Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

Abstract

International audienceWe present here the results of time-resolved absorption and emission experiments for 4-(dimethylamino)benzonitrile in solution, which suggest that the fluorescent intramolecular charge transfer (ICT) state may differ from the twisted ICT (TICT) state observed in transient absorption

The role of the πσ* state in intramolecular charge transfer of 4-(dimethylamino)benzonitrile

“…6,7,12 The TICT state absorption at 330 nm decays in ∼ 4.8 ns, which is significantly longer than the decay time (2.9 ns) of the ICT fluorescence at 490 nm. 7 The rise time of the ICT fluorescence, ∼ 3.1 ps (which is the same as the fast-component decay time of the LE fluorescence), also differs from that (∼ 4.3 ps) of the TICT-state transient absorption (TA) at 420 nm.…”

“…6,7,12 The TICT state absorption at 330 nm decays in ∼ 4.8 ns, which is significantly longer than the decay time (2.9 ns) of the ICT fluorescence at 490 nm. 7 The rise time of the ICT fluorescence, ∼ 3.1 ps (which is the same as the fast-component decay time of the LE fluorescence), also differs from that (∼ 4.3 ps) of the TICT-state transient absorption (TA) at 420 nm. 7 The disparity between the rise and decay times of the ICT fluorescence and those of the TICT-state TA suggest that the fluorescence ICT state differs from the TICT state observed in transient absorption.…”

“…1 We have found that the strong quenching of fluorescence at higher excitation energies in gas phase 2 and at elevated temperature in non-polar solvents 3 can be attributed to the crossing of the fluorescent ππ* state and a dark πσ* state, 1 which arises from the promotion of an electron from the π-orbital of the phenyl ring to the σ*-orbital localized in the C≡X (where X is N or CH) group. In polar solvents, the πσ* state of 4-(dimethylamino)benzonitrile, DMABN, has been proposed [4][5][6] to be the precursor of the twisted intramolecular charge transfer (TICT) state, [4][5][6][7] in which the positively charged dimethylamino group lies perpendicular to the negatively charged benzonitrile moiety.…”

A theoretical study of the intramolecular charge transfer in 4-(dimethylamino)benzethyne

“…For the case of DMABN and some related molecules, the evidence of the participation of ps* states in the relaxation processes leading to the population of ICT states rests upon recent experimental studies carried out by Lim, Gustavsson and co-workers. 24 The results obtained by these authors using time resolved transient absorption and fluorescence up-conversion experiments point out to the existence of two different relaxation pathways for the initial excited population of DMABN. Specifically, the decay time of the LE state was found to be similar to the rise time of the TICT transient absorption, and different from the ICT fluorescence feature measured by fluorescence up-conversion techniques, pointing towards the existence of more than one ICT state present in the molecule on polar solvents.…”

“…9 Recently, however, it has been suggested that a low-lying ps* state arising from the promotion of an electron from the aromatic p to a s*-like orbital localised on the cyano (CRN) group in DMABN could be a major precursor of the ICT state. [21][22][23][24][25][26][27] This idea is based on and influenced by previous ab initio computational studies where a minimum in the ps* ICT state was located. 5,28,29 The involvement of ps* states in different photoinduced processes has been increasingly recognised over the last few years and it is nowadays accepted that this kind of excited state plays an important role in processes ranging from molecular photodissociation to ICT formation.…”