Reactive Intermediate Chemistry

a repetition rate of 75 Hz with an average power of 10 W. These pulses are converted to slightly shorter pulses of the second harmonic at 532 nm with an average power of 1 W that are used to pump a dye laser. This dye laser is cavity dumped to reduce the repetition rate to 1 MHz for the output 700-nm, 6-ps, 20-mW pulses. The pulses from the dye laser are mixed with residual 1053-nm pulses to give, when pyridine 1 dye is used, 420nm pulses for sample excitation. The fluorescence is collected at 90 to the excitation path, directed through a monochromator, and measured with an MCP-PMT whose output is amplified with a CFD. A portion of the 700-nm pulse is sent to a photodiode connected to another channel of the CFD. The output of the CFD start emission and laser stop pulses are used by a time- to-amplitude converter whose output in turn is sent to a computer containing a multichannel analyzer card and software for hardware control and data acquisition and analysis.

1.3. Raman Spectroscopy

In the picosecond regime, Raman spectroscopy takes the kinetic capabilities of transient absorption and fluorescence methods and combines them with more featureful vibrational spectra. The potentially greater detail provided by the bands in a Raman spectrum is offset with the need to assign a group of bands firmly to the structure of a reactive intermediate. Libraries of assignments of vibrational frequencies to functional groups and structural features have been compiled for stable, ground-state molecules. Similar libraries need to be compiled for the new manifestations of bonding and structure that exist within reactive intermediates. Interpretations of the shifts of bands from the ground to an excited state or from one reactive intermediate to a different, but structurally related, reactive intermediate help one to understand the structure of the transient species.

Several types of time-resolved Raman spectroscopies have been reported and reviewed by Hamaguchi and co-workers8 and Hamaguchi and Gustafson.9 These include pump–probe spontaneous and time-resolved coherent Raman spectroscopy of the anti-Stokes and Stokes varieties [coherent anti-Stokes Raman spectroscopy (CARS) and coherent Stokes Raman spectroscopy (CSRS)], respectively). Here we will focus on pump–probe time-resolved spontaneous Raman spectroscopy.

The laser used to generate the pump and probe pulses must have appropriate characteristics in both the time and the frequency domains as well as suitable pulse power and repetition rates. The time and frequency domains are related through the Fourier transform relationship that limits the shortness of the laser pulse time duration and the spectral resolution in reciprocal centimeters. The limitation has its basis in the Heisenberg uncertainty principle. The shorter pulse that has better time resolution has a broader band of wavelengths associated with it, and therefore a poorer spectral resolution. For a 1-ps, sech2-shaped pulse, the minimum spectral width is 10.5 cm 1.9 The pulse width cannot be <10 ps for a spectral resolution of 1 cm 1. An optimal choice of time duration and spectral bandwidth are 3.2 ps and 3.5 cm 1.8 The pump pulse typically is in the UV region. The probe pulse may also be in the UV region if the signal/noise enhancements of resonance Raman

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(RR) scattering are exploited; otherwise the probe pulse may be in the visible to avoid complicating fluorescence. The probe pulse, which is scattered by the sample into a Raman spectrum recorded over the time duration and at the selected time that the probe pulse passes through the sample, has a narrow band of wavelengths associated with it, which differs from the white-light continuum typically used for picosecond electronic absorption spectroscopy and from the need to generate probe pulses over a desired wavenumber range to record a picosecond-resolved IR spectrum.

The pump pulse energy is controlled to minimize two-photon phenomena and to maximize the concentration of the desired excited-state or other reactive intermediate. The optimal average power of the probe pulse changes with a specific experiment but is often maintained at 10 mW; peak powers in the range of 0.1–10 MW with repetition rates of 1 kHz–1 MHZ are best for picosecond spontaneous Raman spectroscopy.

The system described by Iwata et al.8 uses a continuous wave (CW) modelocked Nd:YAG laser to generate 65-ps, 1064-nm laser pulses at a repetition rate of 82 MHz with an average power of 11 W. These pulses are subjected to temporal compression with an optical fiber and a diffraction grating and to second harmonic generation whose average power is kept constant with a feedback loop driven by an acousto-optic modulator (AOM). The 532-nm second-harmonic laser pulses, exhibiting a half-width of 5 ps, a repetition rate of 82 MHz, and an average power of 0.8 W, synchronously pump a rhodamine 6G dye laser that has a three-plate birefringent filter for tuning the wavenumber of the 588-nm output pulses generated at a repetition rate of 82 MHz with an average power of 130 mW. These pulses are amplified with a two-stage dye amplifier pumped with the 532-nm second harmonic from a CW Nd:YAG regenerative amplifier operated at a repetition rate of 2 kHz and seeded with a portion of the 1064-nm output from the CW mode-locked Nd:YAG laser. The use of Kiton Red 620 in the dye amplifier achieves a gain of 10,000 for the 588-nm output pulses, created at a repetition rate of 2 kHz—500 ms between pulses—and with an average power of 25 mW, that are subsequently frequency doubled to 294-nm, 6-mW pump pulses. The residual 588-nm pulses transmitted through the last frequency doubling crystal and separated from the 294-nm pump pulses with a dichroic mirror are used as probe pulses. The probe pulse is optically delayed (0.3 mm ¼ 1 ps) from its partner pump pulse. Then the two pulses are colinearly reunited with the use of a dichroic mirror so that the probe pulse interrogates the flowing, thin jet of sample solution at a selected time after the pump pulse has excited the sample. The light scattered at right angles to the direction of travel of the pump and probe pulses is collected and focused with two lenses, separated by a narrow band rejection filter, also known as a line-rejection or notch filter, onto the entrance slit of a single-grating spectrograph. The dispersed Raman scattering is measured with the use of a liquid nitrogen cooled CCD detector for typical data acquisition conditions of the following: a 200-mm entrance slit width that corresponds to 5.9 cm 1 at 1400 cm 1 from the 3.5 cm 1 half-width pump pulse; a 0.1–10-mW probe pulse average power; a 5-mW pump pulse average power; and exposure time of 10 min.

1.4. Infrared Absorption Spectroscopy

Picosecond IR spectroscopy complements the vibrational information provided via picosecond Raman spectroscopy. While picosecond Raman spectroscopy has a longer history and has been more widely applied, picosecond IR spectroscopy has been undergoing development and has been applied to an increasing number of chemical systems. Although the components comprising a picosecond Raman spectrometer are simpler and fewer in number than those that make up a picosecond IR spectrometer, picosecond IR spectroscopy does have its advantages including the general absence of complications from fluorescence and the ability to perform polarized IR measurements in order to obtain more information about molecular structure.10 From the perspective of experimental design, two different picosecond IR spectrometers will be described herein—one that primarily covers the fingerprint region and another that covers the remainder of the IR region.

1.4.1. The >1500-cm 1 Region. The >1500-cm 1 spectral region is less demanding instrumentationally than the fingerprint region to be considered next and has been studied by several groups (e.g., see Refs. 11–14 and 15a). Developments in IR spectroscopy have lagged far behind those for Raman spectroscopy in the picosecond regime.

Here we will focus in detail on a UV pump–IR probe spectrometer described by Ernsting and co-workers15 the system is based on an excimer laser and a dye laser operating with a pulse repetition rate ranging from 5 to 10 Hz. Pump pulses at 308 nm excite the sample and are followed at a selected time by probe IR pulses that range from 1950 to 4300 cm 1. Absorbance changes can be recorded with a time resolution of 1.8 ps and with an accuracy in absorbance (A) of 0.001.

Initial laser pulse generation is achieved with the use of a twin-tube excimer laser in which one channel is a XeCl* laser oscillator delivering 15-ns, 308-nm, 80-mJ pulses for the driving of two dye lasers needed for difference frequency generation. The second channel is used for amplification of subpicosecond 308-nm pulses that become pump pulses.

The first dye laser generates 10-ps, 2-mJ, 365-nm pulses that pump a microscopic distributed feedback dye laser (DFDL) producing Fourier transform limited 0.7-ps pulses at 616 nm. These DFDL pulses are amplified in two stages to give 100-mJ, diffraction-limited pulses.

The UV pump pulses are generated from the 616-nm pulses passed through a BBO (b-barium borate) crystal for frequency doubling to give 1-mJ, 308-nm seed pulses that are separated from the 616-nm fundamental with the use of a dichroic mirror. These 308-nm pulses are amplified in two passes through the second channel XeCl* amplifier to give 3-mJ, 1-ps, 308-nm pulses. The 616-nm pulse remaining after second harminc generation is split into two unequal portions; 20% is used to pump the second dye laser while 80% is used for difference frequency generation. Tunable red-shifted pulses are needed that overlap in time with the 80% portion of the 0.7-ps, 616-nm pulses in order to generate IR pulses in the 2-5 mm region (4300–1950 cm 1, respectively). This is accomplished through the use of a

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widely tunable prismatic DFDL. The red-shifted spectral region from 700 to 840 nm can be covered with four different dye solutions. The output is amplified to give 0.5-mJ, 0.4-nm bandwidth, 4-ps pulses. The difference frequency is generated with a LiIO3 crystal that yields IR pulses of several nanojoules that are continuously tunable from 1950 to 4300 cm 1 by means of computer controlled adjustment of the phase-matching angle of the crystal, simultaneously with the wavelength of the prismatic DFDL.

After this difference frequency probe pulse generation, the residual visible pulses are separated from the IR pulses by transmission through 3-mm silicon plates, after which the IR pulse is focused with a CaF2 lens behind a 1-mm Ge plate that splits the probe pulse into sample and reference pulses with approximately equal pulse energies. Each probe pulse is directed through its own respective sample and reference flow cells. Ultimately, the sample and reference probe pulses are focused with CaF2 lenses onto liquid nitrogen cooled InSb detectors. A nearly collinear path for the UV pump pulse is used. The time delay between the UV pump and IR probe pulses is selected through the use of a computer-controlled translation stage. One data point for one delay position is typically the combination of 100 laser shots. Difference IR absorption spectra are recorded.

The temporal widths of the IR pulses and the time resolution of this spectrometer are tested with the use of a Ge sample that, when exposed to the pump pulses, results in transient IR absorption at 2290 cm 1. Modeling the risetime of this absorption gives a cross-correlation width (full width at half-maximum, fwhm) of 1.8 ps.

1.4.2. The Fingerprint Region.16–18 The pump–probe approach has been used to study the more difficult, but very informative, fingerprint region of the IR spectrum of a transient species. The 527-nm second harmonic of a mode-locked Nd:YLF (Nd3þ:yttrium lithium fluoride) laser is used to pump synchronously two dye lasers so that tunable pulses between 545 and 578 nm are generated when the laser dye rhodamine 560 is used. Pulses of the fundamental 1053-nm output of the Nd:YLF laser are amplified via passage through a CW Nd:YLF amplifier, are converted to the second harmonic, and are used to pump two rhodamine 590 dye amplifiers that are used to boost the power of the pulses produced by the two synchronously pumped dye lasers to the level of microjoules. To generate an IR pulse in the 1640–940 cm 1—the mid-IR or fingerprint—region, an amplified pulse from one of the dye lasers and a 1053-nm pulse from the Nd:YLF regenerative amplifier are tunably difference-frequency mixed in a b-barium borate (BBO) crystal to give a pulse that can be tuned in the range from 1.15 to 1.30 mm in the near-IR. The residual dye laser pulses are removed with the use of a cut-off filter, and the near-IR pulses are difference-frequency mixed in an AgGaS2 (AGS) crystal to give tunable mid-IR pulses at a repetition rate of 1 kHz. The wavenumber of the IR probe pulse is tuned by changing the wavelength of the dye laser and by phasematching the angles of the BBO and AGS crystals. Unconverted portions of the near-IR pulses are filtered from the IR pulses with the use of a germanium plate. The IR probe pulses are sent to the sample.

Approximately 10% of a mid-IR pulse is reflected, by means of a BaF2 plate, for use as a probe pulse that is focused into the sample cell with an aluminum-plated concave mirror. The mid-IR probe pulse transmitted through the sample is reflected and focused with a concave Al-plated mirror onto a liquid-nitrogen cooled MCT (HgCdTe) detector. The portion of the mid-IR pulse transmitted through the first BaF2 plate is again sent through a second BaF2; the reflected portion is used as a reference pulse that is focused with a CaF2 lens onto another MCT detector. The signals from the two MCT detectors are amplified, and the pulsed components are gated with a boxcar system. The probe signal is electronically divided by the reference signal in order to reduce the noise resulting from instability of the IR pulses. The processed signals are converted from analogue to digital, sent to a computer, and stored.

The pump pulse for sample excitation comes from the output of a second dye amplifier. Its wavelength depends on the dye that is used; for rhodamine 560, the range is from 545 to 578 nm. The polarization angle of this pulse is selected with the use of a Fresnel rhomb prism. An optical delay line is used to change the path length traveled by the pump pulse relative to its corresponding probe pulse. The pump pulse is noncollinearly focused onto as much as possible of the same volume of the sample as the probe pulse. The train of pump pulses is modulated with a mechanical chopper that is synchronized to the regenerative amplifier at one-half of the 1- kHz repetition rate of the probe pulses. This process gives rise to a set of pump–probe–no pump–probe pulses analogous to those described for the picosecond difference electronic absorption spectrum. The detected mid-IR probe pulses for the sample and the reference with the pump pulse on and off are separately stored in the computer, and the difference IR absorption spectrum is calculated. The temporal and spatial overlaps of the pump and probe pulses are checked with a standard such as detecting the rise of a broad transient IR absorption resulting from pump–pulse generated free carriers in silicon.19 The timing jitter between the pulses from the two synchronously pumped dye lasers can be as much as10 ps.

With this spectrometer, a difference mid-IR spectrum at a selected time after sample excitation is recorded by sweeping from 1640 to 940 cm 1 in steps that may be as short as approximately equal to the spectral resolution of the spectrometer—in this case,16 8 cm 1. The sample solution is pumped through a flow cell that has IR-transmitting CaF2 windows set with a 0.1-mm optical pathlength. The BaF2 windows have also been used for the sample cell.18

1.5. Optical Calorimetry

Picosecond-resolved thermochemical information can be extracted from the evolution of a transient grating produced by the crossing of two laser pulses and interrogated with a third short pulse of light. Several groups have applied this method to thermodynamic questions about the decay of excited states and the evolution of excited states into reactive intermediates.20–25

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Pulses of one of the harmonics of an actively–passively mode-locked Nd:YAG such as the third harmonic at 355 nm are split into two parts, and after traveling different paths are simultaneously recombined in the sample noncollinearly.22 An optical delay is used to insure temporal coincidence in the sample. These two pulses traveling along crossed paths generate a transient grating in the sample cell. A third pulse (e.g., at 532 nm) serving as the probe pulse is sent along a variable optical delay and passes through the sample along a counterpropagating path, relative to the two crossed 355-nm pulses, at an appropriate angle from Bragg diffraction. The diffracted pulse is filtered with a 500-nm cutoff filter and reflected onto a vacuum photodiode. The output signal of the photodiode is amplified and fed into a multichannel analyzer board residing in a personal computer. The signals from 20 laser pulses are averaged and analyzed appropriately. Those interested in the details of the data analysis are referred to the cited literature.

2. STUDIES ON REACTIVE INTERMEDIATES

The examples that are presented in this section are not rigorous reviews of reactive intermediates but highlight instead the application of picosecond-resolved methods to selected problems. The symbols and abbreviations used herein may be different from those in the original publications, but if they have been changed, it is to achieve a uniformity of meaning for which they are used in this chapter.

2.1. Excited States of Conjugated Pi Systems

2.1.1. 1,2-Diphenylethene (Stilbene). This molecule has been the subject of many photophysical and photochemical investigations and the subject of several reviews.26–28 It is the prototypical alkene for studies of photoisomerization. Transient spectroscopic measurements in the picosecond time domain have been performed on electronically excited trans-stilbene in a wide range of environments. Selections from these studies are described here.

An absorption band with a maximum near 585 nm resulted from 265-nm, 7-ps

pulsed excitation of a 1-mM solution of trans-stilbene (tS) in hexane at room temperature.29,30 The absorption band was assigned to an Sn < S1 absorption of tS in a

geometry close to that of ground-state tS. Narrowing of this absorption band occurred for 50 ps after excitation. This narrowing has been attributed to intramolecular vibrational relaxation of vibrationally hot tS in its S1 state generated via 265-nm excitation. Information from Raman spectroscopic measurements supports the case for vibrational relaxation.31–37 Pulsed excitation at 306 nm created the tS S1 state with less excess vibrational energy and as a consequence narrowing of the Sn < S1 absorption band was not observed. The lifetime of the tS S1 state is 95 ps in room temperature hexane as measured by means of transient absorption spectroscopy29 and is consistent with streak camera based measurements of the decay fluorescence that competes with the major pathway of twisting about what was the ethylenic carbon–carbon double bond in the ground state.30

Three possible paths have been discussed for the unimolecular decay of the near vertical geometry of the tS S1 state.27,38,39 The commonly accepted, dominant

mechanism has the original ethylenic bond undergo a twist on the excited singlet state surface to a perpendicular geometry—the twisted excited singlet state or phantom state—that partitions upon return to the ground state to a mixture of transand cis-stilbene. A second pathway, taken by some substituted stilbenes after direct excitation, proceeds via intersystem crossing to a triplet surface upon which isomerization occurs followed by return to the ground state. A third mechanism, involving internal conversion to high vibrational levels of the ground state and subsequent trans–cis isomerization, has been found to be unimportant.

The picosecond IR absorption spectrum of the tS S1 state in the fingerprint region is different in n-heptane and in acetonitrile.13 The spectrum recorded for S1 tS in the nonpolar solvent n-heptane is consistent with a species that has a center of symmetry. In acetonitrile, the spectrum exhibits additional weak bands near 1570, 1250, and 1180 cm 1, which are approximately at the same frequencies as strong Raman bands assigned to in-plane vinylic vibrational modes in S1. This result was taken to suggest a molecular structure for S1 that lacks a center of symmetry in acetonitrile. However, because the intensities of these three bands are weak, it was concluded that either the polarization of S1 or the contribution from polarized S1 structures to all of the S1 structures in acetonitrile may be small.

tS

Picosecond Raman measurements have led to the proposal of a dynamic polarization model.40,41 In this model, S1 tS undergoes reversible changes in vibrational frequencies that are induced by solvent fluctuations. The mixing of a perturbing state with S1 interconverts carbon–carbon double bonds with single bonds that leads S1 tS near the vertical geometry to proceed along the pathway for isomerization.

While much has been learned about S1 near the geometry of tS in solution, there have been no reports on the spectroscopic detection of the twisted excited singlet state of stilbene. Information about the twisted excited singlet state of tetraphenylethene will be described in this chapter in Section 2.3. Interesting photochemistry also occurs following the excitation of cis-stilbene; however, because of the much shorter lifetime of S1 cis-stilbene, femtosecond-pulsed lasers must be used.42–45

At higher concentrations in solution, the photodimerization of tS has been studied by means of picosecond electronic absorption spectroscopy.6 The S1 state of tS in benzene at 22 C is quenched with a diffusion-controlled rate constant of 2:03 1010 M 1 s 1 to give a new reactive intermediate exhibiting an absorption maximum at 480 nm. This new species decays unimolecularly with a rate constant of ð2:40 0:37Þ 109 s 1. It has tentatively been assigned to either the excimer or a biradicaloid species located at the pericyclic minimum.

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2.1.2. 1,4-Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review.26 Unlike tS in which the lowest vertical excited singlet state is the 11Bu state and S2 is the 21Ag state in solution, these two excited states lie very close to each other in all-trans- 1,4-diphenyl-1,3-butadiene (DPB). The additional carbon–carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rotamers. Most spectroscopic studies in solution have concluded that the 11Bu state is S1. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically.

Femtosecond electronic absorption spectroscopy was used to study DPB and the following four stucturally related butadienes: 1,4-diphenyl-1,3-cyclopentadiene (DPCP); (E,E)-diindanylidenylethane (stiff-5-DPB); 1,1,4,4-tetraphenyl-1,3-buta- diene (TPB); and 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPCP).46 The related compounds were used to infer additional information about DPB through interpretations of the spectroscopic observations that were made. The absorbance changes of S1 DPB in hexane at 620, 660, and 690 nm within the 640-nm absorption band were measured following excitation at 310 nm. At each of these three wavelengths, the data were fitted well to an instrument limited rise time of 120 fs and a exponential decay time of 465 10 ps. Although only these three wavelengths were

monitored, the fact that each exhibits the same decay kinetics indicates that the entire 640-nm absorption band evolved uniformly. This procedure is not as good as monitoring the entire absorption band, but it is clearly more reliable than merely monitoring the kinetics at a single narrow band of wavelengths. Kinetics studied at a single narrow band of wavelengths, often reported as a single wavelength, must be carefully evaluated because, at short times after excitation, absorbance changes may be the result of intramolecular vibrational relaxation or changes in solvation in addition to an overall increase or decrease in the concentration of a species. In

this femtosecond study,46 no evidence exists for 11Bu–21Ag interconversion as was described in an earlier picosecond spectroscopic study.47

Picosecond Raman spectroscopy provided information about the excited states of DPB.48,49 Raman spectra of the S1 state of DPB in several solvents including

pentane, hexane, heptane, octane, decane, dodecane, and tetrahydrofuran (THF) exhibit bands assignable to both the 11Bu and 21Ag states. Because of a small energy gap of 500 cm 1 between these two states, it was concluded that the lowest excited singlet state in solution is of mixed character. The changes in the Raman bands of S1 with solvent and probe wavelength are consistent with a distribution of s-trans conformers in the S1 state that controls the degree of mixing of the 11Bu and 21Ag states. Changes in the position and bandwidth of the peak assigned to the phenyl–vinyl stretch were attributed to vibrational relaxation within S1.

The excited-state ordering is expected to play an important role in governing the photophysical and photochemical modes of deactivation of DPB following electronic excitation in solution.26

2.1.3. 1,6-Diphenyl-1,3,5-hexatriene. The quantum yield for photoisomeriza-

tion of all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) is much lower than that of trans-stilbene.50–53 The DPH compound is the first in the series of vinylogous stil-

benes for which the 21Ag state is lower than the 11Bu state. Early picosecond fluorescence54 and electronic absorption55 measurements were interpreted as providing

DPH

evidence for the equilibration of the 11Bu state, initially populated via one-photon absorption, with the lower lying 21Ag state. A subsequent picosecond study56 demonstrated that there was no evidence from either picosecond fluorescence or electronic absorption measurements to support equilibration of the 11Bu and 21Ag states. The fluorescence decay recorded with a streak camera56 did not exhibit a <30-ps decay component after excitation of DPH in cyclohexane of the type previously recorded for DPH in hexane.54 It was proposed56 that the previously reported fast decay was actually due to a solvent Raman band induced by the excitation pulse. Transient absorption spectra in the later work56 did exhibit spectral changes reported earlier.55 However, these changes were attributed56 to the effects of chirp and not to equilibration of excited states. Additional complications due to fluorescence from the sample and to multiphoton absorption leading to the ionization of DPH to give the DPH radical cation were also considered56 as factors affecting the appearance of the transient absorption spectra previously.55 Also, the absorbance ratio of the two maxima near 460 and 650 nm were found to be temperature independent56 contrary to the earlier report.55 It was concluded56 that the relaxation of the initially formed 11Bu state to the equilibrium mixture is complete within 10 ps, much faster than previously reported.54,55 This faster

890 THE PICOSECOND REALM

relaxation time is supported by more recent femtosecond absorption57 and picosecond Raman investigations.58,59

Picosecond-resolved CARS has been used to study the excited states of DPH in cyclohexane and methanol solutions58,59 in order to learn about the vibrational

spectroscopy of the excited singlet state, the time scale for equilibration of the two lowest lying singlet excited states, and the structure of the excited state. The vibrational spectrum was assigned to an excited singlet state of both 11Bu and 11Ag character that is produced from the intially populated 11Bu Franck–Condon state on a subpicosecond time scale. In methanol solution, shifts of high-frequency chain modes are observed that indicate changes in bond orders in the chain. It was noted58 that these changes should be linked to altered yields for competing biradicaloid and zwitterionic pathways of DPH photoisomerization reported for solvents of different polarity.53

2.1.4. Diphenylacetylene. The photophysics and photochemistry of diphenylacetylene (DPA), also known as tolan, and several derivatives have been reviewed.60 The unusual photophysical properties, such as a very slow rate of conversion of S2 to S1, a dependence of the S2 lifetime on temperature, and a strong dependence of excitation wavelength on the fluorescence quantum yield of the lowest excited singlet states of DPA were correlated with the S2–S1 energy gap.

Picosecond electronic absorption and fluorescence measurements were performed on DPA in a series of solvents, including isopentane, n-hexane, cyclohexane, methylcyclohexane, trans-decalin, ethanol, and acetonitrile.61 The picosecond fluorescence measurements were performed by means of time-correlated singlephoton counting. Picosecond absorption spectra recorded 295-nm excitation with a 4.2-ps dye laser pulse and corrected for the effects of group velocity dispersion (chirp) associated with the white-light continuum probe pulse. In hexane, a 500-nm absorption band appears immediately after excitation, has a lifetime of 8 ps, and was assigned to an absorption from the S2 state. The fluorescence lifetime agrees well with the 8-ps lifetime of S2. Note that a shorter lifetime is indicated as a result of a femstosecond-resolved study.62 The rapid decay of the 500-nm band gives rise to two absorption bands with maxima at 436 and 700 nm. These two bands have a lifetime of 200 ps and were assigned to absorptions from S1. The decay of the S1 absorption bands are concomitant with the rise of an absorption band at 415 nm assigned to a Tn < T1 transition. No fluorescence with a lifetime of 200 ps is observed; the fluorescent state of DPA in solution is the S2 state, which is an excep-

tion to Kasha’s rule.63 The S2, S1, and T1 states have been assigned to the 11B1u, 11Au, and 13B1u states, respectively.60,64 The temperature dependence of the fluor-

escence lifetime gives an activation energy (Ea) of 890 cm 1 or 2.5 kcal/mol; this is the activation barrier that must be traversed on going from S2 to S1.61 An investigation of the temperature dependence of the fluorescence quantum yield gives an Ea of 14 kJ/mol or 3.3 kcal/mol.64 The difference between these two activation energies was explained64 by the presence of a temperature-independent contribution that was not detected in the earlier investigation.61