mendes thanks.

Conjugated C=C bonds are particularly strong in Raman spectroscopy and there have been many studies of the dependence of the Raman spectrum on conjugation length in homologous oligomer series.

Article Conjugation length dependence of Raman scattering in a serie...

DOI: 10.1021/jp050039w

Perspective: Optical spectroscopy in π-conjugated polymers and how it can be used to determine multiscale polymer structures

J. Chem. Phys. 146, 130902 (2017); https://doi.org/10.1063/1.4979495

ABSTRACT

Exciton delocalization in conjugated polymer systems is determined by polymer conformations and packing. Since exciton delocalization determines the photoluminescent vibronic progression, optical spectroscopy provides an indirect link to polymer multiscale structures. This perspective describes our current theoretical understanding of how exciton delocalization in π-conjugated polymers determines their optical spectroscopy and further shows how exciton delocalization is related to conformational and environmental disorder. If the multiscale structures in conjugated polymer systems are known, then using first-principles modeling of excitonic processes it is possible to predict a wide-range of spectroscopic observables. We propose a reverse-engineering protocol of using these experimental observables in combination with theoretical and computational modeling to determine the multiscale polymers structures, thus establishing quantitative structure-function predictions.

Calculating the average conjugation length of a polymer using the intensity from FTIR spectra is not easy. C=C is usually weak in IR spectrum. As conjugation increases, it is natural that the electric dipole has better 'strength'. Let us assume the oscillator strength doesn't change as much (may be C=C peak shift in IR can be neglected). That means C=C intensity should increase with n (conjugation length). However, there is no reason to assume it should be linear. Under natural circumstances, intensity change with n should saturate at n~15-20 (see literature for UV and Raman studies of Conjugated Polymers). Moreover, the presence of solitons and polarons can have influence: they can lead to response in n vs. IR intensity with slopes greater than 1. Further, it is difficult to guess the distribution of segments with different conjugation length in a polymer. In some polymers smaller n segments may be rich and in some others, they may be absent.

Despite these facts, an empirical equation can be constructed if you have a series of oligomers (dimer, trimer, etc.). This result can then be applied to estimate an average property of a polymer (in solution). In Raman spectroscopy, for instance, there is peak shift with change in n, and the corresponding empirical relationship has been demonstrated.

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Thanks, everyone.