We know it is usually not possible- all good absorbers have to be good emitters and vice versa. But I am looking for controlling the emission of an excited state (thermally or visibly). It may hold in a metastable state but de-excitation is required at my control by a trigger. Can somebody suggest how to hold an excited state of a molecule or material? It basically means engineering a small minima at the excited state which disallows de-excitation to ground state. We may change selection rules or whatever other crazy ideas.
Consider a molecule with a triplet state, an indirect band gap semiconductor, or an H-aggregates. All are able to absorb light and channel it to lower level dark states.
Consider a molecule with a triplet state, an indirect band gap semiconductor, or an H-aggregates. All are able to absorb light and channel it to lower level dark states.
Absorption and emission exhibits same oscillator strength for a given set of energy levels. The meta-stable forbidden states (triplet) or traps are good to hold the emission.
your requirements are similar to the dosimetry where the photons (PSL) or phonons act as triggers. Deeper traps can hold the excitation for longer time and show better response for external stimulation.
Further I wish to add that vibrational excitations may also be included which easily absorb infrared region and manifest either as increased amplitudes or phonon occupancy. I want to retain the occupancy for a delayed time of choice.
And finally, what is in my mind- I share. Designing materials which would absorb solar radiation including heat but do not spontaneously emit. The emission has to hold till we desire. Thus the material placed in the solar radiation is is energized material which will deliver energy as and when required. The rate of absorption has also to be maximized.
@Harry ten Brink soot does not hold. These black stuffs absorb but emit at the same time. Emission has to be stopped like raising some stones to rooftop but holding them there and retaining potential energy, providing them the slope to fall down when we want.
In general we need some energy conversion mechanism - the one Thomas La Cour Jansen pointed out or yet another form of greenhouse effect. Another possibility is employment of smart creature, a close relative of Maxwell daemon. Little friend using its magic wand will reverse emitted photons waiting for your command to release them.
@Vinjay
Soot does not emit at al, that is visible light!? or do you refer to IR heat emission?
What then do you have in mind? when the fluorophore has a high absorption it will emit its extra energy also in the gas-phase in a mater of nanoseconds. Then the only way is to have a molecule with a high enough density of states to have a very efficient intersystem crossing. Think of it you might have a look at a substituted PAH with a high S0-S2 absorption but with an n-p lowest excited singlet state and very efficient ISC and a hold up in the triplet state
Forgot the basic answer: a compound CAN be a good absorber but not an emitter.
This is the case when the absorption is into a higher electronic state from which the molecule more rapidly than emitting transfers to a state with a much lower emission probability.
Substitution of hydrogen by deuterium also can extend the lifetime of excited states
@Harry that sounds interesting. If we can control emission probability in comparison to absorption probability it would help greatly. Any such possibility including deuteration can be considered. Any reference or elaboration?
It is well-known in phosphorescence of PAH molecules. I used it in a lab course on spectroscopy. My example was deuterated pyrene in PMMA.
But what wavelength do you have in mind and how long should the excited state live?
Of course phosphorescence processes are known but we want to hold as long as hours or days. And holding the solar spectrum including IR in particular.
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