58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

·         study the SP-MC mechanism using advanced analytical techniques to better interpret kinetics

·         synthesize and characterize SP-inspired molecular systems that overcome limitations

·           New Photochromic Systems Based on Tetrahydrochromeno Our interest in use of chromic molecules for probing the microenvironment of hybrid systems led us to examine common chromophores such as SP.  Raymo, Sortino and co-workers reported a structurally analogous system based on [1,3]oxazines (OX) that possesses a fused structure compared to the spirocyclic SP.  Comparison of OX and SP suggested that tetrahydrochromeno[2,3-b]indole (CR) may be a potential useful chromic system.  Scheme 1 compares the three chromic systems and the structural relationship between CR, SP and OX. All three systems possess a hemiaminal carbon and generate 4-nitrophenolate upon ring-opening; the structures differ in the indole ring location for the bridging group and presence/absence of C=C bond between indole and phenolate.  The recognition of this structural similarity was the impetus for the current study. Scheme 1. Photoisomerization of spiro[chromene-2,2'indoline] (SP), benzo[5,6][1,3]oxazino[3,2-a]indole (OX) and tetrahydrochromeno[2,3-b]indole (CR); the bold bond shows the bridging connection for the 4-nitrophenolate unit.   We obtained evidence for reversible ring-opening of CR based on H/D exchange, trapping experiments with trimethylsilyl cyanide and transient absorption studies.  These CR compounds readily undergo reaction with tetra-n-butylammonium cyanide.  The cyanide reaction is 10-100x faster when the solution is irradiated with 350 nm light.  Reaction with trimethylsilyl cyanide occurs only with UV irradiation demonstrating photoreactivity.  The rate of tetrahydrochromeno[2,3-b]indole ring-opening is greater for 1) Me substitution at the hemiaminal carbon (compared to Ph), and 2) substitution of fluorine at the 9-position of the indole.  Under acidic conditions, the ring-opened indolium ion is observed.  In collaboration with Professor Joe Perry at Georgia Tech, we observed a femtosecond transient lifetime for the ring-opened CR species.  This rapid ring-opening diminishes the utility of these new compounds for phenomena on longer time scales. The synthesis and characterization of these tetrahydrochromeno compounds have appeared in The Journal of Physical Organic Chemistry.[1]  A second publication[2] will appear on the transient absorption experiments.  Both publications acknowledge the support of the Petroleum Research Fund.   New Mechanistic Insight into the SP-MC System Using Ion Mobility – Mass Spectrometry

The photoinduced and thermal isomerization of SP have been extensively studied.  The first step in the photochemical process is C_spiro-O bond cleavage to generate an excited triplet or singlet state that decays within picoseconds and produces a mixture of geometric isomers of MC that differ in cis/trans (C or T) conformations about the a, b and g bonds linking the indole and chromene subunits (Scheme 2).  For 1',3',3'-trimethyl-6-nitrospiro[chromene-2,2'-indoline] (6-nitro-BIPS, SP-1), the photochemical reaction proceeds via a triplet mechanism followed by intersystem crossing to ³CCC merocyanine and subsequent conversion to ³CTC and ³TTC.  Theoretical investigations suggest ³CCC* decays in picoseconds while the lifetime of the ³CTC and ³TTC species is milliseconds.  Evidence for the CTC form of 6-nitro-BIPS comes from laser-desorption/electron diffraction and excited state dynamics. 

Here we used electrospray ionization (ESI) ion mobility-mass spectrometry (IM-MS) to examine both equilibrated and irradiated samples of spiropyrans 1-3 in methanol (Scheme 2) to gain further experimental insight into the photoisomerization.  IM-MS provides information on the molecules' shapes and sizes based on collision cross-section (CCS), in addition to mass and compositional information. Our interpretation of the IM-MS experimental data argues for presence of a long-lived (>milliseconds) CCX (refers to two of the four MC isomers with cis configuration at the central bond) isomer of MC.  The lifetime of this proposed species is considerably longer than the sub-nanosecond lifetimes reported previously.

Scheme 2. General Structures of SPs 1-3 and their corresponding MCs.

            Based on our results, we conclude that the likely reaction pathway is: SP ⇋ CCT/CCC ⇋ TTC/CTC. Theoretical values for the relative energies of the TTC and TTT isomers revealed a minor difference for calculations in vacuo versus DCM solution.  We feel our results are reflective of SP-MC dynamics and the contribution of field effects does not alter our conclusions significantly.  Mechanistic elucidation of the elementary steps in SP-MC isomerization has been largely limited to transient spectroscopy and model systems. The combination of ESI with IM-MS provides additional information on the structure-dynamics of this important photochromic system.

            This study has been recently published in Chemical Communications.[3]

References

1.         “Reactivity of tetrahydrochromeno[2,3-b]indoles: chromic indicators of cyanide,” Douglas, N.; Neef, C. J.; Rogers, R. A.; Stanley, J. A.; Armitage, J.; Martin, B.; Hudnall, T. W.; Brittain, W. J. J. Phys. Org. Chem. 2013, 688-695.

2.         “Transient absorption studies of          tetrahydrochromeno[2,3-b]indole ring-opening,” Perry, J. W.; Brittain, W. J. J. Phys. Org. Chem. to be submitted.

3.         A study of the spiropyran-merocyanine system using ion mobility-mass spectrometry: experimental support for the cisoid conformation,” Rogers, R. A.; Rodier, A. R.; Stanley, J. A.; Douglas, N. A.; Li, X.; Brittain, W. J. Chem. Commun. DOI: 10.1039/c3cc47697a.