S 2 state photochemistry of diphenylcyclopropenone
2.2 Experimental results
2.2.3 Transient IR spectra of diphenylcyclopropenone
Spectra of diphenylcyclopropenone corresponding to the ones discussed earlier for diphenylacetylene are shown in Figure 2.4. In the stationary spectrum, the most promi-nent features are strong bands at 1632 and 1854 cm−1 assignable to a combination of the carbonyl mode and the central C-C double bond as well as the carbonyl mode and the cyclopropene ring distortion mode (assignment from a quantum chemical calcu-lation using the GAMESS[49] package with the B3LYP functional[50] and 6-311G basis set[51]). Smaller bands at 1340, 1448, and 1498 cm−1 originate from ring vibrations of the phenyl moieties and the cyclopropene ring.
In the stationary spectrum, the absorption at 1632 cm−1has an integrated absorp-tion coefficient of 180 km mol−1. For the spectrum recorded 25 ps after excitation, one
Chapter 2. S2state photochemistry of diphenylcyclopropenone
1400 1500 1600 1700 1800
−6
−4
−2 0
25 ps
∆A/mOD
−6
−4
−2 0
1000 ps
O
∆A/mOD
1400 1500 1600 1700 1800
0 10
5
˜
ν/cm−1
e/100lmol−1cm−1
Figure 2.4:From top to bottom: transient IR spectra of diphenylcyclopropenone with small (top panel) and large (middle panel) delays after excitation at 267 nm and stationary IR spectrum (bottom panel), all measured in CD3CN.
would expect a strong absorption of theS1 state of diphenylacetylene, which is obvi-ously not the case. Rather, the observed absorption corresponds to a few percent of diphenylacetylene in its S1 state relative to the bleached spectrum of diphenylcyclo-propenone. It should be noted that it is imperative to only employ fresh solutions of diphenylcyclopropenone, as has been pointed out by others as well[4, 14, 35], due to the absorption characteristics of the product, as discussed in subsection 2.2.1.
It is obvious, however, that all the characteristic absorptions of ground state diphenylacetylene are also present, albeit covered more or less strongly by the bleached spectrum of diphenylcyclopropenone. In the 1000 ps spectrum, the absorptions at 1602 cm−1 and – even stronger – at 1498 cm−1 are clearly visible, and the intensity of the 1498 cm−1 band is in good agreement with the amount of
diphenylcyclopro-2.2. Experimental results penone consumed in the reaction as determined from its 1632 cm−1 band. The re-maining two absorptions at 1444 and 1574 cm−1 are apparent at closer inspection, but covered strongly by bleached bands of diphenylcyclopropenone. The 25 ps spec-trum, on the other hand, indicates substantial vibrational excitation of the ground state diphenylacetylene, as the 1602 and 1498 cm−1bands are somewhat red-shifted, while the remaining diphenylacetylene bands are not immediately visible.
To take a closer look at the origin of electronic ground state diphenylacetylene, transient spectra of its 1498 cm−1absorption are depicted in Figure 2.5. In the earliest spectrum, only two bleached absorptions are visible, at 1485 and 1498 cm−1, which – by comparison with the static spectra – can be attributed to diphenylcyclopropenone and an impurity of diphenylacetylene, respectively. Subtracting this earliest spectrum from the other transient spectra of Figure 2.5 generates a more precise picture of the evolution of the 1498 cm−1 band and thus of the fate of diphenylacetylene generated in its ground electronic state during the decarbonylation of diphenylcyclopropenone.
The obvious red-shift in the earlier spectra reflects the vibrational excitation of the reaction product, while its decline in the succeeding spectra is caused by vibrational cooling, i.e. transfer of excess vibrational energy to the solvent bath. The integral of this band saturates with a time constant ofτ = (16.6 ± 0.4)ps and thus much slower than the S2 state lifetime of diphenylcyclopropenone determined previously as only a few hundred femtoseconds[6, 35]. This can be interpreted as a decrease in oscillator strength of the 1498 cm−1 absorption upon vibrational excitation of the molecule (see subsection 3.3.1) and explains the smaller appearance of bands in the 25 ps spectrum in Figure 2.4.
With the apparatus used, it was further possible to cover all previously employed wavelengths of excitation. The results of exciting either diphenylacetylene or diphe-nylcyclopropenone at 267 nm or 295 nm and various longer wavelengths are shown in Figure 2.6. Two aspects are particularly noteworthy: First, the 1553 cm−1 absorption of diphenylacetylene ceases to be observable in both scenarios, excitation of the pure substance and photo-decarbonylation of diphenylcyclopropenone, at approximately the same excitation wavelength of 307.5 to 310 nm. This is in sharp contrast to the earlier finding of a wavelength limit of approximately 320 nm[4]. Second, the red-shift of diphenylacetyleneS0 state absorption, hinting at vibrational excitation of the reaction product, appears to reflect the rising amount of excitation energy at shorter wavelengths.
Finally, an experimental test of pathway II of Figure 2.1 – internal conversion from the electronically excited state followed by dissociation from a vibrationally hot
elec-Chapter 2. S2state photochemistry of diphenylcyclopropenone
1460 1470 1480 1490 1500 1510
−0.1 0 0.1 0.2 0.3 0.4 0.5
∆A/mOD
0.4 ps 5.2 ps 10.8 ps 20.7 ps 30.3 ps 41.5 ps 60.0 ps 100.6 ps
1460 1470 1480 1490 1500 1510
0 0.1 0.2 0.3 0.4 0.5
˜ ν/cm−1
∆A/mOD 0 25 50 75 100
0 4
τ= (16.6± 0.4)ps
t/ps
bandintegral /arb.units
Figure 2.5:Transient IR spectra of the 1498 cm−1 absorption of diphenylacetylene. Spectra were recorded in CD3CN after excitation of diphenylcyclopropenone at 267 nm.
The bottom panel shows the spectra from the top panel with the earliest spectrum subtracted. The inset in the bottom panel shows the band integral of the spectra in the bottom panel (including spectra not shown) and an exponential fit.
2.2. Experimental results
260 280 300 320 340
267 λpump/nm
1500 1550 1600
O
25 ps CD3CN
˜
ν/cm−1
1500 1550 25 ps CD3CN
˜
ν/cm−1
260 280 300 320 340
267 λpump/nm
Figure 2.6:Transient spectra of diphenylcyclopropenone and diphenylacetylene after excita-tion at various different wavelengths. Spectra were normalized with respect to the bleach of the 1632 cm−1 absorption of diphenylcyclopropenone or the S1 state ab-sorption of diphenylacetylene at 1553 cm−1, respectively, and the respective exci-tation wavelengths added to the change in optical density. Corresponding UV ab-sorption spectra are shown in the margins.
Chapter 2. S2state photochemistry of diphenylcyclopropenone
1760 1780 1800 1820 1840 1860 1880 1900
−2.5
−2
−1.5
−1
−0.5 0
O
˜ ν/cm−1
∆A/mOD
0.5 ps 2.5 ps 4.8 ps 7.5 ps 10.7 ps
Figure 2.7:Transient IR spectra of the carbonyl absorption of diphenylcyclopropenone.
Recorded after excitation at 267 nm in CD3CN to monitor the possible emergence of the vibrationally hot ground state of the reactant.
tronic ground state – would be to observe vibrationally excited ground state diphe-nylcyclopropenone, i.e. the situation before it undergoes reaction in this postulated scenario. The carbonyl stretching absorption at 1854 cm−1is most appropriate for this test, as obviously no absorptions of any state of diphenylacetylene obscure it. But as is clearly obvious from the spectra shown in Figure 2.7 covering the first eleven picosec-onds after excitation, such vibrationally excited ground state diphenylcyclopropenone could not be observed.
2.3 Discussion
The results of the transient IR experiments rule out both pathways I (adiabatic dis-sociation) and II (hot ground state disdis-sociation) from Figure 2.1, while supporting pathway III (non-adiabatic dissociation). Vibrationally hot diphenylcyclopropenone, as required for pathway II, is lacking, and this supports the argument of the hypothet-ical statisthypothet-ical reaction on the ground state energy surface being much slower than the observed product formation (see section 2.1).
As for pathway I, the amount of S1 diphenylacetylene observed after decarbony-lation of diphenylcyclopropenone is much less than expected and more readily ex-plained as an impurity or by secondary excitation (see below). Additionally, the
depen-2.3. Discussion dence on pump wavelength of its appearance supports this finding, as it does not show any significant difference between the direct excitation of the pure diphenylacetylene and its generation as a reaction product. To the contrary,S0state diphenylacetylene is clearly evident at all excitation wavelengths and at smaller delay times than expected.
At longer delays, the ground state absorption matches with the consumption of the reactant, while its attenuated intensity at shorter delay times is probably caused by a decreased absorption coefficient as a result of vibrational excitation. It should be noted that the non-radiative decay proposed on the basis of fluorescence experiments[36] is much too slow to explain the observation of such large amounts of ground state reac-tion product.
More recent experiments have indicated chain-reaction mechanisms in the decar-bonylation of solid diphenylcyclopropenone, where a quantum yield of 3.3 was deter-mined[52], and, based also on experiments in solution[52–54], this finding was rational-ized resorting to pathway I. It remains to be explained, why these observations deviate from the transient IR results presented. First and foremost, it has to be noted that none of the aforementioned experiments in the visible and near UV spectral range was ever capable of monitoring ground state diphenylacetylene. That is, in all of these experi-ments wavelengths shorter than 390 nm were not monitored. Additionally, substantial differences between the spectra obtained from direct excitation of diphenylacetylene and from photodecarbonylation of diphenylcyclopropenone usually had to be con-ceded[4, 35].
Most of the previous studies employed light with a wavelength of about 267 nm for reaction initiation[35, 54], which inevitably increases the risk of exciting impurities of diphenylacetylene, whose absorption is much stronger at this wavelength than that of the reactant (cf. subsection 2.2.1). While all authors mention this problem and their pre-cautions against it (usually purification of the reactant and high flow rates of the sam-ple solution, as was imsam-plemented for this study), a more immediate danger is hardly ever considered: the excitation of nascent diphenylacetylene by the trailing edge of the pump pulse. This problem is particularly grave for experiments with picosecond excitation pulses[4]or higher excitation intensities[35].
Based on pathway III, a model has been proposed, which rests on the following kinetic sequence:
DPCP(S0) −−−→k1(t) DPCP(S2) −−−→k2
−CO DPA(S0) −−−→k3(t) DPA(S2)
Chapter 2. S2state photochemistry of diphenylcyclopropenone where
k1(t) = eDPCP·I(t)·ln 10 k3(t) = eDPA· I(t)·ln 10
and I(t) is the intensity of the pump pulse, while k2 is the rate of dissociation of the S2 state of diphenylcyclopropenone. This model neglects effects due to stim-ulated emission and changes in absorption coefficient due to vibrational excitation of the product, as they are expected to be very small. For the experiments dis-cussed here, with 10 mM sample solutions, an (idealized) cylindrical pump volume of 0.6 mm length and 0.2 mm diameter, a Gaussian-shaped pump pulse of 150 fs dura-tion and 1µJ energy, decadic absorption coefficients ofeDPCP =104 l mol−1cm−1and eDPA=2×104l mol−1cm−1– corresponding to the situation at 267 nm – and a decar-bonylation rate constant ofk2 = (0.2 ps)−1, almost 5% of diphenylacetylene generated as a result of the decarbonylation of diphenylcyclopropenone are excited to theS2state in this model, as determined by numerical integration.[6]
For other experiments, some of the required parameters are usually not clearly stated, but it can be assumed that especially for the earliest experiments by Hirata[4], although performed at 295 nm, but with pulses of 8 ps fwhm and 600 µJ, this ef-fect should lead to a degree of excitation of reaction-generated diphenylacetylene indiscernible from excitation of the substance itself. It has to be mentioned, how-ever, that newer experiments with detection in the visible spectrum and short pump pulses (35 fs) should not suffer this defect, yet on the other hand they do not monitor the ground state of the product.[54]
Nevertheless, the results and considerations presented here, alongside with ac-companying measurements of the transient visible and near-UV spectra[6], strongly point toward pathway III, the non-adiabatic dissociation process, as the actual reaction mechanism.
Chapter 3
Intramolecular vibrational energy transport
In this chapter, investigations of the transport of vibrational energy along molecular chains shall be presented. To this end, azulene derivatives were used as the internal conversion of azulene after electronic excitation to itsS1state offered a simple means of depositing energy in a molecule. The derivatives contained different side chains, some of which were simple aliphatic chains, others made up of either polyethylene glycol units. Monitoring of the energy flow was accomplished by installing suitable sensor groups with marked IR absorptions.
This chapter is organized as follows: After an introductory review of the research conducted in this field during the last one and a half decades, results obtained for the present study shall be discussed. The discussions will follow a pattern of first focusing on spectral information, to then present the temporal evolution of various signals, and finally compare the different substances used. Subsequently, a rate-equation model of energy transport will be applied to the experimental data and discussed. Finally, an attempt will be undertaken to model the spectral data obtained experimentally.