Photophysical and spectroscopic manifestations of the low-lying πσ* state of 4-(dimethylamino)benzethyne: solvent-polarity dependence of fluorescence and excited-state absorptions

Abstract

A concerted experimental and computational study of 4-(dimethylamino)benzethyne, DMABE, has been carried out to probe the low-lying pisigma* state and the role it plays in the photophysics of the molecule. The subpicosecond transient absorption spectra reveal the presence of a strong excited-state absorption at about 700 nm and a weaker absorption at about 520 nm. The observed absorption maxima are in excellent agreement with the TDDFT calculations that place a strongly allowed pisigma* <--pisigma* transition at 750 nm, and a weaker pipi* <--pipi* (LE) transition at 528 nm. This agreement combined with the differing decay times, and differing solvent shifts of the two transients, allow assignments of the 700 nm absorption to the pisigma* state and the 520 nm absorption to the LE (pipi*) state. The bifurcation of the initially excited L(a) (pipi*) state into the pisigma* state and the LE state, as probed by transient absorption, is strongly influenced by solvent polarity, with polar environments favoring the L(a)-->pisigma* decay channel over the competing L(a)--> LE decay channel. The nanosecond radiationless decay of the LE state to the dark pisigma* state is also strongly enhanced in polar environments, thus accounting for the dramatic quenching of fluorescence in solvents of high polarity.

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

“…3) of the two compounds in n-hexane. It is evident that fluorescence of DMABE in acetonitrile is also entirely from the lowerest-energy ππ* state (LE state), consistent with the absence of the ICT reaction of the molecule even in the polar solvents 9 . Figure 4a presents the excited-state absorption spectra of DMABE in n-hexane and acetonitrile.…”

“…6,8 The vertical TD/BP86/cc-pVDZ excitation energies, calculated at the optimized CIS geometries of the ππ* and πσ* states, show that only one strongly-allowed (with an oscillator strength of about 0.6) excited state absorption of πσ* ← πσ* type occurs at about 640 nm (DMABN), [4][5][6] and 750 nm (DMABE). 9 The ππ* ← ππ* transitions of the lowest-energy ππ* state (L b or locally excited; LE state) to the higher-lying ππ* states are predicted to occur at about 524 nm and 423 nm (in DMABN) with much smaller oscillator strengths. [4][5][6] These predictions are in good accord with the experimental observations of a strong excited-state absorption at about 700 nm and weaker absorptions at about 525 and 450 nm.…”

“…We have proposed that the absence of the TICT state in DMABE supports the πσ*-mediated TICT model, as the TICT state of the molecule lie above the πσ* state, whereas the opposite is the case in DMABN. 9 Figure 5 illustrates a schematic energy diagram of DMABN (in acetonitrile) for the proposed πσ*-mediated ICT reaction mechanism.…”

Intramolecular charge transfer of 4-(dimethylamino)benzonitrile probed by time-resolved fluorescence and transient absorption: No evidence for two ICT states and a πσ∗ reaction intermediate

“…It was recently mentioned that the assignment of the 700 nm transient to the ‫ء‬ ͑C w N͒ state is strongly supported by the fact that 4-͑dimethylamino͒benzaldehyde ͑DMABA͒ and related ketones and ethers, without a cyano group, do not exhibit an absorption around 700 nm ͑spectra not shown͒. 58 This is rather surprising, as among the D/A molecules having a C͑vO͒R substituent, 5-formyl-1-methyl-indoline ͑FMI͒ in MeCN ͑no ICT͒, has a transient absorption spectrum with a maximum at about 610 nm. 7,59,60 Also 5-N , N-dimethylamino-1-indanone ͑DMI͒ in 1-propanol has a pronounced transient absorption maximum in this spectral range ͑shifting from 635 to 620 nm due to relaxation of the polar solvent͒.…”

Synthesis and structure‐activity relationship of several aromatic ketone‐based two‐photon initiators

“…A charge transfer state is populated either via direct excitation, or indirectly, via excitation of a locally excited state. 38 The current data does not allow speculations of possible populations of the triplet state of the PIs. However femtosecond time resolved transient absorption and emission spectroscopy are currently being performed to help shed light onto the photophysics and possible mechanism(s) of photoinitiation for these TPIPs.…”