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What else (besides benzophenone) can both do phosphorescence but not fluorescence, and what else can do both phosphorescence and fluorescence? For example, does pentacene can do both?

I'm reading that there are more compounds that can only do 1, than do both. But is there any inherent contradiction that if it had the properties, the properties will contradict?

For example, if a compound has properties A and B to do fluorescence, and properties C and D to do phosphorescence, then a compound that has A, B, C, and D, should do both fluorescence and phosphorescence, and will not have any contraindicating properties?

And what do we have for fluorescence/phosphorescence in the visible-light range, i.e., absorb violet and emit red.

So far I only have organic compounds examples, what about for inorganic compounds examples? As well as for organometallics.

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    $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Chemistry Meta, or in Chemistry Chat. Comments continuing discussion may be removed. $\endgroup$
    – Karsten ♦
    Commented Oct 13, 2023 at 4:58

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An excited state has initially five possible fates, (1) internal conversion to a state of the same spin, (2) intersystem crossing to a state of different spin (e.g. singlet to triplet) (3) emission to a state of the same or different spin multiplicity, i.e. fluorescence, phosphorescence or in general luminescence,(4) chemical reaction such as bond breaking, isomerisation, electron or proton transfer or other photochemistry. Not all molecules exhibit chemical reaction (there may not be enough energy in an excited state for this to happen) but I add it for completeness. (5) quenching by other species in solution. This could be energy or electron transfer or 'heavy atom' effect which is the same as quenching by paramagnetic species such as dissolved oxygen. Clearly this pathway cannot occur with isolated molecules in the gas phase.

The probability of any one process occurring depends on the nature of the excited state and its closeness in energy to other states, (and the nature of any quencher) but any process always occur in competition with all the others, its just a matter of the yield of each process. If the rate constants are $k_i$ then the yield (probability) of event $i$, say fluorescence, is $\varphi_i=k_i/\sum k_i$ and always $0<\varphi_i <1$ and of course the sum of all different yields is unity.

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    $\begingroup$ (+1) Nice answer! Back in the early 1980s, I used to do laser-excited two photon photoionization of PAHs in solvents such as n-hexane. I used a pulsed nitrogen laser at about 337 nm. So sometimes excited molecules can suffer another fate: get hit again and ionize. ;-) $\endgroup$
    – Ed V
    Commented Jul 24, 2023 at 16:48
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    $\begingroup$ Anyways, a compound I hear that does phosphorescence but not fluorescence (or poorly fluorescence), is benzophenone, I believe, at room temperature. $\endgroup$
    – Neal Conroy
    Commented Jul 24, 2023 at 17:39
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    $\begingroup$ @NealConroy The Hope diamond is blue and famously phosphoresces a deep red color. Phosphoresce is rare compared to fluorescence and especially so in liquid solutions at room temperature and in the presence of oxygen. $\endgroup$
    – Ed V
    Commented Jul 24, 2023 at 18:00
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    $\begingroup$ It is true that Benzophenone has almost 100% intersystem crossing forming triplets and these are used to sensitise reactions. Phosphorescence is observed in O2 free solutions (Parker & Joyce, Chem. Comm, 1968, p749) $\endgroup$
    – porphyrin
    Commented Jul 25, 2023 at 6:40
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    $\begingroup$ @porphyrin Ed V I had only recently found out that the mechanism behind glow-in-the-dark toys, is not phosphorescence, but called persistent luminescence. Which is something in its Wikipedia article said the mechanism is not fully understood, and it is commonly mistaken as phosphorescence. According to an 2009 article written by a chem professor "the challenge of finding an efficient red emitting persistent phosphor is still waiting." So those 3, and chemiluminescence, are like 4 categories of its own. What do you guys call the study of this? I guess either photochemistry or photophysics. $\endgroup$
    – Neal Conroy
    Commented Jul 25, 2023 at 13:31

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