Ultrafast Excited-State Dynamics of 2,4-DimethylpyrroleMichael Staniforth†‡, Jamie D. Young†‡, Daniel R. Cole†, Tolga N.V. Karsili§, Michael N.R.

Ashfold§* and Vasilios G. Stavros†*

†Department of Chemistry, University of Warwick, Library Road, Coventry, CV4 7AL, UK§School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK

Abstract

The dynamics of photoexcited 2,4-dimethylpyrrole were studied using time-resolved velocity

map imaging spectroscopy over a range of photoexcitation wavelengths, 276-238 nm. Two

dominant H-atom elimination channels were inferred from the time-resolved total kinetic energy

release spectra; one which occurs with a time-constant of ~120 fs producing H-atoms with high

kinetic energies centered around 5000-7000 cm-1 and a second channel with a time-constant of

~3.5 ps producing H-atoms with low kinetic energies centered around 2500-3000 cm -1. The first

of these channels is attributed to direct excitation from the ground electronic state (S0) to the

dissociative 11* state (S1) and subsequent N–H bond fission, moderated by a reaction barrier

in the N–H stretch coordinate. In contrast to analogous measurements in pyrrole (Roberts et al.,

Faraday Discuss. 2013, 163, 95-116), the N–H dissociation times are invariant with

photoexcitation wavelength, implying a relatively flatter potential in the vertical Franck-Condon

region of the 11* state of DMP. The origins of the second channel are less clear cut but, given

the ps time-constant for this process, we posit that this channel is indirect, and is likely a

consequence of populating higher lying electronic states (e.g. 21* (S2)) which, following

vibronic coupling into lower-lying intermediary states (namely S1 or S0), leads to prompt N–H

bond fission.

Keywords: Photodissociation, photofragments, time-resolved, ultraviolet.

1

I. Introduction

In order to gain a better understanding of the photochemistry and photophysics of

complex biochemical systems, spectroscopists have, for decades, utilized bottom-up

methodologies, wherein the spectroscopy of isolated subunits of larger biomolecules are

studied.1, 2 This allows for a more intimate understanding of the intrinsic properties of these

subunits, and thus a better foundation on which to develop a wider understanding of biochemical

processes as we build in molecular complexity.3-5 Exemplar works using such methodologies, of

particular interest to the present study, involve the molecule pyrrole,6-17 a subunit found in heme

and tryptophan.18, 19 Such studies of the ultraviolet (UV) photochemistry and photophysics of

pyrrole have gone some way to providing benchmark spectroscopic data to be exploited for more

complex pyrrole-like systems. In this work, we utilize this benchmark data to continue the

bottom-up theme and study a derivative of pyrrole, 2,4-dimethylpyrrole (DMP). In particular, we

explore the role of N–H bond fission mediated by dissociative 1* states of DMP and compare

this, where appropriate, to its parent molecule, pyrrole.16

The role of dissociative 1* states in the relaxation dynamics of photoexcited molecules

was highlighted by Sobolewski et al.,20 and since then has received growing attention both

experimentally and theoretically.1, 11, 16, 21-26 The authors proposed that 1* states localized along

hetero-atom hydride (X–H, where X is commonly O or N) bond coordinates within a subunit

may be accessed through photoexcitation with UV radiation from the ground electronic state (S0)

of a molecule. Traditionally ‘optically dark’, these states can be populated either (i) directly8, 23, 27-

29 or (ii) indirectly through ‘optically bright’ bound 1* states;1, 20, 21 after excitation to 1*

states, population may flow into the 1* state through internal conversion (IC) mediated by a

1*/1* conical intersection (CI). Once populated, 1* states may facilitate non-radiative

2

relaxation back to S0 via a 1*/S0 CI at extended X–H bond distances or undergo X–H bond

fission.11, 20, 21

While pyrrole in S0 has comparatively high (C2v) symmetry, substituting the hydrogen

atoms at ring-positions 2 and 4 by methyl groups results in a reduced Cs symmetry for DMP

(Figure 1a and 1b, respectively). This has direct consequences for the UV absorption spectrum,

shown in Figure 1c, with the highlighted region indicating the photoexcitation window of the

present measurements. Relative to pyrrole, photoexcitation to the first two 1* states of DMP

(i.e. the and transitions)

becomes electric dipole allowed (given the selection rule , where is the

symmetry of the transition dipole moment operator, which transforms as either or in Cs

symmetry). Thus the UV spectrum of DMP shows a discernible absorption feature spanning the

range 285-245 nm that has been attributed to photoexcitation to the S1 state.14 The corresponding

1*-state absorption in pyrrole is much weaker16 and both theory30 and photofragment

translational spectroscopy studies8 suggest that this (weak) transition strength derives from

vibronic interaction with higher lying 1* states. The steep rise in the DMP absorption spectrum

below 245 nm may be attributable either to the transition

intensity-borrowing from the higher-lying electronic and states

or to direct population of these latter states.14

Our previous studies on pyrrole have demonstrated the role of tunneling out of the quasi-

bound 3s Rydberg well of the 11* state (in the vertical Franck-Condon (vFC) region, the

11* state has notable 3s Rydberg character associated with the N atom).16 Measurements on

pyrrole and pyrrole-d1 (selective deuteration of the N-H bond) returned a kinetic isotope effect

(KIE, kH/kD) of ~11. This observation, coupled to the significant reduction in the appearance

3

time-constant for H-atom elimination at excitation energies above the threshold required to

surmount the barrier associated with the Rydberg well, led us to conclude that tunneling was

very likely operative. Electronic structure calculations for DMP return a more pronounced

barrier associated with 3s(Rydberg)/*(valence) mixing in the 11* state of DMP (~0.35 eV,

defined relative to the local S1 minimum at equilibrium N–H bond length, cf. ~0.18 eV in

pyrrole).31 Relative to pyrrole, therefore, we might anticipate a longer appearance time-constant

for H-atom elimination following photoexcitation of DMP in this quasi-bound region.

In the present study, we test this hypothesis by building on our previous work on

pyrrole.16 Specifically, we investigate the role methyl substitution has on the relaxation dynamics

in DMP and compare these to pyrrole. To this end, we present the first comprehensive study of

the dynamics that take place within DMP following excitation between 276-238 nm using a

combination of time-resolved velocity map imaging (TR-VMI) and detailed theoretical

calculations using the complete active space self-consistent field (CASSCF) theoretical method

together with its second order perturbation theory extension (CASPT2).

II. Methods

a. Experimental methods

The experimental setup of the TR-VMI spectrometer has been described in detail

elsewhere32, 33 and, as such, only a brief overview is given here. A commercially available

Ti:Sapphire oscillator and regenerative amplifier system (Spectra-Physics Tsunami and Spitfire

XP, respectively), produces 3 mJ laser pulses of ~40 fs duration centered around 800 nm at a

repetition rate of 1 kHz. Two optical parametric amplifiers (Light Conversion, TOPAS-C), each

pumped with a fraction of the 800 nm fundamental (1 mJ/pulse), produce tunable UV pump and

4

probe pulses (hpu and hpr respectively). hpu is tuned between 276-238 nm (5-7 J/pulse) and is

temporally varied with respect to the probe pulse by means of a hollow gold retroreflector

mounted on a motorized stage, allowing for a maximum temporal delay of t = 1.2 ns. hpr is set

to 243 nm (~7 J/pulse) and is resonant with the two-photon allowed transition in

hydrogen. Both beams are then focused near-collinearly into the VMI spectrometer where they

intersect perpendicularly a molecular beam seeded with the sample molecule.

The molecular beam is produced by passing 2 bar of He gas over the sample molecule

(DMP, Sigma-Aldrich, 99%) heated to 50-55oC, and then expanding the seeded He into vacuum

(~10-7 mbar) via an Even-Lavie pulsed solenoid valve,34 operating at 125 Hz. Typical opening

times for the valve were 12-14 s. The subsequent supersonic jet expansion passes through a 2

mm conical skimmer, into the VMI spectrometer. Temporal overlap of the pump and probe

pulses (t = 0) and characterization of the instrument response function (IRF) are achieved via

multiphoton ionization of methanol yielding an IRF of ~120 fs at full width half maximum

(FWHM), which provides a minimum temporal resolution of ~30 fs (~25% of IRF).

The spectrometer itself, which is a modified version of our previous setup,32, 33 is now in

line with the molecular beam and follows the standard Eppink and Parker design.35 After

photolysis of the parent molecule with hpu, any resulting H-atoms are ionized with hpr using 2

+ 1 resonance enhanced multiphoton ionization (REMPI). The VMI ion-optics project the 3-D

velocity distribution of H+ ions towards a position sensitive detector consisting of a pair of

micro-channel plates and a P-43 phosphor screen (Photek, VID-240). The emitted light is

captured using a CCD camera (Basler, A-312f). The initial 3-D velocity distribution is

reconstructed from the resulting 2-D image using a polar onion-peeling (POP) algorithm,36

resulting in both energy and angular information. Radial pixels on the image can be converted

5

into total kinetic energy release (TKER) using the appropriate Jacobian, calibration factor and

co-fragment mass, enabling us to obtain the desired 1-D TKER spectra. The calibration factor is

obtained through photolysis of HBr at 200 nm, which produces H-atoms with well-characterized

kinetic energies (KEs).37

Comparative TR-VMI studies were carried out on monodeuterated DMP (2,4-

dimethylpyrrole-d1 (DMP-d1)). DMP-d1 was synthesized according to reference 38 by repeated

stirring of undeuterated DMP in excess D2O in a light sealed vessel for ~24-48 hours. The

deuterated product was then separated from the D2O and dried over Na2CO3. Time of flight mass

spectroscopy was used to check for deuteration and returned a 3:1 ratio of deuterated to non-

deuterated photoproduct.

In addition to the above, a vapor phase UV absorption spectrum of DMP was measured

by placing a drop of DMP in a fused silica sample cell and then recording the spectrum using a

commercially available UV-visible absorption spectrometer (Perkin-Elmer, Lambda 25, 0.2 nm

resolution).

b. Theoretical methods

Using the Gaussian 09 computational package,39 the minimum energy geometry of

ground state DMP was optimized using Møller-Plesset second order perturbation theory (MP2)40

coupled with the aug-cc-pVTZ (AVTZ) basis set. Given the optimized MP2 ground state

geometry, the Molpro 2010.1 computational package41 was used to calculate unrelaxed (rigid

body) potential energy curves (PECs) along the N–H stretch coordinate (RN–H). These were

calculated using CASPT2,42, 43 based on a fully state-averaged CASSCF reference wavefunction

(SA5-CASSCF) comprising the lowest five equally weighted singlet states (3 A' and 2 A" states)

and coupled with the AVTZ basis set with additional even tempered sets of s and p diffuse

6

functions on the N atom (ratio = 2). The (10/10) active space comprised 10 electrons arranged in

the following 10 orbitals: three occupied and two virtual molecular orbitals, two and two

* orbitals (one of each of which is centered around the N–H bond) and the 3s Rydberg orbital

centered on the N atom. Using this same active space and basis set, the geometries of the ground

and first four excited electronic states of the parent molecule were then re-optimized using

CASSCF and corrected with CASPT2 in order to determine the vertical and adiabatic excitation

energies. CASSCF/CASPT2 calculations using the AVDZ basis set were also undertaken on the

2,4-dimethylpyrrolyl radical, (C6H8N in Figure 2a, labelled DMPr henceforth), in order to

calculate the adiabatic (and vertical) excitation energies between the ground and first excited

state of this species. The (10,8) active space in this case comprised of the following eight

orbitals: three occupied and two virtual molecular orbitals, one ring centered and one *

orbital and the N centered pz orbital. Equation of motion coupled cluster single and double

(EOM-CCSD) calculations were also performed within Molpro, at the MP2 optimized ground

state geometry with the AVTZ basis set, to calculate the transition dipole moment (TDM)

vectors and oscillator strengths (f) for transitions from S0 to the first four singlet excited states.

III. Results

a. Ab initio calculations

(1) Vertical and adiabatic excitation energies

Table 1 lists the calculated vertical and adiabatic excitation energies and oscillator strengths for

transitions from S0 to the S1 (11*), S2 (21*), S3 (11*) and S4 (21*) electronic states of

DMP. The vertical excitation energies are in good accord with previous calculations (at the

EOM-CCSD level).14 Reference to Figure 1 confirms that the calculated S1–S0 adiabatic energy

7

matches well with the experimentally observed origin (~279 nm (4.44 eV)). The S1–S0 vertical

excitation energy returned by the CASPT2 calculations exceeds the experimental S1–S0 energy

gap, by ~0.47 eV. This is an inevitable consequence of ‘freezing’ all bar the N–H stretch

coordinate at the optimized ground state geometry, and accords with (i) the expectation that

promoting a -bonding electron should result in some expansion of the ring and (ii) the observed

breadth of the S1S0 (*) absorption.

(2) Potential energy cuts (PECs)

Figure 2a shows the calculated 1-D ‘unrelaxed’ PECs along RN-H (i.e. PECs calculated

with all other coordinates fixed at the corresponding ground state value), demonstrating the

dissociative nature of the S1 and S2 states. Both PECs show a small barrier to NH bond

dissociation, formed through an avoided crossing between the diabatic 1s Rydberg and 1*

valence states. The calculated height of this barrier in the 11* PEC is ~0.35 eV (not zero-point

corrected, and defined relative to the S1 minimum in the vFC region). Upon increasing RN–H, both

the 11* and 21* PECs show a CI with the S0 PEC that will control the branching between

ground (X) or excited state DMPr products (plus an H-atom). We note, as is often the case, that

the present ‘unrelaxed’ CASPT2 calculations significantly over-estimate the lowest dissociation

energy (De(N–H) ~36000 cm-1 (cf. the experimentally determined value of 3120050 cm-1 in

reference 14)). Indeed, the nature of these rigid body scans also lends to the apparent radical

splitting of ~0.2 eV (Figure 2) which is significantly smaller than that predicted by relaxed

CASPT2/aug-cc-pVDZ calculations. These latter calculations return an adiabatic excitation

energy of ~0.85 eV between the ground and first excited electronic states of DMP r – a result to

which we return in Section IV.

8

b. Total kinetic energy release (TKER) spectra

Figure 3a shows an example TKER spectrum with the image from which it was extracted

shown inset; the left half corresponds to the recorded H+ ion image and the right half is a slice

through the center of the reconstructed 3-D ion distribution (the black arrow indicates the electric

field polarization, , of the pump pulse). A pump wavelength of 272 nm was used to photoexcite

DMP molecules to the S1 state, with the generated H-atoms subsequently ionized via 2 + 1

REMPI by the probe pulse. The pump-probe delay time used when recording this particular

image was set at t = 1.2 ns. The dominant feature is a single ring with a highly anisotropic

distribution (see Figure 3j and below), corresponding to the high KE feature observed in the

TKER spectrum centered at ~5500 cm-1.

Figures 3b-3i show the TKER spectra obtained from photolysis of DMP over the

wavelength range of 276-238 nm at two different pump-probe time-delays; t = 1 ps (blue) and

1.2 ns (red). In all cases, the images used to obtain the TKER spectra have had a one-color (243

nm probe only) background image subtracted from them to reveal the true two-color pump-probe

signal. Each spectrum possesses a common high KE feature peaking around 5000-7000 cm-1, the

intensity of which remains constant beyond t = 1 ps. A second, lower KE feature is present at

pump wavelengths ≤254 nm, and appears subsequent (in time) to the high KE feature.

The high KE feature can be understood by considering the PEC of the 11* state, which

correlates non-adiabatically to H + DMPr(X) products (Figure 2). TKER spectra obtained at short

time delays when exciting at wavelengths <259 nm (i.e. above the calculated energy of the

barrier to dissociation on the 11* PEC (~0.35 eV, ~2800 cm-1)) are well described using a

Gaussian function (see spectra collected at t = 1 ps (blue), Figures 3b-3e). We can envisage two

contributory reasons for this. First, the spectral bandwidth of the pump pulse (~500 cm -1)

9

prepares a broad wavepacket of Franck-Condon active vibrational modes, which then map

through into the DMPr products. Second, the energy resolution of our VMI spectrometer (E/E

~7%) results in broadened features.24 The mean TKER of this high KE feature, , remains

approximately constant across a broad range of pump wavelengths, as seen in previous time and

frequency domain studies of pyrrole,8, 16 and corresponds to the energy difference between the

barrier associated with 3s(Rydberg)/*(valence) mixing in the 11* state and the dissociation

asymptote to yield H + DMPr(X) products (Figure 2a). Interestingly, at the longest pump

wavelength (276 nm), the high KE feature is clearly distorted, with a sharp cut-off at high KE.

This is a direct result of DMPr being populated in only a limited set of vibrational states,

predominately in v = 0. This result is in excellent agreement with the H Rydberg atom

photofragment translational spectroscopy (HRA-PTS) studies of Karsili et al.14

Through consideration of the above energetics, we can determine the TKER for

dissociation into DMPr and H-atom photofragments according to the relationship:

(1)

In Equation (1), hpu is the pump photon energy, D0(NH) is the adiabatic dissociation energy of

the N–H bond and Eint is the total internal energy of the radical (electronic and vibrational). By

using the experimentally determined value of D0(NH) = 3120050 cm-1,14 and for DMPr

cofragments formed in the X-state and v = 0 (Eint = 0), we obtain maximum TKER (TKERmax)

values spanning the range ~5000-10800 cm-1 with our photoexcitation wavelengths. These values

are indicated by the blue arrows in Figures 3b-3i and match well with the high KE tails of the

TKER spectra. We note that TKERmax sits on or near the peak of the high KE feature in spectra

10

recorded at 276 and 272 nm, consistent with previous observations that the DMP r fragment is

formed predominantly in its X(v = 0) level,14 and that the TKER spectrum recorded at 272 nm

shows a small but obvious shoulder on the low TKER side. The energy separation between this

shoulder and the peak of the high KE feature accords well with the energy spacing between the v

= 0 level and one quantum of in-plane ring breathing mode, v19, in DMPr – as observed in the

earlier HRA-PTS study of DMP around this photoexcitation wavelength.14 This is further

illustrated in the supporting information (SI, Figure S1).

To obtain further insight regarding the origins of the high KE feature, specifically which

electronic state we are initially photoexciting, we turn to the measured anisotropy parameters, 2,

shown in Figure 3j. These have been obtained by averaging the 2 values across the FWHM of

the high KE feature at a delay time of t = 1.2 ns. Formally, 2 can take values ,

where the limiting values of -1 and 2 correspond to scenarios in which the dissociating bond is

aligned, respectively, perpendicular or parallel to the electronic transition dipole moment (TDM),

. The excitation probability is maximal when is aligned parallel to 44 Photoexcitation at

272 nm results in a near limiting value of -1, consistent with prompt dissociation following

photoexcitation to the 11* state, where is aligned perpendicular to the N–H bond, as

indicated by the TDM vectors in Figure 2b. With decreasing photoexcitation wavelength, 2

declines, to ~-0.25 in the spectrum recorded at 238 nm. The trend in 2 does not show a step

change such as might be anticipated if the dominant transition moment switched from being

perpendicular (as calculated for the * excitations) to near-parallel with the N–H bond (as

predicted for the S3S0 (*) excitation – see Figure 2b for TDMs); the gradual decline

(shown, as a guide, by the red line in Figure 3j) is more compatible with a progressive vibronic

enhancement (by the higher lying 1* states) of the S1S0 transition moment.

11

The low KE feature is more difficult to reconcile with the PECs (Figure 1) and we once

again seek insight from the measured 2 values. 2 for this feature is consistently ~0 across all

excitation wavelengths (≤254 nm), with an appearance time-constant of ~3.5 ps (vide infra)

likely ruling out the loss in anisotropy as a result of rotational motion of the parent molecule.

Likewise the striking difference between 2 values for the low and high KE features (2 ~0 at

254 nm for the low KE feature versus 2 ~-0.5 for the high KE feature) suggests the onset of a

second transition, and it is tempting to assign this low KE feature to dissociation following

S2S0 absorption. Given that 2 is ~0, this would imply vibronic enhancement from both S3 and

S4, whose transition moments from S0 are orthogonal to one-another (Figure 2b).

As demonstrated in previous publications (e.g. reference 1 and references therein), a

quantitative measure of the separation between different features within TKER spectra such as

those seen in Figures 3b-3e can be obtained by fitting functions (e.g., Gaussian and Boltzmann

functions) to the TKER spectra. We elect to fit the TKER spectra, based on the quality of the fits

and for consistency, with two Gaussian functions for the low and high KE features in the data

presented in Figures 3b-3e. The fits return a separation between the low and high KE

features of ~3000 cm-1, in very good accord with that determined from analysis of the

corresponding TKER spectrum obtained by HRA-PTS following photoexcitation of DMP at 238

nm (SI, Figure S1).

c. H-atom transients

Collecting a series of TKER spectra at different t and then integrating the high KE

feature results in the H+ transients shown in Figures 4a and 4b for pump wavelengths of 272 and

238 nm respectively. D+ transients were also collected following excitation of DMP-d1 at 272

12

and 238 nm and are shown in Figures 4c and 4d respectively. We specifically focus on these two

photoexcitation wavelengths: 272 nm excitation populates the S1(11*) state directly, whilst

reference to the absorption spectrum (Figure 1c) and preceding discussion suggests that 238 nm

excitation likely populates the S2(21*) state also. Analogous measurements were performed at

selected intermediate wavelengths, the data from which are presented in Table 2 with the fitted

transients given in the SI. These transients are fit to a single exponential rise function at positive

t (black line), a single exponential rise at negative t (green line) and an exponential decay

(blue line), centered at t = 0. All three functions were convoluted with our IRF. The red line

shows the overall fit. It is important to note that, in order to avoid contaminating the high KE

transients collected at ≤254 nm with signal associated with the low KE feature (due to spectral

overlap between low and high KE features), it was necessary to make a judicious choice of the

integration area (from 500 cm-1 below the peak centre of the high KE feature to ~15000 cm-1).

Further details relating to the fitting procedure can be found in the SI.

Two distinct features to the transients shown in Figure 4 are worthy of comment. First,

there is a sharp rise and decay at t = 0 with a decay time-constant returned from our kinetic fit

of dec <30 fs. The minimum temporal resolution of our setup is ~30 fs, so we quote time-

constants returned from the kinetic fitting that are shorter than this value as <30 fs. This is most

likely due to multiphoton absorption and subsequent generation of H+(D+) products through

dissociative ionisation.45-48 Interestingly the feature is clearly present in three of the four

transients (Figure 4a-c) but less so in Figure 4d, and likely highlights the sensitivity to laser

pump intensity in generating these H+(D+) fragment ions. The second is the reverse dynamics and

manifests as a sharp rise with a time-constant rev <30 fs. As in pyrrole,16 this is likely due to N–

H bond fission following excitation at 243 nm and subsequent 2-photon excitation of the

13

transition in atomic hydrogen. The pump laser radiation then ionizes these excited H-

atoms to generate H+ (or D+) ions. Since these features have no consequence to the thrust of the

current study, they are not discussed further.

The 272 and 238 nm H+ transients both show a rise (at positive t) with a time-constant

of hKE = 120 ± 9 fs (272 nm) and 132 ± 23 fs (238 nm) returned from the kinetic fit. The

subscript hKE denotes the exponential rise in the H+ signal attributed to the high KE feature

(~5000-7000 cm-1) as is evident from the TKER spectra at all excitation wavelengths shown in

Figure 3. The D+ transients recorded at 272 and 238 nm also both show a rise, with respective

time-constants hKE = 392 ± 18 fs (272 nm) and 228 ± 37 fs (238 nm). We contrast the >3-fold

increase in the appearance rate of H+ ions following photoexcitation at 272 nm (120 fs, cf. 392 fs

for D+) with the <2-fold difference in the respective appearance rates when exciting at 238 nm,

and return to consider the implications of these time-constants in Section IV.

As is evident in the TKER spectra collected at ≤254 nm (Figure 3b-3e), a low KE feature

emerges subsequent (in time) to the high KE feature. To model the appearance time for the

former, we integrate the TKER spectra between 0-15000 cm -1 and fit the measured H+ (and D+)

transients in an analogous manner to that described for the high KE component, with an

additional exponential rise function to reflect the low KE component. Importantly for these fits

hKE, determined through integrating the high KE feature (vide supra), is held fixed, which

markedly improves the fits (along with errors) during optimisation in parameter space. The

resultant fits (in red) are shown in Figure 5a and 5b for both H+ and D+ respectively, following

photoexcitation at 238 nm. The time-constants returned for the low KE feature,lKE, are lKE =

3.7 ± 0.9 ps and lKE = 5.2 ± 0.9 ps for H+ and D+ respectively. We discuss the low KE feature

further in Section IV.

14

IV. Discussion

Inspection of the unrelaxed PECs displayed in Figure 2a would encourage the expectation

that photoexcitation at wavelengths ~276 nm promotes DMP molecules to quasi-bound levels

within the 3s Rydberg well of the S1 state. The calculated barrier to N–H bond fission (~0.35 eV

defined relative to the S1 minimum) is almost twice that calculated for the corresponding 1-D

PEC for the S1 state of pyrrole (~0.18 eV 14, 16, 31). Within this 1-D picture therefore, and

(temporarily) neglecting modifications arising from proper inclusion of zero-point energy

effects, we would predict that N–H bond fission following excitation of DMP at energies near its

S1 origin would involve tunneling, and that the timescale for H-atom appearance would be longer

in the case of DMP than in pyrrole.

Yet experiment reveals very similar H-atom appearance times when exciting DMP and

pyrrole at wavelengths near their respective S1 origins (i.e. hKE =120 ± 9 fs when exciting DMP

at 272 nm, cf. = 126 ± 28 fs when exciting pyrrole at 254 nm 16). Further, the H-atom

appearance times from DMP vary little with pump wavelength (e.g. hKE =132 ± 23 fs when

exciting at 238 nm – a photon energy that is well above that of the calculated barrier in the 1-D

S1 PEC). Observation of a large KIE is another tell tale signifier of a tunneling process. The

measured time-constant for D-atom elimination following excitation of DMP-d1 at 272 nm (hKE

= 392 ± 18 fs) implies a rate constant ratio kH/kD ~3.3, i.e. a KIE larger than the 2 expected on

the basis of the H/D mass difference and consistent with some (mild) tunneling behavior.49 The

corresponding ratio of decay rate constants when exciting DMP-d1 at 238 nm (hKE = 228 ± 37 fs)

is kH/kD ~1.7 (see Table 2). The measured KIEs thus hint at some possible role for tunneling

when exciting DMP at the longest excitation wavelengths. However, the (small) magnitude of

the effect, especially when one takes into account the uncertainty in the measured time-constants

15

for DMP and DMP-d1 at 238 nm, which returns a KIE that ranges between 2.4 and 1.2, and the

insensitivity of the H+ appearance time from DMP to pump wavelength would imply (at most) a

mild barrier along the 11* state in DMP.

Two salient points are worth recapping here. First, the similar H-atom elimination times

measured when exciting DMP and pyrrole at energies near their respective S1 origins runs

counter to expectations based on the ~2-fold larger (1-D) calculated barrier to N–H bond fission

in DMP. Second, whereas excitation of pyrrole at energies above the 3s/* barrier results in

much faster dissociation (e.g. = 46 ± 22 fs when exciting at 238 nm 16), the H-atom appearance

time from DMP changes little upon tuning to shorter wavelengths (Table 2).

All of the experimental data taken at longer excitation wavelengths imply that the 1-D

PECs (fig. 2) provide a poorer representation of the minimum energy path to N–H bond fission

on the S1 PES than in the case of pyrrole. The effective barrier to dissociation on the full

dimensional PES must be smaller. Zero-point energy (ZPE) effects have been neglected thus far.

The ZPE associated with the three modes (one stretch, two bends) most intimately linked with

the disappearing N–H bond will be similar in pyrrole and DMP, and reduce similarly on

extending RN–H from its equilibrium value out to the barrier maximum, so including such effects

would not be expected to alter the landscape along the N–H coordinate substantially. However,

DMP has 42 vibrational degrees of freedom (cf. 24 in pyrrole), including several low frequency

modes (e.g. torsional modes associated with the methyl groups14, 50). Such low frequency modes

are very likely to be excited when using a broadband fs pump pulse, and their associated energy

could aid passage through (or over) the effective barrier to dissociation.49, 51, 52 Any observed KIE

must reflect contributions from all such factors.

16

The measured hKE values for H-atom elimination from DMP remain ~120 fs when

exciting at energies well above the calculated barrier to dissociation. This, too, is understandable

if the effective barrier to dissociation is smaller than in pyrrole, and the minimum energy path for

N–H bond fission on the full dimensional S1 PES is actually rather flat (along RN–H) in the vFC

region. Given a suitably shallow gradient, a wavepacket prepared near the S1 origin could take

~100 fs to move out of the vFC region and towards the 1*/S0 CI; the measured mean TKER is

determined by the repulsive part of the S1 potential at longer RN–H. Moving to shorter pump

wavelengths, hKE barely changes nor, as the blue shaded regions in Figure 3 show, does the

mean TKER of the prompt dissociation products. These observations, and the deduced KIE

(kH/kD ~1.7 at 238 nm), can be understood if photoexcitation populates parent modes orthogonal

to the N–H dissociation coordinate and the excitation in such ‘spectator’ skeletal modes

(including the low frequency torsional modes of the methyl groups) maps through adiabatically

into the DMPr products – as proposed in the original HRA-PTS study of pyrrole photolysis.8

We close with a brief discussion of the low TKER component, which gains in relative

importance on shifting to shorter pump wavelengths. The origins of this component are less

clear, but several observations from the present data may assist towards identifying its source. (1)

The measured KIE (kH/kD ~1.5 (mean value), Table 2) suggests that the path responsible for the

ps dynamics is unlikely to involve tunneling. (2) The low and high KE components observed

when exciting at wavelengths ≤254 nm show differences in both their measured anisotropy

parameters (see Section IIIb) and temporal behaviors, as demonstrated in Figure 5; following H-

atom elimination to yield high KE H/D-atoms in <500 fs, the low KE component only begins to

plateau beyond 20 ps. Such behavior might be expected if the two components originate from

two different initial states populated in the photoexcitation process (e.g. the 11* and 21*

17

states). A possible alternative, wherein photoexcitation populates a single initial state which then

decays by two (or more) routes to give the different TKER components can be excluded on

kinetic grounds: relative to hKE, the formation time constant for the low TKER component (lKE)

is uncompetitive and its quantum yield should thus be negligible in this scenario. (3) Unintended

multiphoton processes are unlikely to be making any significant contribution to the low TKER

products, given their presence in both the fs TR-VMI and ns HRA-PTS measurements. As Figure

S1 in the SI shows, the TKER spectra taken using the two methods at 238 nm are essentially the

same, notwithstanding the 2-orders of magnitude differences in pump laser intensities (1011-1012

W cm-2 and 109-1010 W cm-2, respectively). (4) Our highest level relaxed CASPT2/aug-cc-pVDZ

calculations predict a splitting between the X and A states in DMPr of ~0.85 eV (~7000 cm-1),

very different from the observed peak separation between the high and low KE components

(which, as Figure 3b-3e show, is consistently 3000 cm-1) and tends to rule out excitation to the S1

state and subsequent dissociation to H + DMPr(A) products as the origin of the low KE

component.

Armed with the above, one could speculate (in accord with point (2)) that excitation at

wavelengths ≤254 nm could be accessing S2 (and possibly S3 and S4). Whilst the present

adiabatic and vertical excitation energies might imply that these states are inaccessible at 254 nm

(where the low KE feature begins to appear), the obvious rise in the UV absorption spectrum by

245 nm (Figure 1c) suggests otherwise and it is tempting to suggest that we are starting to access

the tail end of absorption to these states. Appropriate coupling could then occur between e.g. S2

and S1, which would then lead to rapid N–H dissociation. Indeed, a recent quantum dynamics

study in pyrrole by Neville and Worth53 has shown that both the antisymmetric C–H stretching

modes (Q17 and Q19) and the ring-stretching mode (Q21), vibronically couple the S2 and S1 states.

18

One could envisage an analogous scenario in DMP, with similar high frequency modes coupling

S2 with S1. The low KE feature may then arise from sequential N–H dissociation,

with lKE reflecting the time for IC from (once on the S1 state, dissociation will occur on

a timescale akin to hKE). The red shift of this feature in the TKER spectra is reconciled by the

fact that energy, in these high frequency modes, is retained in the radical cofragment DMP r.

Alternatively, S2 could be coupling to S0 through appropriate CIs, akin to those revealed in recent

calculations for pyrrole.54 If these CIs have a component of N–H stretch, then one could envisage

cases where momentum along N–H cannot be quenched fast enough through intramolecular

vibrational redistribution. Once again, the modes involved in the nonadiabatic coupling at the

CIs could then be retaining the excess energy in DMPr.

V. Conclusions

The dynamics of photoexcited DMP have been studied using TR-VMI following photoexcitation

between 276-238 nm. From the measured TKER spectra and associated H+ transients, two

dominant H-atom elimination channels are deduced. The first channel occurs with a time-

constant of ~120 fs producing H-atoms with high KE (5000-7000 cm-1); the second channel

occurs with a time-constant of ~3.5 ps and produces H-atoms with low KE (2500-3000 cm -1).

The first channel is attributed to direct excitation to the 11* (S1) state. The invariance of the

measured dissociation time-constant of the first channel to excitation wavelength suggests a

shallower potential (along RN–H) in the vFC region of the 11* state of DMP, with a less

pronounced Rydberg/valence barrier than in pyrrole. The origins of the second channel are less

clear. However, the indirect nature of the dissociation (implied by the ~3.5 ps time-constant)

could be explained in terms of initial (vibronically induced) excitation to one or more higher

19

lying electronic states (most probably the 21* (S2) state), which decay by N–H bond fission

following radiationless transfer to the S1 and, possibly, the S0 states.

Importantly, the present studies provide further illustrations of the ways in which simple

chemical substitution, remote from the dissociation coordinate, can influence the excited state

dynamics, viz the substitution of hydrogen atoms at ring-positions 2 and 4 of pyrrole by methyl

groups. These studies also serve to highlight the importance of bottom-up approaches, as there is

still much to be learned about the intrinsic properties of what appears, at first sight, a very similar

system to pyrrole. For instance, a more complete rationale for the observed dynamics following

excitation to the 11* state requires proper treatment of the nuclear dynamics utilizing fully

multi-dimensional potential energy surfaces in DMP. In doing so, this will assess the role of low

frequency modes such as methyl torsions, which may aid passage through (or across) the barrier

to dissociation. Whilst this is beyond the scope of the present studies, such a study is certainly

warranted in order to progress to more complex polyatomic systems.

20

ASSOCIATED CONTENT

Supporting Information

TKER spectra derived from H Rydberg atom photofragment translation spectroscopy, further

details regarding our kinetic fits and H+/D+ transients at intermediate wavelengths. This

information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

*E-mail: v.stavros@warwick.ac.uk, mike.ashfold@bristol.ac.uk

Author Contributions

‡These authors contributed equally to the work.

ACKNOWLEDGMENTS

The authors are grateful to Dr Gareth Roberts (Bristol) for helpful discussions. M.S. thanks the

EPSRC for postdoctoral funding and J.D.Y. thanks the University of Warwick for a doctoral

training award. V.G.S. thanks the EPSRC for an equipment grant (EP/J007153) and the Royal

Society for a University Research Fellowship. The Bristol component of the work was enabled

by EPSRC programme Grant EP/L005913/1.

21

Table 1. Vertical and adiabatic (relaxed) excitation energies from S0 to the S1, S2, S3 and S4 states

of DMP calculated at the CASPT2(10,10)/aug-cc-pVTZ (AVTZ) level. The values in brackets

are in wavelength (nm) to aid direct comparison with the UV absorption spectrum of DMP

(Figure 1c). Associated oscillator strengths calculated using EOM-CCSD.

Transition Vertical Energy / eV Adiabatic Energy / eV Oscillator Strength f

S1-S0 (*) 4.91 (253) 4.58 (271) 0.0019

S2-S0 (*) 5.66 (219) 5.45 (228) 0.0012

S3-S0 (*) 5.87 (211) 5.49 (226) 0.0218

S4-S0 (*) 6.04 (205) 5.51 (225) 0.1447

Table 2. Time-constants extracted from kinetic fits to TR-VMI H+ (D+, in brackets) transients

following excitation at the specified wavelengths. Quoted errors are one standard deviation ().

hKE is assigned to direct N–H(D) bond fission following excitation to the S1(11*) state (i.e. the

high KE feature), while lKE is associated with the low KE feature (see Figure 2).

λ / nm 272 265 254 245 238

hKE / fs 120 ± 9 (392 ± 18)

124 ± 8 (325 ± 19)

118 ± 12 (204 ± 18)

101 ± 24 (175 ± 18)

132 ± 23 (228 ± 37)

lKE / ps - - 4.0 ± 2.3 (5.0 ± 2.2)

3.2 ± 0.8 (6.6 ± 1.9)

3.7 ± 0.9 (5.2 ± 0.9)

22

Figure 1. Molecular structures of a) pyrrole and b) DMP. c) Vapor-phase UV absorption

spectrum of DMP between 200-290 nm. The blue arrow indicates the excitation wavelength

range investigated in this study.

23

Figure 2. a) Calculated unrelaxed PECs for the S0, S1, S2, S3, and S4 states of DMP. The grey

circle highlights the S1/S0 and S2/S0 CIs. b) Calculated TDMs for transitions from S0 to the S1, S2,

S3 and S4 excited states, the last two of which lie in the plane of the ring.

24

Figure 3. a) TKER spectrum recorded after photoexcitation at 272 nm and probing H-atom

photoproducts using a time-delayed 243 nm probe pulse at t = 1.2 ns. The inset shows the VMI

image from which the TKER spectrum was derived (left half) together with a reconstructed slice

through the center of the original 3-D ion distribution (right half). The vertical black arrow

indicates the electric field polarization, , of the pump pulse. b)-i) H-atom TKER spectra over a

pump wavelength range 238-276 nm respectively at t = 1 ps (blue) and 1.2 ns (red). Spectra b)-

e) have been fit to two Gaussian functions (see main text for details). Vertical blue arrows

indicate the predicted TKERmax values for N–H bond dissociation into DMPr (X) + H products. j)

2 values associated with the high KE feature in the TKER spectra shown for b)-i). The red line

serves as a visual guide to the reader.

25

Figure 4. Normalized H+ and D+ signal transients following excitation of DMP at a) 272 nm and

b) 238 nm, and DMP-d1 at c) 272 nm and d) 238 nm. The open circles show the integrated H+/D+

signal around the high KE feature, whilst the solid red line shows the overall kinetic fit.

Components to the overall fit are given by the black (exponential rise, positive t), green

(exponential rise, negative t) and blue (exponential decay around t = 0) lines. See text for

further details.

26

Figure 5. Normalized H+ and D+ signal transients following excitation of a) DMP and b) DMP-d1

at 238 nm. The open circles show the integrated H+/D+ signal between 0-15000 cm-1, whilst the

solid red line shows the overall kinetic fit (see text for details).

27

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