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On the large apparent Stokes shift of phthalimides
Anna Reiffers1, Christian Torres Ziegenbein, Luiz Schubert
1Institut für Physikalische Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, 40225 Düsseldorf, Germany. gilch@hhu.de.
Physical Chemistry Chemical Physics : PCCP
|February 20, 2019
Summary
N-methylphthalimide (MP) shows a large Stokes shift due to ultrafast excited-state depletion. Solvent polarity significantly influences its fluorescence lifetime, ranging from picoseconds to nanoseconds.
Area of Science:
- Photochemistry
- Molecular Spectroscopy
- Quantum Chemistry
Background:
- N-methylphthalimide (MP) is a molecule with interesting photophysical properties.
- Understanding excited-state dynamics is crucial for designing new photoactive materials.
Purpose of the Study:
- To investigate the photophysics of N-methylphthalimide (MP) in various solvents.
- To elucidate the factors contributing to its large Stokes shift and varying fluorescence lifetimes.
Main Methods:
- Steady-state and time-resolved fluorescence and absorption spectroscopy.
- Quantum chemical computations using Density Functional Theory - Multi-Reference Configuration Interaction (DFT-MRCI).
Main Results:
- MP exhibits a large Stokes shift (∼10,000 cm⁻¹) in all studied solvents (cyclohexane, ethanol, acetonitrile, water).
- An ultrafast (<100 fs) depletion of the initially excited state leads to a weakly emitting state, responsible for the large Stokes shift.
- Solvent polarity strongly affects later processes, with fluorescence lifetimes ranging from ∼10 ps in cyclohexane to ∼3 ns in water.
- DFT-MRCI calculations identified two low-lying singlet ππ* excitations with different oscillator strengths, supporting the experimental findings.
Conclusions:
- The large Stokes shift in MP is attributed to an ultrafast excited-state decay mechanism, largely independent of solvent.
- Solvent-dependent variations in fluorescence lifetime are linked to the energetic accessibility of nπ* excitations.
- MP's photophysics are a complex interplay of initial ultrafast dynamics and solvent-influenced later relaxation pathways.
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