Mattanjah S. De Vries, University of California (Santa Barbara)
Polycyclic aromatic hydrocarbons (PAHs)
are a ubiquitous class of molecules, appearing in large abundance in crude oil,
combustion products, and biomasses. Many of the chemical processes in their
formation and reactions involve short lived excited states, which under the
complex conditions of carbohydrate chemistry are often difficult to investigate
in detail. In this project we study excited state dynamics of isolated PAHs in
molecular beams. With this approach we study these molecules not only in the
gas phase, with relevance to many combustion processes, but also under
collision free conditions, to reveal their fundamental, intrinsic properties.In the first year of this project we
have studied the excited state dynamics in two types of cyclic and polycyclic
aromatic compounds, pyrimidines and anthraquinones. At issue is how these types
of compounds react, once they are in an electronically excited state. We
investigate this question by studying how they respond to UV excitation. Aside
from fluorescence, competing photochemical pathways include (i) internal
conversion to the electronic ground state, (ii) coupling to other electronic
states, and (iii) intersystem crossing to a triplet state. The first option
essentially minimizes the likelihood of UV photochemical damage while the
latter two can potentially lead to further reactivity. This is a new direction
of research both for the principle investigator and for the two graduate
students, who have been working on this project, Jacob Berenbeim and Faady
Siouri. We are gaining new insights and developing new ways of approaching this
type of photochemistry for these types of compounds. A number of undergraduate
students have been involved with this work as well, exposing them to
state-of-the art research in chemistry. Two of them have just graduated and, in
part inspired by this experience, are now going to graduate school. These are
Carmen Segura and Samuel Boldisar. Carmen is a first generation college
student who participated in our laboratory in the framework of CAMP, California
alliance for minority participation. (I) Pyrimidine photochemistry For pyrimidines UV
absorptions leads to excitation to the S2 state. Next, subpicosecond de-excitation
back to the ground state, S0, appears to be the dominant process. This process
of internal conversion involves at least one conical intersection. However, a
small portion of excited state molecules appears to populate a “dark state”,
the character of which is unknown and which has been extensively debated in
recent literature.
Vibrational
spectra.
Figure 1 We have obtained
high resolution two photon spectra of U and T. We have also obtained the IR
spectra for these compounds both for the ground state and for the dark state.
Spectra and structures appear in Figure X. The pulse sequences employed to
obtain these spectra appear in Figure 2. For the ground state, an initial IR
pulse is scanned in the NH and OH stretch frequency region and followed by two
step ionization probing. When the IR pulse is resonant with a vibrational
frequency this modifies the ground state vibrational population producing a
modified Franck-Condon landscape. Usually this reduces the ion probe signal,
but in this case it increases the ion probe signal. This suggests a strong geometry
change between S0 and S2, which would also be consistent with the gradual onset
of the UV absorption. For the dark state the pulse sequence starts with
excitation to S2 (purple pulse) followed by rapid relaxation to the dark state.
After 20 ns the IR laser is fired (green pulse) followed after another 30 ns by
the ionization pulse from the excimer laser (black pulse), serving as the
probe. In this sequence the IR laser modifies the dark state vibrational
population, producing the green ion-dip spectra.
Figure 2 Lifetimes and dynamics. We have obtained lifetimes for the dark
state by pump probe measurements. We achieve this by varying the delay between
the excitation laser pulse (purple in Figure 2) and the ionization laser pulse (black
in Figure 2). Lifetimes increasing with excitation energy, varying from 59 to
69 ns for U and from 177 to 275 ns for T. We postulate that this wavelength
dependence reflects the de-excitation dynamics. For T we also observe a very
long timescale component, with a lifetime longer than our experimental window
of several microseconds. We interpret this signal as due to ionization out of
the hot ground state, resulting from the rapid internal conversion out of the S2
state. The resulting model is sketched schematically in the Jablonski diagram
of figure 3. Figure 3 (II) anthraquinonephotochemistry
Anthraquinones are derivatives of the
three ring PAH anthracene. We are studying a systematic series of hydroxyl
anthraquinones, the structures of which ar shown in Figure X: 1,2-dihydroxy and
and 1,4 dihydroxyanthraquinone (1,2-dhaq and 1,4 dhaq, respectively) and 1,2,4-trihydroxyanthraquinone
(1,2,4-thaq). The substitutions in the first two are a subset of those in the
trihydroxy form. Have obtained high-resolution fibrotic spectra for 1,4-dhaq
and 1,2,4-dhaq (shown in figure 4). The linewidths indicate excited state lifetimes
order of nanoseconds. However, by contrast, for 1,2-dhaq, we have only found me
broad spectrum with no vibronic structure, associated with a very short excited
state lifetime. This constitutes me quite remarkable difference in the excited
state dynamics between very similar compounds. Our current hypothesis assumes
an important role for the internal hydrogen bonds, indicated by grey arrows in
figure 4. We are further investigating this intriguing excited state behavior
in the second grant period by further time resolved experiments on these and
related derivatives. Figure 4 In addition to charting new directions
for our research, this work has enriched the careers of the participating
students. They have presented their work in a number of meetings, including a
Gordon Research Conference. Especially noteworthy was the presentation in which
Carmen Segura gave a seminar of her work in a CAMP sponsored symposium. These
opportunities provide invaluable training as well as boosting the self confidence
and motivation of promising future scientists in the field.
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