Christopher A. Haynam

Electronic spectra of the mixed complexes of s‐tetrazine with He and Ar

van der Waals complexes of s‐tetrazine, H2C2N4, with the rare gases He and Ar have been formed in a supersonic free jet and observed by the technique of laser induced fluorescence. The resolution of rotational structure in the π*←n electronic transition (1B3u←1Ag) of the s‐tetrazine chromophore allows the structure of these complexes to be measured. The complex between s‐tetrazine and one argon atom is similar to that observed between s‐tetrazine and He. The argon lies near the out‐of‐plane A rotational axis with a ground state separation of 3.44 A, which decreases to 3.40 A in the excited state. When the number of complexing atoms is two or less they are found to always bind directly above and below the tetrazine ring. Complexes containing three or more rare gas atoms have been observed and their structures have been measured. In the case of three or more rare gas atoms the out‐of‐plane preferential binding sites are filled by the first two atoms and additional atoms must find new sites. As long as one o...

Dimers in jet‐cooled s‐tetrazine vapor: Structure and electronic spectra

Dimers with two distinct conformations form as s‐tetrazine vapor (C2N4H2) is cooled in a supersonic jet expansion. Rotationally resolved laser‐induced fluorescence excitation spectra of both hydrogen and deuterium substituted dimers have been obtained with single frequency scanning dye lasers. Dimer structures were determined by a combined rotational contour generating program and nonlinear least squares fitting procedure. One dimer conformation is planar with two translationally equivalent monomer units. In this conformation one hydrogen of each ring is in close proximity to the nitrogen lone pair of the other, allowing for stabilization by weak hydrogen bonds. The center‐to‐center ring separation is 5.6 Ȧ. The other dimer conformation is T shaped with symmetrically inequivalent rings giving rise to two electronic transitions with different rotational band types. In this configuration, one hydrogen of the first monomer is in close proximity to the π cloud of the second monomer, and the ring centers are s...

Intramolecular vibrational relaxation in jet‐cooled phthalocyanine

We have observed the dispersed fluorescence spectrum of free base phthalocyanine cooled in supersonic free jet. The spectrum consists of a single strong feature and some weaker red shifted structure. The frequencies of the single strong feature and the red shifted structure change only slightly as a function of the exciting frequency. This indicates that the potential surface does not change greatly upon electronic excitation and that the strong fluorescence transitions are Δv = 0. For excitation at frequencies greater than 720 cm−1 above the origin, the features in the fluorescence spectrum are broadened although the absorption features remain narrow. This is taken to indicate a mixing of background vibrational levels leading to intramolecular vibrational relaxation that is fast compared to the fluorescence lifetime. The intramolecular vibrational relaxation in phthalocyanine is compared to that observed in a recent series of experiments on alkylbenzenes.

The fluorescence excitation spectrum of free base phthalocyanine cooled in a supersonic free jet

The fluorescence excitation spectrum of free base phthalocyanine was observed in a supersonic free jet. The spectrum was very intense, and the cooling provided by the supersonic expansion produced a very well‐resolved spectrum showing extensive vibrational structure. This is the first time that vibrational structure has been resolved in the electronic spectrum of the isolated molecule. The separation between the origins of the first two excited singlet states was similar to that observed in the static gas, but in the cold spectrum both origins were at several hundred cm−1 higher energy than the band maxima observed in the static gas. The region near the origin of the first singlet was clean and well‐resolved, while that near the origin of the second singlet was congested due to mixing with excited vibrational levels of the first singlet. No van der Waals molecules were observed.

The spectroscopy, photophysics, and photochemistry of the dimer of dimethyl tetrazine

The dimer of dimethyl tetrazine (DMT)2 has been prepared in a supersonic free jet and its structure and dynamics have been studied by laser induced fluorescence spectroscopy. The binding energy of the excited π*←n electronic state of the dimer was significantly greater than the binding energy of the ground electronic state, and we have called the DMT dimer a mild excimer. The geometry of the DMT dimer was determined from the analysis of the rotational structure in the spectrum of the perdeutero isotopic species and was found to have the two DMT rings stacked and parallel with one ring slipped with respect to the other. At our lowest temperature the perdeutero species was found to have nonrotating methyl groups while the h12 isotopic species had freely rotating methyl groups at all temperatures. The structure of the DMT dimer was consistent with the observation of two exposed aromatic faces as determined by the observation of two van der Waals molecules formed by binding one or two helium atoms to the dime...

The photophysics and photochemistry of weakly bound dimers of s‐tetrazine

Energy redistribution within the excited states of the planar and T‐shaped dimers of s‐tetrazine was monitored by dispersed emission following selective vibronic excitation. When the dimer contained excess vibrational energy above the electronic origin, three processes were observed: (1) energy transfer from the initially populated ν6a vibration (in‐plane stretch) to low frequency dimer vibrations involving motion of one monomer subunit with respect to the other; (2) electronic energy transfer from the high energy ring (∥ polarized) to the low energy ring (⊥ polarized) in the T‐shaped dimer; and (3) photodissociation of the T‐shaped dimer into monomer subunits. Simultaneously observed fluorescence from the initially populated state allows us to estimate the rates at which these processes occur. The observation of photodissociation (τ=38 ps) of the T‐shaped dimer with an input of 1595 cm−1 of excess vibrational energy yields an estimate for the binding energy in the ground state Eb<1552 cm−1.

New S1 vibronic level assignments for s-tetrazine vapor: Polarization and lifetime measurements

Abstract New fluorescence excitation and dispersed SVL fluorescence spectra of s-tetrazine vapor in supersonic expansions of helium and argon are reported. A forbidden in-plane-polarized component of the A 1 B 3u - X 1 A g transition is discovered at (0, 0) + 578 cm−1 with a type-B band contour in rotationally resolved excitation spectra obtained with a single-frequency cw ring dye laser. Linewidth measurements of single rovibronic transitions provide data to calculate lifetimes of low-lying S1 vibronic states of the isolated molecule. Depending on the vibrational mode involved, the lifetime varies from 0.05 to greater than 1 nsec. The number of cold-band assignments in the absorption spectrum of s-tetrazine vapor now confirmed by analysis of SVL fluorescence spectra increases from three to ten.

The spectroscopy of dimethyl‐s‐tetrazine cooled in a supersonic free jet

The fluorescence excitation and dispersed emission spectra of dimethyl tetrazine have been observed in a supersonic free jet. Measurements of vibrational structure in the low resolution fluorescence excitation spectrum and in the single vibronic level emission spectrum have allowed us to confirm and extend vibrational assignments made in the static gas and solid matrix. The spectrum is dominated by progressions in the mode ν6a with strong progressions in ν1 as well. Excited vibrational levels of ν6a in the upper electronic state appear to be perturbed. The progression in ν1 is considerably more intense in dimethyl tetrazine than it is in tetrazine. The vibrational modes ν2, ν8a, ν16a, and ν16b have been assigned. Of these, the assignments of the first three agree with previous work, while the assignment of ν16b is at a much lower frequency. Rotational structure has been resolved at the origin band, and torsionless m=0 transitions have been assigned. Analysis of the m=0 levels indicates that the ring bond ...

Spectroscopy and photophysics of organic clusters

A supersonic free jet expansion has been used to prepare small clusters of aromatic organic compounds, and their structure and energy-transfer properties have been studied by laser-induced fluorescence. The dimer of s-tetrazine was found to have two observable conformations. In the first, the two monomer units were side by side in a planar configuration. In the second, the two molecules were in a T-shaped configuration. In the T-shaped geometry the two rings are not equivalent, and spectra resulting from excitation of one or the other ring could be distinguished. A mixed-gas expansion of tetrazine and benzene in helium produced a mixed tetrazine–benzene dimer, a trimer consisting of two tetrazines and one benzene, and bands that were attributed to larger clusters. The benzene–tetrazine dimer had a stacked, parallel-plate geometry, while the T2B trimer had a benzene stacked parallel over a planar tetrazine dimer. Dimers of dimethyl tetrazine were found to be more tightly bound in the excited electronic state. Excitation of the 6a1 ring mode in the dimethyl tetrazine dimer led to intramolecular vibrational relaxation into the cluster modes. Dissociation was observed following excitation of the 6a20 transition of the tetrazine–benzene dimer.

Mild excimers: The spectrum of the dimethyl‐tetrazine dimer

The fluorescence excitation spectra of a mixture of DMT in helium is studied.(AIP)The fluorescence excitation spectra of a mixture of DMT in helium is studied.(AIP)