Fate of Excited States in Jet-Cooled Aromatic Molecules:  Bifurcating Pathways and Very Long Lived Species from the S₁ Excitation of Phenylacetylene and Benzonitrile

Abstract

Pump-probe delayed ionization studies on phenylacetylene and benzonitrile in a supersonic beam reveal the production of a low-ionization-potential (approximately 5.7 eV) species lasting more than hundreds of microseconds after excitation to the S1 state. Excitation of the molecules was done with a frequency-doubled, Fourier transform-limited, pulse-amplified cw laser, and the rotationally resolved structure of the S1-S0 transition ensures that the excited molecules are monomers. Excited-state photoelectron spectroscopy shows that the long-lived species are formed during the light pulse but not by transfer from the fluorescing S1 population after the pulse, even though the S1 spectral signature is present in the long-lived action spectrum. This behavior differs greatly from that found in benzene and with most commonly held pictures of radiationless transitions in large molecules.

Hydrogen‐Bonded Complexes of Fluorophenylacetylenes: To Fluoresce or Not?

“…To begin with, all the six (fluoro)phenylacetylenes considered in the present investigation are fluorescent following excitation to 1 ππ* (S 1 ) state. [17] Any singlet dark state that can couple and possibly quench the fluorescence is not energetically accessible, at least from the Frank-Condon zone (Figure7, top panel). This description also holds for all the complexes with intermolecular structures that do not involve � CÀ H•••X hydrogen bonding with the acetylenic group, such as structures M1, M3 ( Figure 2) and C1, C9 (Figure 4).…”

“…On the other hand, an increase in the CÀ H•••X hydrogen bond strength moves the dark state away from the Franck-Condon region thereby switching ON the fluorescence emission (see lower panel in Figure 7). Further, the excited state lifetime of the monomers viz., PHA (70 ns), [17] 2FPHA (84 ns), 3FPHA (76 ns), 4FPHA (90 ns) and 26DFPHA (63 ns) are in the range of 63-90 ns, and those of fluorescent complexes are much lower and are within the laser pulse width of about 8 ns, therefore could not be measured accurately with ns laser. On the other hand, the lifetimes of the non-fluorescent complexes are expected to be much lower than the fluorescent complexes.…”

“…On the other hand, the lifetimes of the non-fluorescent complexes are expected to be much lower than the fluorescent complexes. For instance, the excited state lifetime of PHA is about 70 ns, [17] on the other hand, the lifetime of fluorescent PHA-MA M1 complex is less than 8 ns, which suggests that hydrogen bonding to PHA, in general, leads to lowering of the excited state lifetime relative to the monomer, and in some instances results in fluorescence quenching behaviour, which is dependent on the intermolecular structure, as illustrated in the case-I of Figure 5. The experimental results indicate the coupling between the initially excited (S 1 ) state and the dark (S n ) state is operative in the Franck-Condon zone in a narrow � CÀ H•••X hydrogen bond stabilization window.…”

Photo-assisted intersystem crossing: The predominant triplet formation mechanism in some isolated polycyclic aromatic molecules excited with pulsed lasers

“…Secondly, in our laboratory it was found that phenylacetylene and benzonitrile pumped by nanosecond lasers did not follow a classical kinetic scheme at all. 15,16 Their triplet states were entirely populated during the laser pulse--only--and then the triplet populations did not decay at all during the 140 μs we could observe them in the beam. This is completely inexplicable in the generally accepted model of the way larger isolated molecules are supposed to behave.…”

Visualizing competing intersystem crossing and internal conversion with a complementary measurement

“…Since they thought that none of the standard processes (IC, ISC or IVR) were responsible, this unknown phenomenon was named "channel three" and remained mysterious for a long time. [32][33][34][35] In previous work, we investigated the low vibronic S 1 state of o-xylene with one-photon pump at 266 nm. The lifetime of the low vibronic S 1 state with vibronic energy of 0.02 eV is much longer, extrapolated to ∼12.7 ns.…”