Luminescence Spectroscopy of Rhodamine Homodimer Dications in Vacuo Reveals Strong Dye-Dye Interactions.

Being alone or together makes a difference for the photophysics of dyes but for ionic dyes it is difficult to quantify the interactions due to solvent screening and nearby counter ions. Gas-phase luminescence experiments are desirable and now possible based on recent developments in mass spectrometry. Here we present results on tailor-made rhodamine homodimers where two dye cations are separated by methylene linkers, (CH2 )n . In solution the fluorescence is almost identical to that from the monomer whereas the emission from bare cation dimers redshifts with decreasing n. In the absence of screening, the electric field from the charge on one dye is strong enough to polarize the other dye, both in the ground state and in the excited state. An electrostatic model based on symmetric dye responses (equal induced-dipole moments in ground state) captures the underlying physics and demonstrates interaction even at large distances. Our results have possible implications for gas-phase Förster Resonance Energy Transfer.

Photodissociation of protonated tryptophan and alteration of dissociation pathways by complexation with crown ether.

The behavior of protonated tryptophan (TrpH(+)) and its complex with 18-crown-6-ether (CE) after photoexcitation has been explored based on measurements of dissociation lifetimes, fragmentation channels, and absorption spectra using an electrostatic ion storage ring. A recent implementation of pulsed power supplies for the ring elements with microsecond response times allows us to identify the daughter ion fragment masses and to disentangle fragmentation that occurs from excited states immediately after photoexcitation from that occurring on a longer time scale of several microseconds to milliseconds. We find that attachment of crown ether significantly alters the dissociation channels since it renders the pisigma(*)(NH(3)) state inaccessible and hence prevents the N-H bond breakage which is an important fragmentation channel of TrpH(+). As a result, on a long time scale (>10 micros), photoexcited TrpH(+)(CE) decays exponentially whereas TrpH(+) displays a power-law decay. The only ions remaining in the latter case are Trp(+) radical cations with a broad internal energy distribution caused by the departing hydrogen. Large changes in the fragment branching ratios as functions of excitation wavelength between 210 and 290 nm were found for both TrpH(+) and TrpH(+)(CE).

A new technique for time-resolved daughter ion mass spectrometry on the microsecond to millisecond time scale using an electrostatic ion storage ring.

A new method for time-resolved daughter ion mass spectrometry is presented, based on the electrostatic ion storage ring in Aarhus, ELISA. Ions with high internal energy, e.g., as a result of photoexcitation, dissociate and the yield of neutrals is monitored as a function of time. This gives information on lifetimes in the microsecond to millisecond time range but no information on the fragment masses. To determine the dissociation channels, we have introduced pulsed supplies with switching times of a few microseconds. This allows rapid switching from storage of parent ions to storage of daughter ions, which are dumped into a detector after a number of revolutions in the ring. A fragment mass spectrum is obtained by monitoring the daughter ion signal as a function of the ring voltages. This technique allows identification of the dissociation channels and determination of the time dependent competition between these channels.

Dianions of 7,7,8,8-tetracyano-p-quinodimethane and perfluorinated tetracyanoquinodimethane: information on excited states from lifetime measurements in an electrostatic storage ring and optical absorption spectroscopy.

We have developed an experimental technique that allows us to study the physics of short lived molecular dianions in the gas phase. It is based on the formation of monoanions via electrospray ionization, acceleration of these ions to keV energies, and subsequent electron capture in a sodium vapor cell. The dianions are stored in an electrostatic ion storage ring in which they circulate with revolution times on the order of 100 micros. This enables lifetime studies in a time regime covering five orders of magnitude, 10(-5)-1 s. We have produced dianions of 7,7,8,8-tetracyano-p-quinodimethane and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-p-quinodimethane (TCNQ-F(4)) and measured their lifetimes with respect to electron autodetachment. Our data indicate that most of the dianions were initially formed in electronically excited states in the electron transfer process. Two levels of excitation were identified by spectroscopy on the dianion of TCNQ-F(4), and the absorption spectrum was compared with spectra obtained from spectroelectrochemistry of TCNQ-F(4) in acetonitrile solution.

Photophysics of protoporphyrin ions in vacuo: triplet-state lifetimes and quantum yields.

Lifetimes of triplet-state molecules and triplet quantum yields are important parameters in photobiology as they determine the generation of singlet-oxygen upon irradiation with visible light. Here we report lifetimes of protoporphyrin IX (pp) in vacuo measured in an ion storage ring. We find that after 532 nm photon absorption, pp(-) (free base and negatively charged carboxylate) and pp(+) (single protonation of ring nitrogen) have triplet-state lifetimes of 12 and 6 ms, respectively. After 415 or 390 nm absorption the lifetime of the anion is shorter (1.5 and 0.6 ms) as expected from the increase in temperature. Triplet quantum yields of pp(-) and pp(+) are similar, 0.6-0.7, close to values reported for the free base and monocation in solution. The other channel, direct decay to the electronic ground state and subsequent dissociation of vibrationally excited ions, is much faster than triplet-singlet intersystem crossing. We measured lifetimes of 63 micros, 96 micros, and 0.3 ms after 390, 415, and 532 nm excitation, respectively. A fit of a statistical model to the pp(-) decay results in an Arrhenius activation energy of 0.5+/-0.2 eV for CO(2) loss and a low preexponential factor (10(6)-10(10) s(-1)), indicative of an entropic barrier.