Excited state hydrogen transfer dynamics in substituted phenols and their complexes with ammonia: ππ∗-πσ∗ energy gap propensity and ortho-substitution effect

Abstract

Lifetimes of the first electronic excited state (S(1)) of fluorine and methyl (o-, m-, and p-) substituted phenols and their complexes with one ammonia molecule have been measured for the 0(0) transition and for the intermolecular stretching σ(1) levels in complexes using picosecond pump-probe spectroscopy. Excitation energies to the S(1) (ππ*) and S(2) (πσ*) states are obtained by quantum chemical calculations at the MP2 and CC2 level using the aug-cc-pVDZ basis set for the ground-state and the S(1) optimized geometries. The observed lifetimes and the energy gaps between the ππ* and πσ* states show a good correlation, the lifetime being shorter for a smaller energy gap. This propensity suggests that the major dynamics in the excited state concerns an excited state hydrogen detachment or transfer (ESHD/T) promoted directly by a S(1)/S(2) conical intersection, rather than via internal conversion to the ground-state. A specific shortening of lifetime is found in the o-fluorophenol-ammonia complex and explained in terms of the vibronic coupling between the ππ* and πσ* states occurring through the out-of-plane distortion of the C-F bond.

A Multipronged Comparative Study of the Ultraviolet Photochemistry of 2-, 3-, and 4-Chlorophenol in the Gas Phase

“…5 The reported lifetime for 4-FPhOD(S1) molecules is not much longer:  = 3.2 ns. 40 The reported lifetimes for PhOH(S1) and PhOD(S1) molecules are also similar: 2.2 ns and 13.3 ns, respectively.…”

“…This study returns a 2-FPhOH(S1) lifetime  = 2.3±0.2 ns, in reasonable accord with previous estimates from high resolution linewidth studies of the syn-conformer ( = 4.60.2 ns and 2.8±0.2 ns 31 ) and similar to that found when exciting bare phenol at its S1-S0 origin. 5,30 As with phenol, the 2-FPhOH(S1) decay time constant was seen to decrease on tuning to shorter excitation wavelengths (e.g.  =…”

The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects

“…Previous studies of ways in which ring-substituents affect the dynamics and energetics of photoinduced O-H bond fission in phenols 16,17,[31][32][33][34][35][36] and S-H bond fission in thiophenols [37][38][39] have been extended to the cases of 2-and 3-YPhSH (with Y = Me and F). Using a combination of experiment-H (Rydberg) atom PTS, at many different near UV excitation wavelengths-and complementary electronic structure methods, we have been able to gain further insights into the interplay between substituent-induced geometric (i.e., steric effect/intramolecular hydrogen bonding) and electronic (i.e., π (resonance) or σ (inductive)) effects.…”

“…Such fragmentation dynamics have been demonstrated in phenol [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] and thiophenol, [24][25][26][27][28][29][30] in a range of substituted phenols [31][32][33][34][35][36] and thiophenols, [37][38][39] and in the methylated analogues anisole 40,41 and thioanisole, [42][43][44][45][46] in both the gas and condensed [47][48][49][50] phase. The present study focusses on thiophenols and, particularly, how the much-studied S-H bond fission process is affected by asymmetric substitution (i.e., in the 2-and 3-positions) of the aromatic ring.…”

Exploring the Energy Disposal Immediately After Bond-Breaking in Solution: The Wavelength-Dependent Excited State Dissociation Pathways of para-Methylthiophenol

“…23,24 The absorption cross-section for direct excitation to the 1 * state increases at λ < 271 nm, and by 260 nm it becomes the only state populated in the photo-excitation. 15 Our earlier transient absorption work on this system in ethanol and cyclohexane solution 16,17 has…”