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Question

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(a) The absorption and emission spectra were recorded for the molecule shown in the figure below. Explain, using a Jablonski diagram, the likely photophysical processes which gave rise to the bands observed in both spectra. The spectra were recorded in hexane. How would the spectra change if the following experimental parameters changed;
(i) Temperature of the solvent was increased
(ii) If iodide substitution ortho to the methyl groups on the aryl ring was introduced.

Accepted Answer

∻Solution∻ ‖ 1 ⋮ **Jablonski Diagram Explanation**: A Jablonski diagram is a graphical representation of the electronic states of a molecule and the transitions between them. It includes the ground state (S0), excited singlet states (S1, S2, etc.), and triplet states (T1, T2, etc.). Absorption spectra correspond to transitions from the ground state to various excited states, while emission spectra involve transitions from excited states back to the ground state or to lower excited states. ‖ ‖ 2 ⋮ **Photophysical Processes**: The absorption bands are due to electronic transitions from the ground state to higher energy states. Upon absorption of light, the molecule reaches an excited singlet state (S1 or S2). Internal conversion (non-radiative relaxation to the lowest vibrational level of S1), followed by fluorescence (radiative return to S0), or intersystem crossing to a triplet state (T1) and subsequent phosphorescence (radiative return to S0) are the likely processes. ‖ ‖ 3 ⋮ **Effect of Temperature Increase**: Increasing the temperature of the solvent can lead to broadening of the absorption and emission bands due to increased molecular vibrations and collisions. Additionally, higher temperatures can promote non-radiative decay processes, potentially decreasing the intensity of fluorescence and phosphorescence. ‖ ‖ 4 ⋮ **Effect of Iodide Substitution**: Introducing iodide substitution ortho to the methyl groups on the aryl ring can result in heavy atom effect, which enhances intersystem crossing due to spin-orbit coupling. This can increase phosphorescence at the expense of fluorescence, altering the emission spectrum. The absorption spectrum may also shift due to changes in the electronic structure of the molecule. ‖ ∻Answer∻ ⚹ - Absorption spectra are due to electronic transitions from S0 to higher energy states. - Emission spectra are due to transitions from excited states to lower energy states or S0. - Increased temperature can broaden spectra and decrease fluorescence and phosphorescence intensity. - Iodide substitution can enhance phosphorescence and alter both absorption and emission spectra. ⚹ ∻Key Concept∻ ⚹ Jablonski diagram and photophysical processes ⚹ ∻Explanation∻ ⚹ The Jablonski diagram helps explain the absorption and emission spectra by illustrating the electronic transitions and the processes such as fluorescence and phosphorescence that molecules undergo after absorbing light. ⚹ ∻Key Concept∻ ⚹ Effects of temperature and substitution on spectra ⚹ ∻Explanation∻ ⚹ Increased temperature can cause spectral broadening and decrease emission intensity, while heavy atom substitution like iodide can enhance intersystem crossing, affecting both absorption and emission spectra. ⚹◊What is the impact of increasing the temperature of the solvent on the absorption and emission spectra of a molecule?⍭ Generate me a similar question◊

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