Internal Conversion vs. Intersystem Crossing – What Drives the Dynamics of Cyclic α,β-Enones?

Abstract

Abstract. The origin of ultrafast intersystem crossing and its competitiveness with singlet pathways was studied in cyclic α,β-enones by transient absorption, time-resolved photoelectron spectroscopy and ab initio calculations.

Internal Conversion versus Intersystem Crossing: What Drives the Gas Phase Dynamics of Cyclic α,β-Enones?

“…As can be seen, the intense 1 ππ* spectra shown in the main panel exhibit a red shift of the maximum upon increasing substitution. The opposite is true for the 1 nπ* state (inset of Figure ) where increasing substitution lowers the energy of the nonbonding lone-pair orbital, yielding a small spectral blue shift (this is more clearly seen in liquid phase spectra as shown in ref ).…”

“…Most recently, it was shown in femtosecond time-resolved transient absorption studies that excitation of the 1 ππ* state of 3-methyl-2cyclopentenone (3MeCPO) in acetonitrile and 2,3,4,5tetramethyl-2-cyclopentenone (TMCPO) in acetonitrile and methanol led to a ground state bleach in the 230 nm region, with a time independent amplitude within the first nanosecond. This was interpreted as unit triplet quantum yield Φ ISC 23,24 and is consistent with the increased Φ ISC found for acrolein upon solvation. 12 Here we investigate a series of cyclic α,β-enones: CPO, the methylated species 3MeCPO and TMCPO, as well as the open chain molecule MVK, all depicted in Figure 1.…”

“…This renders ISC the dominant solution phase relaxation pathway. 23,24 These results suggest that cyclopentenone and its methylated derivatives form an "intermediate case" between the ultrafast gradient directed dynamics and processes which are better described by statistical theories. The photoinitiated dynamics of smaller polyenes are characterized by the former, displaying electronic relaxation on a subpicosecond time scale, which in general is shorter than the time scales for intramolecular vibrational redistribution (IVR).…”

“…This can be understood as solvent-induced relaxation rapidly bringing the system close to the S 1 minimum, energetically below the barriers which lead to S 0 –S 1 intersection. This renders ISC the dominant solution phase relaxation pathway. , …”

Ultrafast intersystem crossing dynamics in uracil unravelled byab initiomolecular dynamics

“…26,36,38 While ISC is astonishingly fast in this case, cytosine seems to be not the only organic molecule where ISC and IC processes can compete on the same time scale. Experimental fast to ultrafast times scales for ISC have been reported for aldehydes, 39 a number of small aromatic compounds, such as benzene, 40,41 naphthalene, anthracene and their carbonylic derivatives [42][43][44][45][46][47][48][49][50][51][52][53][54][55] as well as nitrocompounds. 46,[56][57][58][59][60][61][62][63][64][65][66][67][68] The substitution of oxygen by sulfur in nucleobases enhances ISC so dramatically that the ultrafast IC responsible for photostability disappears while turning ISC to the lowest triplet states into the most efficient deactivation pathway -with enormous consequences for photodamage.…”