Reactive Intermediate Chemistry

Several vibrational modes in the excited singlet states of DPA have been identified from picosecond CARS measurements.65,66 The central C C bond of DPA

retains much of its triple-bond-like character in the S2 state; however, the central

C C bond has double-bond-like character in the S1 state. By analogy with the trans-bent form of S1 acetylene,67 it was proposed65,66 that S1 DPA has a trans-

or cis-form bent structure that is consistent a previously proposed structure.64 It has been cautioned that, because of the conjugation between the phenyl groups and the central C C bond, the S1 state may assume a structure different from the bent form.60 The interpretation of recent picosecond IR absorption measurements provide support for the trans-bent planar structure for S1 of DPA. The diphenylacetylenic fluorophore has recently been incorporated into several chemosensor structures whose fluorescent signaling properties are controlled by the relative flexibilities of the molecules.68

2.2. Photolysis of tert-Butyl 9-Methyl-9-fluoreneperoxycarboxylate

Picosecond electronic absorption spectroscopy was used to investigate the formation of the 9-methyl-9-fluorenyl radical following 266-nm, 18-ps pulsed excitation of the peroxyester in either cyclohexane or acetonitrile.69 The absorption of the 9- methylfluorenyl radical (3) grows with a rise time of 55 15 ps as monitored at a single, unreported wavelength. The proposed events following 266-nm, pulsed excitation is outlined in Scheme 19.1 in terms of a pathway in which the O O bond is

Scheme 19.1

first broken and then decarboxylation occurs. The electronic absorption spectrum of 1 was described as nearly superimposable on the spectrum of 9-methyl-9-fluorene- carboxyilic acid, methyl ester, except for the broad, weak absoprtion band, extending to wavelengths >400 nm, that is characteristic of peroxides such as 1. Excitation at 266 nm was thought to generate a higher excited singlet state (1**) in which the

892 THE PICOSECOND REALM

excitation is mainly localized in the fluorenyl moiety and that subsequently relaxes to the lowest singlet excited state (1*) in which the excitation resides on the peroxyester group. From the inability to detect fluorescence or transient absorption from 1** or 1*, it was concluded that the slow step giving rise to the 55-ps rise time is due to the decarboxylation of acyloxy radical 2 to give radical 3 as the only reactive intermediate detected spectroscopically.

More information about the sequential pathway versus one in which the O O and C C bond homolyses occur concertedly was obtained by means of picosecond IR absorption spectroscopy.15b While IR spectroscopy, in principle, can provide more structural information, limitations stemming from the weakness of some IR intensities can be detrimental to the detection of a reactive intermediate. In this case, the CO2 photoproduct was monitored (near 2335 cm 1) instead of radical intermediates 2 or 3. It was argued that neither 1, 2, nor 3 absorbed in this region. Excitation of 1 in CCl4 was achieved with a 308-nm, 1-ps pulse. The 1.3-ps probe pulse may be tuned to cover a range from 2000 to 4300 cm 1 with the spectrometer that was used, but in these specific experiments the interrogation window was from 2000 to 2400 cm 1.

2.3. Pi-Bond Heterolysis versus Homolysis

The excited-state behavior of 1,1,2,2-tetraphenylethene (TPE) has been studied by

means of picosecond fluorescence,70,71 absorption,72,73 and Raman74 spectroscopies and picosecond optical calorimetry.71,75–77 It has been shown that, like stilbene,

TPE derivatives substituted with minimally perturbing stereochemical labels such as methyl groups undergo efficient photoisomerization.78 However, unlike stilbene, strong spectroscopic evidence exists for the direct detection of the twisted excited singlet state, S1P herein but traditionally designated as 1p*, of TPE.

Excitation of TPE in hexane with a 305-nm, 0.5-ps laser pulse led to the appearance of absorption bands at 423 and 650 nm during the time duration of the excitation pulse.79 The 630-nm band decays and the 423-nm band shifts to 417 nm with a lifetime of 5 1 ps, and the 423-nm band shifts to 417 nm on this time scale. The 417-nm absorption then decays with a lifetime of 3:0 0:5 ns. The 630and 417-nm bands were assigned to an electronic transition from the vertical singlet excited state (S1V ) and to an electronic transition from the nonfluorescent S1P, respectively.79

Picosecond fluorescence measurements70 made at several temperatures ranging from 4 to 293 K with the use of a streak camera are consistent with results obtained from transient absorption spectroscopy. These fluoresence studies performed on TPE in several solvents, including 3-methylpentane, isopentane, decalin, phenol, ethylene glycol, and triacetin, at several temperatures provided more information about the excited-state behavior of TPE. After excitation with a 355-nm, 5-ps laser pulse to a vibrationally hot TPE vertical excited singlet state designated here as (S1V)*, intramolecular vibrational relaxation (ivr) gives the relaxed vertical singlet excited state S1V, which undergoes further relaxation, proposed to be torsional, to another geometry on the excited singlet state surface to S1R.

A subsequent picosecond electronic absorption spectroscopic study72 of TPE excited with 266or 355-nm, 30-ps laser pulses in cyclohexane found what was reported previously.79 However, in addition to the nonpolar solvent cyclohexane, more polar solvents such as THF, methylene chloride, acetonitrile, and methanol were employed. Importantly, the lifetime of S1P becomes shorter as the polarity is increased; this was taken to be evidence of the zwitterionic, polar nature of TPE S1P and the stabilization of S1P relative to what is considered to be a nonpolar S0P, namely, the transition state structure for the thermal cis–trans isomerization. Although perhaps counterintuitive to the role of a solvent in the stabilization of a polar species, the decrease in the S1P lifetime with an increase in solvent polarity is understood in terms of internal conversion from S1 to S0, which should increase in rate as the S1–S0 energy gap decreases with increasing solvent polarity. Along with the solvent-dependent lifetime of S1P, it was noted that the TPE S1P absorption band near 425 nm is located where the two subchromophores—the diphenylmethyl cation and the diphenylmethyl anion—of a zwitterionic S1P should be expected to absorb light. A picosecond transient absorption study73 on TPE in supercritical fluids with cosolvents provided additional evidence for charge separa-

tion in S1P.

Picosecond optical calorimetry71,75–77 was used to learn about changes in the S1P energy when the solvent is changed. For the calorimetry studies, 634-nm,50-ps pulses from a synchronously pumped dye laser were used. From an analysis75 of the heat released after excitation of TPE in cyclohexane, TPE S1P was estimated to lie 73.1 kcal/mol above the ground state. This put S1P 39.4 kcal/mol above the transition state for thermal isomerization. This value was approximated by analogy to that of 1,2-di(4-methylphenyl)-1,2-diphenylethene, which is known to exist 36.0 kcal/mol above the ground state.80 Additional picosecond optical calorimetric experiments76 with several solvents refined the initial estimate of the energy of S1P above the ground state to 67:0 1:3, 66:2 1:4, and 65:3 1:8 in alkane (pentane, hexane, and nonane), diethyl ether, and THF, respectively, as solvents. Within the context of the dielectric continuum model for solvation, the dipole moment of S1P was estimated to be 6.3 D, which compares well with a dipole moment resulting from the separation of a unit charge by the length of a C C single bond.76 Cautions have been raised concerning the ability to separate contributions from enthalpy and volume changes in these photothermal measurements.76,81 A somewhat larger dipole moment of at least 7.5 D was measured by means of time-resolved microwave conductivity that used 7-ns, 308-nm pulsed excitation.82

A combined picosecond fluorescence, which employed time-correlated single photon counting, and picosecond optical calorimetry investigation71 followed two nanosecond fluorescence studies77,82 and gives a mapping of the TPE S1 surface. The two nanosecond studies demonstrated that S1P is not formed irreversibly from the fluorescent states reported previously70 but is in equilibrium with the fluorescent S1R state in hydrocarbon solvents. In S1,71 S1V is 77 kcal/mol above the ground state and traverses a barrier of 2.6 kcal/mol to get to S1R at an energy of 69 kcal/mol. The S1R state passes over a barrier of 2.4 kcal/mol to arrive at S1P whose energy of 67 kcal/mol lies 2 kcal/mol below S1R.

894 THE PICOSECOND REALM

To this point, twisting to give S1P has been inferred from the time-resolved absorption, fluorescence, and photothermal studies described herein along with product studies and applications of steady-state spectroscopies. Nanosecond Raman spectroscopy provides the needed evidence for the twisting. Both TPE and its analogue with 13C present at the ethylenic positions in heptane were excited at 316 nm with a 10-ns laser pulse and probed with a 417-nm, 10-ns pulse. In S0, strong coupling exists between the vinyl C C stretch and the phenyl C C stretch. In S1, two Raman bands are assigned to the phenyl C C stretch, but none are observed for the vinyl C C stretch. This absence indicates that, in S1, coupling between the vinyl C C stretch and the phenyl C C stretch is absent, which was interpreted as being consistent with the decrease in the bond order of the ethylenic C C bond in S0 on going to S1P. Unfortunately, no Raman band for this C C stretch could be assigned in the spectrum of S1. Further interpretation of the Raman spectra led to following conclusions: (a) that the large difference in the frequencies of the two phenyl C C stretch bands is consistent with the presence of cationic and anionic moieties in S1P and may be considered as manifestations of its zwitterionic nature and (b) that the resonance enhancement of the phenyl Raman bands is consistent with localized electronic transitions from cation and anion subchromophores in S1P as was described previously.72

More recent femtosecond spectroscopic investigations provide interesting information about shorter time scale phenomena associated with excitation of TPE.83,84

3. CONCLUSION AND OUTLOOK

Much useful kinetic and structural information can be provided by means of picosecond spectroscopies. The future may be considered in terms of the development of new picosecond spectroscopic methods and investigations of new chemical systems including the excited states of reactive intermediates. As picosecond electronic absorption and fluorescence methods have evolved to more user-friendly implementations in greater number, one can anticipate the same for the more instrumentally challenging infrared absorption and Raman spectroscopies and the more challenging analyses associated with optical calorimetric and dichroism measurements. For the measurement of transient absorption spectra, the importance of probing wavelengths <400 nm will make necessary an increase in the number of spectrometers capable of measuring further into the UV region.

The outlook is good for applications of these picosecond methods to an increasing number of studies on reactive intermediates because of the limitations imposed by the time resolution of nanosecond methods and the generally greater challenges of the use of a femtosecond spectrometer. The pump–probe technique will be augmented in more widespread applications of the preparation–pump–probe method that permits the photophysics and photochemistry of reactive intermediates to be studied.

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J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 2nd ed., Kluwer Academic/Plenum Publishers, New York, 1999.

M.D. Fayer, Ed., Ultrafast Infrared and Raman Spectroscopy, Vol. 26, Marcel-Dekker, New York, 2001.

J.R. Schoonover and G. F. Strouse, ‘‘Time-Resolved Vibrational Spectroscopy of Electronically Excited Inorganic Complexes in Solution,’’ Chem. Rev. 1998, 98, 1335.

H.-O. Hamaguchi and K. Iwata, ‘‘Physical chemistry of the lowest excited singlet state of trans-stilbene in solution as studied by time-resolved Raman spectroscopy,’’ Bull. Chem. Soc. Jpn. 2002, 75(5), 883.

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900 REACTIONS ON THE FEMTOSECOND TIME SCALE

structure and reactivity. It espoused the ideas associated with ‘‘reactive intermediates.’’ These conceptual insights were scarcely glimpsed by organic chemists in 1920; today these mutually supportive intellectual foundations are generally appreciated as durable and powerful supports for the impressive advances in chemical understanding achieved over the past 40 years.

The modes of thinking about structures and reactions and intermediates facilitated or even demanded by these conceptual innovations meshed productively with new tools of a different sort to drive the progress of recent decades. Advances in electronics and computers, all sorts of spectroscopy, laser optics and physics, chromatography, mass spectrometry, quantum theory and computational strategies, molecular beam experiments, and so on, radically expanded the limits of experimental and theoretical investigations.

By now many sorts of reactive intermediates, such as free radicals, carbocations, carbenes, benzynes, and Bredt’s rule violating olefins, have been generated and isolated under conditions permitting full structural characterizations. Theory-based structural parameters are generally in nearly perfect agreement with experimentally determined values.

Experimental work providing information on reaction kinetics—the time dependence of reactants and products under defined conditions—served indispensably to correlate structure–reactivity data and to provide estimates of transition state energies. Theory-based definitions of transition structures gave some clues as to how reactions might actually take place. But the dynamic aspects of chemical reactions remained inaccessible, or only poorly accessible.

2. REACTION DYNAMICS

The dream or vision of being able to see atomic motions as a chemical reaction takes place has long been an aspiration, one approached with understandable diffidence and with reliance on theory-based modeling. Since the 1930s physical chemists concerned with dynamics have explored through computations and experiments on very simple reactions, involving few atoms, just how reactions take place. But the dynamics of more complicated organic reactions remained unapproachable using the experimental tools at hand before the advent of experimental femtochemistry.

With better electronic structure theory and faster computers, theory-based limitations to gaining deeper insights on reaction dynamics began to recede in the 1970s. In a landmark contribution, Salem and co-workers1,2 calculated just how the thermal cis, trans interconversions of 1,2-d2-cyclopropanes take place—how one C C bond lengthens, how methylene groups rotate to form a diradical intermediate, and how further exquisitely choreographed coupled motions of terminal methylene groups and C C C bond angle variations lead to formation of an isomeric labeled cyclopropane.

The sequence of three-dimensional (3D) structures for this thermal epimerization, a series of virtual photographs in timed sequence (Fig. 20.1), constituted

Figure 20.1. A computationally inferred lapsed-time sequence of structures representing reaction dynamics for a thermal stereomutation of cyclopropane.1,2 [Reproduced with permission from J. A. Horsley, Y. Jean, C. Moser, L. Salem, R. M. Stevens, and J. S. Wright, J. Am. Chem. Soc. 1972, 94, 279. Copyright # 1972 American Chemical Society.] Reproduced with permission from Y. Jean, L. Salem, J. S. Wright, J. A. Horsley, C. Moser, and R. M. Stevens, Pure Appl. Chem. Suppl. (23rd Congr., Boston) 1971, 1, 197.

a chemical equivalent of stop-motion or lapsed-time photography as pioneered by Eadweard Muybridge, Etienne-Jules Marey, and Harold Edgerton.3

The visual and conceptual impact of seeing the timed sequence of structures, a full representation of atomic-scale events as a complex chemical reaction took place, was powerful. This achievement, the product of state-of-the-art calculations applied to an ambitious objective as well as excellent presentation graphics, was not diminished through a repressed awareness that it all depended on theory. Nothing experimentally based provided an anchor for the visually compelling rendition of the reacting system as a cyclopropane cleaved a C C bond, formed a trimethylene diradical intermediate, and executed a net one-center epimerization before reverting to the cyclopropane structure.

As better and better methods for following fast reactions with precision were introduced and exploited, characteristic reaction times faster than a second—times measured in milliseconds (ms, 10 3 s), or microseconds (ms, 10 6 s), or nanoseconds (ns, 10 9 s) and then in picoseconds (ps, 10 12 s)—were measured through stopped-flow techniques (Chance, 1940), flash photolysis (Norrish and Porter, 1949), temperature-jump and related relaxation methods (Eigen, 1954), and then