Enhancement of Triplet Stability in Benzene by Substituents with Triple

Article pubs.acs.org/JPCA

Enhancement of Triplet Stability in Benzene by Substituents with Triple Bonds Philip M. Johnson*,† and Trevor J. Sears†,‡ †

Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, United States Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, United States

‡

ABSTRACT: Excitation of phenylacetylene (PA) and benzonitrile to their lowest singlet states in a molecular beam has previously been shown to immediately (only during the 8 ns laser pulse) result in long-lived species with low ionization potentials (Hofstein, J.; Xu, H.; Sears, T.; Johnson, P.M. J. Phys. Chem. A 2008, 112, 1195−1201). Using the fragmentation of ions produced by photoionization at various times after initial excitation as a diagnostic for molecular geometry evolution, the long-lived species in phenylacetylene is shown to be a PA state (most likely a triplet) rather than an isomer. Delayed fluorescence and a delayed photoelectron signal indicative of S1 are also seen, indicating a singlet−triplet mixing process that is not quite in the statistical-coupling limit and is parallel to the long-lived species channel. Electronic structure calculations indicate that the lowest triplet state of phenylacetylene is nonplanar with the ethynyl group bent in a trans-configuration out of the plane of the ring. The substituent π-electrons are significantly conjugated into the ring, resulting in a tendency toward a quinoidal structure, which may be related to the unusual excited state stability. These molecules constitute the first members of a new class of excited state behaviors.

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INTRODUCTION Due to considerable interest in the development of organic light-emitting diodes (OLEDs) and molecular electronics in general, the understanding of energy evolution in the excited states of larger molecules has taken on a new importance. In particular, OLEDs have the promise to provide much more energy efficient light sources and displays, but their efficiencies are compromised by the dispersal of excitation energy in the emitting molecules into both the singlet and triplet manifolds, where the latter does not easily allow emission. Doping the emitting molecules with heavy metals has enabled the recent development of commercial products by breaking down the spin purity of the triplet levels, making the optical transition to the ground state more allowed.1 However, this is a somewhat expensive and environmentally questionable solution. A more recent attempt at getting emission out of the energy that ends up in triplet states involves creating special molecules (based on dicyanobenzene) with very small singlet−triplet gaps, allowing thermal excitation to move the energy back to the singlet state, where the molecule can fluoresce.2 This takes time, so to have good efficiency one must have molecular systems where the excited states do not rapidly return to the ground state nonradiationally. The source of such stability is not well understood in detail. Pump−probe ionization spectroscopy in molecular beams provides a way to study the energy evolution of single-molecule excited states in a nonperturbing environment.3,4 Typically, a molecule is excited to an excited level by a nanosecond range dye laser and, after an elapsed time, ionized by another laser of the same or different color. For medium-sized molecules such © 2013 American Chemical Society

as substituted benzenes, the intensity of the ionization signal initially follows the lifetime of the pumped state due to fluorescence and radiationless transitions. Then, if the radiationless transition proceeds to a state with a low enough ionization potential and sufficiently large Franck−Condon factors to an accessible ionic state, the signal will show the lifetime of that level. In a collisionless environment, a pumped singlet state will usually cross over to an isoenergetic, hot triplet level, which will then eventually evolve due to intersystem crossing to a very vibrationally excited ground state level. The triplet decay time greatly depends upon its vibrational excitation and thus how far it is above T1. For most substituted benzenes studied to date, with S1−T1 gaps ranging from 2500 to 8600 cm−1, these lifetimes range from hundreds of nanoseconds to a few microseconds.3−8 In an interfering process, the dissociation of clusters formed in the supersonic beams can lengthen the measured triplet lifetimes by removing vibrational energy equal to the dissociation energy, but that effect is not large and lifetimes over 10 microseconds are still not seen.8 Against this background of normal and understandable isolated molecule behavior, the excited state dynamics of phenylacetylene (PA, ethynylbenzene) and benzonitrile (cyanobenzene) stand out.9 These molecules have S1−T1 gaps of over 10 000 cm−1, so they should be in the “large molecule” or “statistical” limit of radiationless transition Received: April 15, 2013 Revised: July 29, 2013 Published: July 30, 2013 7786

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theory10,11 (where the background states coupled to the “bright” nonstationary state are so closely spaced they overlap to form a continuum). Also, compared to other substituted benzenes, they have two strikingly anomalous behaviors: (1) products are formed by excitation to S1 that seem to live indefinitely (>150 μs, a lower limit determined by the flight time through the apparatus), while the formations of these products have action spectra that mirror the absorption and fluorescence excitation spectra of the monomer S1 at the rotational level, and (2) the long-lived products appear immediately during the laser pulse and do not build up during the lifetime of the singlet level, as happens in benzene for example. Questions have therefore arisen: (1) is the secondary product of the S1 excitation the triplet state, as is assumed in all previous work on other molecules, or is it an isomer of some sort? and (2) why is the long-lived species created only during the laser pulse? It is very difficult to answer the first question by directly probing the neutral long-lived species because it is in the presence of a vastly larger number of ground state parent molecules. Instead, in the study reported here the long-lived species is ionized, and the ions are analyzed by mass spectrometry to compare them to those of PA ionized directly. If the fragmentation mass spectrum indicates isomeric ionic structures were produced from the long-lived species, the neutral product would then also be considered to be an isomer. The comparison is made between PA cations and ions derived from the long-lived species, by photodissociating each of them and looking at the distribution of fragments and metastable ions produced by a visible wavelength laser from the two sets of parent cations. The results of the experiments focused on the first question showed that the fragmentation of the cations produced from PA S1 and those from the long-lived species were virtually identical, indicating the latter is an excited state of PA itself. Examination of the fluorescence and photoelectron signals at considerable delays after the excitation showed that singlet state decay (initially with a lifetime of 70 ns) is nonexponential with a weak long-lived tail lasting at least tens of microseconds. This indicates that PA is not acting like a molecule in the statistical limit of radiationless transitions even though it has a very large singlet−triplet gap. However, it sheds no light on question 2, because the channel containing singlet character appears to be decoupled from the one resulting in the long-lived species.

the photophysics was being studied (the pump laser). Second, light from a 193 nm excimer laser was used to ionize the excited species remaining in the sample at various times after the pump laser. Third, visible light from a YAG-pumped dye laser was used to fragment the ions formed in the first two steps. Mechanical shutters were positioned in each beam, setting up a different set of incident beams each second so various background signals could be subtracted while the mass spectra were being accumulated over thousands of laser shots. Lasers were kept at minimum powers necessary to see the signals, to minimize background signals (single-laser ionizations, for example) and nonlinear effects. Exceptions to this were when the mass spectrum from a single laser multiphoton excitation was desired. For reference, multiphoton mass spectra from the pump and excimer lasers alone were measured. Photoelectron spectra using the various pumping schemes were obtained using a simple magnetically shielded time-offlight tube with no accelerating fields. Mechanical optical shutters were also used in those experiments to subtract out single-laser signals. Time-of-flight data were inverted to an energy scale, and intensities were converted using an E−3/2 scaling factor (the Jacobian of the scale change), so the areas are consistent between the two representations. Fluorescence decays were measured in a molecular beam using a filtered photomultiplier, accumulating the decay using photon counting, over about 20 000 laser shots. Excitation was along the molecular beam so the number of excited molecules in the viewing region of the detector remains representative of the population that would be present in a motionless sample. For both the photoelectron and fluorescence experiments, the excitation laser was a CW dye laser centered on a single rotational line of the S1 origin band, followed by 3 dye amplifiers pumped by an 8 ns, seeded YAG laser. These experiments have been described in detail previously.9 For the mass spectra experiments, laser bandwidths of around 0.1 cm−1 were used.

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RESULTS AND DISCUSSION Mass Spectrometry. The various pumping schemes used in this experiment can be understood by referring to Figure 1. There the relevant states of the neutral and ion are plotted on an energy scale, with the deposited photon energies showing the excitation possibilities for the various schemes. The longest length arrows represent the photon energy of 193 nm ArF excimer emission, the medium arrows represent 278.7 nm dye laser photons resonant with the 0−0 S1 ← S0 transition of PA, and the short arrows stand for the 561 nm green photons used to photofragment the variously produced cations. For all the cases except c, the pump and probe photons are overlapped in time. For c, the probe is delayed by 1 μs from the pump. For three cases, photoelectron spectra are superimposed to show the distributions of ionic states formed after the two-laser excitation, due to energy carried away by the departing electron. In case a, the two-photon energy through the S1 0−0 is just barely above the ionization threshold, and the photoelectron spectrum would appear as a narrow line on the scale of this diagram, if given. In case a, the green photons meant to produce a fragmentation pattern merely excite the cation to its C̃ + (D3) state, which is quite stable, and unless enough laser power is used to caused multiphoton excitations, no fragmentation is observed.

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EXPERIMENT The quantum yield of the long-lived species is much larger in PA than in benzonitrile (∼20% versus ∼5%), so this study used PA exclusively. As previously9 PA was added into a twoatmosphere helium stream at its room temperature vapor pressure and expanded through a pulsed valve into a differentially pumped vacuum system. The source chamber had a background pressure of about 10−5 torr when the valve was on. The supersonic beam was skimmed while proceeding to an analyzing chamber where the pressure was about 10−7 torr. Analysis of ions formed by multistep ionization processes was done with a reflectron time-of-flight mass spectrometer having a mass resolution of over 500. In order to minimize cluster formation, measurements were only taken on the leading edge of the molecular beam pulse. Three lasers were used in the mass spectrometry experiments. First, a YAG-pumped, doubled dye laser tuned to the S1 origin of PA transferred population to the excited state where 7787

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In case d, the two-photon excitation is into a region where we have no state information from published photoelectron spectra. However, it provides an indication of what happens when the cation is provided with considerable excitation. The reflectron fragment mass spectra for cases b, c, and d are shown in Figure 2. The parent mass at 102 amu is not shown in

Figure 1. Excitation schemes for the mass spectrometry experiments. Photoelectron spectra (orange) are superimposed vertically on three schemes, with their zeros at the position of combined pump−probe photon energy and extending downward. These provide a picture of the ionic state distribution produced by the ionization process for each photon combination. Scheme a is a simultaneous 279 nm two-photon resonance ionization followed by a 561 nm photon. No fragmentation occurs because the green light excites to a stable state of the ion. Scheme b creates ions directly from the singlet state by employing simultaneous 279 and 193 nm photons, and a distribution of ionic à + and X̃ + ionic state vibrational levels are formed. The 561 nm green photon excites this distribution. Scheme c is the same as b except the 193 nm laser is delayed by 1 μs, well past the decay of the prompt fluorescence. Primarily, higher energy levels of the ion are formed, and the 561 nm photon results in more energy being deposited in total. Scheme d uses two simultaneous photons and reaches even higher into the energy levels of the ions. The reflectron mass spectra resulting from schemes b−d are shown in Figure 2, and the photoelectron spectra for schemes b and c are shown on a larger scale in Figure 3. State types are labeled as S for the singlets, T for the triplets, and D for the ionic doublets.

Figure 2. Reflectron TOF mass spectra resulting from schemes b−d in Figure 1. While the metastable ion formation changes as expected with the increasing deposited in the ion, the prompt ion fragment intensities are much the same in schemes b and c, indicating that phenylacetylene cations are being formed in both cases and thus no isomerization is taking place.

these spectra because they are a result of a subtraction of spectra, with and without the green fragmentation laser. Since parent is not produced by the green laser, and the peak is so large it is often saturated, its magnitude in a subtracted spectrum is noisy and meaningless, thus excluded. The mass regions from 76 to 80 in Figure 2 illustrate a useful feature of reflectron mass spectra. The signal in that region is due to metastable ions which dissociate at various points during the ions’ trip through the apparatus.12 Similarly to a normal linear TOF mass instrument, a decaying tail can occur on the higher side of a mass peak, representing metastable ions dissociating in the acceleration region of the spectrometer. This is seen for the mass 76 peak in these spectra. For a reflectron instrument, another effect can occur when the metastable ion decays between the acceleration region and the reflector. Then, the daughter ions will penetrate a different distance into the reflector, and a new peak will appear. The position of this peak is sensitive to the voltages in the reflector and can move around with respect to the prompt ion mass peaks, depending upon those settings. The peaks in the spectra at around mass 80 are due to that phenomenon. The spectra in Figure 2 show that in each excitation scheme, mass 76 ions are created by a fairly slow parent ion dissociation, to a greater or lesser extent. Referring to the excitation energies shown in Figure 1, it is seen that the metastable production gets larger with respect to the prompt fragment production as the total energy absorbed by the molecule increases. The whole pattern does not change in a qualitative way, however. That is, a different mass is not seen being produced from the metastable parent.

In case b, the pump laser excites a combination of a state that decays with 70 ns lifetime and a long-lived state whose signal does not appreciably decrease for the maximum flight time in our apparatus (∼150 μs). The excimer laser is overlapped in time with the pump, ionizing both of these species, which have distinctively different photoelectron spectra, as shown in ref 9. The spectrum shown in b has had the long-lived contribution subtracted out. The short-lived component emits fairly highenergy electrons, and the cation drops down into the à + and X̃ + states upon ionization. In case c, where the excimer laser ionization takes place 1 μs after the S1 level is pumped, the photoelectrons exhibit mostly low energies, and the resulting ionic states may be vibrational levels of the B̃ + state or highly excited vibrations of the lower two states, depending on the Franck−Condon factors resulting from the vibrational excitation of the intermediate long-lived state. 7788

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set of background states, the fluorescence should be diminished to about 8 × 10−7 of its initial strength at 1 μs. The amplified trace of the 1 μs spectrum (×20) shows that the singlet component has only decreased by a factor of 37. This is substantial evidence that some set of triplet levels is coupled to the singlet for a long period of time, providing a reservoir for continued singlet character. The observed 70 ns fluoresecence decay is consistent with an 8 MHz Lorentzian contribution to the line width in the high-resolution LIF spectrum reported by Riblett et al.;13 see below. Since there is a delayed singlet photoelectron spectrum, one would also expect a delayed fluorescence signal. Figure 4 shows

The prompt fragment distributions do not change much between the spectra. The main fragmentations appearing are the following: (1) loss of an acetylene molecule [mass 76]; (2) dissociation into diacetylene [50] and either cyclobutadiene or adamantane cations [52], and (3) dissociation into cyclopentane [39] and vinylcyclopentane [63] ions. There are occasional losses of hydrogen from these species also. In this experiment, the laser intensities were adjusted so there was little fragmentation without the use of the green laser, so it is assumed that the fragmentation takes place from the parent ion of whatever species is ionized by the 193 nm photons. The similarity between both the metastable behavior and the prompt fragmentation demonstrates that PA cation is the parent species in each case, which in turn indicates that the long-lived species created by S1 excitation is not an isomer. It is very difficult to imagine an isomeric cation that would have both the same metastable behavior and fragmentation pattern as PA. Thus the long-lived species is some combination of excited PA molecules. The only excited states that would have lifetimes of at least hundreds of microseconds are triplet states. Delayed Photoelectron Spectroscopy and Delayed Fluorescence. The verification of triplet states as the longlived species gives merit to a closer analysis of previous9 photoelectron and fluorescence experiments. If a mixed state is produced in the triplet manifold at the S1 energy, it has the possibility of producing delayed fluorescence and a singlet-type photoelectron signal at time delays well after the primary decay of the fluorescence. In Figure 3 photoelectron spectra of S1 excited phenylacetylene are shown with a 193 nm ionization laser pulse

Figure 4. Fluorescence decay curve for phenylacetylene showing the nonexponential nature characteristic of delayed fluorescence.

a nonexponential fluorescence decay at long times characteristic of that phenomenon. The signal persists out to at least 20 μs, but it is not clear at present whether that cutoff is an intrinsic feature of PA or due to the difficulties in accurately measuring very long lifetimes in a set of rapidly moving molecules. This residual fluorescence is very weak compared to the direct S1 emission, but it persists for about 100 times longer, so the integrated intensity measured in this experiment is on the order of 1% of the S1 emission. The Triplet States of Phenylacetylene. Now that it is clear the long-lived species is some kind of a triplet state, the state structure must be analyzed in more detail. Gaussian 09 electronic structure calculations were performed using timedependent density functional theory (TDDFT) to determine the energies and the optimized geometries of the lower excited singlet and triplet states. They were done using Dunning’s augmented correlation corrected triple-ζ basis set (aug-ccpVTZ) and the B3LYP DFT method. The ground state and lowest triplet state were done without using the timedependent augmentation of the DFT method, and vibrational frequencies were determined in order to get zero-point corrections. A zero-point correction was determined for S1 using TDDFT. The energies for the S0, S1, and T1 (Cs) minima of PA are given in Table 1. Energies of T2, T3, and T4 are also given, at their C2v minima. It is illustrative to plot the energies of the PA states as a function of geometry along a line in multidimensional space that intersects the energy minima of S0 and T1 states. This is

Figure 3. Photoelectron spectra from schemes b and c (without the green laser), showing that even after over 14 lifetimes of the prompt singlet fluorescence there is a residual photoelectron signal representative of the singlet state.

coincident in time with the excitation and when the ionization is delayed by 1 μs (Figure 3 is extracted from Figure 4 of ref 9, where spectra at other delay times are shown as well). It is seen that there are two parts to the spectrum, one that mostly decays with the lifetime of the singlet and one that is there immediately, which never decays. Since the experimental initial fluorescence lifetime of S1 is 70 ns, if S1 were not coupled to a 7789

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Table 1. Calculated Electronic Energies and Transition Energies for the Lower States of Phenylacetylene S1−Tn

from S0 state

symm.

energy, au

Δ, eV

Δ, cm−1

λ, nm

S0 S1 T1 T2 T3 T4

A1 B2 A′ A1 B2 B2

−308.509241335 −308.332190987 −308.393654958 −308.357574529 −308.354502990 −308.341191807

4.818 3.145 4.127 4.211 4.573

38858 25368 33287 33961 36883

257.3 394.2 300.4 294.5 271.1

Δ, cm−1

13490 5571 4897 1975

from S0 with ZPC zero pt. corr., au

Δ, eV

Δ, cm−1

λ, nm

exptl, eV

0.109349 0.103649 0.103693

4.663 2.991

37607 24127

265.9 414.5

4.448a 3.154b 4.255b

a

Chang, C.-H.; Lopez, G.; Sears, T.; Johnson, P. J. Phys. Chem. A 2010, 114, 8262−8270. Gas phase origin. bSwiderek, P.; Gootz, B.; Winterling, H. Chem. Phys. Lett. 1998, 285, 246−25. Solid state electron energy loss spectra.

shown in Figure 5, where the x-axis is the generalized distance along that line, with zero being the S0 minimum. Seen in that

of triplet states provides a very large density of vibronic levels in which to distribute the energy from singlet−triplet crossing. The most interesting features to come from the calculations are with respect to T1. As shown in Figure 6, for that state, the

Figure 6. Geometry of phenylacetylene at the energy minimum of the lowest triplet state.

geometry optimization produces a molecular structure where the ethynyl group is significantly bent and with bond lengths that differ considerably from the other π-states of the molecule. From the point of view of the almost planar ring, the ethynyl carbons are bent down by 9° and the terminal hydrogen is bent up from that by 30°. This arrangement is a trans-bend of the ethynyl group, resulting from the conjugation of that substituent into the ring π-structure. Electron density is lost in the two ring bonds adjacent to the acetylene, and they lengthen considerably to 1.48 from 1.41 Å (in the ground state), while the bond from the ring to the −CCH group shortens to 1.35 from 1.43 Å. The overall electronic structure becomes quinoidal in nature, and the triple bond lengthens to 1.27 from 1.21 Å. In order to analyze the effect of PA’s electronic structure on its unusual excited state dynamics, it is useful to compare with benzene, which has a more normal behavior in its pump−probe ionization results.4,9 Using the same TDDFT methods, the excited state pattern is almost the same for benzene as for PA. The same four triplet states are below S1, in the same order. The only difference is that in benzene the middle two of those are degenerate, forming the 3E1u state. Therefore, it does not seem like the pattern itself is the reason for the different behavior. Only in T1 (and T5, a 3A2 state in C2v, and the second triplet at some geometries) is there a massive change in going from benzene to PA. While we are not aware of any experimental evidence concerning the geometry of PA’s triplet state, a related molecule, dicyanobenzene, has had its triplet state studied by both infrared14 and ESR15 techniques in low-temperature solids. A comparison of the infrared spectrum with DFT calculations suggests the same electronic and geometry changes seen in PA, with the bonds to the −CN groups changing from single to double and the electronic structure becoming

Figure 5. TDDFT energies of S0 and five triplet states plotted along a line in geometry space that intersects the minima of the S0 and T1 potentials. Energy distances are referenced to the S0 minimum, and the geometric distance is determined from the square root of the sum of the Cartesian, squared displacements of the atoms.

way, it is apparent that each state has its own function with respect to geometry, there are varying numbers of triplet states below S1, and the state ordering changes often, depending on geometry. It should be noted that this particular cut through geometry space does not intersect the minima of any states other than S0 and T1, so the minima shown in the other curves do not represent the state energies. Except at the origin, the symmetry of the molecule in Figure 5 is Cs, and the states have either A′ or A″ state symmetries. For purposes of clarity, we have suppressed the avoided crossings where two states of the same symmetry cross. When that happens, potential surfaces with multiple minima can be formed. The crossing of the S0 and T1 curves at about 4 eV is of particular note. Although there is probably considerable error in the calculation, this gives an upper limit to the energy above which triplet vibrational levels can easily cross over into S0. At other geometries the crossing may be lower. It is seen that at the geometry of S0 there are four triplet states below the calculated S1 energy, with T1 being over 10 000 cm−1 below and T4 only about 1900 cm−1 down (although this gap is within the error in the energies, so the ordering is not certain). This group 7790

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quinoidal.14 Dicyanobenzene is a commonly used electron acceptor in molecular electronics, and the cyano groups are said to suppress the radiationless deactivation of T1.2 The results here with PA suggest it is primarily the triple bond in the substituent group that changes the nature of the triplet state, and ethynyl groups may generally act similarly to cyano groups in stabilizing the triplet state, not a complete surprise since cyano and ethynyl are isoelectronic. The difference between the present study and previous work is that here the molecules are completely isolated, whereas prior experiments have been done in a condensed phase, where any triplet formed would rapidly cool to the lower vibration levels of T1. The fact that suppression of radiationless deactivation is seen in molecular beams means that it is an intrinsic property of the molecule not dependent upon external perturbations. In the beam, any triplet states formed by crossing directly over from S1 have over 10 000 cm−1 of vibrational energy (about 14 000 K) if one assumes the predominant population is in T1. These very hot molecules are well above the energy of the S0−T1 crossing and are unlikely to last long as triplets. This is probably the reason that the population with a triplet ionization signature does not continue to build up during the fluorescence lifetime. The long life of the population created during the laser pulse will require a different explanation. State Evolution. Notable early work on the theory of radiationless transitions is exemplified by the work of Robinson8 (mostly applied to condensed phases) and Jortner and co-workers11,16 (isolated molecules studied at higher temperatures with incoherent light sources). Building on this foundation, Tric and co-workers17−19 extended the concepts to include short-pulse excitation with coherent sources and treated isolated molecules at low temperatures. This treatment is appropriate to modern experiments using lasers and molecular beams and is in common use, particularly as extended by Kommandeur20 to include the effects of the laser coherence width. Tric treated radiationless transitions such as intersystem crossing (ISC) and internal conversion (IC) as the evolution of a nonstationary state wave function that is a linear combination of quasi-stationary components |n⟩, with a complex energy so both the time-dependent part of the component wave function (energy = εn) and its decay properties (line width = γn) are explicitly included.

pyrazine.20 The faster component is the result of the dephasing of the initially pumped pure singlet state, while the longer component is the composite decay of the mixed eigenstates, which have been diluted by the coupled dark levels. How the ISC of a molecule behaves depends upon the relative bandwidths of the initial singlet (γs, its radiative width) and the decay bandwidths {γl } of the background molecular eigenstates it is coupled to over the width Δ. When γl − γs ≫ Δ the molecule is in the “statistical limit” with a single exponential fluorescence decay time of (γs + Δ)−1. Large-gap aromatic molecules such as benzene are in this limit. With a pulsed excitation, “intermediate case” molecules such as pyrazine have a large Δ, a small ρ, and a very rapid initial decay followed by a slow one with substantial intensity, characterizing γl− γs ≪ Δ. One would suspect from the S−T gap in PA that it would be in the statistical limit, but the long-lived fluorescence seen weakly indicates that it can considered to be in the “almost statistical limit” where γl − γs ≈ Δ. The model of Tric19 is valid when the initial state is excited coherently. That is, the coherent bandwidth of the excitation is larger than Δ, the line width of the optically bright state as perturbed by the semicontinuous set of dark background states. For PA, we have a good measure of Δ from the CW dye laser fluorescence excitation spectrum of Ribblett et al.13 Their lines were measured to have a Voigt profile consisting of a Lorentzian component of 10 MHz and a Gaussian component of 18 MHz fwhm. We have used two different dye lasers in the PA experiments. The pulse-amplified CW dye laser used in many has a bandwidth of 80−100 MHz and a pulse width of 8 ns, making it about double the transform limit. This type of laser is known to have a slight chirp, and that is the likely cause of the wider frequency distribution. A standard multimode YAG-pumped dye laser with a bandwidth of about 0.2 cm−1 was also used. This laser has an unknown coherence width that is much narrower than the bandwidth. It is found that there is little difference in the dynamic properties measured for PA with the two lasers, possibly indicating that their coherence widths are similar at about 50 MHz, and large enough that the Tric model is valid. Looking at only the fluorescence results one would be only mildly surprised that PA does not have completely statistical behavior but satisfied that it fits a model that can be rationalized by assuming there is only small density of more strongly coupled “doorway” states to the triplet manifold at the S1 level. One possibility is that these doorways states are vibrations of the T4 state, which may be only about 1800 cm−1 below S1, a gap similar to that of pyrimidine, which has been studied as an intermediate case example21 and shows a behavior similar to pyrazine. Franck−Condon factors would be very favorable for those states, and triplet internal conversion would provide the broad line width for the coupled state indicated by the fluorescence behavior. Another possibility is that along certain vibrational normal coordinates the effective S1−T1 gap is greatly reduced by the altered geometry of T1, possibly creating a potential crossing (conical intersection) that increases couplings, as described by Englman and Jortner.16 Most previous studies of radiationless processes have only looked at fluorescence properties, and the theories have focused on describing only the singlet emission. By using ionization probes, one can also examine the downstream species, and that is where anomalies start to appear for PA. Still considering the ISC event, if one accepts that triplet production is the only rational way to produce a very long-lived species that

⎛γ ⎞ En = ∈n −i⎜ n ⎟ ⎝2⎠ Ψ(t ) =

∑ cn|n⟩e−i(∈ −iγ /2)t n

n

n

From these functions, an effective Hamiltonian matrix can be formed, with the optically bright singlet component energy, ϵs − iγs/2, in the (1,1) position, other imaginary basis energies ϵl − iγl/2 for the coupled states {l} along the diagonal, and because only the initial decay of the pumped state is being considered, coupling elements vsl in only the first row and column, with zero elsewhere. Some of the subsequent dynamics of the initially coupled states is represented in their line widths. General solutions for this model can only be obtained for some simple cases, such as when all of the coupling elements are the same and the quasi-stationary energy levels are evenly spaced. Under the conditions of strong coupling (vρ ≫ 1, where ρ is the density of states), a characteristic biexponential fluorescence decay is seen, as is classically demonstrated in 7791

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inhibited, but the effects of these differences are in need of further study. Other experiments here show that there are delayed fluorescence and photoelectron signals that would normally be ascribed to a system that is not quite in the statistical limit, even though the density of states at the triplet level is enormous, due to over 10 000 cm−1 of energy and multiple complicated electronic levels between S1 and T1. The long-lived ionization signal is only created during the laser pulse and has no easy explanation within the context of the standard radiationless transition picture.

photoionizes into a PA cation, one must try to explain why the triplet signature is produced only during the laser pulse. In PA the species with the triplet signature in the PES ceases to build up at the end of the light pulse. None appears during the fluorescence decay, as occurs in benzene. Thus the experimental evidence points to a mechanism whereby there is a (possibly coherent) coupling created by the light field that encourages the formation of a long-lived triplet. No model that we are aware of contains such a coupling element, so further work will have to involve an investigation of various possibilities. Of itself, the absence of a “triplet” signal that builds up during the singlet lifetime is not a unique feature of PA and BN, however, since no “triplet” ionization signal appears at all in styrene,9 for example. That can be explained by assuming that IC or Franck−Condon factors in the hot triplet manifold prevent ionization with 193 nm photons. What is unique with PA and BN is that triplet gets formed during the laser pulse, and then stops. Lahmani, Tramer, and Tric19 discuss the applicability of a kinetic scheme to represent the more fundamental quantum mechanical model they produced. A kinetic model has the advantage that it sometimes gives a more intuitive understanding of the processes. Their conclusion was that the differences between a quantum mechanical and a kinetic model are small, and if the rates are chosen properly, use of a kinetic model is justified. Since the state gaps in PA are substantial, the interstate couplings will vary substantially as a function of that gap, so a discussion in terms of a kinetic model may provide a good overall picture of events. One scenario that provides a rational for the experimental observations is the following. In the consideration of ISC in any larger molecule, the initially excited pure S1 state, while fluorescing, dephases into a mixed singlet−triplet entity with a rate ki . The triplet component has an enormous vibronic phase space with various parts, each dominated by a particular triplet electronic configuration. One of these regions (perhaps the T4 electronic configuration predominates since it is nearby and identical to S1 except for spin) is preferentially mixed with S1 and is the product of the dephasing. These coupled states provide a reservoir for the slow singlet PES and fluorescence signals because of a reverse rate kr. They do not give an ionization signal with 193 nm light. Meanwhile, another population of states has been created during the laser pulse, giving a characteristic PES signal. Since T1 has the largest region of available phase space (has the most vibrational energy), whatever states are initially populated by the coupling mechanism, statistically most of the nonemitting excitation going through the other regions will end up there. Due to T1’s unique electronic structure, only its lower levels are weakly coupled to S0 so a mechanism that somehow dissipated enough energy to get below any conical intersections would better fit the available data.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; telephone (631) 6327912. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge G. V. Lopez for his contributions to some of the early experimental measurements and calculations, and valuable discussions with Prof. Thomas Weinacht concerning the dynamics of molecular excited states. Work at Brookhaven National Laboratory was carried out under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences.

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REFERENCES

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CONCLUSIONS The experiments described here show that the long-lived species in PA is the triplet manifold, and therefore that in PA (and by extension, benzonitrile) gas phase triplet states are formed that are extremely resistant to intersystem crossing to the ground state, in contrast to the other substituted aromatics that have been studied. Calculations show that the lowest triplet has an interesting electronic and geometric structure that may be so different from the ground singlet that coupling is 7792

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