Sublimation enthalpy of dye molecules measured using fluorescence

Area of Science:

• Physical Chemistry
• Spectroscopy
• Materials Science

Background:

• Accurate determination of sublimation enthalpy is critical for molecular beam applications.
• Existing literature values for tetraphenylporphyrin (TPP) sublimation enthalpy show significant discrepancies (over a factor of 2).
• Dye molecules like TPP, porphine, and nile red are potential candidates for molecular de Broglie wave interferometry.

Purpose of the Study:

• To develop and apply an in-flight fluorescence detection scheme for precise enthalpy of sublimation measurements.
• To determine the enthalpy of sublimation for TPP, porphine, and nile red.
• To investigate the reasons for discrepancies in previously reported sublimation enthalpy values.

Main Methods:

• Development of an in-flight fluorescence detection technique for molecular beams.
• Application of the technique to measure sublimation enthalpies of selected dye molecules.
• Analysis of molecular flow characteristics as a function of source temperature.

Main Results:

• Enthalpy of sublimation values were determined: H(sub)(TPP) = 142±3 kJ/mol, H(sub)(porphine) = 87±3 kJ/mol, and H(sub)(nile red) = 66±2 kJ/mol.
• The measured sublimation enthalpy for TPP aligns with the lower end of reported literature values.
• Changes in molecular flow characteristics with source temperature were identified as a key factor contributing to data discrepancies.

Conclusions:

• The developed in-flight fluorescence detection scheme provides accurate enthalpy of sublimation data.
• The study resolves discrepancies in TPP sublimation enthalpy, confirming a lower value.
• Source temperature-dependent molecular flow is a critical consideration for accurate sublimation enthalpy measurements.