Experimental data reveal that the incorporation of carbonyl groups into polymer matrix can significantly enhance singlet oxygen (1O2) generation and suppress production of other reactive oxygen species. Excitonic processes investigated by phosphorescence spectroscopy reveal enhanced triplet-exciton generation in the modified g-C3N4, which facilitate 1O2 generation through an energy transfer process. Benefiting from this, the modified g-C3N4 shows excellent conversion and selectivity in organic synthesis. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.